DATE DUE

1 OCT 2 9 1974

i

!

30 501 JOSTEN-S

Organic coatings in the form of paints, varnishes, lacquers, and related products are used for

protective, decorative, and functional purposes on a wide variety of man's products—from chil- dren's toys to guided missiles. On rockets and spacecraft, paints provide identification, environmental protection, and temperature control.

UNITED STATES DEPARTMENT OF COMMERCE"'"'

Alexander B. Trowbridge, Secretary '~~T A

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

A/7

Organic Coatings

Properties, Selection, and Use

Aaron Gene jRobeils

Building Research Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Building Science Series 7

Issued February 1968

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price $2.50
Abstract

This publication was prepared to fill the need for a comprehensive, unifying treatise in the field of organic coatings. Besides presenting practical information on the properties, selection, and use of organic coatings (and certain inorganic coatings), it provides basic principles in a number of important

areas such as polymer structure, coatings formulation, pigment function, use of thinners, coating system

compatibility, and theory of corrosion. Each chapter deals with a major area of the coatings field, including types of coatings, properties of synthetic resins, selection of coating systems, storage and

safety, application methods, and surface preparation and pretreatment. There is also a consolidating

chapter with illustrative examples of solutions to typical coatings problems. Interrelationships among

the various areas of information are indicated through appropriate cross-referencing in the text. Specific references to Federal, Military, and other specifications are given where pertinent, and an entire chapter

is devoted to a quick guide and summary of Federal specifications for organic coating materials.

Finally, a selected bibliography and a comprehensive index are provided. While written primarily to meet the informational needs of the engineer, architect, maintenance superintendent, and responsible

coatings procurement officer, the treatise is sufficiently broad in scope to serve as a general manual,

a concise text, or a convenient reference source in the field of organic coatings.

Key words: application, coatings, coating systems, corrosion-inhibiting. Federal specifications,

fire-retardant, lacquer, latex, multicolor, organic coatings, paint, pigments, polymers, properties,

resins, selection, substrates, surface preparation, varnish, water-thinned.

Library of Congress Catalog Card No. 66-61927

II
Foreword

Since publication in 1945 of the Paint Manual issued as National Bureau of Standards

Building Materials and Structures Report 105, there have been many significant technological advances in the field of organic coatings. Materials and techniques that were

nonexistent or only dimly foreseen twenty years ago are now in daily use. Coatings

with new and improved properties emerge continually from research laboratories and

production lines. The task of choosing wisely from this huge array of materials is a complex and difficult one that requires both a knowledge of what is available and an

understanding of where and how these materials and techniques can best be utilized.

The present publication was written to fill this need.

Through careful selection and orderly presentation of information, it has been

possible to include a broad range of subject matter not previously available within a

single volume. Practical information is given on the properties, selection, and application

of organic coatings, including methods of adequate surface preparation. Basic principles

also are discussed. Thus, a description is given of the polymeric nature of the synthetic

resins that are the basis for most of our modern coating systems, and relationships

between polymer structure and properties are indicated. A unique feature which should

prove useful to those concerned with the procurement or supply of organic coatings for

the Government is a quick guide to Federal specifications for these materials in the form

of a classified table and a series of summarizing charts.

Names and descriptions of some specific proprietary products are included for the

convenience of the user. The omission of any particular product does not imply that it is considered unsuitable or unsatisfactory. Conversely, inclusion of a particular proprietary product does not constitute endorsement. Photographs of proprietary instrumentation and apparatus are included for illustrative and educational purposes and do not

constitute approval or endorsement of the particular product. Use of the information

given herein for advertising or publicity purposes is not authorized.

For the architect, engineer, maintenance superintendent, procurement officer, and

others responsible for the proper selection and application of organic coating systems,

this volume should be useful as a general manual, a unifying text, or a convenient source

of authoritative information.

A. V. AsTiN, Director.

m

Preface

Paints of the pigment-in-oil type have long provided man with durable protective

and decorative coatings for his houses and other structures. Such paints, improved and

embellished with new pigments in a wide variety of colors, are useful even today. In

recent years, however, paralleling the tremendous growth of the plastics industry,

conventional paints have given way more and more to new and often superior coatings

based on the new synthetic resins and the new formulating techniques that have been

developed. The list of available and useful film-forming materials is almost as long as

the roster of different plastics, resins, and cellulose derivatives. These materials, singly or in physical admixture or chemical combination, have been modified with a wide variety

of pigments, plasticizers, extenders, activators, stabilizers, solvents, and diluents to produce an almost endless proliferation of coating materials, each with its own set of

useful properties. The information problem confronting those who must choose wisely

from this bewildering array is indeed a formidable one.

It is a basic purpose of this publication to familiarize responsible personnel in the

Federal establishment and elsewhere with the types of coating materials that are available,

and to organize the essential information concerning such materials in a manner that

will permit the selection and application of an adequate coating system for a particular

job without requiring that the individuals responsible be or become expert coatings

technologists.

The sheer magnitude of the problem precludes a discussion or even merely a listing

of every material; nor would such an approach be desirable in anything but an encyclopedic treatment of the subject. Instead, the guiding philosophy throughout the preparation of this volume has been to anticipate and to meet the practical informational needs

of the engineer, architect, maintenance superintendent, and coatings procurement officer

while keeping in mind the need for conciseness and clarity.

This volume is not intended to take the place of the many excellent manuals tailored

in detail to the specific needs of a specialized Government agency or activity. Rather, it offers a broad survey of the field of organic coatings that will serve to orient the reader

and may be useful in indicating areas in which available manuals might advantageously

be modified to utilize the newer materials and techniques. The information has been

selected and presented in a manner that will provide maximum usefulness when the

monograph is used as an ajunct to the handbooks and manuals of particular agencies

to provide basic information and practical working details not usually found in the more

specialized manuals.

The author wishes to express his deep appreciation to Paul T. Howard, Head, Organic

Coatings Group, National Bureau of Standards, for his encouragement, critical review,

and many constructive suggestions throughout the preparation of this work. Grateful

acknowledgment is made to Dr. S. G. Weissberg of this Bureau for his scholarly review

of the completed manuscript. Other NBS colleagues to whom the author is indebted are

Dr. A, F. Robertson, for his review of the information on fire-retardant coatings;

Dr. R. Florin, for reviewing the section on effects of high energy radiation; W. C. Cullen,

for his review of the section on bituminous coatings; and Dr. W. W. Walton, for his

overall review of the manuscript. The author wishes to thank J. H. Geyer of Amchem

Products, Inc., G. R. Hoover of Armco Steel Corporation, and M. B. Roosa of Parker

Rust Proof Division, Hooker Chemical Corporation, for their valued review of the material on pretreatment of steel, aluminum, and galvanized surfaces. Appreciation is expressed

also to Anthony Gallaccio, Frankford Arsenal, for his constructive review of the section on surface preparation and pretreatment of magnesium and its alloys. Finally, the

author's great debt to the literature on organic coatings is humbly acknowledged.

It is hoped that this volume will contribute tangibly to the knowledge and perspective

so essential for the wise choice and proper application of modern coating systems.

A. G. Roberts.

Washington, D.C.

Contents

Page Abstract ii Foreword III Preface v

1. Introduction 1

1.1. Scope 1

1.2. Basic objectives and arrangement of information 1 1.3. Structure and use of the index 2

1.4. Historical background ^ 3

1.4.1. Early history of varnish and paint 3

1.4.2. Twentieth-century developments 3

2. Types of coatings 5

2.1. General description and basis for classification 5

2.2. Clear coatings 5

2.2.1. Drying oils 5

2.2.1.1. Linseed 6

2.2.1.2. Tung 6

2.2.1.3. Dehydrated castor 6

2.2.1.4. Perilla, oiticica 6

2.2.1.5. Refined fish oils 6

2.2.1.6. Semi-drying oils 6

2.2.1.7. Non-drying oils 6

2.2.1.8. Tall oil 6

2.2.2. Varnish 6

2.2.3. Spirit varnish 7

2.2.4. Lacquer , 7

2.2.5. Sealer 7

2.2.6. Wax polishes 8

2.2.7. Strippable coatings 8

2.2.8. Synthetic resin coatings 9

2.3. Pigmented coatings 9

2.3.1. Pigments 9

2.3.1.1. Pigment function 9

2.3.1.2. White hiding pigments 9

2.3.1.3. Extender pigments 10

2.3.1.4. Color pigments 10

2.3.1.4.1. Natural 10

2.3.1.4.2. Synthetic inorganic 11

2.3.1.4.3. Synthetic organic 12

2.3.1.5. Corrosion-inhibiting pigments 13

2.3.1.6. Metallic pigments 13

2.3.1.6.1. Aluminum 13

2.3.1.6.2. MetalHc copper and its alloys 13

2.3.1.6.3. Zinc dust 14

2.3.1.6.4. Gold and silver flake 14

2.3.1.6.5. Stainless steel flake 14

2.3.1.6.6. Other metallic pigments 14

2.3.1.7. Black pigments 14

2.3.1.8. Fluorescent pigments 14

2.3.2. Oil paints 15

2.3.2.1. Straight oil paints 15

2.3.2.2. Oleoresinous paints . 15

2.3.3. Pigmented lacquers 15

2.3.4. Enamels 16

2.3.5. Latex paints 16

2.3.5.1. General 16

2.3.5.2. Forerunners 16

2.3.5.3. Uses 17

2.3.5.4. Formulation 17

vn
Contents Pae^

2.3.5.5. Film fonnation 18

2.3.5.6. Chemical types 18

2.3.5.6.1. Styrene-butadiene 18

2.3.5.6.2. Polyvinyl acetate 19

2.3.5.6.3. Acrylic 19

2.3.5.6.4. Latex blends and copolymers 19

2.3.6. Water-solution coatings 20

2.3.7. Multicolor finishes 21

2.3.8. Fire-retardant coatings 21

2.3.8.1. General 21

2.3.8.2. Flame-retardant coatings 22

2.3.8.3. Insulative fire-retardant coatings 22

2.3.8.4. Evaluation of fire-retardant coatings 23

2.3.8.5. Selection of fire-retardant coatings systems 23

2.3.8.6. Summary 24

2.3.9. Cold water paints 25

2.3.9.1. Cement-water paints 25

2.3.9.2. Whitewash 25

2.4. Primers 26

2.4.1. Wood primers 26

2.4.2. Metal primers 26

2.4.2.1. Primers for iron and steel 27

2.4.2.2. Primers for galvanized iron and steel 27

2.4.2.3. Primers for aluminum 27

2.4.2.4. Primers for magnesium 27

2.4.3. Masonry primers 28

2.4.4. Primers for plaster and wallboard 28

2.5. Bituminous coatings 29

2.5.1. Hot applied 29

2.5.2. Cut-backs 29

2.5.3. Paints and varnishes 29

2.5.4. Emulsions 29

2.5.5. References 30

3. Properties of synthetic resins for coatings 33

3.1. General nature of synthetic resins 33

3.1.1. Polymer structure and properties 33

3.1.2. Functionality and polymerization mechanisms 34

3.1.2.1. Functionality 34

3.1.2.2. Addition polymerization 34

3.1.2.3. Condensation polymerization 35

3.1.3. Methods of polymerization 35

3.1.3.1. Bulk polymerization 35

3.1.3.2. Solution 35

3.1.3.3. Emulsion 36

3.1.3.4. Suspension 36

3.1.4. Types of polymers 36

3.1.4.1. Homopolymer 36

3.1.4.2. Copolymer 36

3.1.4.3. Block polymer 37

3.1.4.4. Graft polymer 37

3.1.4.5. Stereoregular polymers 37

3.1.5. Classification of synthetic resins 37

3.2. Cellulose derivatives 38

3.2.1. Cellulose esters 38

3.2.1.1. Cellulose nitrate 38

3.2.1.2. Cellulose acetate 39

3.2.1.3. Cellulose propionate 40

3.2.1.4. Cellulose butyrate 40

mi
Contents

3.2.2. Cellulose mixed esters 40

3.2.2.1. Cellulose acetate-butyrate 40

Z.I.I.Z. Cellulose acetate-propionate 40

3.2.3. Cellulose ethers 40

3.2.3.1. Ethyl cellulose 40

3.2.3.2. Methyl cellulose 41

3.2.3.3. Carboxymethyl cellulose 41

3.2.3.4. Hydroxyethyl cellulose 41

3.2.3.5. Benzyl cellulose 41

3.3. Phenolic resins 41

3.3.1. Modified phenolic resins, oil-soluble 42

3.3.2. Pure phenolic resins (100% phenolic) 42

3.3.2.1. Oil-soluble, thermoplastic 42

3.3.2.2. Oil-soluble, heat-reactive 42

3.3.2.3. Alcohol-soluble, oil-insoluble, thermosetting 43

3.3.3. Phenolic dispersion resins 43

3.4. Alkyd resins 43

3.4.1. Composition and properties of alkyd resins 43

3.4.1.1. General 43

3.4.1.2. Effect of oil-modification 44

3.4.1.3. Uses 44

3.4.2. Resin-modified alkyds 45

3.4.2.1. Styrenated-alkyds 45

3.4.2.2. Silicone-alkyds 45

3.4.2.3. Other resins with which alkyds may be combined. _ 45

3.4.3. Polyester resins 45

3.5. Amino resins 46

3.5.1. Properties 46

3.5.2. Formation 46

3.5.3. Uses 46

3.5.4. Comparison of urea and raelamine resins 47

3.6. Vinyl resins 47

3.6.1. Polyvinyl acetate 47

3.6.2. Polyvinyl chloride 48

3.6.2.1. General properties 48

3.6.2.2. Plastisols and organosols 48

3.6.3. Polyvinyl chloride-acetate 48

3.6.4. Polyvinyl alcohol 49

3.6.5. Polyvinyl acetals 50

3.6.5.1. Polyvinyl formal 50

3.6.5.2. Polyvinyl acetal 50

3.6.5.3. Polyvinyl butyral 50

3.6.6. Polyvinylidene chloride and its copolymers 51

3.6.6.1. Polyvinylidene chloride 51

3.6.6.2. Polyvinylidene chloride copolymers (general) 51

3.6.6.3. Vinylidene chloride-vinyl chloride copolymers 51

3.6.6.4. Vinylidene chloride-acrylonitrile copolymers 51

3.6.7. Polyvinyl ethers 52

3.7. Acrylic resins 52

3.7.1. Properties 52

3.7.2. Uses 52

3.8. Polystyrene and its copolymers 53

3.8.1. Polystyrene homopolymer 53

3.8.2. Styrene-butadiene copolymers 53

3.8.3. Styrenated oils and alkyds 54

3.8.4. Other styrene derivatives 54

3.8.4.1. Copolymers 54

3.8.4.2. a-Methyl styrene 54

3.8.4.3. Vinyl toluene polymers 54

IX
Contents ^as*

3.9. Silicone resins 54

3.9.1. Pure silicone resins 54

3.9.2. Modified silicone resins 55

3.9.2.1. Silicone-alkyds 55

3.9.2.2. Other silicone-modified resins 55

3.10. Epoxy resins 55

3.10.1. Unmodified epoxy baking resins 56

3.10.2. Unmodified epoxy air-dry resins 56

3.10.3. Esterified epoxy resins 56

3.11. Polyamide resins 57

3.11.1. Nylon resins 57

3.11.2. Thermoplastic polyamides 57

3.11.3. Thermosetting polyamides 58

3.11.3.1. Epoxy-polyamides 58

3.12. Polyurethanes 58

3.12.1. Two-component urethane systems 59

3.12.1.1. Prepolymer type 59

3.12.1.2. Polyisocyanate type 59

3.12.2. Single-package urethane systems 59

3.12.2.1. Heat-cured 59

3.12.2.2. Air-cured . 60

3.12.2.3. Moisture-cured 60

3.12.3. Properties and uses of the polyurethanes 60

3.13. Polyethylene 60

3.13.1. Properties and uses of low- and high-density polyethylene 60

3.13.2. Irradiated polyethylene 61

3.13.3. Chlorosulfonated polyethylene 61

3.14. Fluorocarbon resins 62

3.14.1. Polytetrafluoroethylene 62

3.14.2. Polychlorotrifluoroethylene (Kel F) 63

3.15. Synthetic rubbers 64

3.15.1. Polychloroprene (neoprene) 64

3.15.2. Chlorosulfonated polyethylene 65

3.15.3. Styrene-butadiene rubber (SBR) 65

3.15.4. Butadiene-acrylonitrile rubber (nitrile rubber) 65

3.15.5. Isobutylene-isoprene copolymer (butyl rubber) 65

3.15.6. Polysulfide rubber 66

3.15.7. Silicone rubber 66

3.15.8. Polyurethane rubber 66

3.15.9. Chlorinated rubber 66

3.15.10. Polyacrylic ester rubber 67

3.15.11. Cyclized rubber 67

3.16. Water-soluble resins 67

3.16.1. Thermoplastic types 67

3.16.2. Thermosetting types 67

3.17. Miscellaneous resins 68

3.17.1. Resin combinations 68

3.17.2. Resins discussed elsewhere in text 68

3.17.3. Natural resins 68

3.17.4. Furan resins 68

3.17.5. Polypropylene 68

3.17.6. Polycarbonate resin (Lexan) 69

4. Selection of coating systems 71

4.1. Basis for selection 71

4.1.1. Service environment 72

4.1.2. Nature of substrate 72

4.1.3. Basic function.. 73

4.1.4. Appearance 73

X
Contents

4.1.4.1. Clarity and whiteness 74

4.1.4.2. Translucent coatings 74

4.1.4.3. Opacity 74

4.1.4.4. Gloss 74

4.1.4.5. Textured coatings 75

4.1.4.5.1. Wrinkle 75

4.1.4.5.2. Crackle 75

4.1.4.5.3. Pebble 75

4.1.4.5.4. Hammertone 75

4.1.4.5.5. Multicolor 75

4.1.5. Application limitations 76

4.1.6. Cost 76

4.2. Coating systems for wood 77

4.2.1. Exterior coatings for wood 77

4.2.2. Interior coatings for wood 78

4.3. Coating systems for metals 79

4.3.1. Coatings for structural steel 1 80

4.3.1.1. Oil-base and oleoresinous systems 80

4.3.1.1.1. Intermediate and finish coats 81

4.3.1.2. Vinyl systems 81

4.3.1.3. Bituminous coatings 82

4.3.1.4. Galvanized steel 82

4.3.2. Industrial coatings for steel 83

4.3.3. Coatings for other metals 83

4.4. Coatings for concrete and masonry 84

4.4.1. Cement-water paints 84

4.4.2. Oleoresinous paints 84

4.4.3. Latex paints 84

4.4.4. Synthetic resin coatings 85

4.5. Coatings for plaster and wallboard 85

4.6. Functional coatings 86

4.6.1. Chemical resistance 86

4.6.1.1. Acid resistance 87

4.6.1.2. Alkali resistance 87

4.6.1.3. Oxidation resistance 87

4.6.1.4. Resistance to organic solvents .. 87

4.6.1.5. Water resistance 87

4.6.2. Heat resistance 88

4.6.3. Abrasion and mar resistance 88

4.6.4. Electrical properties 88

4.6.5. Impermeability 88

4.6.6. Transparency 88

4.6.7. Fire resistance 88

4.6.8. Radiation resistance 88

4.7. Summary 90

5. Storage, safety, and application methods 91

5.1. Storage of coating materials 91

5.2. Safety precautions 91

5.2.1. Toxicity 91

5.2.2. Fire 92

5.2.3. Falls and spillage 93

5.3. Mixing, thinning, and activation 93

5.3.1. Mixing of paints 93

5.3.2. Use of thinners 94

5.3.2.1. Principles of proper thinning 95

5.3.3. Activation of two-package systems 96

5.4. AppUcation methods and equipment 96

5.4.1. Brush application 96

XI
Contents ^^^e

5.4.2. Roller application 97

5.4.3. Spray application 98

5.4.3.1. Conventional cold spray 98

5.4.3.2. Airless spray 99

5.4.3.3. Hot spray 100

5.4.3.4. Flame spraying 100

5.4.3.5. Electrostatic spray 100

5.4.3.5.1. Liquid 100

5.4.3.5.2. Airless 101

5.4.3.5.3. Dry 101

5.4.3.6. Dual-gun spraying 101

5.4.4. Dipping and flow coating 102

5.4.4.1. Dipping 102

5.4.4.2. Flow coating 102

5.4.5. Knife and roller coating 102

5.4.5.1. Knife coating 102

5.4.5.2. Roller coating 108

5.4.6. Fluidized bed coating 103

6. Surface preparation and pretreatment 105

6.1. Importance of surface preparation and pretreatment 105

6.2. Preparation of wood surfaces 106

6.2.1. Bare wood 106

6.2.1.1. General 106

6.2.1.2. Exterior wood 106

6.2.1.3. Interior wood 106

6.2.2. Previously painted wood 107

6.3. Preparation and pretreatment of metal surfaces 108

6.3.1. Cleaning of steel (and iron) surfaces 108

6.3.1.1. Solvent cleaning 108

6.3.1.1.1. Wiping, dipping, or spraying 109

6.3.1.1.2. Vapor degreasing 109

6.3.1.2. Steam cleaning 109

6.3.1.3. Emulsion cleaning 109

6.3.1.4. Alkali cleaning 110

6.3.1.5. Acid cleaning or pickling 110

6.3.1.5.1. Phosphoric acid cleaning 110

6.3.1.5.2. Pickling 111

6.3.1.6. Hand cleaning 111

6.3.1.7. Power tool cleaning 112

6.3.1.8. Sandblasting 112

6.3.1.8.1. Sandblasting to "white" metal 112

6.3.1.8.2. Commercial blast cleaning 113

6.3.1.8.3. "Near-white" blast cleaning 113

6.3.1.8.4. "Brush-off" blast cleaning 113

6.3.1.8.5. Wet sandblasting 113

6.3.1.8.6. Dry sandblasting equipment and

techniques 113

6.3.1.8.7. Vacuum blast cleaning 113

6.3.1.8.8. Cost and production rate 113

6.3.1.8.9. Safety 114

6.3.1.8.10. After sandblasting 114

6.3.1.9. Flame cleaning 114

6.3.2. Chemical pretreatment of steel (and iron) surfaces 115

6.3.2.1. Cold phosphate pretreatment (phosphoric acid type) 115

6.3.2.2. Cold phosphate pretreatment (zinc phosphate type) 116

6.3.2.3. Hot phosphate pretreatment (zinc phosphate type) 116

6.3.2.4. Hot phosphate pretreatment (iron phosphate type) 117

6.3.2.5. Organic pretreatment coating (wash primer) 117

xn
Contents

6.3.2.5J.. Two-component wash primer 117

6.3.2.5.2. Single-package wash primers 118

6.3.3. Galvanized surfaces 118

6.3.3.1. New galvanized surfaces 118

6.3.3.2. Old galvanized surfaces 119

6.3.3.3. Cadmium surfaces 119

6.3.4. Aluminum and its alloys 119

6.3.5. Magnesium and its alloys 120

6.3.5.1. Cleaning 120

6.3.5.1.1. Electrochemical cleaning 120

6.3.5.1.2. Alkaline cleaning 120

6.3.5.1.3. Acid pickling 121

6.3.5.1.4. Mechanical cleaning methods 121

6.3.5.2. Chemical pretreatments 121

6.3.5.2.1. Type I chrome-pickle treatment 121

6.3.5.2.2. Type II sealed chrome-pickle treat- ment 121

6.3.5.2.3. Type III dichromate treatment 122

6.3.5.2.4. Type IV galvanic anodizing treat- ment 122

6.3.5.2.5. Type V caustic anodizing treatment. 122

6.3.5.3. Anodic or electrochemical treatments 122

6.3.5.3.1. The HAE process 122

6.3.5.3.2. Acid chromate anodic process 123

6.3.6. Other metals 124

6.3.6.1. General 124

6.3.6.1.1. The cleaning process 124

6.3.6.1.2. Wash primer type of pretreatment... 124

6.3.6.1.3. Proprietary chromate treatments 124

6.3.6.2. Zinc and cadmium 124

6.3.6.3. Copper, brass, and bronze 124

6.3.6.4. Silver 124

6.3.6.5. Tin 124

6.3.6.6. Terneplate 125

6.3.6.7. Lead 125

6.3.6.8. Titanium 125

6.3.6.9. Stainless steel, nickel, and chromium 125

6.4. Preparation of concrete and masonry surfaces 125

6.4.1. Aging and weathering 125

6.4.1.1. Latex (water emulsion) paints 126

6.4.1.2. Cement-water paints 126

6.4.1.3. Alkali-resistant synthetic resin coatings 126

6.4.1.4. Oil-base and oleoresinous paints 126

6.4.2. Cleaning 126

6.4.2.1. Dust, dirt, and loose material 126

6.4.2.2. Form oil 126

6.4.2.3. Efflorescence 127

6.4.2.4. Mildew or mold 127

6.4.3. Patching, grouting, and filling 127

6.4.4. Roughening 127

6.4.5. Pretreatment 127

6.4.6. Conditioning of chalky surfaces 128

6.4.7. Summary 128

6.5. Preparation of plaster and wallboard surfaces 128

6.5.1. Plaster 128

6.5.1.1. New Plaster 128

6.5.1.2. Old plaster 128

6.5.2. WaUboard 129

6.5.2.1. Gypsum wallboard 129

6.5.2.2. Fiberboard, hardboard 129 xm
Contents

6.6. Preparation of previously painted surfaces 129

6.6.1. Maintenance painting 129

6.6.2. Previously painted wood surfaces 130

6.6.3. Previously painted metal surfaces 130

6.6.4. Previously painted concrete and masonry 130

6.6.4.1. Latex (emulsion) paint 130

6.6.4.2. Solvent-thinned paints 131

6.6.4.3. Cement-water paints 131

6.6.5. Previously painted plaster and wallboard surfaces 131

6.7. Summary 132

7. Consolidation of coatings information 135

7.1. A glance backward and forward 135

7.2. Supplementary information 135

7.2.1. Painting conditions 135

7.2.2. General coating requirements 136

-• ' 7.2.2.1. Thickness 136

7.2.2.1.1. Spreading rate 136

7.2.2.1.2. Thickness measurement 137

7.2.2.2. Drying time 138

7.2.2.3. Coating system compatibility. 139

7.2.2.4. Workmanship and general appearance 140

7.2.3. Application of corrosion-inhibitive primers 140

7.2.4. Helpful hints 141

7.2.4.1. Wetting properties of common coatings 141

7.2.4.2. Sources of specifications 141

7.2.4.2.1. Federal specifications 141

7.2.4.2.2. Military specifications 141

7.2.4.2.3. Federal test methods 141

7.2.4.2.4. ASTM test methods 141

- 7.2.4.2.5. ASA standards 141

7.2.4.3. Application hints. . 141

7.2.4.3.1. When applying multicoat paint sys- tems 141

7.2.4.3.2. When painting metal 142

7.2.4.3.3. When applying latex paints 142

7.2.4.3.4. Vinyl coating systems 142

7.2.5. Test equipment and test methods 142

7.2.6. Paint film defects and failures 142

7.2.7. Basic information (table) 142

7.3. Use of the text to solve specific coating problems 143

7.3.1. Selection and application of a suitable coating system for new

exterior wood siding on a home in an ordinary urban environ- ment 143

7.3.2. Selection and application of a coating system for structural steel to be exposed in a moderate industrial environment 143

7.3.3. Selection and application of a coating system for galvanized

steel to be exposed in an ordinary environment 144

7.3.4. Selection of a coating system for the inside of a potable water tank 144

7.3.5. Selection and application of a coating for a cinderblock base- ment wall 144

7.3.6. Selection and application of a coating system for exterior concrete 144

7.3.7. Coating system for gypsum wallboard (dry wall) 144

7.3.8. Selection and application of a coating system for aluminum

exposed to a moderately corrosive atmosphere 145

7.3.9. Coating for wooden gymnasium floor 145

7.3.10. Acid-resistant coating 145

7.3.11. Fire-retardant coating system 145

7.4. Summary and conclusion 145

XIV
Contents

8. Summary of Federal specifications for organic coatings.- 149

8.1. Quick guide to Federal specifications for organic coating materials 149

8.1.1. Enamels 149

8.1.2. Lacquers 149

8.1.3. Paints 149

8.1.4. Primers 149

8.1.5. Sealers 150

8.1.6. Stains 150

8.1.7. Varnishes 150

8.1.8. Miscellaneous coatings 150

8.1.9. Accessory materials 150

8.2. Tabular summary of important Federal specifications for organic coatings _ 150

8.2.1. Description and use 150

8.2.2. Quick-glance summary tables 152

9. Selected bibliography , 169

9.1. Specific references 169

9.2. General references 172

10. Index 175

XV
List of Illustrarions

Frontispiece. Rocket in flight A Figure 1.1. Early varnish-making 4 Figure 1.2. Modern phthaHc anhydride plant 4 Figure 2.1. Integrometer increases speed and precision of adhesion measurements 30 Figure 2.2. Radiant panel flame spread apparatus 31 Figure 2.3. Explosion of cellulose nitrate film in storage 31 Figure 2.4. Bubble viscometer 32 Figure 2.5. Washability machine 32 Figure 3.1. Jet abrader 69 Figure 3.2. Parallel-grove adhesion tester 70 Figure 3.3. Infrared spectrophotometer 70 Figure 4.1. Xenon-arc accelerated weathering machine 90 Figure 4.2. Carbon-arc accelerated weathering machine 90 Figure 5.1. Coil-coating inspection station 104 Figure 5.2. Electrostatic spray gun paints chain-link fence 104 Figure 6.1. Glossmeter 132 Figure 6.2. Portable slipperiness tester 133 Figure 6.3. Testing machine for stress-strain measurements 133 FiGtmB 6.4. Spreading grout coat onto concrete surface 134 Figure 6.5. Trowel finishing mortar topping on concrete surface 134

Figure 7.1. Eddy-current non-destructive thickness gage 146 Figure 7.2. Gage for thickness measurement on magnetic substrates 146 Figure 7.3. Wet film thickness gage 147 Figure 7.4. Balanced-beam scrape adhesion tester 147 Figure 7.5. Indentation hardness tester 148 FiGtmE 7.6. Hardness rocker 148

Figure 8.1. , Improved dip-coater 150

Figure 8.2. Modern exposure racks 151 Figure 8.3. Cinder-block test house 168 Figure 8.4. Masonry wall specimen 168

XVI
1. Introduction

1.1. Scope

Surfaces are painted to protect them from

harmful agents in their environment, to decorate them, or to provide some functional purpose such as light diffusion or reflection, heat

reflection or absorption, abrasion resistance,

fire retardance, cleanliness, imperviousness, or

identification. Whatever the objective of painting, it is successfully achieved only to the extent

that the protective, decorative, or functional

coating remains bonded to its substrate and

retains its essential properties. To ensure this

requires careful selection of the proper coating

system, scrupulous attention to surface preparation, skilled application, and adequate maintenance. It is the purpose of this manual to provide sufficient information relating to these

requirements to enable those responsible for the

maintenance of structures and equipment to

select the proper coating material for a particular application, and to provide those who must

apply and maintain these coatings with suffi- cient detailed information to do the job in a manner that will ensure adequate adhesion and

durability.

The subject matter is not limited to conventional paints but also includes pigmented and

clear coatings based on a wide variety of cellulosic and resinous film-forming materials that

have become available in recent years. The

term "paint" is often used in its broadest sense

to denote a liquid material that has the property, when applied in a thin layer, of hardening

into a continuous film that adheres to the substrate; in this sense, any organic coating

material may be considered a paint. A few

inorganic coating materials, such as cementwater paints and phosphate coatings, also have

been considered because of their important

uses in the coatings field.

1.2. Basic Objectives and Arrangement of Information

This volume seeks to provide coverage in

depth of the organic coatings field by giving

basic principles and information as well as

practical working details. Through treatment

of both the fundamental and practical aspects

of the subject, it is hoped that the volume will

be found useful as a working manual, as a concise textbook, or as a convenient reference

source on the properties, selection, and use of

organic coatings.

It has been neither feasible nor desirable to specify a particular coating system for every

application or to give individual instructions,

without explanatory background, for handling

every situation. Such a presentation, while

appropriate and useful for specialized manuals

of limited scope, would involve almost endless

repetition of detail if applied to the entire field of organic coatings. Instead, the objective has

been to provide an adequate basic understanding of the subject that would make repetition

of detail unnecessary. Toward this end, each

chapter deals with a major area of the coatings

field. Cross references within the text show

interrelationships among areas of information

while serving to avoid repetition and conserve

space. Specific references to Federal, Military,

and other specifications are given at appropriate places, and an entire chapter has been

devoted to a quick guide and summary of

Federal specifications for organic coating materials. A consolidating chapter provides supplementary information and furnishes examples

of use of the volume in solving typical coating

problems.

With the wide variety of coating materials

available today, the solution to a particular

coatings problem is seldom a unique one. This

publication has not presumed to make those

decisions which are the prerogative of the

1
responsible agency or coatings official. Instead,

it has sought to provide reliable and comprehensive information upon which such decisions

might be based. By treating each broad aspect

of coatings technology in one unified portion of

the text, repetition of detail has been kept to a minimum and it has been possible to broaden

the scope of information that could be included.

Given such information in a well-indexed and

accessible form the reader can readily extract

the information he needs and arrange it himself

in whatever sequence best suits his purpose.

1.3. Structure and

The paramount objective of any index should

be to lead the user quickly to all the information

that pertains to his problem. Toward this end,

entries have not merely been listed but have

been organized under a system of headings and

subheads that not only direct the user to the

immediate subject matter but also make the

index itself a source of integrated information

that can lead him to pertinent related areas. To achieve adequacy of coverage and cross- referencing without undue repetition, a "pyramidal" arrangement of entries has been

employed in which the user is directed to successively more detailed branches until the subject

has been fully covered. Two examples will serve to illustrate how this arrangement provides maximum utility with minimal redundancy. It is suggested that the reader actually

follow the examples through the index.

Example 1 : Suppose it is desired to obtain

detailed information on clear coatings. One

might first look for this under the general heading "Coatings", with the expectation of finding "clear" as one of the sub-entries under it. This

is actually the way it appears in the index; reference is made to a single set of pages on clear coatings, with no further breakdown at this point

(to subdivide the index further at this point

would make the already long listing under "Coatings" very cumbersome, especially if such sub- dividing were applied generally to the other

entries). The main heading, however, directs the user to "See also . . . under specific coating type",

in this instance under "Clear coatings", where he

finds a listing of the various types of clear coat- ings (drying oils, lacquer, varnish, etc.) as well as a general reference to their "durability compared to pigmented" types. Again, the user is directed to "See also under specific type" if he wishes further details; thus, he can look under

"Drying oils" to find page references to types of drying oils such as boiled or raw oils, or to specific

oils such as linseed or tung oil. Here, again, the user is told to "See . . . under specific oil" where

he is given a final breakdown of detailed information; for example, under "Linseed oil" he will find references to its use in alkyd resins and in house

paints, and he will learn (if he is not already

In presenting information on the properties,

selection, and use of organic coatings, the volume has endeavored to be a comprehensive

guide rather than a specification. It should be

understood, therefore, that when the general

or basic information presented herein differs

from the stipulated 7equirements of a particular

specification or from the specific instructions of

the manufacturer of a particular product, the

requirements of the specification or the manufacturer's instructions normally should take

precedence.

Use of the Index

aware of it) that a water-soluble type of linseed

oil is available; under "Tung oil" he will be di- rected to information on its use in spar varnishes,

in wrinkle finishes, etc. It is seen from the preceding example that even

the user with only a general knowledge of clear coatings would be led by the index to all the rele- vant information in the text, through a series of expanding steps; and that he would learn about

the basic types of clear coatings and of many

specific materials along the way. Alternatively, the user with specific informational needs could enter the index at any stage in the interrelated chain of entries with equal success. Example 2: Let us consider briefiy the general way in which the index deals with the complex and voluminous subject of surface preparation. In this case, it has been desirable and

feasible to employ the "pyramidal" index struc- ture largely under the main heading "Surface

preparation for painting", and the bulk of the information on this subject is developed here. Major subheads provide references to general

principles and methods and to each type of substrate (concrete, metals, wood, etc.), and these

categories are further developed through detailed

sub-entries. Cross references direct the user to "See also Surface pre-treatment, specific clean- ing method, and specific substrate". The comprehensive listing under "Surface preparation"

together with the cross references guide the user

to all the pertinent information.

Attention is invited to the detailed treatment

given to the subject "Coatings for" in the index.

While neither the list nor the accompanying

page references are exhaustive in nature, they

provide an entree to places in the text which

mention coating materials for many specific

applications.

A formal glossary was considered beyond the

scope of the present volume ; however, the index

will often serve in lieu of one to direct the user

to places in the text where paint terms or concepts are defined or discussed.

Although practical considerations have necessarily limited the space that could be devoted

to indexing, it may be apparent from the foregoing that the index is not a perfunctory one. The reader is urged to make full use of it.

2
1.4. Historical Background

1.4.1. Early History of Varnish and Paint

Varnish much like that which we know today was made by the Egyptians as early as 500

B.C. [1].^ Some of it still endures today as a coating for their mummy cases. Their varnish was made of resin and oil and evidently was applied in a viscous, unthinned form with

a blade or with the fingers. A durable varnish

made by cooking amber resin with linseed oil

is described in the works of Theophilus in the

tenth century A.D. Theophilus ascribed his

formulas to earlier compilations dating back to the eighth and the first centuries. His varnish

formula remained the standard of the art for

another 800 years. Turpentine, though known

since early times, was used for other purposes

(for example, as an embalming fluid) rather

than as a varnish component or thinner until

about the fourteenth or fifteenth century. The

first technical treatise on the paint and varnish

industry as it is known today was prepared by

Watin in 1772. He described the formulation

and manufacture of oleoresinous varnishes,

spirit varnishes, and resin dispersions in turpentine. His book went through many editions

with only minor changes and remained the

standard of the varnish-making art until the

turn of the twentieth century.

Paint in the form of finely divided solid

particles (pigment) suspended in a fluid me- dium has been used since prehistoric times for

decorative purposes. Protective and decorative

paints using egg yolk or albumen, gum arable,

and beeswax as vehicles or binders were made

for many centuries. Pigments often were sus- pended in water, with or without binder. For more durable paints, the pigment was sus- pended in a varnish vehicle like that described

by Theophilus. Straight linseed oil paints,

though slower drying than varnish-base paints, were lower in cost and easier to apply, and

found considerable use during the middle centuries. During most of the nineteenth century,

paints based on white lead-in-oil were widely

used for both interior and exterior purposes.

For many centuries paint formulation was

an art that was carefully guarded and handed

down from one generation to the next. As the

paints were made in small batches using crude

grinding mills and laborious mixing methods,

they were expensive and available only to the

affluent segments of society. Not until the

establishment of the first paint and varnish

factories during the nineteenth century did

paints begin to find their way into general use.

' Figrures in brackets indicate the literature references at the end

of the text.

1.4.2. Twentieth-Century Developments

As with many of the arts of man, the paint

and varnish industry, which had undergone

little change for many centuries, felt the impact

of the scientific and technological awakening

ushered in with the twentieth century. New

pigments, improved drying oils, cellulosics and

synthetic resins, and a variety of modifying

agents began to flow from research laboratories

and production lines in ever-increasing numbers and became the basis for a never-ending

stream of new coating materials.

A few milestones may be mentioned here.

Titanium dioxide was developed as a superior

white pigment with outstanding hiding power

and durability. Tung oil, when heat-treated

at 500 °F (260 °C), acquired excellent properties useful in the manufacture of fast-drying,

durable paints and varnishes. Phenolic resins were developed and in combination with tung

oil yielded varnishes of exceptional water re- sistance and outdoor durability. Linseed and

soybean oil-modified alkyd resins were used in

the manufacture of fast-drying paints with

excellent gloss and weathering properties. The

introduction of quick-drying nitrocellulose lac- quers revolutionized the automobile and furniture finishing industries. Baked urea- and

melamine-formaldehyde resins provided durable, gleaming finishes for automobiles, household appliances, and table tops. Recent years

have seen the development of abrasion- and

water-resistant vinyls, tough and chemically

resistant epoxies, durable acrylics, heat-resistant silicones, hard high-gloss polyurethanes,

and many others. The advent of water emulsion latex paints and convertible water-solution

paints has added yet another dimension to the

variety, usefulness, and complexity of the or- ganic coatings field; the elimination of the

hazards, inconvenience, and expense of organic

solvents in such paints has given tremendous

impetus to their further development.

Most of these new materials are discussed

in the pages which follow. In the interests of an unencumbered and concise presentation, free use is made, throughout this publication, of

common paint and technical terms, e.g., plasticizer, polymerization, copolymer, pigment volume concentration (PVC), extender, gloss,

thermoplastic, emulsion, etc. The meaning of

many of these terms is made clear through discussion at appropriate places in the text, and

the Index will serve in lieu of a glossary to direct the reader to them. Standard definitions

of many terms relating to paint, varnish, lac- quer, and related materials are given in ASTM

D16-59 [115].

3

2. Types of Coatings

2.1. General Description and Basis for Classification

The fundamental constituent of all coating

materials is the film-forming base which at- taches itself to the substrate to which it is applied, and into which are incorporated all the other ingredients that impart to the film

the special properties required for a particular

purpose. In order to adhere to the substrate,

the film must be applied in the form of a liquid

that wets the substrate (see Chapter 6 on sur- face preparation) and then cures to a solid

state. Some film-formers, such as linseed and

other drying oils, are liquids of usable viscosity

initially. Others, such as uncured phenolic or epoxy resins, are viscous liquids which may

require thinning with suitable solvents and

diluents before use. Many (e.g., cellulose derivatives) are solids in fibrous, granular, or powder form; these must be dissolved, dispersed, or suspended in a volatile liquid medium

(vehicle) before they can be applied. Uipon

evaporation of the volatile portion, the solids are deposited in the form of a continuous,

adherent, transparent or translucent film. The film-forming base (often referred to as

the binder in pigmented paints) need not be a simple or single resin. Often it is a copolymer

(see sec. 3.1.4.2), such as polyvinyl chlorideacetate. Or the film-former may be a chemically reacted combination, such as a drying

oil-alkyd. It may also be a physical mixture

of two or more film-forming ingredients, such

as an alkyd-nitrocellulose blend.

The film-forming solution constitutes a liquid

vehicle in which insoluble pigment particles

may be dispersed to give color, opacity, and

increased outdoor durability to the film. Many

other substances may be added to the vehicle

(and if dissolved in it become a part of it) to achieve specific properties in the film—driers to hasten the film curing process, plasticizers to impart flexibility or moisture resistance or

other properties, stabilizers to lessen the deteriorative effects of heat and sunlight, fillers and

extenders to modify gloss, or improve mechanical properties or reduce cost, thinners

(solvents, latent solvents, diluents) to adjust

viscosity and to control the evaporation rate for proper application and flowout, etc. The wide variety of coating materials that are available may be conveniently classified on

the basis of their composition, behavior, and

function, as discussed in the sections which

follow.

2.2. Clear Coatings

2.2.1. Drying Oils

The drying oils used in organic coatings are

derived chiefly from the nuts or seeds of certain

plants and from certain species of fish. Chemically, these are mixed glycerides (glyceryl

esters) of saturated and unsaturated fatty

acids. They have the property, when exposed

in a thin layer to the atmosphere, of "drying"

or hardening into a clear or translucent film by

a process of oxidation and polymerization (see

sec. 3.1.1). An indication of their drying ability (chemical unsaturation) is given by their

Iodine Number.^ When combined with a hard

resin, the drying oils impart their inherent

flexibility, elasticity, and durability to the derived film. The drying oils are useful in paints in both

their raw and variously processed forms. Raw

OILS, besides being the least expensive, possess

the best brushability and the greatest penetrating ability; however, their flow and leveling

properties are inferior to that of the processed

forms. Refined oils have almost the same

brushing and penetrating properties as the raw

oils; however, because they have been purified

by chemical and mechanical treatment, their

clarity and color are improved and they can be

heated to varnish-cooking temperatures without the formation of a sludge that would impair

the drying properties and gloss of the varnish

film. "Boiled" oils are not actually boiled, but

are drying oils which have been heated with

lead, cobalt, and manganese compounds to produce a solution that will have the necessary

drying properties; for practical purposes, the same drying results are achieved by adding

soluble organic salts of these heavy metals to the cold untreated oil. Heat-bodied oils are

refined oils (usually alkali-refined) that have

been held at an elevated temperature (about

500 to 600 °F [260 to 316 °C]) until suflRcient

polymerization has taken place to produce the

desired viscosity. While such bodied oils lack

the easy brushability and excellent penetrating

ability of raw oil, they possess superior flow

and leveling properties, convert to a dry film

more rapidly, and yield films of higher gloss

5
and paler color. Blown oils are those through

which air has been blown at temperatures

around 200 °F (93 °C), resulting in partial

oxidation of the oil with a consequent increase

in its viscosity and in its tolerance for water.

Blown oils are darker in color and less durable

than the heat-bodied oils, but they have excellent flow, leveling, and gloss properties—in

fact, they are so free-flowing that difficulties may arise from pigment settling in containers

and from a tendency to run or sag when applied. Their brushability, penetrating properties, and drying rate resemble those of the

heat-bodied oils.

2.2.1.1. Linseed oil, derived from the seed of

the flax plant, is the most important and most

widely used of the drying oils. In addition to being the only drying oil which is used extensively by itself as a paint vehicle (in exterior

house paints), it is an important constituent of

many high-grade varnishes and oleoresinous

paints. In various combinations with alkyd

resins (see sec. 3.4), it forms one of our most

versatile and useful classes of coating material.

Recently, a water-soluble from of linseed oil has been developed [121].

2.2.1.2. Tung oil is invaluable in imparting

fast drying qualities, excellent water resistance,

and exterior durability to varnishes. Its drying

ability is the result of a highly unsaturated,

conjugated double-bond molecular structure.

Its tendency to form wrinkled films when used

alone can be utilized in the manufacture of

wrinkle finishes (see sec. 4.1.4.4).

2.2.1.3. Dehydrated castor oil somewhat

resembles tung oil in its drying characteristics

but forms a softer, more flexible film with some

tendency (controllable) to develop after-tack.

It has excellent color and color retention properties and is widely used in the paint and

varnish industry.

2.2.1.4. Perilla and oiticica are important

plant-derived oils with fast-drying properties.

Oiticica oil has a conjugated double-bond type

of unsaturation similar to that of tung oil.

2.2.1.5. Refined fish oils have found extensive use in the low cost paint field and in the

formulation of heat-resistant paints for such

applications as smoke-stack exteriors, boiler

fronts, and roofing, where their softness and

elasticity can be used to advantage, and their

tendency to yellow and to develop after-tack

and undesirable odor is not objectionable. Current research on fish oils holds a promise of

better utilization of these materials in the

future.

^ For a description of Iodine Number and a discussion of its uses and limitations, refer to Federal Test Method 5061 of Fed. Std. No. 141 (see general bibliography).

2.2.1.6. Semi-drying oils are in a slower drying category than the oils just discussed. Typical of these are soybean oil, safflower oil, and

tall oil (see sec. 2.2.1.8). Semi-drying oils dry

to soft, tacky films at ordinary temperatures,

but they may be blended or baked with fasterdrying oils or hard resins to produce excellent

coatings. When combined with a hard resin,

the semi-drying oils, like the drying oils, impart

their inherent flexibility, elasticity, and durability to the resultant film.

2.2.1.7. Non-drying vegetable oils, such as

cottonseed, cocoanut, castor, corn, and peanut

oil should not be confused with the drying and

semi-drying oils. Although widely used in the

organic coatings field, the non-drying oils function as plasticizers rather than as film-formers.

2.2.1.8. Tall oil is a semi-drying oil derived,

unlike the oils already described, as a byproduct of the sulfate wood pulp process used in

paper-making. Acidification of the black liquor

skimmings from the pulping process yields an

incompatible liquid mixture from which, upon

separation, the tall oil is drawn off. The oil consists chiefly of a mixture of resin acids re- lated to abietic acid, and of fatty acids related

to oleic acid, together with a small proportion

of nonacidic substances. Because of its ready

availability, low cost, and good properties when

combined with various resins, it is finding

steadily increasing use in alkyd resin manufacture and in combination with other resins

such as epoxies, phenolics, and styrenated

resins (see Ch. 3).

2.2.2. Varnish

Although a varnish has sometimes been defined as any liquid composition which is con- verted to a transparent or translucent film

after application as a thin layer, the term is most often used in a more specific and descriptive sense to denote a homogeneous mixture of

resin, drying oil, drier, and solvent which dries

by a combination of evaporation, oxidation, and

polymerization to give a transparent or trans- lucent film. The resin imparts certain desirable properties, such as hardness or faster

drying, to the film. Such a varnish is often

called an oleoresinous or oil varnish. It is made by cooking the drying oil and resin

together until chemical combination and partial

polymerization have taken place, and then adding the drier and solvent to the cooled ingredients. The ratio of oil to resin is customarily

stated as the "oil length" (number of gallons

of oil per hundred pounds of resin) of the

varnish. Varnishes are further classified as "short oil" varnishes if their oil length is below

15 gallons, "medium oil" varnishes if the'r oil length is between 15 and 30 gallons, and "long

6
oil" varnishes if their oil length is over 30

gallons. Short oil varnishes dry more rapidly

than long oil varnishes and form a harder film,

but the long oil varnishes have greater elasticity and better exterior durability.

Interior varnishes often are based on linseed

oil in combination with estergum or a maleicmodified rosin resin. Rosin-modified phenolic

resins form varnishes with fairly good water

and alkali resistance, but for exterior durability a long oil length is required. The most

durable spar (wood) varnishes for exterior

exposure are usually made from tung oil combined with a 100 percent phenolic resin (substituted phenol-formaldehyde, oil soluble—as described in section 3.3.2.1). The phenolics, as a group, yield varnishes of relatively dark

color. Terpene-phenolic resins are lighter in

color and lower in cost than the comparable

phenolic resins ; they have very good water and

alkali resistance, but only fair exterior durability. The terpene resins themselves are low

in cost and make good light-colored interior

varnishes with good color retention and good

water and alkali resistance.

Coumarone-indene resins are thermoplastic

resins available in a wide range of hardnesses.

Although poor in color and outdoor weathering, they contribute excellent water and alkali

resistance and good dielectric properties to varnishes.

Petroleum hydrocarbon resins are soluble in

drying oils and form varnishes with good

water, alkali, and alcohol resistance, but only

I fair color.

j Natural resins (sec. 3.17.2) such as congo,

; kauri, pontianak, and batu are often used in

varnishes because of their low cost and their

I ability to impart specific properties such as

j hardness, gloss, and moisture resistance.

I So-called alkyd varnishes differ somewhat

1 from the conventional type of varnish in that

I the drying oil is co-reacted with the resin at

I the time the alkyd resin itself is being formed.

Because of the wide versatility and importance

of alkyd resin vehicles, these are discussed

j separately in section 3.4 on Alkyd Resins.

2.2.3. Spirit Varnish

A spirit varnish consists of a resin or gum

(e.g., shellac or damar) dissolved in a volatile

solvent (e.g., alcohol). Such a varnish dries

primarily by evaporation rather than by oxidation or polymerization. A shellac varnish is covered by Fed. Spec. TT-S-300.

2.2.4. Lacquer

j Much confusion has resulted from the diversity of meanings given to the term "lacquer."

Some restrict the term to solutions of cellulose

I derivatives in volatile solvents, which dry by

evaporation to form a solid film. Others, in a

yet narrower sense, limit the term solely to solutions of nitrocellulose (plus modifying

agents). Such restricted definitions ignore

the existence of many modern coating materials

which are, quite properly, called lacquers. For

example, wide reference is made in the industry

to vinyl lacquers, acrylic lacquers, butyrate

lacquers, polyurethane lacquers, and others.

(The long-known "Chinese lacquer" is a different type of material that dries slowly by absorption of oxygen in a cool, moist atmosphere.

It is somewhat toxic even when dry and is little used in this country.)

The definition of a lacquer, to be generally

acceptable and useful, should not exclude modern materials to which the term has been

reasonably applied. Present usage strongly

suggests that the properties most often asso- ciated with the term "lacquer" by those in the

industry are : (1) rapid air-dry by solvent evaporation, (2) ease of application by spraying

(although other means, such as brush or dip, are not necessarily excluded), (3) solubility of

the dried film in the original solvents, making

it relatively easy to touch up or repair. A

lacquer may therefore be appropriately and

usefully defined as a quick-drying, film-forming

solution, especially well-suited for spray application, which dries by evaporation of its volatile constituents to form a coating that can be

redissolved by the original solvents. (The

latter requirement is met only by thermoplastic resins.)

Within the above definition, lacquers may be

either clear or pigmented. In general, they are

set to the touch (tack-free) within a few minutes after application, can be over-coated in

about a half hour, and dry to a firm film in a few hours. The drying process can be con- siderably hastened, and adhesion and durability

often improved, by baking the film at a moderate temperature.

Among the most widely used lacquers are

those based on cellulose derivatives in combination with a modifying resin and a suitable

plasticizer. The volatile solvent in which these

film-forming ingredients are dissolved is a

carefully balanced mixture formulated to give

the proper application viscosity and drying

characteristics. A discussion of the principles

involved in formulating such a balanced solvent

is given in the section on Use of Thinners

(sec. 5.3.2).

2.2.5. Sealer

A sealer is a relatively thin liquid composition that is applied to porous surfaces to prevent excessive absorption ("suction") of the

finish coats. Sealers for wood are essentially

thinned "versions of a varnish or a clear lac- quer; their low viscosity and good aflfinity for

7
the substrate result in good penetration and

sealing of the pores, so that good hold-out

(uniform appearance) of topcoats is obtained.

Sealers for plaster formerly were usually thin

solutions of water-soluble substances such as starch or glue, sometimes containing a small

amount of pigment to help seal the pores ; such

sealers are often referred to as "sizes." They

are quite inferior to the oleoresinous and latexbase primer-sealers now available. (See sec. 2.3.5 on Latex Paints, and sec. 2.4.4 on Primers

for Plaster and Wallboard.)

In the absence of a sealer on porous surfaces,

the absorption of the finish coat often occurs

unevenly, resulting in variations in the gloss,

thickness, and durability of the topcoat. In

the case of pigmented materials, the uneven

absorption also causes variations in hiding

power. In addition, the liquid vehicle may be

selectively absorbed into the substrate, thus

increasing the pigment volume concentration

in the remaining topcoat material ; this process

reduces gloss and also may leave the pigment

"starved" in binder and therefore prone to excessive chalking.

2.2.6. Wax Polishes

Wax polishes, although seldom used alone

as a finish, are widely used to protect and give

luster to other finishes for floors, furniture,

automobiles, and appliances because of the relative ease with which they can be applied and

renewed. Most wax polishes are mixtures of waxes of various degrees of hardness in a suitable solvent or dispersing agent, together with

natural or synthetic resins and other ingredients to impart specific desirable properties.

Hardness and luster are derived chiefly from

the carnauba wax content (or other hard waxes

such as candelilla, ouricury, esparto, or sugarcane). The semi-hard waxes (e.g., beeswax,

parafin, montan, ceresin, and oxidized microcrystalline wax) contribute other properties,

such as ease of application, flexibility, and

lower cost. Shellac and other resins are often

incorporated into wax floor polishes to reduce

slippage, to increase gloss, or to increase durability. Colloidal silica also is effective in imparting anti-slip properties.

From the standpoint of proper selection and

use, wax polishes are of three basic types: (1)

The organic solvent type, in which the mixture

of waxes is dissolved in a volatile medium

such as a turpentine-mineral spirits mixture;

this type may be in either paste or liquid form,

depending on the amount of solvent present.

(2) The aqueous emulsion type, in which the waxes are dispersed in water by use of an

emulsifying agent such as morpholine or tri- ethanolamine to produce a "self-polishing"

liquid wax preparation. (3) An emulsion type

in which the emulsified waxes are diluted with

organic solvents, such as naptha, turpentine, or mineral spirits, so that the final liquid wax

preparation contains both water and organic

solvents.

The organic solvent type of wax polish, particularly in paste form, is generally considered

to be the most durable and lustrous type, and

the kind best suited for floors, although application is more difficult and considerable rubbing is required to bring out the full luster;

this type cannot be used on asphalt tile or on some types of rubber tile, because these materials (especially asphalt) are attacked by the

solvents usually employed. The water-emulsion type of wax preparation must be used on

such surfaces. The water-plus-solvent type of

wax emulsion finds application in furniture

and automobile polishes and in emulsion polishes for vinyl tile and linoleum; the organic

solvent helps to clean and brighten the surface

and reduces the time required for drying to a

tack-free state. Care should be taken that

such solvent-containing emulsion types are not

used on solvent-susceptible surfaces such as asphalt tile. Emulsion type waxes may incorporate a mild abrasive and possibly a soap or detergent to provide a cleansing action as well

as a waxing operation. The abrasive should

not be used in furniture polishes, but is useful

in automobile polishes.

2.2.7. Strippable Coatings

Strippable coatings are coatings of low adhesion and considerable toughness designed to afford temporary protection against corrosion

and abrasion of metal surfaces, parts, equipment, and structures. They are usually applied

as a continuous envelope that is strong enough

to remain intact and in place until the coating

is slit and peeled off. Strippable coatings may

be formulated for application by hot dip [133] or by spray [134]. The film-forming base is usually ethly cellulose, cellulose acetate butyrate, or a copoljmier of vinyl chloride with vinyl

acetate or vinylidene chloride (these resins are

described in Chapter 3). In the hot-dip type,

the film-forming base is dissolved in a hot

plasticizer or oil into which the part to be

coated is dipped; setting takes place as soon

as the film cools. Spraying types are formulated with organic solvents in addition to

plasticizers, and cure by evaporation of solvent.

In both hot-dip and spray types, strippability

is achieved by incorporating into the formulation a suitable proportion of an incompatible

plasticizer or oil which will migrate to the

coating-substrate interface to form a very thin

liquid layer that prevents the development of

coating-to-substrate adhesion.

Strippable coatings are used for the packaging and protection of items ranging from small

tools and machine parts to large completely

8
assembled machines and equipment such as military aircraft, ship-board gun batteries, and

field artillery pieces. Besides use for packaging a variety of small consumer articles

(jewelry, kitchen gadgets, hardware, etc.),

strippable coatings have become important to the industrial builder for the on-site protection

of building hardware (ornamental railings,

grilles, door knobs, and fixtures of brass, copper, and aluminum) during the period of

construction.

Small articles are generally coated by the

hot dip method. "Mothballing" of the large

items is accomplished by first spraying a fi- brous "cobweb" coating onto a framework of

tape around the equipment to form a "cocoon",

after which additional strippable coating material is sprayed on until a coating of sufficient

thickness to provide the necessary strength

and sealing properties has been built up. The

"cocoon" coating is usually a polyvinyl chloride-acetate type of high vinyl chloride content.

to which some polyvinylidene chloride has been

added to promote cobwebbing. The final spray

coating is generally a polyvinyl chloride-acetate

type. The cocoon may be slit to permit flushing

of the interior with dry air and the insertion

of a desiccant or preservative, after which it is resealed. Major pieces of equipment may

thus be stored for long periods, in readiness

for immediate operation as soon as the cocoon

is cut away.

2.2.8. Synthetic Resin Coatings

Among synthetic resin coatings, the distinctions between the various general types of coat- ings (i.e., whether a varnish, lacquer, enamel,

etc.) and between clear and pigmented materials are secondary in importance to the nature

of the resin or resins employed. The properties of the main classes of synthetic resins are therefore discussed separately in chapter 3.

2.3. Pigmented Coatings

Pigmented coating materials fall basically

into two classes: (1) those based on clear

solution-type vehicles, to which the pigment or blend of pigments has been added in the form

of finely divided particles that are essentially

insoluble in the vehicles, and (2) those consisting of a suspension of insoluble, finely

divided pigment particles in a resin-water

emulsion or latex (sec. 2.3.5.1). The latter

type is of comparatively " recent development,

and many of the principles of formulation are

quite different from those employed in conventional solution-type vehicles. Before detailed

consideration of pigmented coating materials, a description of the pigments themselves is given.

2.3.1. Pigments

2.3.1.1. Pigment function.—The pigment

not only adds opacity and a decorative or

functional color to the film but also increases

its durability and protective character by

screening out harmful light rays, controlling

the transmission of moisture and gases through

the film, imparting desirable mechanical properties, and contributing various other properties (e.g., corrosion inhibition, chalking, control

of gloss), depending on the nature of the pigment and the concentration in which it is used.

The number of available pigments, particularly the colored ones, is very large, and it is not a purpose of this manual to list them all. However, a representative group of the most

important types of pigments are described in

the following sections. A list of Federal Specifications for pigments is given in the Index

of Federal Specifications, Standards, and Handbooks [67]. A comprehensive list of the commercially available pigments appears in the

Pigment Section of the Raw Materials Index

of the National Paint Varnish and Lacquer

Association [2].

A primary purpose of the pigment, in all pigmented coatings, is to hide the substrate

by preventing the transmission of light through

the film to the substrate (and out again). Colored pigments accomplish this, to a greater or

lesser extent, by absorbing some of the light

rays and reflecting others. White pigments,

however, absorb relatively little light, so that

their hiding power depends primarily on their

ability to scatter and reflect the incident light.

This in turn depends on the particle size and

size distribution and on the difference in re- fractive index between the pigment and the

surrounding medium. The higher the refractive index of the pigment, the greater is its hiding power, other factors such as particle

size and shape being equal. The greatest hiding power among available pigments is ex- hibited by titanium dioxide; the rutile form has

a refractive index of 2.76 as compared with an index of about 1.5 for the common paint

vehicles.

2.3.1.2. White hiding pigments.—The number of white pigments having good hiding

power is quite limited. The chief ones are

basic carbonate white lead, basic silicate white

lead, basic sulfate white lead, titanium dioxide

(both anatase and rutile forms), zinc oxide.

9
leaded zinc oxide, zinc sulfide, antimony oxide,

and lithopone. The latter is a co-precipitated

mixture of zinc sulfide and barium sulfate.

The above pigments are often referred to as prime pigments. They are relatively high in cost and are most often used in combination

with less expensive extender pigments.

White lead is a very durable pigment which

contributes to the formation of flexible, durable

films. It suffers, however, from a tendency to darken in industrial atmospheres. Although

this tendency does not impair the protective

character of the film, the decorative properties

suffer. Titanium dioxide and zinc oxide are widely used in these environments to prolong

the decorative life of exterior white paints.

Titanium dioxide is superior to white lead in

hiding power, whiteness, and brilliance; the

anatase form, because of its chalking tendency,

imparts self-cleaning properties to white paints

(not desirable in tints). Since the rutile form

is chalk-resistant, the degree of chalking of

titanium dioxide-pigmented paints can be controlled by the ratio of anatase to rutile pigment.

Zinc oxide contributes improved mildew re- sistance, increased durability, and other valuable properties to the paint film.

2.3.1.3. Extender pigments are white pigments of relatively low refractive index approaching that of the common paint vehicles, so that they have little hiding power at ordinary

pigment volume concentrations (below the

critical PVC). However, they are very useful

in combination with the prime pigments.

Among the most important extender pigments

are

:

hydrated aluminum silicates (china clays)

magnesium silicate (talc)

silica (usually in diatomaceous form, but

sometimes as quartz)

calcium carbonate (natural chalk and

limestone ["whiting"] , or chemically precipitated)

barium sulfate (natural barytes or precipitated blanc fixe)

calcium sulfate

powdered mica

Although the extender pigments are low in

cost, their use does not necessarily result in an

inferior paint; in fact, the extenders usually

contribute many valuable properties. They aid

in producing the proper paint consistency for

good non-settling characteristics and smooth

application. They help to control the penetration

of priming paints. They are useful for diluting colored pigments of great tinting strength.

They assist in obtaining the desired surface

appearance (degree of gloss or flatness) in the

dried film. They may inhibit the tendency to- ward film cracking on aging, and they can improve various other mechanical properties of

the film (e.g., washability and abrasion resistance) through reinforcement of the film structure.

2.3.1.4. Color pigments comprise a w^ide

variety of natural and synthetic materials, both

organic and inorganic. They may conveniently

be classified on the basis of their source and

general nature as: (1) natural pigments, (2)

synthetic inorganic pigments, or (3) synthetic

organic pigments. Only the more important

and widely used pigments in each class can be

discussed here. From the standpoint of the

paint user, the chief properties that govern the

usefulness of a color pigment are

:

Color (hue, brightness, and saturation)

Durability (lightfastness, heat resistance,

chemical resistance—depending on the

requirements of the application)

Bleeding resistance (in oils and solvents)

Tinting strength

Hiding power (or, in some instances,

transparency)

.

Properties which are primarily the concern of

the paint manufacturer, such as ease of grinding and oil absorption, will not be considered

here.

2.3.1.4.1. Natural pigments comprise the in- organic earth colors or mined products and a few organic materials of vegetable and animal

origin. Of chief importance are the iron compounds, composed mainly of iron oxides in

combination with siliceous material and smaller

percentages of the oxides of manganese, aluminum, calcium, and/or magnesium, together

with some carbonaceous matter. These ferroferric oxide pigments include the yellow ochers,

the dark yellow siennas, the brown umbers,

the red hematites and burnt siennas, and the

black magnetite or magnetic oxide. With the

exception of magnetite, which is 94-95 percent

iron oxide (Fe304), the natural iron oxide pigments vary widely in composition. The yellow,

brown, and red iron oxides are largely in the

ferric form, and the ferric oxide (Fe203) content may range from as low as 11 percent in some of the ochers to as high as 80 percent in some of the hematites. The iron oxide pigments lack brilliancy, but they are widely used

for their low cost, good durability, and excellent non-bleeding properties. However, the

natural pigments, because of their variable

composition, have been replaced in many applications by synthetic iron oxide pigments of

greater uniformity, strength, and brilliance.

The natural organic color pigments are of

only minor importance. Their chief use is in

stains, artists' colors, and printing inks. In

the vegetable group are Vandyke brown and

various precipitated dyes. In the animal group

are cochineal (carmine) and sepia.

10
2.3.1.42. Synthetic inorganic pigments of

uniform composition and properties and in a

variety of chemical types have largely replaced

the natural pigments in modern paint technology. The most important types are discussed in the follow^ing paragraphs.

Synthetic iron oxide pigments of high purity are available as single oxides or blends of iron

oxides in a range of colors that include yellow,

orange, tan, red, maroon, brown, and black.

The shade of the pure colors depends on the

particle size and shape as well as on composition. Pure yellow iron oxide is hydrated ferric

oxide (Fe203 • H2O) prepared by precipitation

and oxidation from solutions of iron salts. Pure red iron oxide is ferric oxide (Fe203)

obtained by calcining the hydrated form. Pure

-^lack iron oxide is a precipitated ferro-ferric

oxide (Fe304) of bluish-black hue. The other

iron oxide colors are made by blending yellow,

red, and black forms. Venetian reds are cal- cium sulfate base iron oxide pigments containing from 25 to 40 percent of red iron oxide.

The iron oxide pigments as a group are chemically inert, nonfading, bake-resistant, nonbleeding, and low in cost. They find wide use

in applications where durability rather than

brightness of color is required ; for example, in

paints for railway freight cars, in industrial

paints, barn paints, floor paints, and metal

protective paints.

Iron blues are complex ferric-ferrocyanides

known by a variety of names. Chinese blue

and Milori blue have a greenish tint tone (i.e., the tone perceived when the colored pigment is added in small quantities to a large quantity

of light pigment) ; Prussian blue is reddish in

tint tone. However, in mass tone (where the

colored pigment predominates), Milori is reddish and Prussian is greenish. The iron blues are non-bleeding in oils and solvents but are somewhat reactive with certain organic vehicles. Although stable to acids, they have poor

alkali resistance. They may be baked at 350 °F

(177 °C). Their hiding power and tinting

strength are excellent, as is their outdoor durability when used full shade. They are relatively low in cost and are very widely used in

paints, lacquers, and enamels.

Chrornate pigments comprise the chrome

yellows and oranges, and molybdate chrome

orange. Medium chrome yellow is almost pure

normal lead chromate; the lighter shades contain lead sulfate. Blends of normal and basic

lead chromate yield the chrome oranges, the

higher proportions of basic lead chromate producing the deepest shades. Molybdate chrome

orange is a coprecipitated mixture of lead

chromate, lead sulfate, and lead molybdate; it is a strong, bright orange pigment with excellent hiding power. In general, the chromate

pigments provide good hiding and durability

at relatively low cost; however, the chrome

yellows and molybdate orange are poor in al- kali resistance while the chrome oranges have

poor acid resistance. All except the deep

chrome oranges tend to darken somewhat on

exposure. All have excellent bleeding resist- ance. The deeper chrome oranges of high

basic lead chromate content have been effec- tively utilized in metal protective primers.

The chrome yellows enjoy wide general use in

the coatings field. Chrome greens are composite pigments consisting of an intimate blend of iron blue and

chrome yellow. By varying the proportions of

yellow and blue, a wide range of hues is obtainable. Chrome greens have high tinting

strength and hiding power and very good color

permanency and outdoor durability. Although

sensitive to alkalis, they are resistant to acids

and are quite stable on baking. Their cost is relatively low, and they find wide application

in the coatings field. Chromium oxides and their hydrates are green pigments of exceptional stability and

color permanence. They are lower in tinting

strength and hiding power than the chrome

green pigments described above, but they resist

acids and alkalis and withstand high temperatures. The pure chromium (chromic) oxide

will withstand even ceramic heats. Chromium

oxide pigment is relatively low in brightness;

however, hydrated chromium oxide has good

brightness. Their chief use is in applications

where their stability to light, heat, and alkali

justifies their higher cost. Ultramarine blue is a dark, brilliant blue

pigment with high decorative appeal. It is made by calcining mixtures of china clay, soda

ash, silica, and sulfur in the presence of a re- ducing agent such as charcoal, pitch, or rosin.

The calcined material is washed to remove

soluble salts, wet-milled, filtered, dried, and

ground into a range of particle sizes yielding

diff'erent shades and tinting strengths. Although low in hiding power and tinting

strength, ultramarine blue finds wide use as a

tinting pigment in interior protective and decorative coatings, where it offers good heat stability, non-bleeding character, excellent resistance

to most alkalis (except lime), and extreme

lightfastness. It is very sensitive to acids, however, and since most vehicles develop some acidity during outdoor exposure, ultramarine blue

is not usually used in exterior coatings.

Cadmium yellows and reds are essentially

sulfides (yellow) or sulfo-selenides (red) of

cadmium, usually combined with an inert base

of barium sulfate. They are available in a wide range of colors, from a delicate light

yellow to a deep maroon. They possess good

color brightness, lightfastness, and resistance

to acids. Their excellent alkali resistance

makes them suitable for use in emulsion paints

and special alkali-resistant finishes. Although

11
the yellows tend to fade and discolor on outdoor

exposure, the reds and maroons exhibit good

exterior durability. The cadmium pigments

have excellent heat resistance and find important use in high temperature baking

enamels.

2.3.1.4.3. Synthetic organic pigments comprise a very large and important group of

colors that are prepared by the insolubilization or precipitation of organic dyestuffs. As

with inorganic pigments, particle size and

shape are important factors in determining the

color shade obtained with a particular chemical

type, the smaller particles generally producing

the lighter shades and greater brightnesses.

The most widely used synthetic organic pigments are discussed in the paragraphs which

follow.

Copper phthalocyanine blues and greens are

chemically inert pigments of high tinting

strength and excellent lightfastness. They

are highly resistant to acids and alkalis and

are completely free from bleeding in oil and

organic solvents, even at elevated temperatures.

Although not in themselves readily wetted with

water, they may be treated with surface active

agents to render them easily dispersible for use in water base paints. Because of their

strong, dark mass tone, the phthalocyanines are used chiefly in relatively light tints, yielding

clean, bright blues and greens of outstanding

durability. They have found wide acceptance

in high quality paints, lacquers, and decorative

enamels for exterior exposure. Phthalocyanine greens differ in composition from the blues

in that most of the hydrogen atoms present in

the blues have been replaced with chlorine

atoms. The greens are free from the tendencies toward flocculation and crystallization

exhibited by the blues; however, phthalocyanine blues with good resistance in these re- spects have been developed.

Toluidine reds are dyestuffs belonging to the

insoluble azo group of pigments that are inherently insoluble in water without containing a metal precipitant. A range of light red to me- dium red shades is available. The toluidines

have good hiding power and excellent brightness, durability, and color permanence when

used full shade (undiluted), but the tints tend

to fade. Although resistant to acids and alkalis,

they bleed in many organic solvents, fats and

oils. Toluidine reds find wide use in industrial

and decorative enamels where brilliance of shade

and durability are of prime importance.

Para reds also belong to the insoluable azo

group. Though not as durable or lightfast as the toluidines, they are lower in cost. Their

bright color and adequate lightfastness when

used full shade make them useful in trim

paints and in porch and deck paints. They

are not suitable in tints because of fading.

Lithol reds are members of the soluble azo group of pigments. They range in shade from

a light yellow red to a deep red, depending on

the metal precipitant used to insolubilize them.

By resination, maroon shades are obtained.

The lithols show good resistance to bleeding in

oils, hydrocarbons, and water, but they may

bleed considerably in lacquer thinners, and

their alkali resistance is poor. Although

neither durable nor lightfast, they are the least

expensive of the organic red pigments and find

considerable use in interior applications where

durability is unimportant and a strong color is desired.

Hansa yellows (toluidine yellows) are members of the insoluble azo group. Like the toluidine reds, they have good acid and alkali resist- ance but bleed in organic solvents and oils. They are durable and lightfast in full shade.

Because of their alkali resistance, they are

suitable for use in water emulsion paints, being superior to the chrome yellows in this respect. They are lower in cost than the cadmium

yellows sometimes used in this application. A

growing field of use for these pigments is in toy

enamels where pigments free of toxic metals are

required. Hansa yellows find wide use in paints,

lacquers, and enamels requiring a durable, brilliant yellow pigment.

Benzidine yellows belong to the insoluble azo group. They are two to three times as strong

tinctorially as the Hansa yellows but are some- what less durable. They are more than ten

times as strong as the chrome yellows. Like

the toluidines, they have good acid and alkali

resistance but bleed in lacquer solvents. They

are of general usefulness in coatings where

strong color and fairly good durability are

required.

Tungstated and molybdated pigments are organo-metallic precipitates of the fugitive but

brilliant basic dyestuffs such as malachite

green, methyl violet, and the rhodamines.

These pigments are strong and brilliant in

color but are not sufficiently lightfast to withstand exterior exposure. They are fairly stable

at moderate baking temperatures and fairly

resistant to acids, but their resistance to al- kalies is poor, and they bleed in lacquer solvents.

Their chief use is in specialty coatings where

permanence is unimportant and a striking

decorative efi'ect is desired. They are available

in a variety of colors including blue, violet,

green, yellow, and red.

Indanthrenes are strong, bright pigments

possessing exceptional lightfastness even in

light tints. They are non-bleeding in oils and

solvents and resistant to acids and alkalis.

Their use is limited by their high cost, so that

they are used chiefly for toning and tinting

other colors. However, indanthrene maroon

has become a preferred pigment for high quality maroon automotive finishes. The blue also

12
finds use in the automotive field. Indanthrene

greens and yellows are less used.

2.3.1.5. Corrosion-inhibiting pigments.—

A small but important group of pigments

worthy of special mention are those used almost

entirely for their ability to inhibit metallic

corrosion. These include red lead (Pb304),

sublimed blue lead (basic lead sulfate, blue) , cal- cium plumbate, basic lead chromate, zinc chroI mate (zinc yellow), zinc tetroxychromate, and

[ strontium chromate. Unique among the rust- inhibiting pigments is a recently marketed

j basic lead silico-chromate pigment consisting of

j a shell of basic lead chromate fused to an inert core of silica. The economy effected by limiting

the costly basic lead chromate to the effective,

outer reactive shell, together with the high

bulking value of the light and inexpensive silica core, makes it feasible to incorporate a high

volume concentration of this pigment into intermediate and finish coats as well as in primers, thus providing rust-inhibitive properties

throughout the entire coating system.

The effectiveness of these corrosion-inhibiting pigments derives chiefly from their very

limited solubility in water and their ability to chemically combine with the metal surface to produce an impervious, insulating film thus

rendering the metal passive. When such pigments are incorporated in a metal priming

paint, their limited solubility furnishes a small

but adequate supply of corrosion-inhibiting

ions without leaching enough pigment to render

the film porous.

Zinc dust, when used in a primer in a suffici- ently high concentration (92 to 95% of the dry

film weight) to ensure particle-to-particle contact in the dry film, is an effective corrosion

inhibitor for steel surfaces, producing the

so-called "zinc-rich" paints [128]. It is now

a generally accepted view [117, 118] that the

zinc serves initially as the sacrificial (anodic)

material in a galvanic reaction in which the

steel, as cathode, is afforded protection at the

expense of the zinc. The period of cathodic

protection is limited by the build-up of zinc

corrosion products at the metal-coating interface, which stifles the galvanic action; however, the deposited corrosion products form a

hard, impervious, adherent barrier layer that

is combined with both the coating and the substrate and affords excellent long-term protection against corrosion.

The formulation of metal primers incorporating corrosion-inhibiting pigments is discussed

in section 2.4.2. Important principles and precautions which should be borne in mind when

applying corrosion-inhibiting primers are discussed in section 7.2.3.

2.3.1.6. Metallic pigments for use in paints

are, with the exception of zinc dust, usually in

flake form. The flakes are obtained by rolling

out the metal particles in a ball mill or by hammering thin sheets of the metal in a stamp

mill, in either case in the presence of a lubri- cant. They are either leafing or non-leafing,

depending on whether or not they have been

coated with a leafing agent (usually stearic

acid) and polished during the manufacturing

process. In a suitable vehicle, the flakes coated

with leafing agent float to the surface of the

paint, where they orient and interleave to form

an almost continuous, bright metallic surface

that is highly resistant to the passage of moisture and gasses, high in hiding power and

specular light reflectance, and durable on exterior exposure. The coarse particle sizes produce the brightest, most highly reflective surfaces, while the fine particles are preferred in

decorative coatings, printing inks, and aerosol

paints.

In the non-leafing type, the metal flakes, in- stead of floating to the surface and forming an

interleaved layer, remain suspended throughout the film. Although a high degree of re- flectance is not possible with this type, a wide

array of different colored metallic shades (including hammertone finishes) can be achieved,

depending on the particle size and size distribution. The larger particles produce the greatest brightness or sparkle, while the fine particles

yield the highest opacity and deepest shades.

2.3.1.6.1. Aluminum is the most widely used

metallic pigment. It is available both as a powder and as a paste, in a variety of grades ac- cording to particle size. Aluminum paints are

lightfast, heat-resistant, water-resistant, and

durable on outdoor exposure except in acidic or

alkaline environments. The leafing types are high in hiding power and in moisture resistance

and provide an effective barrier against the

transmission of water vapor and gases through

the film. Important applications for aluminum

paints include marine coatings, aircraft coatings, solvent tanks, transmission line towers,

highway bridges, furnace fronts, and interior

decorative finishes.

2.3.1.6.2. Metallic copper and its alloys with

aluminum, tin or zinc yield the popular bronze

finishes that range in shade from a light brass

to a dark antique copper, depending on composition and on the heat treatment received during manufacture. Care in formulation is required to counteract the tendency of the

copper bronze pigments to retard the drying

of oil, varnish, and other vehicles that cure by an oxidative process. These pigments are high

in hiding power but tend to discolor on exterior

exposure. They are used chiefly in interior

decorative finishes, specialty lacquers, printing

inks, and anti-fouling paints. The latter often are made with a dendritic, non-leafing type of

copper pigment.

13
2.3.1.6.3. Zinc dust is made by distilling zinc

and condensing it under conditions that prevent coalescence. It has very high hiding

power, being almost completely opaque. It is used chiefly in corrosion-inhibiting paints for

steel, where it serves initially as the sacrificial

anodic material providing cathodic protection

to the steel substrate and later as a protective

barrier (see sec. 2.3.1.5), It is used also in

paints for galvanized surfaces, where it provides good adhesion. It is not resistant to acids or alkalis.

2.3.1.6.4. Gold and silver flake are used principally in decorative applications such as sign

writing, book-binding, picture frames, and glass

and chinaware.

2.3.1.6.5. Stainless steel flake has recently

been used as the main pigment, along with some

organic pigment, in a metal protective paint

reported [3] to have shown excellent durability

in corrosive exterior environments. The effec- tiveness of the stainless steel pigment is attributed not to rust-inhibitive properties but to its own chemical resistance plus its exceptional

resistance to moisture. The aged film takes on

the appearance of stainless steel itself.

2.3.1.6.6. Other metallic pigments used to a

limited extent in the coatings field are nickel,

tin, and lead.

2.3.1.7. Black pigments comprise the synthetic carbons manufactured by various fume

and incomplete combustion processes, natural

and synthetic graphite, pyrolyzed vegetable

and animal blacks, and the naturally occurring

combinations of carbonaceous and siliceous materials known as mineral black. Black iron

oxide has already been discussed under Color

Pigments (sec. 2.3.4).

Carbon black (sometimes called channel

black) is manufactured by the closely-controlled burning of natural gas. It is the finest

of the black pigments, the particles being of

colloidal dimensions. It is the blackest of pigments and the highest in tinting strength and

opacity, and it finds wide use in the coatings

field. Other synthetic blacks are lampblack,

furnace black, and thermal black. Graphite is a soft crystalline form of carbon that occurs

in natural deposits of varying carbon content;

it can also be manufactured by heating coke

or charcoal with about three percent of iron

in the high temperature of an electric furnace.

Compared with other black pigments, graphite

is relatively low in blackness and tinting

strength and is used chiefly for its protective

qualities and its electrical conductivity. The

various synthetic blacks (carbon black, lampblack, etc.), although differing in amount of

fixed carbon, particle size, oil absorption, and

tinting strength, are all stable to light, heat,

acid, alkali, and atmospheric conditions. « Vegetable blacks may contain from 50 to 90 j percent of fixed carbon, depending on the vege- j table material from which they are made. Ani- ? mal blacks (boneblack) contain only 10 to 20

percent of carbon, the remainder being chiefly

calcium phosphate. Although they are much

lower in tinting strength than the carbon , blacks, they also have a much lower oil-absorption, so that larger proportions can be incorporated into a formulation to produce low

sheen black finishes. Boneblacks also find use , in artists colors and in latex paints. Mineral

blacks are sometimes used in place of boneblack but are more variable in composition and

properties. Mineral and vegetable blacks are used mostly to color cement and mortar.

2.3.1.8. Fluorescent pigments are finding

increasing use in high visibility paints. Although formerly too expensive and short-lived

for practical use, specially formulated fluorescent paints are now available that will retain

satisfactory color and fluorescent properties

during a year or more of exterior service in

temperate climates. Fluorescent substances

have the property of absorbing energy in one

spectral region and re-emitting it in another

spectral region of lower energy (longer wavelength) . A number of organic and metalorganic dyes yielding a variety of fluorescent

colors are available. For high visibility aircraft

paints [4, 4a] , a red-orange fluorescent pigment

has been preferred because this color affords

high contrast with outdoor surroundings and

is in the spectral region to which the eye is quite sensitive. This pigment absorbs energy in

the ultraviolet and blue regions and re-emits it in the red-orange region. The re-emitted light

is added to the light ordinarily reflected from

the paint surface, greatly increasing its brilliance. The total light radiated and reflected

from a fluorescent paint may be three to four

times as great as that of the same color without the fluorescence.

So-called "daylight" fluorescent paints utilize

chiefly the visible, blue portion of the spectrum

to produce fluorescence, the harmful invisible or "black" ultraviolet (which would soon deterio- rate the fluorescent pigment) being largely

screened out by the use of an ultraviolet absorber or inhibitor in the paint formulation.

The initial reduction in brightness from not

utilizing the ultraviolet is offset by a substantial gain in exterior durability.

Fluorescent pigments are low in opacity.

The fluorescent paints made from them, to be

fully effective, must be applied over a white

undercoat that will reflect the light which

would otherwise be transmitted through the

coating and lost by absorption into the substrate [139].

14
The use of high visibility, daylight fluorescent paints on some aircraft is believed to have

significantly lessened the risk of mid-air collisions. Other applications include warning

signs for hazardous areas or equipment, explosives- and fuel-carrying vehicles, advertising

signs, and eye-catching decorative effects both

indoors and out, in artificial light as well as

daylight.

2.3.2. Oil Paints

Oil paints consist basically of pigment particles dispersed in a drying oil or varnish vehicle,

together with thinners to provide a suitable

consistency and evaporation rate for proper

application, and driers to give the desired cur- ing characteristics. Oil paints dry by a combination of solvent evaporation, oxidation, and

polymerization. The several types of oil paints

may be distinguished as follows

:

2.3.2.1. Straight oil paints are those utiliz- ing a drying oil or combination of drying oils as the nonvolatile vehicle. Of the various drying oils available (see sec. 2.2.1), linseed oil is by far the most widely used because of its excellent brushing characteristics, wetting and

penetrating properties, adhesion, elasticity,

and durability on exterior exposure, as well

as its general availability and relatively low

cost. The linseed oil film is fairly permeable

to water vapor—a property that is useful for

reducing the tendency toward blister formation

in paints on wood and other porous surfaces,

but one which limits its usefulnes in applications where low permeability and good chemical

resistance are required (e.g., an industrial

metal paint) . Oil paints are relatively poor

in abrasion resistance, in resistance to water

immersion, and in resistance to corrosive (especially alkaline) environments.

Both raw and bodied (heat polymerized)

linseed oil are used in paints. The raw oil, be- cause of its superior wetting ability, is used

extensively in metal primers; the bodied oil

is used in finish coats because of its faster drying and better leveling characteristics. Highquality exterior house paints are still based

largely on the time-tested linseed oil vehicle.

Pigmentation consists of various combinations

of titanium with lead and/or zinc pigments and

extender pigments.

2.3.2.2. Oleoresinous paints consist of a

dispersion of pigment particles in a varnish or

oleoresinous vehicle, i.e., a vehicle in which the

drying oil and resin have been combined by a

cooking process. The addition of the resin re- duces the permeability and increases the chemical resistance and hardness of the film as compared to a straight oil paint. The resin, depending on the choice, may also contribute

other desirable properties, such as faster drying, increased gloss, improved adhesion, and

greater durability. The types of resins employed have been discussed in sec. 2.2.2 on

Varnish. Of particular importance are oleoresinous paints ultilizing phenolic resins and

alkyd resins.

Phenolic 'paints based on an oil-soluble phenolic resin (section 3.3.2.1) combined with tung

oil have very good water resistance and weathering resistance. Their fast drying and poor

wetting characteristics, however, necessitate

very thorough surface preparation to obtain

adequate adhesion. Since phenolic paints con- tinue to cure and harden on aging, failure from

a lack of intercoat adhesion can occur if too much time elapses between coats. Ordinarily,

successive coats of phenolic paint should be

applied within 24 hours after the preceding

coat. This time may be extended somewhat

provided a small amount of a strong active

solvent is added to the follow-up coat. Alkyd paints are among the most versatile

and useful materials in the entire field of or- ganic coatings. The oil-alkyd vehicle on which

such paints are based can be varied over a wide

compositional range to give a broad range of

coating properties for numerous applications

in the consumer, manufacturing, and industrial

maintenance fields. Among the valuable properties imparted to paints by alkyd resins are

fast dry, hardness, and durability, with excellent chalk resistance, gloss, and color retention.

A comprehensive discussion of alkyd resins is given in section 3.4. Long-oil alkyds of the air-drying type are widely used in high quality interior architectural paints and enamels. The long-oil alkyds

are also the basis for durable exterior trim

enamels for both wood and metal surfaces.

Blister and stain resistant exterior house paints

based on long-oil alkyds plus added bodied oil are available but have not yet achieved wide

use. Medium-oil alkyds are found in many good

quality floor, porch and deck enamels. Bakingtype medium- and short-oil alkyds, often in

combination with urea- or melamine-formaldehyde resins, are the basis for premium coating

materials of exceptional durability that find

wide use in automotive enamels, appliance fin- ishes, and industrial equipment coatings. The

uses of alkyd resins are discussed in greater

detail in section 3.4.1.3.

2.3.3. Pigmented Lacquers

By dispersing suitable pigments in a lacquer

vehicle, opaque and colored lacquer-type paints are obtained which possess the characteristic

sprayability, fast dry, and repairability of clear

lacquers as described in section 2.2.4. Since

the solids content of lacquer formulations is limited by the viscosity and solubility of the

film-forming base in the volatile solvent medium,

15
it is necessary to use prime pigments with max- imum hiding power and low oil absorption to achieve good hiding without an undue increase

in consistency. Extender pigments are not used

in lacquers, since they increase consistency

and reduce the proportion of film-forming base

that can be used, without contributing to hiding power.

One of the first and most important pigmented lacquers was based on nitrocellulose

plasticized with camphor or castor oil. Later,

the combination of nitrocellulose with an alkyd

resin and suitable pigments and plasticizers

yielded the widely used "Duco"-type automotive finish. After many years nitrocellulosealkyd combinations are still the basis for many

of our present-day lacquers. Other film-forming materials used in the formulation of pigmented lacquers are cellulose acetate, cellulose acetate butyrate, ethyl cellulose, vinyl resins,

and polyurethanes—to name only a few. Very

recently, acrylic lacquers having excellent hardness, durability, and gloss have been developed

and are being widely used in the automotive

finishing industry. The properties of many of

the resins used in lacquers are given in chapter 3 on Properties of Synthetic Resins for

Coatings.

2.3.4. Enamels

Enamels are pigmented coating compositions

that yield films characterized by smoothness

and freedom from brush or other tool marks.

Although most enamels are glossy, flat enamels

also are available. Enamels are usually thought

of as hard coatings. They are often of the baking type, as in the case of automobile and

appliance finishes in which a short-oil alkyd

is combined with a melamine or urea resin to produce a hard, durable finish. Floor, porch,

and deck enamels are air-drying types that are

often based on a medium-oil alkyd resin. Exterior architectural enamels are usually based on long-oil alkyd resins modified with drying

oils such as linseed, soya, or dehydrated castor. Pigmented varnishes often fall into the enamel

classification. Rutile type titanium pigments

are preferred in exterior enamels because of

their excellent hiding power and resistance to chalking. The pigment volume concentration

(ratio of pigment to binder) of enamels, though

dependent on the nature of the pigments and

extenders employed, is generally lower than

that of ordinary paints (see sec. 4.1,4.4).

2.3.5. Latex Paints

2.3.5.1. The various types of pigmented coat- ings that have been discussed thus far,

whether oil paint or lacquer or enamel, have consisted basically of insoluble pigment particles

dispersed in a homogeneous liquid vehicle consisting primarily of drying oil or resin solution. In these coatings the film-forming in- gredient has been dissolved in organic solvents

to give a vehicle having a single, continuous

solution phase. In latex paints, however, the

structure of the system is very different. The

resinous or film-forming ingredient,^ instead of

being dissolved in a solvent medium, is dispersed (in the presence of an emulsifying

agent) in the form of fine particles in a liquid

(water) which is not a solvent for the resm. The pigment, also, is suspended in the water

medium.

The use of water as the volatile vehicle in

latex paints offers many advantages. It is readily available, inexpensive, noncombustible,

nontoxic, odorless, and colorless. Storage facilities for a variety of organic solvents are not

needed. Its evaporation rate is well suited for

brush or roller application. The paint may be

applied on damp surfaces or in damp weather

and is not harmed by rain an hour or two after

application. Surfaces may usually be recoated

the same day, or under favorable conditions

after an hour or two of drying. Brushes, rollers

and containers can be cleaned by washing in

warm, soapy water after completion of the

work.

The disadvantages of water as the volatile

vehicle stem from: its high surface tension

which may cause foaming, crawling, or crater- ing of the applied film ; its high heat of vaporization which slows evaporation and lengthens

the flash-off time necessary to avoid blistering

in baked coatings; its essentially fixed boiling

point which makes it difficult to adjust evaporation rate in the manner that is possible with

mixtures of organic solvents; and its susceptibility to freezing under adverse storage or

application conditions.

2.3.5.2. Forerunners of our modern latex

paint vehicles were the alkyd resin emulsions

that first appeared around 1929, some twenty

years before the advent of commercial latex

paints. The vehicles of these early resin emulsion paints consisted of a dispersion of a longoil alkyd resin in water in the presence of con- siderable amounts of casein and other emulsifying agents. Paints made from these vehicles

had a number of faults, including poor emulsion stability, clogging of paint brushes, poor

water resistance, and a general lack of the

outstanding properties usually associated with

alkyd resins. Subsequent developmental work

greatly improved the properties of alkyd resin

emulsions, and their usefulness for interior

paints was recognized in 1940 by a Federal

Specification (TT-P-88). This was followed

in 1947 by a Federal Specification (TT-P-18)

for an exterior masonry paint based on an

1 Contrary to a popular misconception, a latex paint does not

necessarily or even usually contain rubber, as will be apparent from

the discussion of types of latex paints given in subsection 2.3.5.6.

16
alkyd resin emulsion. Alkyd emulsion paints are supplied in the form of a paste that is thinned with water before use. Since the longoil alkyds on which they are based are liquid

resins that have been only partly polymerized

during manufacture, the paint film laid down

must harden by a process of oxidation and

polymerization after the water phase has evaporated, in the same manner as when deposited

from an organic solvent solution. With the in- troduction of latex paints in 1949 and their

subequent wide acceptance, interest in resin

emulsion paints dwindled. Today, alkyd resin

emulsions find their chief use in blends with

the latexes.

Actually, a latex paint is also a resin emulsion paint; however, it is a special type that

is characterized by the higher molecular weight,

smaller particle size, and more uniform size

distribution of the resin particles as compared

to the older, so-called resin emulsion paints.

These properties of the latex are achieved by an

emulsion polymerization process (sec. 3.1.3.3)

which not only permits better control of the

polymerization than is possible by bulk or solution methods but also permits the development

of a high molecular weight polymer without

problems of high viscosity or insolubility.

2.3.5.3. Uses.—Latex paints are particularly

well-suited for use on porous, absorbent sur- faces such as plaster and masonry. The latex

particles, although very small, are still large

compared to the size of a dispersed polymer

molecule in solution; hence, they do not penetrate into porous surfaces to so great an ex- tent as do solution-type coatings, and their

sealing properties are excellent. The inherent,

good sealing properties of the latexes are utilized to the maximum in the formulation of

primer-sealers (see sec. 2.4.4) . Although latex

paints may be successfully applied in most in- stances without requiring a primer-sealer, use

of the latter provides maximum holdout of topcoats and is particularly useful in securing

uniform gloss, hiding, and color over substrates

(especially plaster) of uneven porosity or composition.

Latex paints are not alkali-sensitive and may

be applied even over unaged (but not fresh,

uncured) masonry or plaster surfaces. It should be noted, however, that when latex

paints are modified with other materials (e.g., an alkyd resin) to achieve some of the advantageous properties of the modifier, the alkali

resistance of the latex may be impaired. Also,

the same non-penetration properties of the

latexes which give them their excellent sealing

properties causes diflRculties when they are

applied over chalky surfaces, because of their

inability to penetrate the chalk to adhere to the substrate.

The water demand of masonry surfaces must

be met either by diluting the paint with water

or, more desirably, by dampening the surface

to be painted. Excessive dilution of the paint

itself will break the emulsion.

Although the widest and most effective use

of latex paints has been for interior coatings on plaster and wall-board, and for exterior

masonry surfaces such as Wick, cinder block,

stucco, cement, and weathered asbestos shingle, some latex paints have been formulated for use

as exterior house paints on wood, as metal

primers, and as industrial alkali-resistant coatings. Most latex paints give a flat or matte

finish; however, acceptable gloss and semigloss types have been developed. Industrial

baking-type latex finishes are also under development. More specific information concerning

uses of latex paints is given later in this section

in the discussion of the individual chemical

types of latexes that are available.

2.3.5.4. Formulation.—The film-forming

resin on which latex paints are based is a longchain polymer of high molecular weight that

has been produced, by an emulsion polymerization process (section 3.1.3.3), in the form of

very small spherical particles (not over 0.2

micron nor below 0.01 micron in diameter).

Even a small proportion of large particles will act as seeds to accelerate coagulation of the

smaller ones. The emulsified or dispersed state

is achieved by surrounding each resin particle

with a film of emulsifying agent. Dispersion

of the resin particles in the water vehicle constitutes an oil-in-water type emulsion, which

is most stable when both phases are present

in about equal proportions. Excessive thinning

with water breaks the emulsion by reducing

the thickness of the emulsifying film around

each particle. At high solids content (about

70 percent) , the emulsion inverts to the waterin-oil type and is no longer thinnable with

water.

Latex emulsions differ from resin solutions

in a number of important respects. In vehicles

of the resin solution type, the solids content is limited by the viscosity and the solubility of

the resin in the solvents employed. These

properties in turn depend on the molecular

weight of the resin, the viscosity increasing

and the solubility decreasing with increasing

molecular weight. Although durability and mechanical properties generally improve with in- creasing molecular weight (increasing polymer

chain length), a practical upper limit is imposed by the necessity for maintaining a practical viscosity and solubility. Latex emulsions

do not suffer from these limitations. Since the

latex particles are suspended, not dissolved, in

the water phase, insolubility at high molecular

weights is a desirable property rather than a problem. Moreover, since the latex particles are not dissolved in the water, they contribute

17
little to the viscosity of the vehicle. Thus latex

paints are able to utilize the inherent advantages of high molecular weight and high solids

content without the viscosity and solubility

limitations imposed upon solution-type vehicles.

The formulation of latex paints, however,

involves characteristic problems of its own.

Before a pigment is incorporated into the resin

emulsion, the pigment must itself be dispersed

in water in the presence of an efficient dispersing agent. Titanium dioxide, particularly the

rutile form, is the preferred white hiding pigment. Extender pigments (e.g., calcium car- bonate, china clay, mica, etc.) are used to help

control flow, leveling, film build, and gloss ; ex- tenders may also have a pronounced effect on

film properties such as cleanability, scrub re- sistance and appearance. Good dispersion is essential for high solids content, package stability, freeze-thaw resistance, good flow and

leveling with minimum floating and flooding,

and for maximum hiding power and color uniformity. Thickeners, in the form of watersoluble protective colloids such as casein or methyl cellulose, are often necessary to stabilize

the emulsion and to produce a suitable consistency for good application properties. These

water-soluble colloids are used in minimal

amounts (or not at all) in exterior formulations because they tend to reduce the water

resistance of the film in which they remain.

Bacterial contamination during manufacture

must be guarded against, and a preservative

(e.g., one percent of a phenyl mercuric salt)

must be incorporated to inhibit the growth of

bacteria, mildew and mold in the container and

in the film. An anti-foaming agent must be

present to suppress the tendency toward foaming during manufacture, filling of containers,

and application. The pH of the system is an

important factor in stability. Styrene-butadiene and acrylic latexes must be maintained on

the alkaline side; for best results, a pH range

of 9.0-9.5 is usually maintained. Polyvinyl

acetate latexes are slightly acidic. Finally,

when the paint is applied to a surface, the in- dividual resin particles must somehow fuse to form a continuous film upon evaporation of the

water vehicle.

2.3.5.5. Film formation results from co- alescence of the latex particles under the influ- ence of driving forces generated when the resin

particles are brought into close contact as the

water evaporates. As the last portions of water

are vaporized, capillary forces act strongly to deform the latex (polymer) spheres, thus filling

the voids left by the vacating water, so that

fusion of the particles takes place and a con- tinuous film is formed. If the polymer is too

rigid to deform sufficiently under the forces

generated, then coalescence does not occur and

a spongy or powdery structure rather than a continuous film remains after evaporation of

the water.

The polymer deformability upon which film

formation depends has its basis in the physicochemical structure of the polymer that determines the modulus of elasticity and the glass

transition temperature, Te. At temperatures

above Te, a rigid polymer acquires rubber-like

characteristics which permit it to deform appreciably under relatively low stresses. The temperature at which a latex paint is applied is therefore the most important environmental

factor involved in film formation. Application

at low temperatures (below 50 °F or 10 °C)

should be avoided, since drying and coalescence

suffer. Porous surfaces require higher temperatures for film formation than non-porous

surfaces. Film formation is also influenced by

the size and the size distribution of the latex

particles, the quality of the dispersion (freedom from flocculation) , the pigment volume

concentration, and the presence of coalescing

aids. A small quantity of coalescing agent, in

the form of a high-boiling solvent or plasticizer

that acts to soften the polymer spheres during

the final stages of drying, is sometimes incorporated as an aid in film formation. The time

required for film formation depends on the rate

of water removal as determined by the temperature, the relative humidity, the porosity

(water demand) of the substrate, and the film

thickness. The final latex film, while continu- ous and water-resistant, remains permeable to water vapor (i.e., it "breathes") and therefore

does not tend to blister when applied over damp

surfaces.

2.3.5.6. Chemical types.—Three different

types of latex paints, based on the general

chemical nature of the polymer employed, are

presently in wide use. These are: (1) the

styrene-butadiene type, (2) the polyvinyl acetate type, and (3) the acrylic type. The most

important properties of each type are briefly

considered below.

2.3.5.6.1. Styrene-butadiene latex was the

first commercially available type, is the lowest

in cost, and has the largest market. The chemical unsaturation imparted by the butadiene

portion of the copolymer allows initial use of a soft yet strong film-former to obtain maximum pigment binding and film continuity,

while laying down a film which subsequently

hardens rapidly (through a curing process similar to that which takes place in oleoresinous

paints) to a tough, durable coating. However,

this reactivity also results in yellowing and

eventual embrittlement on aging. Protective

colloids such as methyl cellulose and casein, despite their adverse effect on water resistance, are usually necessary in styrene-butadiene la18
texes to stabilize the emulsion. The latex is compatible with a wide range of long-oil alkyds,

heat-bodied drying oils, and long-oil varnishes.

It may be blended with these (after first emulsifying the modifier) to obtain a wide

variety of end-use properties. Rutile titanium

dioxide is the preferred pigment for maximum

hiding and good non-chalking properties.

Styrene-butadiene latexes have found wide

application in flat and low sheen interior paints,

exterior masonry paints [5], high-solids tex- ture paints> and in primer-sealers. The some- what objectionable odor of the wet film disappears soon after drying. Specialized uses

include coating the interiors of tank cars carrying hot caustic soda, the dip-coating of mattress

springs, and as a baking-type primer for automobiles. Industrial gloss and semi-gloss paints

have been developed which are suggested for use on metal furniture, automobile interiors,

drums, and motor shells. One of the latest

developments in styrene-butadiene latexes is an industrial type primer in which the re- sidual chemical unsaturation is utilized to ef- fect further polymerization in situ through the

use of catalysts and heat to produce a hard,

solvent-resistant film having improved adhesion

to metal and excellent salt spray resistance.

Suggested end uses for this primer include

automotive parts and bodies, metal furniture,

electrical equipment, refrigerators, and airconditioners.

2.3.5.6.2, Polyvinyl acetate (PVA) latexes are intermediate in cost and usage among the

three main types of latex. Polyvinyl acetate

itself is usually plasticized in order to yield a

film having adequate flexibility. For this purpose it may be internally plasticized (preplasticized) by copolymerization with a suitable

monomer, or it may be externally plasticized

(post-plasticized) by blending with 10-15 percent of a chemical plasticizer such as dibutyl

phthalate or a polymeric plasticizer such as an

alkyd resin. Maximum durability is obtained

either by pre-plasticizing or by the use of polymeric plasticizers that remain in the film

throughout its life. The chemical plasticizers

gradually volatilize, and their loss eventually

results in embrittlement of the film. PVA emulsions are slightly acidic and may

cause corrosion of unprotected metallic containers with resultant impairment of emulsion

stability by the metallic ions liberated.

PVA latexes have excellent sealing properties and find wide use in primer-sealers. They

have performed well in interior finishes and

have found use also in exterior coatings [6]

for masonry, asbestos shingle, and wood such

as yellow pine and red cedar. The PVA film

has a relatively high permeability to water

vapor that permits "breathing" and hence

minimizes blistering. Like other types of latex,

it does not penetrate chalky surfaces sufl[lciently to provide a good bond to the substrate. Although PVA formulations said to adhere to chalky surfaces have been offered recently, not

enough experience with these is available at

this writing to reliably assess their effective- ness. In addition to the usual flat finishes,

PVA emulsions for use in gloss and semi-gloss

interior paints have recently been developed.

These have been formulated at pigment volume

concentrations ranging from 10 to 25 percent

and at a solids content of about 50 percent.

2.3.5.6.3. Acrylic latexes are based on a filmforming acrylic copolymer that is tough, adherent, flexible, color-retentive, and resistant

to ultraviolet light, alkalies, oil, grease, and

moisture. The latex emulsion may be formulated into emulsion paints with a minimum of

additives, thus preserving all the desirable

properties of the acrylic resin base. Although

the acrylics are highest in cost among the three

types of latex available, their excellent combination of properties continues to win them an

increasing share of the latex paint market. In

addition to their excellent durability, they are

easily applied, are practically odor-free, dry

rapidly, and develop excellent resistance to water within an hour or two after application.

Acrylic latex paints [7] have shown excellent

exterior durability in a wide variety of cli- mates, particularly when applied over masonry

surfaces. Recently, they have been offered also as exterior house paints on wood surfaces that

have first been sealed with a zinc-free oil-type

primer. Weathered oil paints on exterior wood

surfaces also can be successfully recoated with

acrylic latex paint. Excellent primer-sealers

for porous surfaces can be formulated from

acrylic latexes, over which virtually any type

of topcoat may be used, including water paints,

oil paints, enamels, varnishes, and lacquers. A

new, thermosetting acrylic emulsion polymer is now available which, it is claimed, yields in- dustrial baking finishes of excellent hardness,

impact resistance, adhesion, and durability

superior to that of premium melamine-alkyds,

while equaling the latter in gloss and stain

resistance.

2.3.5.6.4. Latex blends and copolymers seeking to combine the most desirable properties

of each component into a single superior material are receiving a great deal of attention

from latex paint manufacturers. Among the

more promising recent developments reported

are vinyl-acrylic copolymer dispersions that

produce films having very good clarity, gloss,

flexibility, and water resistance. The vinylacrylic latexes are said to possess high pigmentbinding ability, excellent thermal and mechani19
cal stability, insensitiveness to electrolytes, and

good film-forming ability at relatively low

temperatures. They have been formulated into

fiat wallpaints, semi-gloss enamels, baking coatings, and primer-surfacers. The latter is a sandable first coat which serves both a priming

and a surface-smoothing function.

Latex copolymers may be further modified

by blending with various compatible resins to obtain other desirable properties, such as improved adhesion, greater toughness, or lower

cost. The formulating possibilities, as with

conventional resinous coatings, are virtually

limitless, and the full potential of versatility

and usefulness of modified latex copolymers

has only begun to be realized.

2.3.6. Water-Solution Coatings

A natural outgrowth of the wide acceptance

of latex paints in the consumer field has been

the increased attention being given by the paint

industry to the development of water-thinnable

coatings for industrial applications, where the

protection and durability requirements are more severe. In such coating material

combining the convenience and safety of latex

paints with the film integrity of solution-type

coatings would be highly useful.

The use of water paints goes back to early

civilizations. The ancient Hebrews and Egyptians mixed milk curds and freshly burned lime

"on the job" to make a binder (calcium

caseinate) for the earth colors with which they

decorated the interiors of their houses. This

type of painting still persists in some of the

rural villages of middle Europe and elsewhere.

Colonial Americans often mixed the clabber

from skim milk into the whitewash, made of

slaked lime and water, which was their main

coating material ; oil paints, though known and

used during this period, were an expensive

luxury. Whitewash (see sec. 2.3.9.2) remained

in favor as both an interior and exterior coat- ing until the latter part of the nineteenth

century, when powder paints for mixing with

water before use were introduced. These were

the kalsomines, consisting of glue-bound clay

and whiting. Although factory-made casein

paints in powder form were introduced at about

the same time, it was not until the 1920's that

casein paints reached large volume usage.

Later, to improve the water resistance and

durability of casein paints, they were modified

with alkyd emulsions. Soon casein assumed a secondary role as emulsifying agent for the

alkyd resin in emulsion paints (see sec. 2.3.5.2)

.

The latter gave way to the latex emulsion

paints which, ever since their introduction in

1949, have dominated the water-based paint

market.

Meanwhile, the quest for a durable watersolution type coating has continued. The basic

problem has been to render water-insensitive

the film formed from an initially water-soluble

resin. In recent years, a number of synthetic,

water-soluble resins have been developed which

have a built-in chemical reactivity that permits

them to be insolubilized (see sec. 3.16.2) . These

resins will dissolve in water to form a singlephase solution that is infinitely dilutable with

water (unlike latex emulsions which are twophase dispersions of resin-in-water that are capable of only limited dilution). Conversion

(insolubilization) of the film laid down from

such a solution is accomplished by a chemical

cross-linking reaction between polar groups in

the water-soluble molecule and an added re- active resin or drying oil, under the influence

of heat, oxidizing agents, and/or driers. Although some air-drying types of water-solution

coatings have been made, baking is generally

required to develop optimum film properties.

Baking schedules of about 30 minutes at 300 °F

(149 °C) are typical.

All the advantages (and disadvantages) of a water vehicle inherent in latex paints (see

sec. 2.3.5.1) apply also to the water-solution

type. The fire hazard, toxicity, odor, and

expense of organic solvents are eliminated

while the advantages of a true solution-type

coating, such as good film clarity, high gloss,

and excellent film continuity are retained. The

latter property is reflected in desirable performance characteristics such as low moisture

and gas permeability, and excellent water and

salt spray resistance—properties of particular

importance in coatings for metals.

The formulation of water-solution type paints

does not require the use of wetting agents,

thickeners, stabilizers, and bactericides such as are needed in latex paints. The only ingredi- ents generally necessary are pigment, resin,

water, and (in some instances) driers. A

small amount of high flash aromatic solvent or odorless mineral spirits may be added in some cases to function as a defoamer.

Application of water-solution coatings is usually accomplished by dipping, flow coating, or conventional cold spray ; however, hot spray,

airless (high pressure) spray, and electrostatic

spray also have been employed to some extent. While the number of satisfactory types of

water-solution coatings presently available is not large, technological progress in this area

continues. Durable types have been developed

which are reported to combine the outstanding

gloss and color-retention characteristics of a nondrying alkyd with the hardness, toughness,

and adhesion of a drying-oil alkyd; others are

reported to possess the hardness, gloss, and

durability of the best melamine-alkyds [123]

(cf. sec. 3.16.2).

A water-soluble linseed oil that can be formulated into house paints having properties

comparable to those obtained with conventional

20
linseed oil also is available [121].

It is probable that the next few years will see the development of a greater variety of

water-soluble convertible resins, including sat- isfactory air-drying types for use on wood and

other surfaces that do not permit baking.

2.3.7. Multicolor Finishes

Multicolor finishes—sometimes called "polkadot paints"—consist essentially of discrete particles of viscous coating material suspended in

water containing a suitable stabilizing agent.

The stabilizer is used in an amount sufficient to prevent coalesence of the particles but insufficient to cause their emulsification. A variety of colors and particle sizes and shapes can

be incorporated in a single multicolor finish by

first preparing separate dispersions of each

color and then mixing them in the desired proportions. Each color retains its particular identity in the final mixture for the same reasons

that it remains in discrete particle form in its own individual dispersion. The stabilizing

agent, when properly chosen and used in the

correct concentration, forms a protective film

around each coating particle which effectively

isolates it from neighboring particles in the

aqueous medium, irrespective of their color,

provided that certain basic requirements are

fulfilled during manufacturing. The literature

[8, 9] indicates the following to be the most

important factors:

(1) The solvents used in the coating material phase must have a low solubility in water,

and the water must have a low solubility in the

solvents.

(2) Pigments employed should be non-bleeding either in the solvents used or in water.

(3) The specific gravity of the coating material and of the stabilized aqueous medium must

be closely controlled ; too great a difference will

result in coalescence during storage.

(4) Particle size is largely controlled by the

speed of stirring, the size decreasing with in- creased speed of agitation. The addition of

wetting agents or an increase in the amount

of stabilizer also reduces the particle size. The

viscosity and the temperature of the coating

material phase also have an effect, the size of

the particles increasing as the viscosity or temperature-is increased.

(5) Particle shape depends primarily on the

viscosity of the pigmented coating phase but is also affected by the type and amount of stabilizer employed. Higher viscosities, in addition

to increasing the particle size, yield globules

and filaments instead of spheres.

Although the particle size depends strongly on the speed of agitation, the time of agitation

appears to have relatively little effect provided

sufficient time for adequate dispersion is al- lowed. Only gentle stirring is needed after

storage; rapid stirring or violent shaking will

alter the physical characteristics of the multicolor material and should be avoided.

Most present-day commercial multicolor fin- ishes are based on cellulose nitrate as the chief

film-forming component; however, finishes

based on alkyd, acrylic, vinyl, and other resins as well as on varnish or oil-base vehicles are

possible or available. A Federal specification

[10] describes a multicolor lacquer based on a

blend of cellulose nitrate with an alkyd resin.

A typical commercial interior type blends cellu- lose nitrate with a rosin ester. Water-soluble

organic polymers (protective colloids) such as methyl cellulose or polyvinyl alcohol are the preferred type of stabilizing agent and are effec- tive at relatively low concentrations (0.3 to 5.0%). Various other materials—organic and

inorganic, soluble and insoluble—in finely di- vided form may be employed; the insoluble

types must be used in much greater concentration (perhaps as high as 25 percent of the

aqueous phase) to be effective.

Multicolor finishes are usually applied as heavily sprayed coats that completely mask the

surface. If desired, they may be lightly sprayed

to produce a "spatter" coat over a colored base

coat. They have also been used as a sprayed

coat on paper or textiles subsequently drawn

under a doctor blade to produce a striped effect. The large size and viscous character of the

dispersed coating particles in multicolor finishes

permits their application to highly porous sur- faces without excessive penetration. Thus, dissimilar adjoining surfaces such as masonry

and wood, and rough or imperfect surfaces, can

be masked completely to give a uniform appearance. Heavy films, 5 mils or greater in

thickness, can be sprayed in one application

without sagging or wrinkling. Multicolor fin- ishes can be apphed to damp (but not fresh

alkaline) surfaces without difficulty because

their porous nature permits substrate moisture

to evaporate without blistering the film. No

primer is required for porous surfaces such as wood, masonry, plaster, wallboard, cloth or

paper. A primer may be useful, however, over non-absorbent surfaces such as metal.

The limitless color combinations and wide

variety of patterns provided by multicolor fin- ishes, coupled with the good durability exhibited by many of the types now available, have

led to their use in continually increasing quantities in both interior and exterior applications

for hotels, public buildings, industrial plants,

homes and furniture.

2.3.8. Fire-Retardant Coatings

2.3.8.1. The primary function and effective- ness of present fire-retardant coatings is in re- ducing the surface flammability of substrates over which they are applied. To accomplish

21
this, a fire-retardant coating must itself be

essentially nonflammable (non-flame-propagating). Additionally, it may provide a limited

degree of shielding and insulation, against the

heat generated by a fire, thereby delaying the

time for the substrate to reach the ignition

temperature or—in the the case of a non-combustible substrate—the time for the heat to pass

through the substrate to combustibles on the

other side.

Not even the best fire-retardant coating can

be expected, however, to provide protection for

long under the onslaught of a full-scale fire,

particularly in a confined area.^ After the few

minutes of initial delay, the chief value of a

fire-retardant coating is in reducing the rate

of further flame spread rather than in preventing substrate ignition or structural failure in

the immediate area under attack by fire. However, while fire-retardant coatings have not

proven capable of adding significantly to the

fire endurance of buildings and other structures

as measured in terms of hours of fire exposure

before collapse, the few minutes delay they

afford in the early stages of a fire can be of

critical importance to the escape of personnel

and to the ability of the fire department to control the blaze and minimize damage. It has

been aptly said, from the viewpoint of the fire- fighters who must battle a blaze, that "the first five minutes are worth the next five hours."

The fire-retardant coatings discussed herein

should not be confused with heat-resistant coat- ings (see sec. 3.9.1) of the type employed on

surfaces that become or remain hot in normal

service (e.g., stoves, boilers, hot pipe liners,

etc.). The effectiveness of the heat-resistant

coatings depends on their ability to withstand

more or less continuous exposure to elevated

service temperatures without igniting and without undue impairment of their protective and

decorative qualities, rather than on their ability to withstand direct attack by fire; hence they

are not properly classed as fire-retardant.

The term "fireproof" is not meaningful when

applied to coatings, since it implies a complete

and permanent resistance to fire that is not

possessed by any organic coating material, no

matter how formulated or modified. Reliance

upon so-called "fireproof" coatings to the exclusion of other recognized fire-preventive measures and fire-fighting devices can have disastrous consequences.

As for fire-retardant coatings—these are a

supplement, not a substitute, for adequate struc- tural design, good housekeeping, care in handling materials, fire extinguishers, sprinkler

systems, fire exits, and the fire department.

From the standpoint of formulation and use,

^ A fire-retardant material incorporating chopped glass fibers is available which does significantly increase the fire endurance of wood or steel structures; however, this material is applied at a

thickness (about one-quarter inch) far in excess of that normally

designated as a coating.

and for purposes of discussion, it is desirable

to distinguish between fire-retardant coatings

that are merely flame-retardant (subsection

2.3.8.2) and those which provide the additional

function of heat insulation (section 2.3.8.3),

while bearing in mind that the distinction between the two vanishes, for practical purposes,

after the first 5 to 15 minutes of exposure to a

large-scale fire. A discussion of these two

types follows.

2.3.8.2. Flame-retardant coatings may be

described as those which, though decomposed

by exposure to fire, serve to retard the spread

of flame, although they may not significantly

insulate the substrate from the heat generated.

They have application in situations where a

fire of only short duration is to be expected

(e.g., in an area where only small amounts of

combustible material are stored), or to keep a

localized ignition (such as that from momentary contact with a torch or heated object)

from spreading across the exposed surface.

Flame retardancy in such coatings is achieved

by the use (to the extent feasible) of non-flamepropagating resins and plasticizers, by the use

of fusible inorganic salts and pigments (e.g.,

borax) that form an air-excluding glaze on

heating, and/or by incorporating special agents

(e.g., brominated compounds) that inhibit the

tendency to ignite.

The ignition temperature of a flammable

solid is the temperature at which flammable

gases are generated and ignited. By incorporating into the material to be protected chemical

agents that will convert flammable gases to non-flammable ones as they are generated, an

effective degree of flame retardancy can be

achieved. Lewis acids, such as hydrogen halides or phosphorus oxides, are effective agents

that appear to function in this manner. Although these cannot be used directly in a paint

because of their hazardous, corrosive properties, compounds have been developed that are capable of giving off the desired active agent

upon heating. Among the most successful of

these have been brominated compounds. The

bromine can be incorporated directly into the

molecule of resins such as alkyds and epoxies.

A brominated phenol is compatible with, and

may be physically blended with, phenolic resins

to render them flame-retardant. Vinyl coat- ings can be rendered self-extinguishing by using

a small percentage of a brominated organophosphate compound, such as tris (dibromopropyl) phosphate, as part of the plasticizer

in the formulation.

2.3.8.3. INSULATIVE FIRE-RETARDANT COATINGS

also possessing flame-retardant properties have

proven to be the most effective type in impeding

the progress of a fire. Of these, intumescent

coatings that bubble and swell on heating to

22
form an insulating mat between the fire and

the substrate have been the most successful.

The blowing agents employed in such coatings are sometimes water-soluble compounds that are susceptible to leaching; the coatings therefore may be somewhat water-sensitive and may

not retain their full intumescent properties

after periods of prolonged (particularly exterior) service.

The degree of protection afforded by a particular fire-retardant coating depends not only on

its intrinsic fire-retardant properties (i.e., nonflammability, intumescence, insulative character, strength of charred matrix, etc.) but also on the thickness at which it is applied and on

the nature of the substrate. On completely

non-combustible substrates, such as ceramics

and most metals, application of any organic

coating, even a nonflammable one, will increase

to some extent the rate of fire spread along the

surface—the rate of spread increasing with

coating thickness, at least within the practical

range. However, since metal surfaces generally require such a protective coating to resist

corrosion, the somewhat reduced resistance of

the surface to fire must be accepted. In such

instances, the reduced surface resistance to fire with increasing thickness may be offset to some

extent by the greater heat-insulating effective- ness of the thicker coating. As already pointed

out, the heat barrier furnished by the insulative

type of fire-retardant coating, though only temporary, can be of vital importance in the first few minutes of a fire. On combustible substrates, application of a

fire-retardant coating of either the flame-retardant or insulating type affords increasingly

greater fire protection as the coating thickness

is increased [19] . In fact, any coating that is less flammable than the substrate will afford some degree of protection to that substrate.

This latter fact sometimes results in erroneously

attributing significant fire-retardant properties

to a coating material that in reality is contributing only the normal effect of added thickness.

Such misevaluation may not only involve the

needless expense of paying for a fire-retardancy

that is not achieved, but can also create a dangerous sense of false security about the protection afforded.

2.3.8.4. Evaluation of fire-retardant coatings.—Although the ultimate and only fully

reliable test for the effectiveness of a particular

fire-retardant coating is its actual service performance, several laboratory and large-scale

test methods have been developed which give a good indication of the performance that may

reasonably be expected from a given material

under specified conditions. Such methods are

particularly useful in screening out inferior

materials that do not merit the expense of

large-scale tests or the risk of actual service

tests. Among test methods recognized as useful by those concerned with problems of fire protection are the following:

(1) The standard "tunnel" test developed

by Underwriters Laboratories [11, 12].

(2) The radiant panel test devised by the

National Bureau of Standards [13, 14].

(3) The flame spread test described in Federal Specification SS-A-118b [15].

(4) The "cabinet method", in which measurements are made of the weight loss and char

volume produced by a standard flame that con- sumes 5 ml of absolute alcohol [16].

(5) The "stick and wick method", in which

the weight loss, rate of vertical flame spread,

and char height are measured [17].

(6) Various test methods developed by commercial firms, such as the tests with standard

bowling pins and with load-bearing steel members used by the Fire Research Laboratory of

Union Carbide Chemicals Company [18].

(7) Full-scale experimental fire tests on ex- pendable buildings or structures. Among the test methods (other than full-scale

tests) cited above, the tunnel test is the most

generally accepted means available for evaluating the flame-spread characteristics of building materials. The more recently developed

radiant panel test was designed to be less elaborate and expensive and has given highly re- producible results, while appearing to correlate

fairly well with tunnel results. However, the

radiant panel test, largely because of its small

scale, has not yet been widely accepted by code

officials. Full scale fire tests, while highly in- formative, are too expensive for routine evaluations.

In interpreting fire tests, it should be borne

in mind that different test methods may give

diflferent results with different types of materials. For example, a highly reflective coating

(e.g., an aluminum-pigmented material) may

perform better than a matte coating in the radiant panel test and yet appear inferior by a flame-impingement test. Similarly, tunnel test results may not coincide with results obtained

in a load-bearing test or with a test house.

Nevertheless, such tests are a necessary prerequisite in the screening of fire-retardant materials. Appropriatelj' applied and wisely in- terpreted, the results obtained are very useful.

2.3.8.5. Selection of fire-retardant coating SYSTEMS.—From the information that has

been presented on the nature and effectiveness

of fire-retardant coatings, a basis for their

selection may be given.

Flame-retardant coatings (and impregnants,

with which this volume does not deal) are useful in the following situations:

(1) To prevent the ignition of combustible

fabric surfaces (e.g., draperies or upholstered

23
furniture) that may inadvertently come into

brief contact with a flame or heated object.

(2) To avoid the rapid spread of flame along

combustible surfaces such as cellulosic wallboard or ceiling materials.

(3) In combustible enclosures where only

small quantities of flammable materials are

present, since in such cases the fire (fuel) is likely to burn itself out before the blaze becomes

widespread.

(4) To reduce the danger of deadly flash

fires in susceptible public areas such as schools,

theatres, night clubs, tents, and exhibit buildings, where the loss of life can be very great

although such coatings should by no means be

considered to have entirely eliminated the hazard of fire. Insulative (intumescent) fire-retardant coat- ings are useful: (1) Where a brief delay in

transmission of heat from a fire to a combustible substrate may prevent ignition or serious

damage. For example, utility and telephone

poles protected with a weather-resistant coat- ing (applied to glass screening stapled to the

pole) have withstood repeated brush and grass

fires without suffering significant structural

damage ; such fires usually will burn completely

past a stationary object in two or three minutes, an interval during which the coating is capable

of providing full protection to the structure.

(2) In aircraft, particularly on large compartmentalized types. Although the efi'ective- ness of fire-retardant coatings for aircraft has

not been fully evaluated, there is reason to believe that in some kinds of fires aboard aircraft

in flight, personnel might have time to bail out

or possibly make a safe forced landing if vital areas of the aircraft could be insulated from the

heat of the fire for only a few minutes. Despite the existence of a definite interest in such

coatings, their use on aircraft has not been

widespread.

(3) On ground structures within quick range

of fire-fighting equipment, as at airfields and

military installations, which may be saved from

severe structural damage if ignition of combustible substrates can be delayed long enough

for the fire-fighting equipment to be brought

into action.

(4) To provide heat insulation as well as flame retardance on walls and ceilings of

schools, office buildings, and other heavily populated compartmented buildings, where every

minute of delay in the transmission of heat

from one room to another can be of vital importance in permitting personnel to escape, even though the ultimate fate of the structure

itself may be little affected.

A number of Federal and Military Specifications relating to fire- and flame-retardant

coatings are available. Although all of these

specifications refer to "fire-retardant" coatings,

many of the coatings may be more aptly described as flame-retardant according to the

criteria set forth in the preceding paragraphs,

since their primary function is to reduce flame

spread rather than to insulate the substrate

from the fire. For example, flame-retardant

coatings of conventional coating thickness

rather than heavy intumescent fire-retardant

coatings are specified for use on metal bulkheads and overheads of navy ships [19, 20, 21,

22, 23, 24], where the primary concern is to minimize flame spread via the paint (both directly and through heated bulkheads) and to

limit the production of toxic decomposition

products in the confined space aboard ship.

On the other hand, fire-retardant coatings of

the intumescent type [25, 26] have generally

been the most effective and are usually specified

for wood surfaces, where the combustible nature

of the substrate requires the greater protection

that is afforded by the insulative type of fire- retardant coating in order to avoid quick ignition.

2.3.8.6. In summary, it should be pointed out

that while there exists general agreement as to the ability of fire-retardant coatings to impede

the progress of a fire, there is considerable question as to the extent and effectiveness of the

protection provided. It must be emphasized that

the protection afforded by even the best fire- retardant coating is limited to the early stages

of a fire. However, to the extent that the delay

afforded enables personnel to escape and fire- fighting equipment to be brought into action,

valuable protection is achieved. Once the fire reaches full-scale proportions, no coating will

appreciably affect the course of the blaze.

In many applications, flame-retardant and

insulating types may perform equally well. In

fact, it may often not be known whether a given

fire-retardant paint is one type or the other

unless specific information is provided by the

manufacturer. There are situations, also, in

which a coating of demonstrable fire-retardant

effectiveness may be unsatisfactory for certain

applications because of deficiencies in conventional paint properties such as appearance, durability and weathering resistance ; or the thick- ness required may be too great for some applications (as aboard high-speed aircraft, where

weight must be kept to a minimum) . It is probable that the shortcomings of fire-retardant

coatings with respect to some desirable paint

properties have been a strong factor in slowing

their widespread adoption.

Although there remains much room for improvement in the formulation and efficacy of

fire-retardant paints, continuing developmental

work is being carried on by a number of paint

manufacturers, and it is likely that the role of

24
such paints will become increasingly important

as their effectiveness is increased.

2.3.9. Gold Water Paints (Inorganic)

Water-base paints utilizing organic binders

of the latex type and water-soluble type were

discussed in sections 2.3.5 and 2.3.6, respectively. The present section briefly describes

two other types of cold-water paints which,

though inorganic or largely inorganic in nature,

have important application in the coatings field.

2.3.9.1. Cement-water paints are very useful for concrete and masonry surfaces (see sees. 2.4.3 and 4.4) . They are composed chiefly of

white Portland cement together with some hydrated lime or siliceous aggregate or both, depending on the requirements of the application.

Cement-water paints of two types (medium or high cement content) and two classes (without

or with siliceous aggregate) are described by

Federal Specification TT-P-21 [32]. The lime

contributes easier brushability and, if not used

in excess (not more than 30 percent by weight)

,

assists in obtaining a hard, weather-resistant

coating. The aggregate (usually a white or

light-colored silica sand) is very useful in ce- ment-water paints that are to be applied to open-textured masonry surfaces where filling

is needed; two parts by volume of portland

cement to one part of aggregate produces a good

fill coat composition. For use as a "grout

coat" on very coarse surfaces such as cinder

block, equal parts by volume of cement and sand

are most effective. Proprietary formulas for

general-purpose cement-water paints often add

small proportions of other ingredients to the

white Portland cement and lime. For example,

calcium chloride may be added for its hygroscopic effect in drawing moisture from the air,

which promotes proper curing and hardening.

Titanium dioxide or zinc sulfide pigments may

be added to improve the wet opacity, and col- ored pigments may be added to produce tinted

paints. About one percent of calcium stearate may be incorporated to provide some degree of

water repellency in the dried paint.

The mixture of solid ingredients is furnished

as a dry powder that needs only to be mixed

with water to prepare it for use. The dry material should first be made into a stiff paste by

adding small portions of water with continuous

stirring, after which additional water should

be gradually stirred into the mix until a consistency resembling that of rich cream is obtained (except that a slightly thinner consistency is desirable for the first coat applied on

open-textured masonry surfaces) . Cementwater paints normally remain in usable condition for three or four hours after being prepared for use; however, small additional

amounts of water need to be added (with thorough stirring) during this period to counteract the tendency of the paint to stiffen as a re- sult of its reaction with the water and the loss

of water by evaporation.

Concrete and masonry surfaces that are to receive cement-water paints should be cleaned

free of dirt, oil, grease, and efflorescence as described in section 6.4. Since cement-water

paints are completely resistant to the moisture

and alkalinity in concrete, little or no aging of

the surface is required. Previously peinted

concrete or masonry surfaces must be cleaned

free of all masonry coatings, whitewash, or weak, crumbly cement paint (see sec. 6.6.4.3).

Application of cement-water paints is best

accomplished with a stiff fiber brush which permits the paint to be scrubbed into the voids of

the surface. The surface should be uniformly

damp but not wet with water before the paint

is applied. The paint should be applied in two

coats with at least 24 hours allowed between

coats. The first coat should be dampened with

water before applying the second coat. Excessively thick or thin coats are undesirable. Thick

coats are somewhat prone to crack or chip

from the substrate ; also, it is more difficult to apply them uniformly. Thin coats lack opacity

and durability. Too wet a mix will produce

a powdery texture when dry, as well as too thin

a coating. The proper spreading rate for ce- ment-water paints depends on the texture of

the surface; as a rough guide, the spreading

rate may be about 100 square feet per gallon

for two coats on smooth masonry and about 50

square feet per gallon for two coats on rough

masonry.

Proper curing and hardening of cementwater paint into a durable, stress-free coating

that will not be prone to cracking, crumbling,

or powdering depends on the continual availability of adequate moisture for reaction with

the Portland cement, especially during the first few days after application. To accomplish the

necessary damp curing, the painted surface

should be wet down with a fog spray two or three times a day, starting within 6 to 12 hours

after application of the first coat and continuing for at least 48 hours after application of

the final coat. Properly applied and damp-cured, cementwater paints provide a durable coating for ex- posed concrete or masonry surfaces above or below grade, inside or outdoors.

2.3.9.2. Whitewash, for centuries before

the advent of modern coating systems, was

widely used as a readily available, inexpensive,

general-purpose coating for interior use on wood, plaster, concrete, masonry, glass, and

metal. While lacking the durability and the

protective and decorative qualities of many

present-day coatings, whitewash still finds some

use as a low-cost, easily applied coating for

25
sheds, fences, tree trunks, concrete curbs, gasoline station islands, brickwork, cinder block,

and other noncritical applications in mild en- vironments where its flat, extreme chalky-white

appearance gives a clean, pleasing effect. In

particular, whitewash still finds favor as a coat- ing for the interior walls and ceilings of dairy

barns, for farm implements and structures, and

as a light- and heat-reflective coating for coal

tar pitch or asphaltic surfaces to reduce their

tendency to soften, sag, wrinkle, or run when

exposed to direct sunlight in hot climates.

Whitewash in its simplest form is prepared

by slaking quicklime or soaking hydrated lime

in water, the latter method being preferable be- cause of its greater safety, convenience, and

uniformity of product. Approximately 8 gallons of stiff" lime paste are produced by slaking

38 pounds of quicklime with 8 gallons of water,

or by soaking 50 pounds of hydrated lime in 6

gallons of water. Before use, sufficient water is stirred into the paste to obtain a thin consistency resembling that of whole milk. To improve

durability, other materials are nearly always

added to the mix. For general use on wood,

glass, and metal, the additives most often used

are common salt and calcium chloride, which

are dissolved in the water to be used for diluting

the original lime paste. The use of calcium

chloride rather than salt reduces the tendency

of the whitewash coating to chalk. For use on

concrete and masonry, more satisfactory results are obtained with a whitewash consisting of

equal parts by weight of hydrated lime and

white Portland cement (a composition approaching that of the cement-water paints described in the preceding section) with sufficient

water to produce a mixture having the consistency of heavy cream. Other materials which

may be added to whitewash for various purposes include casein plus formaldehyde for in- terior walls of dairy barns, casein plus borax

to produce a fire-retardant coating, animal glues

for use on plaster, titanium or zinc sulfide pigments to improve opacity, and iron oxide pigments for tinted coatings. More detailed information on whitewash is available in a bulletin

by the National Lime Association [126].

2.4. Primers

A primer is applied as the first coat of paint on the substrate (sometimes preconditioned)

to be painted. Its chief function is to provide a

satisfactory bond between the finish coats and

the substrate. The primer may also be called

upon to impart other important properties to the system, such as corrosion resistance, water

resistance, substrate sealing, enamel hold-out,

and some degree of substrate filling and hiding.

The choice of vehicle and the proper pigmentbinder ratio are important considerations in obtaining the required degree of wetting, penetration, and adhesion. The formulation is usually such as to yield a satin surface to which

topcoats will adhere well ; pigment volume con- centration is intermediate between that for

gloss and dead flat finishes. The pigments employed are similar to those used in finish coats,

except for the addition of corrosion-inhibiting

pigments in metal primers.

2.4.1. Wood Primers

Primers for wood that is to withstand exterior exposure are usually of the oil or oleoresinous type. The primer (or undercoater, as

it is often called in wood painting) serves to

seal the wood pores, so that good hold-out and

adhesion of topcoats is obtained. The vehicle is usually a blend of heat-bodied and raw or re- fined oils (see sec. 2.2.1), usually linseed, modified with a small amount of resin. Brushing

characteristics, control of penetration into the

wood, and leveling of the paint film are largely

dependent on the ratio of raw to bodied oil. With

larger proportions of raw oil brushing properties are improved and degree of penetration is increased, but leveling is poorer. Conversely,

larger proportions of bodied oil improve level- ing properties, but reduce ease of brushing and

penetration. A 1 :1 blend of raw oil with bodied

oil of consistency approximately "Z" on the

Gardner Scale (equivalent to about 23 poises)

has been widely used.

Zinc oxide is sometimes used in the pigment

portion in place of part or all of the resin in the

vehicle, although its use in wood primers is controversial. Chalk-resistant titanium pigment,

white lead, and extenders usually comprise the

pigment portion. An excellent primer for exterior wood is covered by Fed. Spec. TT-P-25

[111].

Exterior oil paints may be made into so- called "self-primers" by thinning with turpentine or mineral spirits and/or by adding raw

linseed oil. Water-based coatings have not, as yet, proven

satisfactory as wood primers. However, some

water-based paints (e.g., an acrylic or a polyvinyl acetate latex paint) can be successfully

applied on wood that has been primed with an

oil paint.

2.4.2. Primers for Metal

Primers are nearly always required for adequate adhesion of topcoats in coating systems

for metals. They may be either air-drying or baking types. Most metals need extra protection against corrosion which necessitates the

26
incorporation of corrosion-inhibiting pigments

into the primer. The manner in which such pigments function is discussed in section 2.3.1.5. The choice of pigment depends on the metal to be protected. A metal conditioning pre-treatment (see sec. 6.3.2), such as phosphatizing,

when used prior to application of the primer,

greatly improves primer adhesion and corrosion resistance; under these conditions, corrosion resulting from scratches or other breaks

in the coating is confined largely to the immediate area of the scratch or break.

2.4.2.1. Primers for iron and steel are based on a variety of rust-inhibitive pigments

in a suitable vehicle, such as linseed oil, an alkyd

resin, or a phenolic resin-drying oil varnish

[27, 28] (also see list in sec. 8.1.4 and summary

charts in sec. 8.2). Raw linseed oil possesses

the best penetrating properties and is essential

where complete removal of surface corrosion

products has not been feasible. Among pigments

often used in primers for iron and steel are red

lead, basic lead chromate, blue lead, and zinc

yellow, together with iron oxide pigments and

silicate extenders. The time-honored superiority

of red lead is believed to result from its ability to combine chemically with the iron or steel substrate to form a hard, highly adherent, moisture-impervious coating that passivates the substrate against corrosion. The use of a basic lead

silico-chromate pigment to impart corrosioninhibiting properties not only to primers but

also to intermediate and finish coats is discussed

in section 2.3.1.5. The choice of pigment and vehicle depends on

the nature of the application. For outdoor steel structures, primers based largely on red lead

[27] in a linseed oil or long oil-alkyd vehicle are widely used (also see 4.3.1.1). In marine

long oil-phenolic vehicle is often preferred. Under severely corrosive conditions,

vinyl primers [143, 144, 145] in an all-vinyl

coating system give durable service (see sec. 4.3.1.2). Automotive primers often contain

iron oxide as the chief pigment in a short or medium oil-alkyd baking vehicle; amino resinalkyd vehicles are also used. Zinc-rich paints

containing high percentages of zinc dust (see

sec. 2.3.1.5) in an epoxy, epoxy-polyamide,

polyurethane, or other alkali-resistant vehicle are being successfully used as primers for the

undersides of automobiles, for underwater

structures, and for other types of severe marine service where their excellent corrosion-resistant properties justify their higher cost. In- dustrial primers for machinery, refrigerators,

washing machines, and other appliances are

generally baking types based on short oil-alkyd

vehicles or newer vehicles such as epoxy ester- modified alkyds. (A discussion of alkyd resins

is given in sec. 3.4).

Water-base metal primers, mostly baking

types, have been successfully formulated for some applications. For example, a styrenebutadiene latex containing iron oxide pigments

plus china clay and barytes extenders is used

as the primer on some automobiles. A watersolution type primer is available for use underneath a recently-developed, water-solution type

industrial topcoat (cf. sec. 2.3.6.). In these

types, the water-soluble resin is converted to a

cross-linked, insoluble coating by baking.

2.4.2.2. Primers for galvanized iron and

STEEL are most effective when formulated with

high percentages of zinc dust as the chief pigment [29] . The zinc dust, in addition to aiding

adhesion, may provide some degree of cathodic

and barrier layer protection to the underlying

iron or steel (see sec. 2.3.1.5).

2.4.2.3. Primers for aluminum are based

largely on zinc chromate and zinc tetroxychromate as the corrosion-inhibiting pigments in a

variety of vehicles, including phenolic alkyds,

styrenated alkyds, and polyvinyl butyral. The

latter is combined with phosphoric acid and al- cohol just before use to yield a so-called wash

primer or metal pretreatment coating [30]

which provides both a phosphatizing and a priming action that gives excellent results (see

sec. 6.3.2.5). A conventional spray-applied,

fast-drying zinc chromate primer [88] or a low

moisture sensitivity type [54] may be used

[122] over the wash primer to provide an

anchor coat for the finish coats. A relatively

slow drying alkyd-base zinc chromate primer,

for brush or spray application on chemically

cleaned bare metal, is covered by Fed. Spec.

TT-P-645 [89] (see summary chart in sec. 8.2) ; this primer is particularly suitable as an

undercoat for alkyd enamels and for marine

service. Lead-containing pigments are not con- sidered satisfactory in primers for aluminum

because of the possibility of a galvanic action

between deposited lead and the aluminum

which would result in pitting of the surface.

2.4.2.4. Primers for magnesium must be

carefully formulated to obtain adequate adhesion and corrosion resistance. Zinc chromate

is the preferred corrosion-inhibiting pigment

[31]. Pigments such as red lead and other

lead pigments (except lead chromate) that perform excellently in primers for steel may promote the corrosion of magnesium. Acidic vehicles may cause gassing at magnesium sur- faces and so destroy adhesion and accelerate

corrosion. Satisfactory vehicles have been

based on phenolic-modified alkyd resins [54,

88], phenolic-tung oil varnishes, epoxy baking

resins, chlorinated rubber, vinyl copolymer

resins, and polyvinyl butyral resin. The latter

is usually a non-acid type, although a polyvinyl

butyral wash primer with reduced acid content

27
is sometimes used on adequately pretreated surfaces. Whenever magnesium is to receive an

organic protective coating system, proper sur- face pretreatment (usually chromating) before

application of the primer is of great value (see

sec. 6.3.5).

2.4.3. Masonry Primers

Primers for masonry are not a well-defined

class of specially formulated materials as are primers for metal. They are usually made

simply by addition of an appropriate thinner

to the topcoat paint. Latex-base paints [5, 6, 7]

,

which are now the most widely used type for

masonry surfaces, are thinned with suitable

amounts of water. Paints made with portland

cement and lime [32] are also thinned with

water. Oleoresinous [33] and synthetic resin

types, including the various solvent-thinned

rubber-base products [34, 35], require specific

organic solvents which are usually identified

in specifications for the materials and on the

container labels. As a general rule, ordinary

mineral spirits can be used to thin oleoresinous

paints whereas thinners of greater solvent

power, such as aromatic and chlorinated hydrocarbons (along with some aliphatic hydrocarbon diluent), are required for such products

as the styrene-butadiene copolymers (non-latex, synthetic rubber type) , chlorinated rubber,

and others.

Apart from primers made as indicated above,

there are a limited number of products made

expressly for priming purposes. For example,

special primers have been developed to combat

mottling of deep-colored masonry finishes.

Others are designed to alleviate some of the

difficulties encountered in the painting of

chalked masonry surfaces. Although removal

of the chalk is the only sure answer (see sec. 6.4.6), some benefit can be obtained by the use

of special oleoresinous anchor coats (e.g., a

phenolic resin-tung oil varnish) , by use of an

emulsion type surface conditioner containing

special wetting agents, or by stirring some oil (e.g., boiled linseed oil) into latex-base paints

just before applying them.

Latex primers and paints are relatively nonsensitive to water and alkali and provide excellent sealing of concrete, brick, cinder block,

stucco, and other porous masonry surfaces.

They may be applied over damp, relatively un- aged masonry surfaces without difficulty.

(Fresh, uncured surfaces should not, of course, be painted). Although permeable to water

vapor, latex primers are not aff'ected by passage

of the vapor through them or through the sur- face on which they are applied. In addition

to thinning latex paints with limited amounts

of water to make them suitable for masonry

priming purposes, it is good practice also to

dampen the porous surface and thereby satisfy

its water demand.

Oleoresinous primers and paints for masonry

surfaces are generally capable of good performance only when applied to well-aged, clean,

dry surfaces that can be expected to remain

reasonably dry during service. Otherwise, discoloration, blistering, peeling, and other defects

may occur. Resin-emulsion paints used as masonry

primers in the past have been largely replaced

by the more alkali-resistant latex types. However a few alkyd resin emulsion paints are

available [36], and some developmental work

on these materials is continuing.

Cement-water paints [32], despite their low

strength and chalking tendency as often formulated and applied, are used on open-textured

masonry surfaces because of their low cost and

their complete freedom from difficulties arising

from dampness or alkalinity in the substrate.

Because they are highly permeable to water

vapor but are virtually unaffected by water,

they are suitable for application to interior

masonry below grade (e.g., basement walls of

cinder block, etc.). With proper formulation

and wet-curing, their durability is quite good.

(See sec. 2.3.9.1).

Under conditions of prolonged immersion, as

in swimming pools, primers and paints based on a chlorinated rubber vehicle [35] or on a

solvent-type styrene-butadiene rubber vehicle

have been oflfered and have achieved a considerable measure of success.

2.4.4. Primers for Plaster and Wallboard

Unlike masonry primers which usually are simply thinned topcoat paints, primers or primer-sealers for plaster and wallboard are

usually specially formulated materials that differ from finish coats in pigment-binder ratio

and vehicle composition. The term "primersealer" emphasizes the important sealing function to be performed on porous surfaces (see

sec. 2.3.5.3). Both latex-base and oleoresin- ous [37] types of primer-sealer are available.

Either type may be over-coated with virtually

any type of finish coat—latex, oil-base, or alkyd.

A thinned latex-base finish coat paint such as that described by Federal Specification TT-P29 [38] is sometimes used as a primer.

Although the latex-base primer-sealers are

relatively new materials, they are widely used

because of their fast dry, excellent sealing properties, and superior water and alkali resistance.

They may be applied on only briefly aged plaster

surfaces ; however, a drying period of at least

two weeks is highly desirable. They are particularly well-suited for application on gypsum

wallboard [127] because of their nonsensitivity

to the alkalinity in the plastered tape joints

employed and because they do not raise the nap

28
of the outer paper layer as do oleoresinous

types. A phenolic emulsion-type dry wall

primer also is available for use on gypsum

wallboard.

Oleoresinous primer-sealers, while less re- sistant to water and alkali than the latex types,

give satsfactory results when applied to dry,

well-aged plaster surfaces (60 to 90 days of

aging are usually required) . The oleoresinous

primer-sealers offer the good brushability, good

flow-out and leveling characteristics, and good

adhesion typically associated with oil-base

paints. On inferior plaster having a powdery

or chalky surface, oleoresinous primers provide

better wetting and adhesion than is obtained

with the latex types. As always, as much of

the chalk as possible should be removed before

painting.

Composition-board and other non-alkaline

wallboards (see sec. 6.5.2) may be sealed with

either the latex or oleoresinous type of primersealer with good results.

2.5. Bituminous Coatings

2.5.1. Hot Applied

Bituminous coatings based on coal-tar pitch or on natural or petroleum-derived asphalt find

wide use in underground and underwater applications, in roofing", and for general waterproofing and damp-proofing applications on concrete

and metal in buildings, dams, penstocks, reservoirs, tanks, sewage disposal plants, etc. Their

usefulness stems chiefly from their virtually

complete imperviousness to water and moisture

in the thick coats (3/32 to 1/2 inch) that can

be economically applied. As hot melts or hotapplied mastics, they are the most widely used

coating materials for the protection of buried or submerged steel pipe and other ferrous metalwork.

Coal-tar pitch enamels and asphalt mastics

for hot application are made by compounding

the bituminous base material with mineral

filler. The mineral filler permits application

of a thicker coating, raises the softening temperature, and reduces the tendency of the coat- ing to crack and disbond at low temperatures.

Coal-tar pitch, while superior to asphalt in

water imperviousness and dielectric strength,

is more susceptible to temperature changes and

to the deteriorative effects of atmospheric

weathering. Therefore, pitch is the preferred

bitumen for underground and underwater applications, while asphalt is more widely used

in above-ground and roofing applications. The

low temperature working limit for coal-tar

coatings has been extended down to about —20 °F by incorporating plasticizer into the

pitch to impart some flexibility.

2.5.2. Cut-Backs

Pitch and asphalt may be "cut back" with

suitable coal tar or petroleum hydrocarbon

solvents to give coatings that may be readily

cold-applied. While the straight "cut-backs"

afford little protection because of their susceptibility to cracking from hardening and shrinkage during atmospheric exposure, they are useful as primers on surfaces that are to receive

hot melt coatings of the same substance. Also,

when cut-backs are compounded with suitable

mineral fillers, such as asbestos, durable products are obtained which are used extensively as

protective coatings in roofing and waterproofing applications. An asphalt roof coating of

this type is covered by Fed. Spec. SS-A-694

[112].

2.5.3. Bituminous Paints and Varnish

Useful yet relatively inexpensive coatings are obtained by combining pitch or asphalt

with resins and/or drying oils in a suitable sol- vent medium to yield bituminous paints and

varnishes. Such coatings provide good protection against industrial and corrosive environments at low cost. Although dark in color and

quick to dull upon weathering, they can maintain their protective qualities for long periods

provided they have been carefully applied (at

least three coats) in a manner that ensures freedom from pinholes and holidays (non-wetted areas). An asphalt varnish of general util- ity is described by Fed. Spec. TT-V-51 (see

summary chart in section 8.2) ; while this var- nish is particularly suited for painting interior

water and gas pipes, it may also be used out- doors if sheltered from the sun. Bituminous paints and varnishes may be applied over old oil paints, but oil paints should

not be used over the bituminous types. Oil

paints tend to crack and flake off from bitumi- nous undercoats because of their inability to follow the greater thermal expansion and contraction of the bitumen. Furthermore, the darkcolored bituminous material will eventually

bleed through an oil or oleoresinous topcoat

(see sec. 7.2.2.3).

2.5.4. Emulsions

Bituminous emulsions are available which

possess many of the desirable properties of both

bituminous and water-base coatings. Certain

asphalt and coal tar emulsions are superior in

weathering characteristics to their "cut-back"

counterparts. In fact, clay type asphalt-base

emulsions, such as that included in Mil. Spec.

29
MIL-R-3472 [113], when used as roof coatings, exhibit the best durability among bitumi- nous coatings. Asphalt-base emulsions for use

as thick protective coatings for metal are cov- ered by ASTM D 1187-56 [114] ) ; the type A

material is a quick-setting type for use where

the coating is to be subjected to water immersion, while the type B material is for use on

surfaces exposed to the weather only.

A coal tar emulsion that may be used by itself or as a topcoat over coal tar mastics or coal tar enamel to improve their resistance to sunlight is covered by Military Specification MILC-15203 [116]. This coating is intended primarily for atmospheric exposure; it should be

allowed to dry for several months in good

weather before immersion in water.

2.5.5. References

For a more detailed treatment of the subject

of bituminous materials and coatings, the reader

is referred to the works of Abraham [39] and

to the manuals [40, 41] and specifications [42,

43] of those Departments and Associations in

which these materials have an important role.

Figure 2.1. The NBS Integrometer (center) increases the speed, ease, and precision of adhesion measurements

with the Adherometer (left), by converting the variable stripping force into electric impulses and adding them to give an output that is indicated on the recorder.

(Roberts, A. G., and R. S. Pizer, Protective coating adhesion measurement using an electronic averaging device for the Adherometer, ASTM

Bulletin No. 221, April 1957, pp. 53-58.)

30
Figure 2.3. Explosion of cellulose nitrate film from decomposition in storage.

31
Figure 2.5. Washability machine for measuring scrub resistance of -paints as described in ASTM Method D

2U86-66T.

32
3. Properties of Synthetic Resins for Coatings

3.1. General Nature of Synthetic Resins

Synthetic resins are poljoneric substances of

high molecular weight made up of repeating

molecular structural units, usually in the form

of chains, with a terminating group at each

chain end. As many as a hundred thousand or more repeating units may be combined into a

single polymer molecule having a molecular

weight in the millions. The most useful high

polymers, however, are those with average

molecular weights ranging from a few thousand to several hundred thousand, corresponding to a degree of polymerization (average

number of repeating units per chain) ranging

from about a hundred to several thousand.

The polymer chains may be linear, branched,

or crosslinked—or some combination of these —depending on the functionality and reactivity

of the monomers from which they are formed

and on the manner of polymerization. Whatever the basic chain structure, the solid polymer consists of a closely entangled mass of

kinked, coiled, sometimes folded, and variously

oriented and ordered polymer chains. The physical properties of the bulk polymer are largely

determined by the structure of the polymer

chains, their molecular weight (chain length),

and the nature and magnitude of the intermolecular forces (van der Waals forces) which

are operative between chains in close proximity. The chemical properties of the polymer depend on the chemical nature, i.e., the reactivity

and bond energies, of the linkages between

chain segments, and on the reactivity of side

groups and end groups.

3.1.1. Polymer Structure and Prop)erties

In linear polymers the molecular chains, al- though closely entangled, are separate entities. Hence, they are relatively free to kink

(shorten), uncoil (stretch), or slip past each

other (undergo plastic flow or permanent deformation) under the influence of applied

stresses; they acquire greatly increased mobility (soften) as the temperature is raised ; and

they can be molecularly dispersed (dissolved)

in suitable solvents. Linear polymers, therefore, are permanently fusible and soluble, and

are described as thermoplastic materials. (A

notable exception to this generalization is polytetrafluoroethylene which, though linear and

classified as thermoplastic, is neither fusible nor soluble in the ordinary sense.)

When the polymer chains have a regular molecular configuration that is conducive to alinement and close packing, the bulk polymer exhibits crystalline properties to a degree determined by the number, size, and arrangement of

the crystallites (highly ordered regions.)

While polymers may be simply characterized

as crystalline (ordered) or amorphous (disordered) on the basis of whether or not they are

capable of yielding definite X-ray difi'raction

patterns, seldom if ever is a polymer completely one or the other. Instead, highly ordered re- gions, amorphous regions, and regions of in- termediate order or disorder are randomly interspersed, and the ratio of crystalline to amorphous material strongly influences the mechanical properties of the polymer. This is because

the close proximity of the polymer chains in

crystalline regions gives rise to large intermolecular forces which are manifest as greater

stiffness, greater tensile strength, a higher softening temperature, and increased resistance to swelling and solvation in the crystalline polymer as compared to its amorphous counterpart.

Crystallinity in a polymer mass may be induced or extended by stretching (cold drawing.) Stretching an amorphous polymer causes an alinement of polymer chains which may produce some oriented crystallites. The drawing

of a crystalline polymer causes the existing

crystallites to become oriented along the axis

of draw and may form still more crystallites.

Orientation substantially increases the tensile

strength of the polymer, greatly enhancing its suitability as a fiber. Typical crystalline poly- mers are polyethylene, polypropylene, polyvinyl

chloride, polyvinylidene chloride, polytetrafluoroethylene, and linear polyamides. Among

typical amorphous polymers are polystyrene,

acrylic resins, and copolymers in general (the

latter are amorphous because the random and

irregular spacing of the component monomeric

species prevents the close packing of polymer

chains)

.

In crosslinked polymers, the molecular chains

are tied together and their mobility is reduced

by the many primary valence bonds between

chains. Such polymers are described as thermoset materials and are infusible and insoluble,

except under chemical attack or degradation.

It is evident that the degree of stiffness and as- sociated properties exhibited by a crosslinked

material will depend on the extent of the crosslinking. Thus, a material with few cross links

between chains will deform relatively easily.

The cross links serve to limit chain slippage,

permitting the material to exhibit rubbery or

elastic properties; such a material may swell

markedly in certain solvents yet not dissolve in

them. Highly crosslinked materials, on the

other hand, exhibit a high degree of stiffness,

infusibility, and insolubility.

33
Branched polymers are those in which basically linear molecular chains acquire long-chain

side groups as a result of the random activation of sites along the already polymerized main

chain or by the introduction of occasional tri- functional groups during the polymerization

process. The side chains are similar in molecular structure to the parent chain and may grow

to considerable length, with side chains of their

own, so that a highly branched structure with

properties resembling crosslinked polymers

may be built up.

Crosslinked or thermosetting polymers, be- cause of their infusible and insoluble character, are generally superior in overall chemical

resistance and dimensional stability to the thermoplastic types. The thermoplastic polymers,

however, offer a wide range of properties and

a versatility of formulation and application that

find wide use in the coatings field. Coatings

based on thermosetting resins require heat and/

or chemical curing agents to develop their full crosslinking and cure. Coatings based on thermoplastic resins are deposited as continuous,

cured films upon evaporation of the solvents in

which they are dispersed or dissolved.

Coatings based on drying oils (see section

2.2.1) cure by a combination of oxidation and

polymerization to form a loosely crosslinked

and fairlj' flexible structure in which neighboring molecules are tied together through the re- active unsaturated groups occurring toward the

middle of the fatty acid radical. This process

by which drying oils harden is often described

as auto-oxidative polymerization.

3.1.2. Functionality and Polymerization

Mechanisms

3.1.2.1, Functionality.—The type of struc- ture obtained when a given material or group

of reactants is polymerized depends on their

chemical reactivity and functionality. Functionality may be defined as the number of active

sites (primary valence bonds) per molecule

available for reaction under the conditions imposed. In order for polymerization to take

place, the functionality of each reacting molecule must be at least 2; that is, there must be

two or more functional or reactive groups per

molecule. A functionality of 2 results in the

formation of linear polymers. Branched poly- mers also may be formed when the functionality is essentially 2, through the random activation of sites as discussed previously. When

the average functionality of the reactants is greater than 2 (not necessarily an integer),

crosslinked polymers are formed. The manner

in which functionality determines the type of

polymer structure obtained will be apparent

from the following discussion of the main types

of polymerization.

3.1.2.2. Addition polymerization.—Bifunctionality arising from the opening of a double

bond in an unsaturated monomeric material,

such as ethylene (CHo=CH,^ -CH0-CH2-),

or styrene (CH=CH2->-CH2-CH2-) , affords

an opportunity for chain growth by a simple

addition process in which successive monomer

radicals become attached end-to-end to form a

linear polymer molecule, without the splitting

off of a by-product molecule. Occasional ran- dom activation of sites along a polymer chain

permits the growth of side chains, again by an

addition process, so that various degrees of

branching may occur in otherwise linear poly- mers. If the monomer is polyfunctional (functionality greater than 2), then the addition re- action will proceed between growing chains as

well as by end-to-end addition, so that a threedimensional, crosslinked structure will be built up. For example, the tetrafunctional p-divinyl

benzene CH=CH2 quickly forms a highly crossCH=CH2

linked, rigid, insoluble polymer which has little practical use. If, however, a very small quantity of p-divinyl benzene (as little as 0.002 percent) is incorporated into styrene monomer,

then a lightly crosslinked polymer is formed in

which the linear polystyrene molecules are tied

together through an occasional divinyl benzene

cross link. Even with so few cross links, complete dispersion of the polymer is no longer possible, so that solvents that would dissolve the

linear polystyrene will only swell the cross- linked type. The limited degree of crosslinking also raises the heat distortion point of the

polymer and reduces the tendency toward cold

flow, without seriously affecting the basic desirable properties of the material.

In addition polymerization the growth of in- dividual chains is propagated by a free radical mechanism and is very rapid—an activated

monomer molecule may grow to a terminated

polymer chain in a small fraction of a second.

However, complete conversion of monomer to polymer may require several hours. During

this time, monomer and polymer coexist while

the monomer material is gradually used up in

the production of polymer chains. The molecular weight obtained depends on the conditions

of the reaction and on the presence of inhibitors, monofunctional reactants, and other chain

terminating mechanisms. Careful control of

monomer purity, catalyst, and reaction conditions is required in order to obtain a polymer

of sufficient uniformity to be useful.

34
In a bifunctional (linear) polymerization,

there is no propagation between growing

chains, and high molecular weights of a hundred thousand or greater are readily attained.

During the course of a given addition polymerization reaction, the average molecular weight

of the polymer chains remains practically the

same; however, the polymer yield (i.e., the ratio of polymer to monomer) continues to in- crease, and the viscosity of the monomer-polymer mixture continues to rise, until all tlie monomer is consumed. Because of the great chain

length of addition-type polymers, the presence

of even small amounts of polyfunctional substances quickly produces a rigid, crosslinked,

insoluble polymer.

3.1.2.3. Condensation polymerization.—

Condensation polymers are formed through the

chemical combination of reactive groups of bi- functional or polyfunctional molecules, with the

splitting out of a small molecule, usually water,

as a by-product. Either linear or crosslinked

polymers may be formed, depending on whether

the functionality of the reacting molecules is 2

or greater than 2. The reactive groups that

take part in polycondensation reactions are the same as those (e.g., hydroxyl, carboxyl, amino,

etc.) that take part in the ordinary condensation reactions between small molecules, as for

example when an alcohol and an organic acid

react to form an ester. A polycondensation reaction proceeds at the same characteristic rate as the reaction between similar groups in a monofunctional re- action. Unlike addition polymers in which long

chains form rapidly by a free radical propagation mechanism, condensation polymers have

a reaction activation energy that requires time

and heat for the formation of long chains.

The polycondensation reaction proceeds ran- domly among all functional groups, whether

they are on a monomer molecule or on a short

or a long polymer chain. That is, reactions

between partly grown chains, between a growing chain and a monomer molecule, and between two monomer molecules are all equally

probable. Consequently, the monomer molecules are soon consumed in the production of

dimer, trimers, and short polymer chains. The

reaction mixture now consists largely of low

molecular weight polymer chains which con- tinue to grow by combining with each other

for as long as they are permitted, or until the

viscosity of the mixture increases to an extent

that limits the mobility of reactive groups and

becomes the rate-determining factor in further

polymerization.

Even when polycondensation reactions are

forced to completion, molecular weights higher

than 25,000 are seldom obtained. However, if the reacting substances are polyfunctional,

then cross-linking reactions between even relatively low molecular weight chains will soon

build up a three-dimensional structure and lead

to gelation and polymer insolubility.

3.1.3. Methods of Polymerization

Four different methods of polymerization are

in general use: bulk, solution, emulsion, and

suspension. Each process is briefly discussed

below.

3.1.3.1. Bulk polymerization.—This is the

simplest type of polymerization process, since

the reaction mixture consists of a single phase

with nothing usually added, except catalyst or initiator if required. Thus the polymerized

product is obtained in an inherently pure chemical state that eliminates the need for subsequent purification steps. However, the bulk

process is difficult to control from the standpoint of obtaining a product of reasonably

uniform molecular weight and density. The

reaction mixture soon becomes very viscous,

causing difficulties in agitation and heat re- moval. Since most polymerizations are exo- thermic (addition polymers, in particular,

usually have a high exotherm) , local variations

in temperature and extent of reaction occur, and the final product may vary considerably

throughout the polymerized mass. The high

viscosities encountered in bulk polymerizations

often limit the degree of conversion that can

be obtained within a reasonable length of time.

The bulk method is used chiefly for cast plastics

and for in situ polymerizations in which the

time element is relatively unimportant, so that a slow but controllable reaction (perhaps at low temperature) is feasible.

3.1.3.2. Solution polymerization.—In the

solution process, the monomeric substances

(with added catalyst or initiator if required)

are dissolved in an inert liquid which may or may not be a solvent for the polymer. If the

polymer is insoluble in the liquid, it precipitates

out as it forms. Otherwise, when the polymerization has proceeded to the extent desired, the

polymer is recovered by precipitating it with

a suitable nonsolvent, or less often by evaporating off the solvent and residual monomer.

The solution polymerization process is more

easily controlled than the bulk process, and a more uniform product is obtained. The lower

viscosity and better heat transfer properties

of the solution medium permit easier agitation

with better control of temperature and a more

uniform reaction mixture. Although applicable

to either addition or condensation polymerizations, the solution method is particularly wellsuited for the condensation type of reaction.

Alkyds, phenolics, epoxies, and many other con- densation polymers are prepared by the solution method.

35
3.1.3.3. Emulsion polymerization.—In the

emulsion process, the monomer (which is in- soluble in water) is emulsified in water with

the aid of suitable emulsifying- agents. The

method is used almost exclusively for addition

polymerizations, since the moderate or low

temperatures employed are unsuitable for most

condensation reactions. The polymerization

proceeds very rapidly in the presence of an

initiator, yielding a high molecular weight

product in finely divided form. Excellent control of the process is possible, and a very uniform polymer is obtained. Since the polymer

is obtained in the dispersed phase, there is no

appreciable increase in the viscosity of the re- action mixture as the polymerization proceeds.

Many difficult polymerizations and copolymerizations that take place very slowly or not at

all by bulk or solution methods are readily

carried out by the emulsion method. The econ- omy and nonflammability of the aqueous dispersion medium are also distinct processing

advantages.

The very small particle size and good uniformity of emulsion p(51ymers permits their use

(after removal of processing agents where

necessary) directly in the manufacture of latex

paints, plastisols (vinyl dispersion resins), and

special molding powders.

3.1.3.4. Suspension polymerization.—This

is the newest method of polymerization. The

polymer is obtained in the form of tiny beads

or pearls that are readily washed free of processing contaminations. The suspension polymerization process is somewhat similar to an

emulsion system in that there is a dispersed

phase and a dispersion medium, usually water.

However, no emulsifying agent is used and an

emulsion does not exist. Instead, monomer

droplets (much larger than those in an emulsion

system) are mechanically dispersed in the

water medium by vigorous agitation. The droplets remain suspended only so long as the agitation continues. A suitable suspension agent

(e.g., an insoluble inorganic salt or oxide, or a

soluble organic polymer) is usually used to prevent the coalescence of partly polymerized

particles and to help control the particle size

developed.

The mechanism of suspension polymerization

is analogous to that of the bulk method rather

than the emulsion method. Each monomer

droplet or bead acts, in effect, as a small, dis- crete bulk polymerization system. Hence, only

those substances capable of bulk polymerization can be polymerized by the suspension method.

However, whereas the bulk process is very diffi- cult to control, excellent control of the sus- pension polymerization process is possible. As

in the emulsion method, there is virtually no

increase in the viscosity of the reaction mixture

with polymerization. The moderate or low

temperatures employed are readily maintained

throughout the thoroughly agitated reaction

mixture, and a very uniform polymer is obtained. Again, the economy and nonflammability of the water dispersion medium are important processing advantages. The suspension

method, like the emulsion method, is used

primarily for addition polymerizations.

3.1.4. Types of Polymers

The complexity and diversity of chemical

composition and physical structure among the

many synthetic polymers that have become

available has resulted in a growing descriptive

nomenclature by which to classify them. Besides the broad distinctions already made in

the preceding sections, such as addition or con- densation polymers, linear or cross-linked,

thermoplastic or thermosetting, etc., we may

now consider, from the point of view of structure, types of addition polymers variously described as homopolymers, copolymers, heteropolymers, and stereo-regular polymers. A familiarity with the basic structure and properties of the materials described by these terms

will be of value to those in the coatings field who are interested in the potential usefulness

of recent types of polymers.

3.1.4.1. A homopolymer is formed from a

single monomer, so that all the structural re- peating units in every chain are identical. All

the polymer chains are therefore similar in

chemical composition, but they may vary in

chain length and structure (and in the associated physical properties), depending on the

degree and type of process control that has

been exercised. Typical homopolymers are polystyrene and poly (methyl methacrylate)

.

3.1.4.2. A copolymer is formed from two

(or more) different monomers. The definition

of copolymer sometimes includes the stipulation that each of the constituent monomers

must be capable of undergoing polymerization

by itself. In this school of thought, if one (or

more) of the monomeric constituents is itself incapable of polymerization (although it will

polymerize in the presence of the other, true

monomer) , then the resulting polymer is termed

a heteropolymer. The author believes that

the distinction made between "copolymer" and

"heteropolymer" tends to create more confusion than order, and finds that the term "heteropolymer" is not favored in the writings of most

leaders in the high polymer field. In ordinary copolymers, the comonomers are combined into the polymer chain randomly

rather than in a regular sequence, so that

different chains vary in chemical composition

as well as in length and structure. Practically,

however, it is the average composition that

36
largely determines the bulk properties of the

polymer produced. The average copolymer

composition may be expressed in terms of the

over-all ratio of one component to the other,

and it depends on the relative amounts and

reactivities of the comonomers present in the

reaction mixture. To achieve a uniform aver- age composition throughout a particular co- polymerization, it is necessary to add appropriate amounts of the faster reacting monomer as the reaction progresses in order to maintain

the original reaction environment.

Through copolymerization it is possible to obtain polymers with properties intermediate

between those of the individual homopolymers.

Homopolymers that cannot be mechanically

blended because of incompatibility often have

compatible monomers that can be successfully

combined through copolymerization. It should

be pointed out, however, that the properties

of a copolymer may be very different from those

of a physical mixture of the two homopolymers

in the same proportion. For instance, a ran- dom copolymer, under any given set of conditions, behaves as a single compositional entity,

whereas each component in the physical mixture may "go its own way."

A good example of copolymerization is that

of vinyl chloride and vinyl acetate. Vinyl

chloride alone forms a hard, horny, difficultly

soluble homopolymer, while vinyl acetate alone

forms a soft, tacky, easily-soluble homopolymer. Copolymerization of the two monomers

in various ratios yields a series of copolymer

resins with highly useful intermediate chemical

and physical properties (see sec. 3.6.3).

3.1.4.3. A BLOCK POLYMER is a special type

of copolymer in which the different monomeric

repeating units, instead of occurring randomly

along the polymer chain as in ordinary co- polymers, occur in relatively long sequences

(blocks) of one constituent alternating with

relatively long sequences of the other. The

alternating runs or blocks of each repeating

unit may be of varying length and are usually

linked together in linear fashion (although the

possibility of making crosslinked block polymers, through the use of polyfunctional monomers, exists).

Because they contain long runs of each constituent monomer, block copolymers tend to exhibit the chemical properties of each constituent rather than a single set of intermediate

properties as in random copolymers, where the

closer alternations of the different monomeric

units impart a pseudo-homogeneous character

to the polymer. Thus, a random copolymer of a hydrophobic and a hydrophilic monomer

would ordinarily exhibit an intermediate, less- ened hydrophobic or hydrophilic character, depending on the average monomer ratio ; for

example, it might have only limited solubility in

a few organic solvents and be virtually insoluble

in water. The block copolymer of a hydrophobic and a hydrophilic monomer, on the other

hand, would tend to have a dual character and

show solubility in both organic and aqueous

media. Such a block polymer would be expected

to make a good emulsifying agent. Block co- polymers can be prepared either by addition

polymerization (e.g., poly (ethylene-propylene

oxide) ) or by a polycondensation reaction

(e.g., epoxy-phenolic copolymer).

3.1.4.4. A GRAFT POLYMER is a Special type of

branched copolymer consisting of a linear main

chain of one monomeric repeating unit with

side chains of another. It is made by grafting

side chains composed of one polymeric species

onto a main chain of another polymeric species.

Since each copolymer molecule thus has a distinctly dual structural character, graft poly- mers (like the block polymers described above)

tend to exhibit the properties of each polymeric

constituent rather than a single set of intermediate properties.

Graft copolymers are prepared by dissolving

the already polymerized main polymer in the

monomer to be grafted onto it. The grafting

reaction usually proceeds by an addition TJrocess at active sites (double bonds, free radicals, etc.)

activated or induced by heat, catalysis, or radiation. Graft polymers that have been successfully prepared include grafts of vinyl acetate

onto poly (vinyl alcohol), polyethylene onto

poly (vinyl acetate), and styrene onto butadiene-styrene copolymer.

3.1.4.5. Stereoregular polymers are a re- cent development in linear, addition polymers

that followed the discovery of solid, heterogeneous-type catalysts capable of exerting a highly stereospecific effect on the polymerization of olefin-type monomers, such as ethylene

and propylene. It is thought that the catalyst,

through a highly specific selectivity for particular isomeric forms, activates only certain

sites in the monomer molecule in a manner that

leads to an orderly polymerization in which the

monomer molecule always adds to the growing

chain in the same steric configuration. The

resulting polymer chains have a predetermined,

regular structure that permits close packing

with formation of a high-density, crystalline

polymer. Such a polymer possesses a higher

softening point, greater mechanical strength,

and greater chemical resistance than the analogous amorphous polymer (see sec. 3.13.1 on

Polyethylene)

.

3.1.5. Classification of Synthetic Resins

Synthetic resins may be classified in various

ways : according to chemical or structural type,

manner of formation, properties, applications, or other criteria, depending on the objective to

37
be served. Categories such as condensation

and addition polymers, thermoplastic and thermosetting resins, linear and cross-linked substances, etc. were discussed in the preceding

section. Such information is of basic useful- ness in understanding the nature of synthetic

resins and the relationships between their composition, structure, and properties. It is neces- sary now to consider these materials from the

point of view of their selection for coatings

applications. For this purpose they will be

discussed under headings that refer to the com- monly accepted designations for these classes

of resins. Broad structural generalizations

that would lead us, for example, to classify

polystyrene, acrylic, and other familiar resins as vinyl polymers will be avoided.

Only the main types of resins that are important or promising for coatings applications can be discussed in the space available. It should be borne in mind that, in general, a combination of resins will yield a material with

intermediate or combined properties. These

properties may be further altered upon formulation of the resin into a coating material. The

ultimate properties of the coating are determined also by the manner in which the film is deposited and cured. Thus an almost infinite

variety of coatings formulations and properties

3.2. Cellulose

The chief sources of cellulose for the manufacture of "cellulosic" coating materials are wood, cotton, and cotton linters. Although

cellulose itself is insoluble except in a few specialized reagents (e.g., cuprammonium solution), it can be converted by suitable chemical

treatment, involving esterification or etherification of the hydroxyl groups with or without

subsequent hydrolysis, into a variety of useful,

soluble cellulose derivatives, including cellulose

esters, cellulose mixed esters, and cellulose

ethers. Through control of the degree of substitution (esterification or etherification) and of

the processes of hydrolysis and degradation,

the chemical composition and the molecular

weight of the cellulose derivative can be controlled to give the desired solubility, viscosity,

and mechanical, physical and chemical properties. The most important and widely used cellulose

derivatives are the esters, particularly cellulose

nitrate and cellulose acetate. Cellulose acetate- butyrate is the most widely used mixed ester and finds important use in the coatings field. Ethyl cellulose is the most widely used of the

cellulose ethers.

The cellulose derivatives are thermoplastic

materials and therefore are permanently soluble and heat-softenable. The different cellulose

derivatives differ considerably in their solubilare possible even with only a limited number

of basic resins. Nevertheless, it is possible to predict much about the properties and potential

performance of a given coating material from

a knowledge of the properties of the basic in- gredients, particularly the film-former, of which

it is composed.

The following main classes of synthetic resins are discussed in the sections of this chapter designated below:

Section

3.2. Cellulose derivatives

3.3. Phenolic resins

3.4. Alkyd resins

3.5. Amino resins

3.6. Vinyl resins

3.7. Acrylic resins

3.8. Polystyrene

3.9. Silicone resins

3.10. Epoxy resins

3.11. Polyamide resins

3.12. Polyurethanes

3.13. Polyethylene

3.14. Fluorocarbon resins

3.15. Synthetic rubbers

3.16. Water soluble resins

3.17. Miscellaneous resins and information

Derivatives

ity in particular solvents and in their softening

temperatures, depending on chemical type, degree of hydroxyl substitution, molecular weight

and viscosity. Some are widely soluble in or- ganic solvents, others only in water. Softening

points may range from about 285 to 570 °F

(about 140 to 300 °C). Cellulose nitrate is an

exception in that it decomposes (sometimes

with explosive violence) when heated, before

reaching a melting temperature.

The cellulose derivatives, being thermoplastic, form films from solution by simple evaporation of solvent. As a group, they are tough,

horny materials that must be plasticized to achieve adequate flexibility and adhesion for

coating purposes. They are fast drying because

they release easily the quick-evaporating liquids in which they are soluble. It should be

noted, however, that both diluents and higherboiling solvents are used along with the lowboiling active solvents in their formulation into

lacquers, to control the evaporation rate and

solution viscosity (see sec. 5.3.2 on Use of

Thinners)

.

3.2.1. Cellulose Esters

3.2.1.1. Cellulose nitrate (or nitrocellulose, as it is often but incorrectly called) is the

oldest of the cellulose derivatives, its preparation having been reported by Bracconot [44]

38
in 1833. It was not until 1923, however, that a cellulose nitrate lacquer was developed with

the durability and fast-drying qualities that

revolutionized the automobile finishing industry by cutting the finishing time from weeks to hours. Shortly thereafter, the furniture industry adopted cellulose nitrate lacquers, and soon

its use had spread to a wide variety of products.

To this day, as the film-forming base in most

quick-drying lacquers, and as a versatile material that is widely compatible with other resins,

oils, and plasticizers, cellulose nitrate is extensively used in the coatings field. Cellulose nitrate is produced by the nitration

of purified cellulose with a mixture of nitric

and sulfuric acids. The degree of nitration obtained is determined largely by the ratio of the

mixed acids. The fully nitrated product (containing 14.14 percent nitrogen) is insoluble and

unstable. Soluble cellulose nitrate for coatings

purposes [45] usually has a nitrogen content

between 11.5 and 12.2 percent. The solubility

also depends on the molecular weight or viscosity of the cellulose ester. Although strength

and durability improve with increasing molecular weight and increasing viscosity, the solubility becomes poorer. To secure the good

solubility and low solution viscosity needed for

practical high-solids coatings formulations, the

cellulose nitrate as first produced is subjected

to controlled degradation by digestion in water

under heat and pressure until the molecular

weight has been reduced to the desired value.

Thus the viscosities of commercial cellulose

nitrates represent a compromise (as with many

processes) between the conflicting requirements

of product formulation and product performance. Both nitrogen content and viscosity must

be controlled within fairly narrow limits to

yield a material that is sliflftciently uniform for

coatings formulation; otherwise, less soluble

portions may not dissolve completely, while

very low molecular weight portions may reduce

tensile strength. To obtain a material of specified average viscosity, it is common practice to blend different batches of cellulose nitrate that are not too widely apart in this property.

Cellulose nitrate itself is a tough, horny

material that soon discolors, embrittles, and

decomposes on exposure to heat or sunlight.

However, when compounded with suitable plasticizers, resins, stabilizers, and pigrnents in a properly formulated solvent vehicle it yields a very wide variety of useful and durable coat- ing materials. Cellulose nitrate is soluble in

esters, ketones, nitrohydrocarbons, and etheralcohols. Alcohols alone swell and soften it but

do not dissolve it (an exception is a special

alcohol-soluble grade containing 10.7 percent

nitrogen) . It is insoluble in aliphatic and aro- matic hydrocarbons but will tolerate large proportions of these diluents in the presence of

suitable active solvents. It has very good moisture resistance and dimensional stability. It is resistant to weak acids and alkalis but is decomposed by strong ones. Pure, dry cellulose nitrate is a dangerously

flammable material. It is therefore handled

and shipped only in a wet state. For shipping,

it is usually wet with alcohol as 30-35 percent

of the total weight. After compounding with

other ingredients, the flammability of the final

film is greatly reduced but still high compared

to other film-forming substances.

Although cellulose nitrate is available in viscosity grades ranging from about 0.05 second

(20 centipoises) to 1000 seconds or more [45],

the majority of lacquers utilize a 14-second or a i/o-second material as a good compromise between tensile strength and solubility requirements. For airplane dopes, where extra tensile

strength and the fabric-tautening property are

necessary, cellulose nitrate of higher viscosity

(5-20 seconds) has been found to give the best

results, as reflected in Government specifications for such materials [45, 46].

While cellulose nitrate has been combined

with many different resins to produce useful

coatings, perhaps the most outstanding have

been those in combination with alkyd and amino

resins to produce tough, hard, durable lacquers

capable of withstanding the severe service re- quirements of automotive, aircraft, and other

industrial finishes. Successful coatings have

also been based on blends of cellulose nitrate

with natural resins and drying oils, resin derivatives, phenolic resins, acrylic resins [138],

certain vinyl resins, and others. Applications

include coatings for metal, wood, paper, fabrics, leather, and cellophane. These are available in a wide variety of colors and surface

textures. A recent successful application for

cellulose nitrate has been as the film-forming

base in multi-color finishes (see sec. 2.3.7).

3.2.1.2. Cellulose acetate, although used in

large quantities in the plastics and textiles

fields, finds only limited use in the coatings

field because of its poor solubility in many com- monly used organic solvents, its limited compatibility with plasticizers and resins, and its relatively high moisture absorption which results in poor dimensional stability. However,

it is resistant to discoloration and degradation

by heat and ultraviolet light, has a high softening temperature (approximately 446 to 572 °F

[230-300 °C], depending on the composition

and molecular weight) , and is relatively slowburning.

Cellulose acetate is manufactured by acetylating cellulose with acetic anhydride and glacial acetic acid in the presence of sulfuric acid

as a catalyst. The fully acetylated material

(cellulose triacetate) thus obtained is virtually

insoluble except in chloroform and a very few

other solvents. It is therefore partially hydro39
lyzed to yield more soluble materials of wider

usefulness. Various commercial grades are

available for particular applications. These

differ in degree of hydrolysis (or esterification), molecular weight, and viscosity; therefore, they differ in solubility, softening point,

tensile strength, and other properties.

Solvents useful with cellulose acetate include

acetone, methyl ethyl ketone, methyl acetate,

ethyl acetate, dioxane, and the glycol-ethers.

Methyl alcohol and ethyl alcohol are good co- solvents in combination with methylene chloride, ethylene dichloride, nitromethane, and

1-nitropropane. The number of plasticizers

useful with cellulose acetate is very limited,

and a careful choice for particular applications

is necessary. Methyl- and ethyl phthalyl ethyl

glycollate are generally useful, and a few

phthalates, phosphates, and citrates are of limited usefulness as plasticizers. Among the few

resins with which cellulose acetate is fairly

compatible are certain phenolics, alkyds, and

ester gums.

Cellulose acetate is resistant to everyday

chemicals such as dilute acids, animal fats,

vegetable and mineral oils, gasoline, and ethyl

alcohol. It is decomposed by strong acids and

by alkalis. It is used in lacquers for paper,

fabrics, wood, metal and wire, particularly

where good resistance to heat and light is required.

3.2.1.3. Cellulose propionate is prepared

by the esteriflcation of cellulose with propionic

anhydride, in a manner similar to the preparation of cellulose acetate. The larger propionyl

radical yields a softer, lower-melting material

than does the acetyl radical. Cellulose propionate is more soluble and requires much less plasticization than the corresponding acetate.

It is tough, shock-resistant, heat-stable, and

weather-resistant. It has only recently become

commercially available. Although at present

used chiefly as a plastics molding powder, it has

found some application in strippable coatings

(sec. 2.2.7) and gel lacquers [158].

3.2.1.4. Cellulose butyrate can be made by

processes analagous to those for the acetate and

the propionate. The bulkier butyryl radical

results in a still softer, lower-melting, and more

soluble material that is not in itself very useful. However, the properties of the butyryl

radical have been utilized to excellent advantage in the coatings field by combining butyryl

with acetyl in the manner described under

Cellulose Mixed Esters.

3.2.2. Cellulose Mixed Esters

By esterifying cellulose with a mixture of

acetic and propionic or butyric anhydrides,

mixed esters can be made that combine many

of the most desirable properties of each constituent. The ratio of acetyl to propionyl or

to butyryl can be varied and the degree of es- teriflcation (or hydrolysis) can be controlled

to produce a wide range of materials. As with

other polymeric materials, the physical and

mechanical properties also depend on molecular

weight and viscosity.

3.2.2.1. Cellulose acetate-butyrate combines the good light and heat resistance of cel- lulose acetate with the better moisture resist- ance and dimensional stability of the butyrate.

The butyryl radical imparts to the mixed ester greater flexibility, better solubility, and wider

compatibility than is obtainable with the acetate alone. Cellulose acetate-butyrate requires

much less plasticization than cellulose acetate

and is compatible with a much broader range

of plasticizers and resins. It is soluble in ketones, esters, chlorinated hydrocarbons, and

nitrohydrocarbons. Alcohols and aromatic hydrocarbons are well-tolerated diluents. Because of its versatility of composition, moisture

resistance, dimensional stability, and weathering resistance, cellulose acetate butyrate finds

considerable use in both clear and pigmented

lacquers for metal, wire, paper, plastics, wood

and fabrics. Its slow-burning characteristics

combined with good tensile and tautening properties and excellent outdoor durability have

been the basis for its use by the Navy and Air

Force as a dope [47, 48, 49] for the fabric sur- faces of military aircraft (training planes and

slower transport planes) in place of the hazardously flammable cellulose nitrate dope. Cellulose acetate butyrate has been notably successful in the formulation of strippable coatings,

hot melt lacquers, and gel lacquers [158].

3.2.2.2. Cellulose acetate-propionate, al- though its utility as a film-forming base has

been established through a comprehensive study

[50], has been used to only a limited extent in

the coatings field. This is largely because of

the earlier and adequate availability of cellu- lose acetate-butyrate, and the similarity in

properties of the two mixed esters, which has

made it uneconomic to manufacture the acetate- propionate on a large scale.

3.2.3. Cellulose Ethers

In general, cellulose ethers are prepared by

treating alkali cellulose with an appropriate

etherifying agent, usually an alkyl chloride,

under heat and pressure. By control of the

reaction conditions, it is possible to control the

degree of substitution and the viscosity, upon

which the physical and chemical properties of

the cellulose ether depend.

3.2.3.1. Ethyl cellulose finds extensive use

in the coatings field [51]. It is tough, flexible

40
at low temperatures, and stable to heat, light,

and weathering, particularly when properly

plasticized and stabilized. Its wide compatibility with plasticizers, resins, gums, and waxes

permits an almost endless variety of useful

formulations. It is completely compatible with

cellulose nitrate and is often used to augment

the properties of cellulose nitrate lacquers [52]

.

Though widely soluble in organic solvents, ethyl

cellulose is resistant to alkalis, dilute acids, and

aliphatic hydrocarbons. Although somewhat

permeable to water vapor, it is low in water

absorption and has good dimensional stability.

Its electrical properties (insulative, dielectric,

etc.) are excellent.

Ethyl cellulose is used extensively in coatings

for wire, paper, fabric, leather, and plastics.

It is also used in coatings for metal, wood,

glass, and ceramics. Hot-melt strippable coat- ings (sec. 2.2.7) for metal parts are readily

formulated with ethyl cellulose.

3.2.3.2. Methyl cellulose can be produced

with a wide range of solubility properties, from

solubility in alkalis to solubility in organic sol- vents, depending on the degree of methylation.

However, the usual commercial product has a

degree of substitution of about 2 and is soluble

in cold water but not in hot. The water soluble

product has very poor acid and alkali resistance

but is insoluble in organic solvents and has excellent resistance to oils and greases. An important use has been as a grease-proof coating

for paper. Methyl cellulose is used extensively

as a thickening agent and protective colloid in

emulsion paints and as a thickener in paint removers.

3.2.3.3. Carboxymethyl cellulose as usually manufactured is a water-soluble cellulose

ether that is insoluble in organic solvents. It has poor resistance to acids and alkalis. Its aqueous solutions are highly viscous, making it a useful thickening and stabilizing agent for

latex paints. It finds some use also as a fabric

finishing agent and as a grease-proofing coating

for paper, leather, and other materials.

3.2.3.4. Hydroxyethyl cellulose, as usually prepared, is a water soluble product. It

is very resistant to organic solvents, oils, and

greases. It has fair resistance to dilute acids

and alkalis. It finds some use in coatings for

paper, fabrics, and leather where a greaseproof or water-soluble coating is desired. Its chief use in the coatings field is as a protective

colloid in latex polymerization and as a thickening agent and stabilizer for latex paints.

3.2.3.5. Benzyl cellulose resembles ethyl

cellulose in general properties but is softer and

lower in melting point because of its bulkier

molecular structure. It is widely soluble in

organic solvents and is compatible with most

common resins and plasticizers. However, it has found only limited use in the United States,

where ethyl cellulose has met with far more

favor. The chief uses for benzyl cellulose are

in chemical-resistant coatings and electrical

insulating coatings.

3.3. Phenolic Resins

Phenolic resins were the first truly synthetic

resins to become commercially available. They

were introduced to the coatings field by Baekeland about 1909 [53]. Their characteristic fast

dry, moisture resistance, toughness, and durability, and their reasonable cost have led to their wide use in coatings.

Phenolic resins are made by the condensation

of a phenol with an aldehyde. A variety of

types is available, depending on the nature and

proportions of the phenol and aldehyde, the

type of catalyst employed, and the reaction

conditions. Formaldehyde, because of its high

reactivity and low cost, is by far the most used

aldehyde. Besides phenol itself, many substituted phenols have been of wide commercial

importance. Both heat-reactive and non-heatreactive types of phenolic resins are available.

Heat-reactivity, with the associated capability

for cross-linking and insolubilization, is favored

by high proportions of formaldehyde and by

alkaline catalysts. Acid catalysts generally

yield thermoplastic resins (novolacs) that are permanently fusible and soluble, although a high formaldehyde ratio can impart cross-linking characteristics to these resins also. Phenolic resins have a yellow color which

tends to darken on aging; hence they are not

usually employed in white finishes. The darkening tendency is greatest with the resinmodified phenolics and least with the parasubstituted pure phenolics. In general, phenolic resins have good resistance to acids and

to weak bases but poor resistance to strong

alkalis. For use as coatings under the weakly

alkaline conditions encountered in ordinary en- vironments and on common alkaline surfaces

such as plaster or concrete, phenolic resins are very satisfactory.

Phenolic resins used in the coatings field may

be classified on the basis of their composition

and properties into the following main cate- gories : (1) Modified phenolic resins, oil-soluble

(2) Pure phenolic resins (100% phenolic)

a. Oil-soluble, thermoplastic

b. Oil-soluble, heat-reactive

c. Alcohol-soluble, oil-insoluble, thermosetting

(3) Phenolic dispersion resins.

41
Each of the above types is briefly discussed in

the sections which follow.

3.3.1. Modified Phenolic Resins, Oil-Soluble

The first phenolic resins used in the coatings

field were of the modified phenol-formaldehyde

type, the usual modifier being rosin or its glyceryl ester (ester gum). The modification was

necessary to render the phenol-formaldehyde

resin soluble in drying oils. The use of these

modified phenolic resins in varnishes in place

of the natural resins, ester gum, maleic and

other resins used up to that time resulted in

varnishes with much faster drying properties

and greatly improved water and alkali resist- ance and durability. Other phenolic resins

rendered oil-soluble by modification are those

based on m-cresol and 3,5-xylenol. It may be

noted that, as a rule, it is the meta-substituted

phenol derivatives that must be modified to render them oil-soluble.

Though not as highly resistant and durable

as the pure phenolic resins that were developed

later, modified phenolic resins are lower in cost,

have better adhesion properties, and are very

well-suited for many important coating applications. They are widely used for interior

applications where fast drying, water and

alkali resistance, and durability are essential, as in furniture finishes and floor varnishes.

They make good primer-sealers for plaster and

wood. They find considerable use in long-oil

exterior coatings such as spar varnishes, and

porch and deck enamels, and in short-oil, fastdrying, traffic paints. As already mentioned,

modified phenolics are not ordinarily used in

white finishes because of their strong yellowing

tendency. Their electrical properties (insulative, dielectric, etc.) are poor. In addition to being very useful in themselves, modified phenolic resins are of general

usefulness in upgrading the drying speed,

water resistance, and durability of lower cost coating materials. Also, they are frequently

used to improve the adhesion properties of the

otherwise superior pure phenolic resins.

3.3.2. Pure Phenolic Resins (100% Phenolic)

Pure phenolic resins (or 100% phenolic resins, as they are often called) are among the

most chemical-resistant and durable resins in

the coatings field. They have been available

since about 1928. The oil-soluble types are made by the condensation of para-substituted

phenols with formaldehyde. Occupation of the

highly reactive para position by the substituent

group reduces the reactivity (and crosslinking

tendency) of the molecule sufficiently to render

it soluble in drying oils without rosin modification. These oil-soluble phenolic resins may be

either thermoplastic or heat-hardening types.

Alternatively, by using other phenols and suitable phenol-formaldehyde ratios and appropriate catalysts, oil-insoluble but alcohol-soluble

resins can be formed which are completely

thermosetting. The pure phenolic resins as a group are characterized by excellent exterior

durability and outstanding water, solvent, and

chemical resistance. Although better than the

modified phenolics in color stability, they still tend to yellow and hence are not usually used

in white finishes. Another limitation is their

relatively poor adhesion; this is usually over- come by combining them with small portions of a modified phenolic. Drying properties range

from slow to very fast, with most of the pure

phenolics in a fast-drying category.

3.3.2.1. Oil-soluble, thermoplastic, pure

phenolic resins are made by condensing approximately equimolar proportions of a parasubstituted phenol with formaldehyde in the

presence of an acid catalyst. This is the type

of phenolic resin most used in air-drying varnishes and oleoresinous paints. Such a resin is covered by Fed. Spec. TT-R-271 [119]. Such

phenolic varnishes [135, 137] and paints have

excellent outdoor durability and are used ex- tensively in structural and marine applications

such as bridges, locks and dams, and storage

tanks. They find use also as exterior trim

paints, automotive primers, and appliance coatings. Some of the finest exterior spar varnishes are made with this oil-soluble, thermoplastic type of pure phenolic resin. (For more

information on phenolic paints and varnishes,

see sec. 2.3.2.2.)

3.3.2.2. Oil-soluble, heat-reactive, pure

phenolic resins are made by condensing a parasubstituted phenol with formaldehyde in the

presence of an alkaline catalyst, using a higher

proportion of formaldehyde (about 2 moles of

formaldehyde to one mole of substituted phenol) than is used in making the thermoplastic

resins. The condensation reaction is interrupted at an intermediate stage at which the

resinous condensate is still oil-soluble though

reactive. The partly polymerized resin is subsequently heat-reacted with drying oils to produce varnishes with very rapid bodying properties. Short-oil industrial baking varnishes of

this type make excellent insulating and impregnating varnishes for electrical equipment be- cause of their fast, deep-curing characteristics

and their excellent insulative and dielectric

properties. Even small proportions of an oil- soluble, heat-reactive type of pure phenolic

resin have a marked effect in increasing the

drying speed, water resistance, hardness, and

durability of other, lower grade resin formulations. This "upgrading" function is an important use for this type of phenolic resin.

42
3.3.2.3. Alcohol-soluble, oil-insoluble,

THERMOSETTING, pure phenolic resins are based

on phenols other than the oil-soluble parasubstituted types. The condensation reaction

between the oil-insoluble phenol (usually a meta-substituted phenol or phenol itself) and

formaldehyde is carried out in alkaline solution

at a fairly high formaldehyde ratio that imparts a high degree of reactivity to the resulting resin. The reaction is stopped at an intermediate stage of polymerization while the resin

is still alcohol-soluble and reactive. The partially polymerized resin is supplied as a solution

in an organic solvent. The applied coating,

when heated, converts to a completely polymerized, thermoset, insoluble resin film that is highly resistant to corrosion, abrasion, and

chemical attack. In resistance and durability, the alcoholsoluble, thermosetting phenolic resins are superior to other types of pure phenolic resin. The

completely thermoset resin is highly resistant

to organic solvents, organic acids, mineral acids

(except strong oxidizing acids such as nitric)

,

and atmospheric corrosion. Special types re- sistant even to strong alkali also are available.

The thermosetting phenolics may be cured at room temperature by adding amine-type or other alkaline catalysts; however, maximum

chemical and solvent resistance is obtained with

heat curing.

Extensive use is made of the thermosetting

pure phenolic resins in coatings for the insides

of cans, drums, and tanks carrying organic solvents, foodstuffs, and chemicals. They find

wide use on military equipment [125, 136] such

as ammunition boxes, aircraft parts, water and

gasoline tanks, and food containers. They are used for steel structures and fabricated parts

subject to severe atmospheric exposure. And

they have proved durable as coatings for submerged steel parts and for buried iron pipe.

3.3.3. Phenolic Dispersion Resins

Phenolic dispersion resins are highly polymerized phenolic resin-oil complexes dispersed in

volatile organic solvents. They are non-oxidizing and dry quickly by evaporation of solvent

to form highly weather-resistant films that have

a high degree of impermeability to moisture,

excellent resistance to water and dilute alkali,

good adhesion to metals, and good resistance to strong solvents such as alcohols, ketones, esters,

and ethers. Because of their resistance to such

solvents, they make good non-lifting primers

under lacquers based on vinyls and on cellulose

derivatives. Although useable alone, they are

usually blended with an alkyd or other varnish

resin. The military specification for zinc chromate primer for aircraft use [54] calls for a

vehicle based on a phenolic dispersion resin

blended with an alkyd resin. Phenolic dispersion resins find use in quick-drying industrial

primers and undercoats, as shop coats for structural and fabricated steel parts, in traffic paints

where they contribute fast dry and durability,

in deck and floor coatings, and in sealers for

wood, paper, and wallboard.

3.4. Alkyd Resins

Alkyds are the most important and most

widely used resins in the coatings field. In versatility of formulation and application, in overall economy, and in general durability, they are

unexcelled. They have been aptly characterized as "the work-horse of the coatings industry". Alkyd resins have been available commercially since 1927.

3.4.1. Composition and Properties of

Alkyd Resins

3.4.1.1. An alkyd is a synthetic resin made

by the esterification of a polyhydric alcohol

with a polybasic acid. Although a wide variety

of resinous products of polyhydric alcohols and

polybasic acids is included in the general term

"alkyd", the most important group (and the

type generally meant when speaking, without

other qualification, of an alkyd resin) are the

oil-modified glyceryl phthalate resins. These

are the reaction products of glycerol and

phthalic anhydride (phthalic acid), modified

with either drying, semi-drying, or non-drying

oils (see sec. 2.2.1). Pentaerythritol is the

most important of the lesser used polyhydric

alcohols ; it yields resins that are faster setting

and more durable than glyceryl alkyds, but

which are more expensive to manufacture and

formulate. Isophthalic, maleic, sebacic, and

other polybasic acids also find use in alkyd resin

manufacture. Long chain dibasic acids, such

as sebacic or adipic acids, yield soft resins that

make good polymeric plasticizers for harder

materials.

The unmodified glyceryl phthalate resin is a

hard, brittle solid of only limited solubility.

Through reaction with fatty acids or oils at the

time the alkyd resin itself is being formed in

the cooking kettle, a series of modified alkyd

resins having a wide range of specific desired

properties intermediate between those of the

resin and the oil can be produced. The resin can be prepared by either bulk or solution polymerization methods (see sec. 3.1.3) . In either case the cooking or refluxing is continued until

the viscosity and acid number of the reaction

mixture reach a predetermined value (when the

43
resin is close to the gel stage), whereupon the

mixture is quickly cooled to stop the reaction.

The resin is usually supplied as a solution in an

organic solvent. Long-oil alkyd resins are

soluble in mineral spirits; short-oil alkyds re- quire stronger solvents such as xylene or other

aromatic solvents ; the unmodified resin is sol- uble only in strong solvents such as ketones

and esters and other oxygenated solvents. Several types of long- and medium-oil alkyd resins are covered by Fed. Spec. TT-R-266 [120].

Oil-alkyds may be further modified with a

variety of other resins to achieve certain properties. In general, most of these doubly-modified resins (amino resins excepted) tend to have

a darker color than the straight oil-alkyd.

Some resins can be incorporated into the alkyd

during the original cooking process ; for example, a modified natural resin like congo ester to impart faster drying properties, or a rosinmodified phenolic resin to improve water and

alkali resistance—although the exterior durability may be lessened by such modification. Or

the alkyd, particularly non-drying types, may

be physically blended with tough resins such as a vinyl chloride-acetate [55] or cellulose nitrate

[52] to plasticize them and to improve their

gloss and chalk resistance. Alkyds may also

be chemically combined with reactive monomers

such as styrene or a silicone to obtain copoly- mers with intermediate properties. Non-drying (non-oxidizing) alkyds can be cured to exceptionally durable films by baking with

amino resins (sec. 3.5) such as urea- or melamine formaldehyde. The air-drying alkyds re- quire the use of metallic driers as used in ordinary drying oil paints in order to dry properly

within a reasonable length of time.

3.4.1.2. Effect of oil-modification.—Oilmodified alkyd resins may be either air-drying

or baking types, drying or non-drying (oxidizing or non-oxidizing) , yellowing or non-yellowing, weather- or chemical-resistant, flexible or hard, etc., depending on the type of oil and the

oil length. Drying, semi-drying, and non-drying oils (see sec. 2.2.1) may be used, singly or

together, in either long, medium or short oil lengths. The best air-drying properties are obtained with long oil alkyds made with good

drying oils such as linseed and dehydrated castor, although air-drying alkyds of medium oil length also are available. The palest color and

best color-retention are obtained with non-drying oils such as cocoanut and castor; however,

these non-drying alkyds must be blended or baked with a film-forming resin (e.g., ethyl cel- lulose or urea-formaldehyde) in order to form

a suitable film. Excellent intermediate drying

and color-retention properties are achieved by

use of a semi-drying oil such as soybean or

safflower ; soybean oil, in particular, is very

widely used in alkyd resin modification because

of its good non-yellowing properties, ready

availability, and low cost. Good brushability,

surface wetting and penetration, flexibility,

and elasticity are favored by a long oil length.

Short-oil alkyds have the best baking properties, greatest hardness and chemical resistance,

highest gloss and chalking resistance, and best

general durability. Intermediate properties for

specific applications may be obtained with me- dium oil lengths. Solubility in polar solvents

(e.g., ketones and esters) and compatibility

with polar resins (e.g., cellulose nitrate and

urea-formaldehyde) is greatest at the higher

alkyd ratios (short oil length). With increasing oil length, the solubility and compatibility

with polar solvents and resins decreases, and

the alkyd resin becomes increasingly compatible with less polar resins (e.g., polystyrene and

chlorinated rubber) and increasingly soluble

in nonpolar solvents, first in aromatic hydrocarbons such as xylene and finally even in ali- phatic hydrocarbons such as mineral spirits.

An alkyd need not be greater in oil length than

65 gallons, since beyond this point the modified

alkyd becomes freely compatible with drying

oils, and oil can be added as required for particular applications. Long-oil alkyd resins for

metal or wood priming paints often have some raw linseed mixed in to promote good wetting

and penetration.

3.4.1.3. Uses of alkyd resins.—Huge quantities of oil-modified glyceryl phthalate alkyds

are used in interior and exterior enamels, architectural paints, wall paints, automotive finishes,

appliance enamels, industrial finishes, and marine coatings. Long-oil, air-drying alkyds have

very good elasticity and exterior durability,

making them suitable for architectural trim

paints (trim and trellis paints), garden furniture coatings, porch and deck enamels, and au- tomotive refinishing enamels. In marine applications, alkyd finishes are often used above the

water line for their good durability, while a

phenolic varnish is used below the water line

because of its better water resistance. Between

the high and low water lines a vinyl-alkyd coat- ing (sec. 4.3.1.2) gives good service. The longoil alkyds also make excellent interior wall

paints, either flat or gloss, that brush on easily

and provide a long-lasting finish. Pentaerythritol type alkyds have fast air-drying characteristics despite a long oil length; although more expensive than glyceryl phthalate alkyds,

they are more durable and make excellent white

architectural trim paints and marine enamels.

Medium-oil and short-oil alkyds are often

combined with other resins to make fast airdrying or baking-type colored finishes for toys,

tools, and sundry items. Resin-and-oil modified

alkyds find use in corrosion resistant primers

for metal ; for example, a phenolic-modified oil44
alkyd employing zinc yellow pigment is the

basis for Fed. Spec. TT-P-666 zinc chromate

primer [88] for use on aluminum and magnesium surfaces. Phenolic-modified alkyds are

also used in interior finishes requiring improved

alkali (soap) resistance.

Automotive finishes have progressed from the

early alkyd-modified cellulose nitrate finishes to the highly durable present-day melamine-alkyd

finishes in which a short-oil alkyd is combined

with about 30 percent of melamine-formaldehyde resin to produce a fast-baking enamel with

excellent hardness, gloss, color retention, and

exterior durability. White baking enamels for

kitchen cabinets, refrigerators, and other appli- ances are often based on short-oil, soybean

alkyds combined with urea- or melamine-formaldehyde resin. For maximum whiteness and

non-yellowing with heat or aging, non-oxidizing

alkyds modified with non-drying oils, such as cocoanut or castor oil, are employed. Nonoxidizing alkyds in combination with melamineformaldehyde resin make good heat-resistant

finishes for lighting fixtures and stove enamels.

The non-drying alkyds are also used as plasticizers in cellulose nitrate lacquers for furniture,

automobiles, aircraft [56] , machinery, and tools.

3.4.2. Resin-Modified Alkyds

3.4.2.1. Styrenated-alkyds are being used

effectively in very fast air-drying and baking

finishes. The resin is prepared either by styrenating the oil with which the alkyd is to be

reacted, or by direct styrenation of the oil- alkyd; in either case, the reaction is with sty- rene monomer. The polystyrene resin, besides

being relatively low in cost, contributes its own

good water and alkali resistance and good color

retention properties to the styrenated-alkyd.

However, styrenated-alkyds are not as durable on exterior exposure as the pure oil-modified

alkyds, and the sensitiveness of polystyrene to

oils and hydrocarbon solvents also manifests it- self to some extent in the modified alkyd. The

hydrocarbon sensitivity can be reduced, in the

case of baking finishes, by adding an amino

resin. Air-drying styrenated-alkyds usually

contain metallic driers. With high pigment

content and a solvent such as xylene, they will

dry hard in about 10 minutes to a lusterless finish. For best flow in high gloss finishes, the

styrenated-alkyd is dissolved in the slowerevaporating mineral spirits, and it dries hard

in 3 to 6 hours. Styrenated-alkyds are used in

fast-drying finishes for wood and metal furniture, household implements, toys, and interior

applications requiring resistance to alkaline

cleaners (e.g., washing machine coatings) but

not subject to contact with oils and greases.

Best resistance is obtained with baking finishes

incorporating amino resins.

3.4.2.2. SILICONE-ALKYDS are copolymerized

silicone and alkyd resins which combine the

excellent heat resistance, water and chemical

resistance, and electrical properties of the sili- cone resins with the good brushability, gloss,

hardness, and durability of the alkyd resins

(also see sec. 3.9.2.1) . Silicone-alkyds are used

in high-grade heat-resistant baked coatings for

stoves, boilers, machinery, and electrical equipment. Because of their good water, alkali, and

chemical resistance, they are also used to advantage in coatings for washing machines, in- dustrial equipment, and kitchen equipment.

Air-drying silicone-alkyds for industrial maintenance paints also have been developed.

3.4.2.3. Other resins with which alkyds

MAY BE CHEMICALLY COMBINED (in addition to the amino, phenolic, and other resins already

mentioned) include epoxies," acrylics, and polyurethanes. The use of such resin combinations

is just beginning to be exploited for coatings

purposes. In general, the properties of the dual

resin may be expected to be a combination of, or intermediate between, those of the individual

constituents (see sec. 8.1.4.2).

3.4.3. Polyester Resins

Although in a general sense all alkyd resins are polyesters, since they are the product of an

esterification reaction between a polyhydric al- cohol and a polybasic acid, the term "polyester"

is commonly used in a narrower sense to denote

a linear-type alkyd possessing carbon-to-carbon

double-bond unsaturation in the polymer chain.

These unsaturated polyesters may be cross- linked by reaction with a monomer such as styrene or diallyl phthalate (often with a peroxide added also) to form an insoluble and infusible resin, without the formation of a byproduct during the curing reaction. Although

the chief use for the unsaturated polyester res- ins in the past has been in the low-pressure

laminating field, the polyesters are now among

the most promising of the newer resins finding

application in the coatings field as attractive

and durable finishes for concrete, masonry,

wood, plaster, wallboard, and metal. On porous

or rough surfaces (all the aforementioned ex- cept metal), reliance is placed primarily on

mechanical adhesion ("bite" or penetration)

rather than on specific adhesion (chemical at- traction) to keep the coating in place. The good

specific adhesion needed on smooth, nonporous

surfaces of metal has been obtained by combining the polyester with an isocyanate polymer

(see sec. 3.12.1.2) in addition to a styrene monomer hardening agent. Thorough surface preparation based on the principles given in chapter

6 and in accordance with the manufacturers instructions for filling, cleaning, sealing, and

priming is very important for the successful

45
application of polyester coatings, and welltrained experienced personnel are required.

Since the uncured polyester resin can be made

in a liquid form suitable for application without the addition of volatile solvents or thinners, a 100 percent solids coating system capable of

high film build (10 to 20 mils per coat) is possible, and tile-like polyester coatings up to 40

mils in overall thickness are available for heavy

duty applications. The hardening agent is mixed into the polyester resin just before it is applied. Curing proceeds throughout the coat- ing by an exothermic, chemical crosslinking

reaction at a rate dependent primarily on the

coating temperature and on the type and con- centration of hardening agent. A hard film is obtained in a few hours. The thickness of the

coating affects the cure indirectly through its effect on the quantity of heat released and re- sultant temperature, and through its effect on

the relative amount of hardener lost by volatilization at the surface, as determined by the

ratio of surface area to coating volume. On the

basis of these factors, a coating thickness of 6

mils is considered to be the minimum necessary

to ensure adequate cure. The tile-like polyester coatings are hard, nonporous, durable coatings capable of withstanding dilute alkalies and acids, organic solvents,

hot water, and a high order of nuclear radiation

(see sec. 4.6.8), as well as mechanical abuse,

impact, and abrasion. The mechanical strength,

toughness, and flexibility of polyester coatings can be greatly increased by incorporating glass

fibers or by laminating glass fabric into the

coating system. The glass fiber reinforcement

is accomplished by chopping and blowing glass

roving into a layer of wet coating by means of an electrical or air-driven chopper and then

rolling in the fibers with another layer of coat- ing material. Appearance is enhanced by depositing colored tracer threads along with the

regular glass roving. Finally, a topcoat of

polyester resin may be applied to provide a smooth finish. For the glass fabric reinforcement, the fabric is laid on the primed, dry surface, all wrinkles are worked out, and the fabric

is saturated with the polyester applied by brush

or roller to wet the fabric but not fill the weave. After the saturation coat is cured, it is sanded

lightly and a heavy topcoat of polyester resin

is applied to complete the coating system. The

glass fabric reinforcement imparts maximum

strength to the coating system.

Uses for polyester resin coatings include hard

lustrous clear finishes for furniture, water- and

weather-resistant finishes for boats, durable

and attractively colored coatings for the exteriors of concrete or masonry buildings, tough

impact- and abrasion-resistant coatings for cor- ridor walls and floors of schools and hospitals, wear- and wash-resistant sanitary coatings for

cafeteria and hospital kitchens and for dustfree rooms, decontaminable and radiation-resistant coatings for walls and floors of atomic

facilities, hot water- and radiation-resistant

coatings for canal walls and floors of nuclear

reactors, tough wear-resistant coatings for

stairways, bowling alleys, and gymnasium

floors, coatings for swimming pools, and industrial coatings for metal structures and equipment.

3.5. Amino Resins

The amino resins (or aminoplasts, as they are sometimes called) that are of chief importance

are the urea- and the melamine-formaldehyde

compounds. The urea resins have been available since 1930 and the melamine resins since

1940.

3.5.1. Properties

The amino resins are thermosetting materials

that readily form hard, brittle, colorless, chemically resistant films on baking. Because of

their brittlenes and poor adhesion when used

alone, they require proper plasticization for use

in coatings. This is usually accomplished by

combining them with short- or medium-oil

alkyds. The amino-alkyd blend provides an

excellent combination of coating properties in

which the adhesion, toughness, gloss, and durability of the alkyd are complemented by the

baking speed, hardness, color retention, and

chemical resistance of the amino component.

The amino resin is used as the minor component

and usually constitutes from 15 to 35 percent of

the resin solids, although it may be as low as

5 to 10 percent in some automobile finishes

and as high as 50 percent in some low-bake

furniture finishes.

3.5.2. Formation

The formation of amino resins proceeds in

three stages. In the first stage methylol amine

monomers are formed by reaction of urea or melamine with formaldehyde under mildly alkaline conditions. In the second or intermediate

stage, these monomers condense to form linear , water-soluble polymers which are partially > etherified with alcohols (usually butanol) as the

polymerization proceeds, to confer organic-solvent solubility. In the third and final stage,

the resin is converted by heat to a cross-linked,

thermoset, infusible, insoluble resin.

3.5.3. Uses

The excellent combination of coating properties (see above) possessed by the urea and

melamine alkyd resins has led to their wide use, ;
singly or together, in high-grade, fast-baking,

durable, mar-resistant, chemically resistant fin- ishes for automobiles, refrigerators, washing

machines, stoves, electrical appliances, hospital

equipment, counter tops, furniture, and other

industrial and household applications.

3.5.4. Comparison of Urea and

Melamine Resins

Although there are many similarities in the

preparation, properties, and uses of urea and

melamine resins, there are also important differences which influence the selection of one

or the other (or a combination of both) for a

particular application. The urea-formaldehyde

resins are lower in cost, have better adhesion

properties, are more amenable to the repair of

coating defects, and yield (in combination with an alkyd) excellent white enamels that are as good or better in color retention than the melamine-alkyds at moderate baking temperatures

(below 300 °F [149 °C]) and in indoor service

at moderate temperatures. The melaminealkyds, although considerably more expensive, are much faster-curing, undergo less shrinkage

during cure, have better color and gloss retention on overbaking and in elevated temperature

applications, are more chemical-resistant, have

superior hardness and mar resistance, and

possess better exterior durability. Melamine

resins, though capable of very fast cures at high

temperatures without color impairment, may

also be cured quite rapidly at lower temperatures and therefore are very adaptable to infrared baking schedules. Because of their greater

functionality and faster cure, the melamine res- ins are generally used in lower proportions than

the urea resins. In general, then, it may be

stated that the urea resins yield coatings having an excellent combination of color retention,

chemical resistance, and durability properties

at relatively low cost. The melamines, though

considerably higher in cost, extend the range of

desirable properties still further and are used

to particular advantage in applications requiring very fast cure, prolonged resistance to ele- vated temperatures, superior alkali resistance,

maximum hardness, and outdoor durability. As

a single example, a urea-alkyd produces a gleaming white refrigerator enamel of excellent durability at relatively low cost; on the other hand,

for a heat-resistant stove enamel or an alkaliresistant washing machine finish, the melaminealkyd would be the preferred coating material.

3.6. Vinyl Resins

In a broad structural sense, all resins derived

from the vinyl radical (CH2=CH-) may be

classified as vinyl resins. However, such a broad classification would include many types

of resins that are better known by more famil-

! iar individual designations such as polyethyl-

, ene, polystyrene, acrylic, and others. The term

I "vinyl resin" is applied here in the narrower

I sense commonly employed in the plastics and

I coatings field. Thus, vinyl resins include chiefly

j the polymers and copolymers of vinyl acetate,

1 vinyl chloride, and vinylidene chloride, and the

polyvinyl acetals and polyvinyl alcohol that are

j derived from polyvinyl acetate. Vinyl resins are thermoplastic, addition poly-

! mers that may be prepared by solution, suspen-

' sion, emulsion, or bulk polymerization methods

(see sec. 3.1.3). Structurally, they may be

thought of as substituted ethylenes in which

one or two hydrogen atoms have been replaced

by a chlorine atom, an acetate radical, an hydroxide group, or an acetal ring structure. In

properties, they range from soft, tacky, widely

soluble polymers to tough, horny, difficultly sol- uble ones, depending on their composition, molecular weight, and molecular structure. Since

first achieving commercial importance in the

early 1930's, their use has grown steadily, and

vinyl resins constitute one of the most versatile

and widely used classes of resins in the coatings

field.

3.6.1. Polyvinyl Acetate

Polyvinyl acetate is produced either as a granular resin or as a dispersion in water

(latex). The latex was discussed in section

2.3.5 on Latex Paints. The granular resin is usually prepared by solution polymerization;

the vinyl acetate monomer is heated, in a sol- vent such as benzene or toluene, in the presence

of a peroxide catalyst. The resulting polymer

is a colorless, odorless, tasteless, nontoxic resin

that is widely soluble in most of the common

organic solvents such as the lower ketones,

esters, methanol, higher alcohols containing some water, aldehydes, aliphatic acids, aromatic

and chlorinated hydrocarbons, and nitrohydrocarbons. Polyvinyl acetate is less soluble or

insoluble in the higher homologs. It is insoluble in aliphatic hydrocarbons, in animal and

vegetable fats and oils, and in water—but it swells and loses strength upon prolonged immersion in water. Polyvinyl acetate is compatible with many of the common ester-type

plasticizers such as dibutyl phthalate (but not

dioctyl phthalate), phosphates, abietates, and

glycolates. Other materials with which it is partially or completely compatible include cel- lulose nitrate, cellulose ethers, chlorinated rubber, some phenolic resins, and a few acrylic

resins and natural resins.

Polyvinyl acetate is light-stable, transparent

to solar ultraviolet and visible light, abrasion47
resistant, flexible, and very resistant to outdoor

aging. However, it softens at temperatures

above 150 °F (66 °C), swells and weakens upon

prolonged immersion in water, is hydrolyzed by

strong acids or alkalis, and dissolved by many

organic solvents. Its many desirable properties, coupled with good ahesion and ease of formulation and application, have led to its extensive use in the coatings field, as well as in the

related field of adhesives. The latex form has

found wide acceptance in primer-sealers, interior wall finishes, and exterior masonry paints.

In emulsion or solution form, polyvinyl acetate

is used as a water- and grease-resistant coating on paper, fabrics, and leather. Such coatings can be easily heat-sealed or solvent-sealed if desired, and they are widely used in food packaging. Lacquers based on polyvinyl acetate

solutions find use as decorative and protective

coatings for paper, wood, and metal. Suitably

formulated PVA coatings have shown very good

adhesion, durability, and corrosion resistance

when applied to metals. In addition to its compatibility with other

resins as noted above, vinyl acetate can be co- polymerized with various vinyl derivatives such

as vinyl chloride, styrene, and acrylics to yield

other materials with very desirable properties.

3.6.2. Polyvinyl Chloride

3.6.2.1. General properties. —Polyvinyl

chloride is a tough, horny, transparent, thermoplastic resin having high inherent strength and

excellent chemical resistance. Its properties

stem in part from its crystalline nature. It is highly resistant to strong acids and alkalis,

water, alcohols, aliphatic hydrocarbons, fats,

and oils—but it is swelled by aromatic hydrocarbons. It is soluble in only a few special

types of solvent, chiefly unsaturated ketones

(e.g., mesityl oxide and isophorone) and ali- cyclic compounds (e.g., cyclohexanone and tet- rahydrofuran) . It is soluble also in nitroben- zene and in dimethyl formamide. Because of

this very limited solubility in organic solvents,

polyvinyl chloride is not widely used in sol- vent-type coatings; however, in the form of a

plastisol or organosol (discussed below) that

is readily applied by dipping or knife-coating

methods, it finds extensive use in severe service

applications.

Polyvinyl chloride is made chiefly by suspension or emulsion polymerization. The resin is available as a general-purpose granular white

powder, as a fine-particle paste dispersion for use in plastisols and organosols, and as a waterbase latex. In each case, the key to its effective use is suitable plasticization and stabilization.

The unmodified resin, though tough and chemically resistant, is not flexible enough for coat- ings use unless plasticized, and it is degraded

by heat and by ultraviolet light unless properly

stabilized. The degradation reaction produces

HCl which catalyzes further degradation unless

the acid is removed as it forms. Fortunately, a

large number of very effective heat and light

stabilizers for polyvinyl chloride have been developed. These function as HCl acceptors or

neutralizers, ultraviolet absorbers, and antioxidants to impart excellent stability to the resin.

The general-purpose polyvinyl chloride resin

finds wide use in high-grade electrical insulating coatings for wire and cable; it combines

excellent dielectric strength with flexibility and

toughness, resistance to moisture and chemicals,

and fire resistance. The plasticized resin may

also be calendered onto paper or fabric to provide tough, water-proof and grease-proof

coatings ; however, polyvinyl chloride surface

coatings are most conveniently applied in the

form of plastisols or organosals (see below).

The use of plasticized polyvinyl chloride film

and sheeting for shower curtains, inflatables,

upholstery, etc., is so extensive as to merit

mention even in a discussion of coatings.

3.6.2.2. Plastisols and organosols.—Plastisols are dispersions of flnely divided polyvinyl

chloride resins (dispersion resins) in suitable

liquid plasticizers. The dispersion resins are powders of very fine particle size (0.05 to 1.0

micron in diameter) made by emulsion polymerization. Among suitable plasticizers are compounds such as dioctyl phthalate, tricresyl

phosphate, other alkyl and aryl phthalates and

phosphates, adipates, sebacates, and azelates.

Plastisols may be applied in a thick coat, by

dipping or knife-coating, to paper, cloth, wood,

and metal. The deposited coating is then fused

by flash-heating at 300-350 °F (149-177 °C) to yield a tough, durable, chemically resistant film. Organosols are similar to plastisols, except that

they contain some organic solvent to facilitate

application of the coating for purposes requiring a lower viscosity. Upon evaporation of the

organic solvent, the deposited coating is fused

by flash-heating just as in the case of the plastisol. Organosols and plastisols are used for

coatings on lawn furniture, office equipment,

industrial equipment, and home appliances.

They give excellent service in such severe applications as coatings for plating baths, chemical

tanks, and dish-washer racks.

Aside from the external plasticization of polyvinyl chloride as in plastisols and organosols,

the resin can be modified for use in solvent-type

coatings by copolymerization with vinyl acetate

to provide permanent internal plasticization, as described in the following section.

3.6.3. Polyvinyl Chloride-Acetate

Polyvinyl chloride-acetate is made by the co- polymerization of vinyl chloride and vinyl acetate, the chloride constituting from 85 to 97

48
percent of the copolymer. Solution or suspension polymerization methods are usually employed. The vinyl chloride-acetate copolymers

retain much of the toughness and chemical re- sistance of polyvinyl chloride while partaking

of the greater flexibility and solubility of polyvinyl acetate. The copolymers are easily formulated into solution-type coating materials for

j either air-drying or baking applications. A few

types are made as dispersion resins for use in plastisols and organosols (see previous section)

.

Like the parent polyvinyl chloride, the copoly- mers must be stabilized against the deteriorating effects of heat and ultraviolet light. Plasticizers, pigments, and fillers are incorporated as required for particular coatings applications.

Polyvinyl chloride-acetate copolymers are sol- uble in ketones, esters, ethers, and chlorinated

hydrocarbons. Aromatic hydrocarbons such as

toluene and xylene are well-tolerated diluents

and are nearly always part of the composite

i solvent formulation. In addition to their availability in a range of chloride-to-acetate ratios

1 and a variety of molecular weights, vinyl chloride-acetate resins may be copolymerized with

a small proportion (about 1%) of maleic resin

to impart improved adhesion of air-drying coat- ings to smooth metal surfaces. Vinyl copoly- mers without the maleic resin must be baked to develop good ahesion to smooth metals, but they

adhere well to porous surfaces of paper, fabric,

leather, and concrete. Most unmodified vinyl chloride-acetate co- polymers are poorly compatible with other resins. However, when modified by incorporating

hydroxyl groups, the copolymer becomes compatible with various alkyd, phenolic, amino, and

I other resins. Compatibility with nitrocellulose

' can be obtained with copolymers of high acetate

|j content.

i Vinyl chloride-acetate copolymer coatings are

j odorless, tasteless, and nontoxic (unless renii dered otherwise by modifying ingredients)

.

! The coatings are tough and flexible, resistant to

I water and chemicals, non-flammable, abrasionI resistant, and weather-resistant.

Uses. Polyvinyl chloride-acetate resins find

wide and important use in the coatings field.

; Their lack of taste, odor, or toxicity makes them

well suited for coating food and beverage containers. Their flexibility and adhesion permit

their use on flat sheet metal that must withI stand subsequent deep drawing or stamping

operations. Their chemical resistance makes

them suitable as coatings for the insides of

drums and tanks and pipes that must carry corI rosive chemicals. Their alkali resistance per-

] mits their use over concrete, plaster, and alkaline wallboards without special precautions,

j Wrinkle and textured coatings based on these

i resins can be applied to paper, cloth, wood, and

j metal to produce durable and attractive finishes; textured finishes for high-quality radio

and amplifier cabinets, which impart a furniture-like appearance to the metal cabinets, have

become popular. Other uses include coatings

for garden furniture, metal signs, hospital and

kitchen furniture that must withstand wear

and frequent washing, refrigerator parts, col- lapsible tube coatings, perspiration-resistant

articles, and specially-formulated strippable

coatings. The combination of toughness and

resistance to weather, water, and chemicals

possessed by the vinyl-chloride acetate copoly- mers makes them valuable coatings for use in

durable, corrosion-resistant coating systems for severe industrial and marine applications (see

sec. 4.3.1.2) ; for example, the coating system

selected for protecting the locks of the St. Lawrence Seaway is based on vinyl copolymer

resins.

3.6.4. Polyvinyl Alcohol

Since vinyl alcohol does not exist in monomeric form, polyvinyl alcohol cannot be prepared by direct polymerization. Instead, it is obtained by the hydrolysis of polyvinyl acetate

in an essentially water-free alcohol medium

(usually methanol) in the presence of small

amounts of acid or alkaline catalysts. The

properties of the final resin depend largely on

the degree of hydrolysis and on the molecular

weight. With increasing hydroxyl content (increasing polar character) the polymer becomes

less hydrophobic and more hydrophilic. When

about 65 percent of the original acetate groups

have been hydrolyzed (replaced by hydroxyl

groups), the product loses its solubility in or- ganic solvents and becomes soluble in cold

water but precipitates on heating. With further hydrolysis, when about 80 to 90 percent

of the acetate groups have been replaced with

hydroxyl groups, the product is soluble in both

cold and hot water. When the hydrolysis is virtually complete (95 percent or more of the

acetate groups replaced), the polyvinyl alcohol

will no longer dissolve in cold water; but if dissolved in hot waterp-it will remain in solution on cooling.

Coatings deposited from solutions of polyvinyl alcohol in water are tough and flexible and

highly I'esistant to water-insoluble organic sol- vents such as aromatic, aliphatic, and chlorinated hydrocarbons, and vegetable and animal

fats and oils. Polyvinyl alcohol coatings are highly impermeable to most gases, have high

tensile strength, and are odorless and tasteless. Polyvinyl alcohol resins can be applied to paper, fabrics, and leather to provide a highly

grease- and gas-resistant coating. In combination with a water-resistant film or coating, an

effective barrier against water, gases, greases,

and most organic solvents can be obtained.

Polyvinyl alcohol is a versatile emulsifying

agent and finds use as an emulsifier, thickener.

49
and stabilizer in emulsion polymerizations and

latex paints.

The flexibility of polyvinyl alcohol films and

coatings can be increased by incorporating

water-soluble plasticizers such as glycerol, the

lower glycols, and various amines. A number

of processes have been patented for insolubilizing polyvinyl alcohol coatings through metal

complexing or through reaction with other resins such as phenolics, melamines, and diisocyanates. A weather-resistant polyvinyl alcohol

water-base coating has been described [57]

which is formulated so as to form insoluble

acetals (without baking) that resist solution in

water while retaining good resistance against

penetration by gases and by organic solvents

and vapors.

3.6.5. Polyvinyl Acetals

Polyvinyl acetals are the products of the con- densation reaction between polyvinyl alcohol

and aldehydes. The aldehydes of commercial

importance for this use are formaldehyde, acet- aldehyde, and butyraldehyde. The reaction can

be controlled to yield acetals having various

ratios of acetate, hydroxyl, and acetal groups.

In general, solubility decreases while viscosity

and strength increase with increasing degree of

acetalization. The resins are usually supplied

as powders which may be compounded with

plasticizers and other resins to produce tough,

elastic, water-insoluble coatings.

3.6.5.1. Polyvinyl formal is the toughest of

the vinyl resins. It has a tensile strength of

about 20,000 psi, a softening temperature of

about 375 °F (191 °C) , and a water absorption

of about 1 percent. It is unaffected by aliphatic

hydrocarbons, gasoline, oils, fats, waxes, and

alkalies, but it is soluble in chlorinated hydrocarbons, dioxane, phenol, and aliphatic and

cyclic ethers. It has excellent abrasion resist- ance and electrical properties, and a chief use has been in combination with phenolic resins as a tough, flexible, electrical insulating coating

for wire. It has also been used as the inner

gasoline-resistant liner of self-sealing "bulletproof" gasoline tanks, as an abrasion-resistant

coating for wooden propellers of aircraft, for can linings, and in metal primers.

3.6.5.2. Polyvinyl acetal is a slightly yellowish, odorless and tasteless resin that is widely soluble in ketones, esters, alcohols, and

chlorinated hydrocarbons, but insoluble in aro- matic and aliphatic hydrocarbons, oils, and

water. Its tensile strength (about 8000 psi)

and its softening temperature (about 320 °F)

[160 °C] are considerably below that of polyvinyl formal, and it has a somewhat higher

water absorption (about 2 percent) . Although

used chiefly in adhesives, it finds limited use in

coatings for fabrics and, in combination with

cellulose nitrate, in interior coatings for household articles.

3.6.5.3. Polyvinyl butyral is best known

for its wide use as the clear, tough interlayer

in safety glass. However, it also performs excellently in the coatings field and has proved

particularly useful as a base for wash primers

(see sec. 6.3.2.5) and metal conditioners. As

made commercially, polyvinyl butyral resins

contain a considerable percentage of hydroxyl

groups (about 7 percent) which impart a hydrophilic character to the resin and contribute

to its excellent specific adhesion to nonporous

surfaces such as glass, metal, and phenolic plastics. Polyvinyl butyral coatings also have very

good adhesion to porous surfaces such as wood,

paper, and fabrics. Baking is not required to develop good adhesion.

Polyvinyl butyral is soluble in non-anhydrous alcohols, glycol ethers, and some esters and ketones. Water is an effective latent sol- vent ; in fact, 3 to 8 percent must be present to achieve good solvent properties. Mixed thin- ners containing about 40 percent alcohol with

esters, ketones, aromatic hydrocarbons, and a

small amount of water are excellent solvents

for use in coatings formulations based on polyvinyl butyral. Although flexible enough for

many coatings applications without added plasticizer, polyvinyl butyral resins may be com- pounded with various chemical plasticizers,

such as trioctyl phosphate, triethylene glycol di

(2-ethylbutyrate) , and dibutyl sebacate, to in- crease their flexibility and to decrease their

viscosity in hot melt applications (e.g., in highspeed book-binding)

.

Polyvinyl butyral is compatible with phenol-,

urea-, and melamine-formaldehyde resins and

can be cured or cross-linked by heating with

these resins. Air-dry cure and crosslinking can be accomplished by the addition of glyoxal.

Crosslinked polyvinyl butyral coatings are

tough, heat-resistant, and solvent-resistant;

they have been used for gasoline tank linings, can coatings, laboratory equipment, and as waterproof cloth coatings replacing rubberized

fabrics.

An effective knot sealer (WP-578) developed

by the Western Pine Association is based on

a combination of polyvinyl butyral with a phenolic resin in ethanol (95%) solution [149].

The sealer may also be applied over asphaltic

materials to prevent bleeding into enamel topcoats. Polyvinyl butyral may be used as a heatsealable coating for foil and other packaging

materials. Durable wood finishes can be made

from combinations of polyvinyl butyral with

shellac or with phenolic resins. The most important application of polyvinyl butyral in the

coatings field, however, has been as the base

for corrosion-inhibiting metal primers (wash

50
primers) utilizing chromate pigments for cor- rosion inhibition, and phosphorc acid to achieve

metal conditioning and cross-linking of the

coating [30]. Such wash primers have excellent adhesion to metal substrates and to virtually any type of topcoat lacquer or paint (except a few types of nitrocellulose and vinyl

lacquers) . Excellent all-vinyl-base coating sys- tems for severe marine applications employ a

polyvinyl butyral wash primer as the first coat

applied to the metal (see sec. 4.3.1.2).

3.6.6. Polyvinylidene Chloride and its Copolymers

3.6.6.1. Polyvinylidene chloride is a tough,

nonflammable, crystalline resin characterized

by extremely low water vapor permeability and

absorption, and outstanding resistance to oils,

greases, acids, alkalis (except strong ammonium hydroxide) , and most organic solvents

and reagents. It is soluble only in cyclic ethers

and cyclic ketones, but is attacked and weakened by halogens and strong organic amines.

Because of its extremely limited solubility and

compatibility with other materials, polyvinylidene chloride itself has not in the past been

used in the coatings field ; however, a polyvinylidene chloride latex for coatings applications

requiring exceptional water and chemical re- sistance is now available. When polyvinylidene

chloride is internally plasticized by copolymerization with vinyl chloride or acrylonitrile, very

useful coatings resins are obtained.

3.6.6.2. Polyvinylidene chloride copoly- mers (general) . The copolymers (commonly

known as saran resins in the trade) have a

less crystalline and less closely packed struc- ture than the polyvinylidene chloride homopolymer because of the irregularities introduced into the polymer chains by the random

presence of the copolymerized vinyl chloride or

acrylonitrile groups. The reduced crystallinity

of the copolymers increases their solubility and

compatibility and permits their use in solutiontype coatings and in plasticized latexes. It is of general interest to note that the irregular

arrangement of vinyl and vinylidene groups in

the copolymer chains produces a fairly soluble

and compatible resin even though both polyvinyl

chloride and polyvinylidene chloride are themselves only narrowly soluble and compatible

materials (cf. discussion of crystallinity in sec.

3.1.1. A range of properties is achieved by

varying the copolymer ratio and system. In

general, commercial polyvinylidene chlorideacrylonitrile copolymers for coatings contain a low proportion of acrylonitrile, whereas polyvinylidene chloride-vinyl chloride copolymers

may vary in composition from low to high

ratios of vinyl chloride. The copolymer la- texes, although convenient to use, are not quite

as water-impermeable or chemically resistant as the solution-type coatings, largely because

of the water-sensitive emulsifiers, stabilizers

and other additives that remain in the cured

latex film.

3.6.6.3. Vinylidene chloride-vinyl chloride copolymers possess their maximum solubility, compatibility, and flexibility when they

contain approximately equal proportions of

vinyl and vinylidene groups. Tensile strength

and softening temperature are higher when

either monomeric group predominates. Color

stability is better with higher vinyl chloride

content; as with straight vinyl chloride resins,

the copolymer resins require heat and ultraviolet stabilizers for best results. A typical co- polymer of medium or somewhat higher vinyl

chloride content is soluble in ketones, cyclic

ethers, and amides ; it has a high tolerance for

aromatic hydrocarbon diluents and exhibits

good compatibility with the common vinyl plasticizers, as well as with many alkyds and a few

other resins; it has excellent resistance to water, chemicals, and many organic solvents, in- dicating a potential usefulness for maintenance

coatings and marine finishes. A copolymer of

high vinylidene chloride content is available

which is soluble in aromatic hydrocarbons and

in mixtures of aromatic hydrocarbons with

ketones, and which is compatible with many

common vinyl plasticizers and a few other

resins. Another type, a latex, does not form

films alone but is compatible with a number of

water dispersable materials and oleoresinous

and alkyd resins to which it contributes toughness and water resistance. In general, vinylidene chloride-vinyl chloride copolymers are used in applications requiring exceptional re- sistance to water, acids, alkalis, and many

solvents, and a high degree of impermeability

to water vapor and gases.

3.6.6.4. Vinylidene chloride-acrylonitrile

copolymers of high vinylidene chloride content

are available as solution or emulsion types. The

resin is soluble in methyl ethyl ketone and in

various cyclic and unsaturated ketones and will

tolerate limited amounts of diluents. It has

suflficient flexibility to permit its use without

plasticization in coatings where maximum

chemical and solvent resistance are needed.

Where greater flexibility is required, the resin

may be plasticized with various phthalates,

phosphates, glycolates, or sebacates. Although

vinylidene chloride-acrylonitrile copolymers

have a limited compatibility with a few other

resins, they are usually used alone because of

the better chemical resistance of the unmodified resin. Coatings based on these resins find use in the packaging industry where they are

applied to paper, fabrics, and metal foil to provide heat-sealable, chemical-resistant.

51
grease-proof, and water-proof coatings. Other

uses include cap linings, coatings for the in- terior of gasoline storage tanks, drum coatings, chemical-resistant maintenance paints,

and strippable cobweb coatings (see sec. 2.2.7)

for protecting instruments and- machinery

against moisture and corrosion during shipment and storage. A water- and fuel-resistant

lacquer [150] based on a vinylidene chlorideacryonitrile copolymer is covered by Mil. Spec.

MIL-L-18389.

3.6.7. Polyvinyl Ethers

Among the polyvinyl ethers, only the alkyl

types are of sufficient importance to warrant

brief mention here. These include the methyl,

ethyl, isobutyl, and n-butyl polyvinyl ethers,

and a copolymer of vinyl methyl ether with

maleic anhydride. The polyvinyl alkyl ethers

are linear, thermoplastic, tacky polymers that are widely soluble in organic solvents and widely compatible with other resins. The vinyl

ether/maleic acid copolymer is soluble in water

as well as in organic solvents ; the vinyl methyl

ether homopolymer is soluble in cold but not in

hot water.

The chief application for the polyvinyl alkyl

ethers has been in pressure-sensitive tapes,

polyvinyl ethyl ether being particularly useful

in this respect in combination with phenolic

varnish resins. In the coatings field, the polyvinyl ethers are finding some use as plasticizing agents in chlorinated rubber paints and cel- lulose nitrate lacquers, and as binding agents

in paints and impregnating solutions. Polyvinyl methyl ether can be used as a nonionic

heat-sensitizing or softening agent in latex

paints to improve film coalescence and adhesion.

3.7. Acrylic Resins

3.7.1. Properties

The acrylic resins of chief importance in

coatings are the polymers and copolymers of

acrylic and methacrylic acid esters. They are

thermoplastic, addition-type polymers that

range in physical properties from soft, sticky

semiliquids to hard, machinable solids, depending on their composition and molecular weight.

The acrylates are softer, more extensible, and more soluble than the corresponding methacrylates. Softness, extensibility, and solubility are

also greater with increasing number of carbon

atoms in the alcohol side-chain. With the wide

range of properties obtainable by varying the

alcohol side-chain, and with the additional opportunities for variation afforded by copolymerization of various acrylates and methacrylates, it is possible to produce resins having al- most any desired degree of flexibility without

the use of plasticizers. The latter is an important advantage, since the unmodified acrylic

ester polymers possess a clarity and a resist- ance to discoloration by aging, heat, and ultraviolet light that is unsurpassed by any other

resin. Other desirable properties of acrylic

resins which have led to their wide use in coat- ings and plastics since their introduction

around 1931 are: wide solubility in common

organic solvents, permitting ease and economy

of formulation; excellent resistance to acids

(except concentrated oxidizing types), alkalis,

water, alcohols, aliphatic hydrocarbons, oils,

and greases ; low acid number and low reactivity with pigments ; and good adhesion to a variety of surfaces. Although acrylic coatings airdry satisfactorily, optimum film properties are developed by baking.

While the degree of solubility (and the so- lution viscosity) of the acrylic resins depends

on their composition and molecular weight, as noted above, they are, in general, soluble in

aromatic and chlorinated hydrocarbons, esters,

and ketones. A few special types are available

which are soluble in alcohol and in mineral

spirits, but in most instances the alcohols and

aliphatic hydrocarbons are non-solvents. The

acrylic resins have only limited compatibility

with each other and with other classes of

resins ; however, their monomers are readily co- polymerized with other addition-type monomers. Some types of acrylic resin are compatible with nitrocellulose, ethyl cellulose, and some of the vinyl resins. They are not compatible with drying oils, oleoresinous varnishes,

or oil-modified alkyds. Where plasticization is desired for special purposes, the acrylic ester resins may be combined and are compatible

with some of the common chemical plasticizers

such as dibutyl phthalate, tricresyl phosphate,

and dibutyl sebacate.

The acrylic resins are available in emulsion

form as well as solution-type resins. In emulsion form they have found wide acceptance in

latex paints and primer-sealers for both interior and exterior applications. The characteristic heat and light stability and chemical re- sistance of the solution-type acrylic resins are

also found in the latex form. A discussion of

acrylic latex paints is given in section 2.3.5.6.

3.7.2. Uses

Because of their versatility and excellent

combination of properties, the acrylic resins

find many uses in the coatings field. In addition to their large-scale use in latex paints, they

are used in protective and decorative lacquers

for paper, fabrics, leather, plastics, wood, and

metal. Clear acrylic coatings are used to pro52
tect paintings and other art objects, to pre- serve important documents, to prevent the tar- nishing of silverware, to protect bright metal

against outdoor corrosion, and as electrically

insulating coatings. White baking enamels

having excellent resistance to chemicals and

chemical fumes can be made v^^ith acrylic resins.

Acrylic vehicles are also used in luminescent

paints. Recently a high-gloss, hard, weatherresistant finish based on acrylic resins has been

developed and adopted for use on automobiles.

Thermosetting types of acrylic resins have been

developed [124] which cross-link upon heating

to form hard, insoluble and infusible coatings

that are suitable for appliance finishes that

must withstand severe service, such as those

3.8. Polystyrene

The styrene monomer is readily polymerized

by bulk, solution, suspension, or emulsion

methods (see sec. 3.1.3) in the presence of a peroxide catalyst and mild heat. Very high

molecular weights (greater than 1,000,000) can

be obtained; however, most commercial polystyrenes are considerably lower in molecular

weight (about 125,000 for molding grades and

about 35,000 for grades used in surface coat- ings). Styrene is also readily copolymerized

with a variety of other materials, including

drying oils, butadiene, phenolics, vinyls, acrylics, maleic esters, and various unsaturated di- carboxylic acids. Substituted styrenes, such as a-methyl styrene, vinyl toluene, etc., also polymerize readily. The polystyrene family of resins

is therefore a large one, and a variety of properties is obtainable depending on the composition and the degree of polymerization.

3.8.1. Polystyrene Homopolymer

Polystyrene itself is a hard, glasslike, colorless, transparent, thermoplastic resin. It is soluble in aromatic and chlorinated hydrocarbons and in some esters and ketones. It is highly resistant to acids (except strong oxidizing types), alkalis, alcohols, and vegetable

fats and oils. Although somewhat permeable

to water vapor, its water sorption is extremely

low, and it is highly resistant to and dimensionally stable in water. It has excellent elec- trical properties. The chief use for unmodified

polystyrene is in the plastics molding field. Polystyrene is not widely used alone as a coat- ing because of its brittleness, although it is readily applied from solution for coatings applications in which flexibility is not a requirement (e.g., as a grease-, stain-, and perspiration-resistant protective coating on hard book

covers, stiff leather and fabrics, labels on rigid

substrates, and as an electrically insulating

coating) . The copolymers of polystyrene with

on washing machines, stoves, and dishwashers.

A water-thinnable vehicle for industrial baking

finishes utilizing a new thermosetting acrylic

emulsion polymer [123] is reported to have a hardness, impact resistance, adhesion, and durability superior to that of premium melaminealkyd baking finishes while being equal to melamine-alkyds in gloss and stain resistance.

Since the acrylic ester monomers are readily

copolymerized with other vinyl monomers, an

important use for the acrylics has been to provide internal plasticization for polymers of

vinyl chloride, vinylidene chloride, vinyl acetate, acrylonitrile, styrene, and others. An

acrylic-nitrocellulose gloss lacquer [138] is covered by Mil. Spec. MIL^L-19537.

and Its Copolymers

butadiene and with drying oils, however, find

extensive use in coatings.

3.8.2. Styrene-Butadiene Copolymers

Styrene-butadiene copolymers range from

hard, brittle solids to soft, rubbery materials,

depending on the ratio of styrene to butadiene.

High butadiene types constitute the widely used

GR-S (now termed SBR) synthetic rubbers.

The high styrene types, containing about 60-80

percent styrene, are used in coatings. Being

non-polar hydrocarbon resins, they are insoluble in water, alcohols, and other strongly polar

solvents, but they are soluble in aromatic and

chlorinated hydrocarbons and in mixtures of

aromatic hydrocarbons with aliphatic hydrocarbons and ketones. They are resistant to

acids, alkalis, mineral oils, and animal and

vegetable fats and oils. Although thermoplastic and possessing a low softening temperature

(about 125 °F [52 °C] for the solution types),

they are heat stable at temperatures as high

as 300 °F (149 °C). Both flexibility and adhesion are greater than that of the polystyrene

homopolymer.

The chief application for styrene-butadiene

copolymers in the coatings field has been in the

water-base latex form. The first latex paints

to be developed were of the styrene-butadiene

type, and these are still among the leading materials in the huge gallonage of interior latex

paints now marketed (see sec. 2.3.5). To a

lesser, but still considerable, extent the styrenebutadiene copolymers also find use in exterior

latex paints. They are also used as greaseproof, chemical-resistant, heat-sealable coat- ings for paper and fabrics.

Solution-type styrene-butadiene resins also

find wide use in coatings. Their combination

of excellent water and chemical resistance,

toughness and abrasion resistance, good adhesion, and fast drying properties have led to their use in traffic paints, masonry paints, con53
Crete floor enamels, swimming pool paints,

metal primers, industrial maintenance paints,

and general-purpose exterior coatings.

3.8.3. Styrenated Oils and Alkyds

Although polystyrene cannot be physically

mixed with drying oils because of incompatability, the styrene monomer is readily copolymerized with drying oils having conjugated unsaturation (e.g., tung oil and dehydrated cas- tor oil). Non-conjugated drying oils, such as

linseed or soybean oil, may be successfully

styrenated if they are first mixed with oils of

the conjugated type. The styrenated oils yield

fast-drying, hard, moisture-resistant films that

have fair durability.

Improved durability is obtained by copolymerizing styrene with alkyds. The alkyd may be

styrenated directly or a styrenated oil may be

used (see sec. 3.4.2.1). Styrenated alkyds,

while exhibiting some of the sensitiveness of

polystyrene to aromatic solvents, have better

acid and alkali resistance than ordinary oil- alkyds. Although not as durable on exterior

exposure as the pure oil-modified alkyds, the

styrenated alkyds find considerable use in low

cost household and industrial finishes where

fast drying is required along with good protective properties. Long-oil styrenated alkyds

also find use as plasticizers for vinyl, amino,

and cellulose nitrate lacquers.

3.8.4. Other Styrene Derivatives

3.8.4.1. Copolymers of styrene with phenolic

resins are used in coatings for porous surfaces

and in household maintenance finishes. Styrene-isoprene copolymers with properties some- what similar to styrene-butadiene copolymers

have been produced. Styrene-maleic anhydride

copolymers find use as emulsifying and stabilizing agents in latex formulations. The polymers

of styrene and acrylonitrile are resistant to aro- matic hydrocarbons but soluble in ketones ; they

have not been used much as coatings.

3.8.4.2. u-Methyl styrene is useful as a moderating comonomer in the mass copolymerization of styrene with the more reactive drying oils ; by slowing the polymerization rate, it permits better control of the process.

3.8.4.3. Vinyl toluene polymers and co- polymers are a promising and increasingly useful class of resins for coatings. The name

"vinyl toluene" refers to the ortho-, meta-, and

para-substituted methyl styrenes, as distinguished from a-methyl styrene. The vinyl

toluene polymers are more flexible and more

widely soluble and compatible than their polystyrene counterparts. Like styrene, vinyl tol- uene can be copolymerized with drying oils, oil- modified alkyds, and various addition-type monomers, yielding a very versatile and useful class

of resins having a wide range of desirable

properties. The reaction of vinyl toluene with

drying oils in the presence of divinyl benzene

as a cross-linking agent produces alkyd-like

resins that are more stable and alkali-resistant

than ordinary alkyds because they have a car- bon-to-carbon instead of an ester-type cross- linkage. Among the more recent developments

in vinyl toluene resins are copolymers with butadiene and other dienes ; these are soluble in

organic solvents and constitute a promising

class of coatings resins.

3.9. Silicone Resins

3.9.1. Pure Silicone Resins

Silicone polymers are organosilicon compounds of high molecular weight, consisting of an inorganic backbone chain of silica (alternate

Si and 0 atoms) with organic radicals attached

to this main chain through C-Si linkages. The

basic silicone (siloxane) structure is: H H H

I I I -Si-O-Si-O-Si-0-

I I I H H H

Replacement of the hydrogen atoms with vari- ous alkyl and aryl groups yields a very large

and very useful class of polymers (organopolysiloxanes) that are characterized by outstanding heat resistance, excellent water resistance

and water repellency, excellent electrical properties, and very good weatherability. The sili- cones have been available since about 1944.

Both linear and crosslinked types are produced.

The low molecular weight linear polymers are

heat- and oxidation-resistant fluids. The high

molecular weight linear polymers are elastomers. The crosslinked types, in a range of molecular weights, are the silicone resins used as

coatings. Crosslinking is generally accomplished by introducing trifunctional silicon

atoms (having a third oxygen atom attached

in place of one of the organic substituent

groups) at intervals along the siloxane chains

to form oxygen bridges between trifunctional

atoms on different chains. Ways have been developed also for accomplishing the crosslinking

through polyfunctional organic side-groups.

Although the solubility, compatibility, and

chemical resistance of the silicone resins vary

with the nature of the side groups and the molecular weight, they are generally soluble in

chlorinated and aromatic hydrocarbons, in mixtures of aromatic and aliphatic hydrocarbons,

and in many ketones, esters, and glycol ethers.
They may be cold-blended or copolymerized

with a variety of other resins as described

later. They are resistant to acids, alkalis, water, corrosive atmospheres, greases, and oils. Silicone resins require baking at elevated

temperatures (300-500 °F [149-260 °C]) to develop their optimum film properties, and they

are capable of thousands of hours of service

at such temperatures. The decomposition temperature for most silicone resins is around

650 °F (343 °C). However, when pigmented

with heat-resistant pigments such as aluminum

or zinc, they afford protection against weathering and corrosion at operating temperatures

up to 1000 °F (538 °C) . The silicone resin will

gradually volatilize on prolonged heating at this

temperature, but a silica matrix remains which

holds the pigment in place. Incorporation of

ceramic frits into the formulation permits even

higher operating temperatures—to 1400 °F

(760 °C).

Uses. Because of their exceptional heat re- sistance, the silicone resins find their main use

in heat-stable maintenance coatings and industrial finishes for severe high-temperature applications such as hot stacks, oven walls, combustion engine mufflers, incinerator interiors, arc light reflectors, and wall and space heaters.

Several Government specifications [58, 59, 60,

61] for high temperature-resistant paints are based on, or can be met by the use of, silicone

resin vehicles. Although used primarily for

their high temperature resistance, silicone resins also perform well at low temperatures. Applied from dilute solution to masonry surfaces,

the silicones provide excellent water repellency.

Some silicone fluids are used as additives in

protective coatings to promote flow-out and

prevent bubbling, and to produce hammertone

finishes (sec. 4.1.4.5) ; these fluids should not

be confused with the film-forming silicone

resins.

3.9.2. Modified Silicone Resins

Where the maximum heat resistance obtainable with the pure silicone resins is not re- quired, more economical yet heat-resistant coat- ings can be made with modified silicone resins

that have been blended, or copolymerized, with

organic resins such as alkyds, phenolics, epoxies, acrylics, amino resins, and cellulose derivatives. Both baking enamels and air-drying

coatings are available. Suitable driers may be

used in the baking types to permit lower baking schedules if necessary. In air-drying types,

the organic constituent is usually the major

component. As little as 25 percent of silicone

resin can considerably increase the heat resist- ance and weatherability of organic resins that

would quickly deteriorate if used alone. At the same time, some physical properties (e.g., adhesion, toughness, color, solvent resistance)

may be enhanced by the organic resin.

3.9.2.1. SiLicONE-ALKYDS.—Silicone-modified

alkyd resins have been particularly useful in the coatings field. Baking enamels of this type

combining a silicone with a non-drying oil alkyd retain their color and gloss for long periods of continuous service at temperatures up

to 400 °F (204 °C). Air-drying aluminum

paints based on silicone-modified medium-oil

alkyds have excellent corrosion resistance and

weatherability, and will withstand temperatures up to 800 °F (427 °C) for limited periods.

Short-oil alkyds containing about 30 percent

silicone have been used to make durable, glossretentive automotive finishes. Exterior trim

paints having excellent chalk resistance can be

made with silicone-modified long-oil alkyds.

Heat- and corrosion-resistant primers for use on metals under silicone-modified topcoats are

usually based on a silicone-alkyd vehicle with

suitable heat- and corrosion-resistant pigments,

such as zinc chromate, aluminum paste, or zinc

dust (also see sec. 3.4.2.2).

3.9.2.2. Other silicone-modified resins

that are the basis for durable coatings combining many of the desirable properties of each

component include combinations of silicones

with phenolic resins to produce weather- and

water-resistant maintenance paints ; with acrylic resins or melamine-formaldehyde resins for

durable white enamels ; with chlorinated rubber to make tough, heat- and corrosion-resistant industrial maintenance paints; with spoxy

resins for strong, heat-resistant electrical in- sulating coatings; and with cellulose derivatives, such as ethyl cellulose and cellulose nitrate, to make heat- and weather-resistant

lacquers.

3.10. Epoxy Resins

Epoxy resins, since their introduction in 1949, have found important and steadily in- creasing use in the coatings field because of

their unsurpassed combination of excellent adhesion, toughness, and chemical resistance.

They are the condensation products of epichlorohydrin with &is-phenol. The chain structure

consists only of carbon-to-carbon and ether

linkages and is therefore quite stable, with a considerably greater degree of chemical resist- ance than is obtained with ester linkages.

Epoxy resins have two types of reactive groups,

the terminal epoxide groups and the hydroxy!

groups along the polymer chains. They are

therefore readily crosslinked with heat-reactive resins or polyfunctional amines to form

55
highly resistant, thermoset polymers. The hydroxyl and epoxide groups may also be reacted

with organic acids; esterification with longchain fatty acids such as linseed, soy, or cocoa- nut fatty acid yields alkyd-like resins with the

inherently more stable ether linkage of the

epoxy chain as the backbone instead of the

ester linkage of the alkyd.

Because of the wide spacing between the re- active epoxide and hydroxyl groups, even the

crosslinked epoxy polymer retains an unusual

degree of flexibility along with its strength and

chemical stability. Etherification of the chromophoric phenolic hydroxyl group during the

condensation with epichlorohydrin greatly re- duces yellowing tendencies in the epoxy polymer. The polar nature of the epoxy molecule

accounts for its strong adhesion to a wide variety of surfaces. It also accounts for its solubility in oxygenated solvents such as ketones,

esters, and ether-alcohols and in chlorinated

hydrocarbons. Aromatic hydrocarbons and al- cohols are latent rather than active solvents

for epoxy resins.

The various grades of epoxy resins offered

commercially differ primarily in molecular

weight, the solubility and compatibility usually

decreasing with increasing molecular weight.

In general, the epoxies are compatible with

pure phenolic resins, urea-formaldehyde resins,

chlorinated biphenyls, and polyvinyl formal.

They are incompatible with rosin-modified phenolics, cellulose derivatives, and most alkyds.

The lower molecular weight epoxies are compatible with polyvinyl acetate and have limited

compatability with a few natural resins and

ester gums.

From the standpoint of composition and use, the epoxy resins fall into three categories: (1)

unmodified (unesterified) baking types, (2) unmodified (unesterified) air-drying types, and

(3) esterified types for either air-dry or baking finishes. These are discussed below.

3.10.1. Unmodified Epoxy Baking Resins

The maximum adhesion, durability and re- sistance properties of the epoxies are realized

with the higher molecular weight, unmodified

(unesterified) epoxy baking resins. These are

dissolved in suitable solvents and blended with

certain urea-formaldehyde or phenol-formaldehyde resins. Upon baking, they cure to tough,

hard, thermoset coatings that exhibit outstanding adhesion, fiexibility, and chemical resist- ance (especially to alkalis) . These coatings

find extensive use in applications such as cor- rosion- and abrasion-resistant tank and drum

linings, can coatings, appliance primers, re- frigerator and washing machine finishes, hospital furniture, collapsible tube linings, wire

enamels, clear metal finishes, and industrial fin- ishes for equipment that must withstand corrosive atmospheres. Thick epoxy coatings can

be applied to irregularly shaped objects and

grill-work by the fluidized resin bed technique

(see sec. 5.4.6)

.

3.10.2. Unmodified Epoxy Air-Dry Resins

The amine-cured epoxy resins combine most

of the advantageous properties of a baked film

with the convenience of an air-dry coating.

Reactive amines, such as ethylene diamine or diethylene triamine, reduced with a suitable

amount of thinner, are stirred into the vehicle

shortly before use at a concentration of about

6 to 8 parts per hundred parts (by weight) of

resin. Once the amine hardener has been

added, the pot life of the mixture is from 8 to 60 hours depending on the nature of the resin

and the amine used. Where possible, an induction period of a half-hour to an hour should be

allowed after mixing before applying the coat- ing [62]. Because of the caustic, volatile, and

toxic nature of reactive organic amines, care must be exercised to avoid contact with the

skin and to provide adequate ventilation during their use. The lower molecular weight

epoxy resins are well suited for cold-curing applications. Their better solubility permits a high solids vehicle which can be applied in a heavy coat in one operation. Since curing proceeds by direct crosslinking rather than by

oxidation, thick coats are evenly hardened

throughout without difficulty, provided the substrate is porous enough to permit escape of

solvent. On nonporous surfaces, thickness

should be limited to about 2 mils and sufficient

time (1 to 2 days) should be allowed before

over-coating, to avoid entrapment of solvent

that might migrate to the interface and impair

adhesion. The cured coating is almost colorless and is satisfactory for white and pastel

finishes as well as for deep colors and clear

coatings.

The amine-catalyzed cold-cured epoxy resins

possess very good chemical resistance, toughness, and durability and are finding increasingly greater use in industrial maintenance

paints, architectural paints and enamels, spar

varnishes for marine applications, and clear

coatings for floors, furniture, concrete, and

metal. They may be applied by brush, spray,

or flow-coating methods. For spray application a special type of spray gun [63] is available which mixes the resin and hardener in the head of the gun at the instant of use (see

sec. 5.4.3.6).

Epoxy-polyamide coatings are discussed un- der Polyamide Resins (sec. 3.11.3).

3.10.3. Esterfied Epoxy Resins

Epoxy resins can be esterified by reaction

with fatty or rosin acids in heating kettles.

56
in a manner somewhat similar to that by which

alkyd resins are made. By the use of different

types and amounts of organic acid, a variety

of air-dry or bake-type vehicles can be prepared. The baking types are often blended

with amino resins to improve their chemical

resistance. The epoxy esters are less expensive and more widely soluble and adaptable

than the unmodified epoxy resins, but they are not as chemical- or weather-resistant. They

soon chalk on outdoor exposure, but their ulti- mate film life is greater than that of mediumlength alkyds, and they have better resistance

to mildew and dirt collection.

Where the maximum chemical resistance obtainable with the unmodified epoxies is not re- quired, the epoxy esters offer a very useful

combination of adhesion, flexibility, toughness.

and durability for coatings applications. Longoil epoxy esters find use in air-dry furniture

and floor finishes, trim and trellis paints, and

general-purpose interior and exterior enamels.

Short-oil baking types combined with urea- or melamine-formaldehyde resins are used for appliance finishes, industrial baking enamels and

primers, metal cabinet enamels, and decorative

enamels.

Styrenated epoxy ester resins can be prepared by adding styrene monomer to the partially esterified epoxy ester and refluxing the

mixture. The resulting resins, while somewhat

similar to styrenated alkyds, are superior to the latter in outdoor durability, chemical re- sistance, and mar resistance. They find use in fast-baking enamels, force-dry wood finishes,

and quick-drying varnishes.

3.11. Polyamide Resins

Polyamide resins are the polymeric condensation products of dibasic organic acids and

polyamines, with recurring amide groups as an

integral part of the main chain. Although the

different polyamides constitute a large family,

those of chief importance are of three types:

(1) the nylons, (2) thermoplastic fatty aciddiamine polymers, and (3) thermosetting fatty

acid-polyamine polymers. These types are discussed below.

3.11.1. Nylon Resins

The nylon resins are linear polyamides made

by the condensation of di-carboxylic acids with

diamines. They are capable of being formed

into strong, oriented filaments on stretching.

The oldest and most widely used type of nylon

is that based on adipic acid and hexamethylene

diamine (called nylon 6/6 because both the

acid and amine components have six carbon

atoms) . Another major type is nylon 6/10, in

which sebacic acid is used instead of adipic

acid to yield a softer, more flexible polymer

with lower water absorption and greater stiff- ness at high humidities. A newer type, nylon

6, is made by heating caprolactam (a six-carbon ring-type amide) under conditions in which

the ring opens to form a linear polymer; this

type has a lower softening temperature and a wider solubility than the other types of nylon.

Although the nylons are thermoplastic resins,

their heat-distortion temperature (about 270-

360 °F [132-182 °C]) is considerably higher

than that of most other thermoplastic resins

and many thermosetting ones. In addition to their heat resistance, they have fairly good re- sistance to sunlight (although they yellow

slightly) , and they are resistant to weak and

strong alkalis, dilute acids, and most organic

solvents. Among the very few good solvents

for ordinary nylons (nylon 6/6 and 6/10) are

phenol, m-cresol, and formic acid. Nylon 6 is soluble also in aqueous isopropanol and higher

alcohols. Nylon resins are strong, tough, and

abrasion-resistant. Their low coefficient of

friction gives them excellent wear resistance even in the absence of lubrication. The excellent combination of properties possessed by the

nylon resins has led to their wide and important use as a textile fiber and for molded plastic articles and mechanical parts. However,

they have found only limited use in the coat- ings field because of their poor solubility and

high softening temperature. Nylon 6, because

of its wider solubility and lower softening temperature, is finding use in both solution and

dispersion form as a coating for textiles,

leather, wire, and heat-sealing foils.

3.11.2. Thermoplastic Polyamides

Linear, thermoplastic polyamide resins can

be made hy the condensation of dimerized fatty

acids with diamines (e.g., ethylene diamine)

.

Unlike the nylons, these are fairly soluble and

compatible with a variety of solvents, plasticizers, and resins. Combinations of alcohols

and aromatic hydrocarbons are the best sol- vents. They are soluble also in isopropanol

and higher alcohols, in hot aromatic hydrocarbons, in chloroform, and in acetic acid.

They are not soluble in glycols, aliphatic hydrocarbons, ketones, or esters. They are compatible with phenolic, maleic, and rosin resins,

but are incompatible with most vinyl resins, urea- and melamine-formaldehyde resins, petroleum resins, coumarone-indene, and cellu- lose derivatives. Although they may be blended

and are to a limited extent reactive with epoxy

resins, the more reactive, thermosetting types

of polyamide are more often used for this purpose, as discussed later.

57
The properties of the various polyamide

resins can be varied over a wide range of flexi- bility and hardness by blending with each other

or with various liquid plasticizers or compatible resins. Polyamide coatings offer a good

combination of properties, including toughness, water and chemical resistance, grease re- sistance, good electrical properties, and excellent adhesion to a variety of surfaces, including difficult ones such as cellophane, glassine,

and polyethylene. They are useful as heatsealing coatings for paper, fabrics, metal foils,

and many plastics. They also find use in high

gloss, overprint varnishes and lacquers, and in

lacquer-type printing inks. A recent and in- creasingly important use for the thermoplastic

polyamide resins is in the formulation of thixotropic paints; for this purpose, they appear to be superior to the metallic stearates and soap

solutions used in the past. The polyamides

also make good general-purpose adhesives.

Polvamide coatings may be applied as a hot

melt, from solution, or as an aqueous dispersion.

3.11.3. Thermosetting Polyamides

The thermosetting polyamide resins are the

polyfunctional condensation products of dimerized fatty acids with polyamines (e.g., diethylenetriamine) . They range from soft, tacky

resins to pourable fluids. They may be re- acted with epoxy, phenolic, or other resins to produce thermoset resins with very useful

properties.

3.11.3.1. Epoxy-polyamide resins are of particular importance. They are obtained by

blending reactive polyamide resins with epoxy

resins, the polyamide serving as both curing

agent and modifier for the epoxy resin. The

resulting resin blend yields coatings having excellent gloss, hardness, flexibility, impact re- sistance, and abrasion resistance, plus very

good resistance to solvents, chemicals, and out- door weathering. They are superior to straight

epoxies in resistance to continuous water immersion. Although resembling amine-cured

epoxies in their ability to cure at room temperature without requiring oxygen, the epoxypolyamides have a considerably longer pot life. In many cases they may be applied as soon as mixed without the induction period recom- mended for epoxy resins. Epoxy-polyamide

coatings exhibit outstanding adhesion to almost

any type of surface—metals, plastics, glass,

paper, fabrics, leather, wood, ceramics, rubber, masonry, etc. Effective corrosion-inhibiting primers using zinc chromate pigment can

be made with epoxy-polyamide vehicles provided

the pigment is dispersed in the epoxy resin before adding the polyamide. The epoxy-polyamides also make very good vehicles for zinc-rich

paints (see sec. 2.3.1.5). Epoxy-polyamides

are used as chemical-resistant can coatings,

drum and pipe linings, weather- and water-resistant marine finishes, abrasion- and corrosionresistant coatings for industrial equipment, floor coatings, and in many other applications where

their combination of excellent adhesion, toughness, and chemical resistance are utihzed to advantage. Several Military specifications for

epoxy-polyamide coatings are cited in the bibli- ography of chapter 8 [129, 130, 131, 132]. In addition to their usefulness in the coat- ings field, epoxy-polyamide resins also find use

as general adhesives, as plastic solders and seal- ants, and in casting, potting, and laminating

applications.

3.12. Polyurethanes

The polyurethanes are among the most re- cent and promising resins to achieve prominence in the coatings field. First developed in (Germany during World War II, the knowledge was transferred to the United States in 1945,

and manufacture of the necessary diisocyanate

raw materials started in this country in 1950.

Since 1954, with the establishment of several

large companies for the manufacture and development of polyurethane resins, the use of

this versatile class of polymers has grown

steadily, with applications in the fields of flex- ible and rigid foams, elastomers, synthetic

fibers, adhesives, and coatings.

The fundamental urethane reaction, discovered by Wurtz in 1848, is that of an isocyanate

with an alcohol:

H

I R-N = C = 0 + R'-O-H ^R-N-C = 0

0-W

isocyanate alcohol urethane

The active hydrogen atom of the hydroxyl

group is transferred to the nitrogen atom of

the isocyanate group, while the alcohol residue

58
(alkoxy group) becomes attached to the car- bon atom of the isocyanate group. In the

general urethane reaction, the active hydrogen

atom can be furnished by any hydroxyl-bearing compound, including castor oil polyols, car- boxylic acids, water, and selected polyesters

and polyethers—or by analogous compounds in

which the active hydrogen is attached to a nitrogen or sulfur atom instead of an oxygen

atom. Through the reaction of polyfunctional

isocyanates (usually toluene diisocyanate)

with polyols, either linear or crosslinked poly- mers can be formed, depending on the functionality and ratio of the reactants. The properties of the polyurethane are dependent also on

the structure of the polyol employed; for ex- ample, a simple triol such as glycerol will yield a hard, brittle polymer, while a castor polyol

or a polyester, with greater spacing between

hydroxyl groups, will yield a flexible polymer.

The ultimate film properties are affected also

by the solvent system employed and by the

curing temperature and humidity. Since iso- cyanate groups are reactive with water and

hydroxyl-bearing materials, care must be exercised to use solvents which are water- and

alcohol-free. Pigments, fillers, and dyes also

must be chosen with care if good package stability is to be maintained.

The combination of different isocyanates with

the large number of available polyols, in vari- ous NCO/OH ratios, makes possible the formation of a wide variety of polymers with

properties "tailored" to specific applications.

Polyurethane coatings formulated for optimum

performance are characterized by an outstanding combination of desirable properties, including hardness with flexibility, high gloss, and

excellent resistance to abrasion and chemicals.

They may be applied by dipping, knife, or

spray,

Polyurethane coatings are available either

as two-component systems that are mixed shortly before use and cure by direct crosslinking, or as single-package materials that cure when

exposed as a film to moisture, oxygen, or heat.

The reaction between isocyanate groups and

active hydrogen atoms proceeds readily at room temperature without catalysts; however,

the reaction is greatly accelerated by the addition of small amounts of tertiary amines, so- dium alkoxides, or Friedel-Crafts catalysts, or by heating. The two-component polyurethane

systems at present provide the maximum versatility and best properties obtainable with this

class of resins, although continuing research on one-component and on solventless systems

may change this situation.

3.12.1. Two-Component Urethane Systems

Two-component polyurethane coating sys- tems are of two types: a so-called Prepolymer

type, and a so-called Polyisocyanate type. Both

are formulated for room temperature cure, but

curing may be hastened and hardness and

toughness of the coating increased by baking.

3.12.1.1. The PREPOLYMER TYPE of vehicle consists of isocyanate-terminated polymers to- gether with a substantial proportion (about 2- 15%) of free toluene diisocyanate (TDI), the

usual isocyanate raw material. Since TDI is very volatile and highly toxic, such prepolymer

vehicles retain some toxicity until cured. Curing is accomplished by mixing the prepolymer

with polyfunctional, reactive amines and polyols shortly before use. The pot life of the

mixed ingredients and the time required for

the coating to cure depend on the nature and

concentration of the curing agents and on the

temperature. Normally, a pot life of 8 hours

or more can be expected. The usual polyurethane prepolymer coatings dry in an hour or two and may be recoated in four to six hours.

Maximum hardness and resistance properties are developed in about a week.

3.12.1.2. The so-called polyisocyanate type

is a prepolymer that has been specially processed to produce a largely tri-functional polymer with little if any residual, unreacted TDI.

Since there is virtually no free TDI in such

vehicles, their toxicity is no longer a problem.

They are cured by mixing with suitable reactive polyols, including the castor polyols, and

various hydroxyl-bearing polyethers and polyesters [146]. To a lesser extent, polyamines

are used to effect the curing. Depending on

the nature of the polyol and the drying conditions, about 2 to 15 hours may be required for

drying. As with all polyurethanes, the properties of the final coating are determined largely by the nature of the modifying polyol ; that

is, by the number and spacing of hydroxyl

groups in the polyol. As has already been

pointed out, either linear, branched, or cross- linked polymers can be obtained, depending

on the functionality and ratio of the reactants. As a matter of convenience in mixing and use, the proportioning of the polyisocyanate prepolymer and polyol curing agent can readily

be worked out in advance by the manufacturer

to permit the mixing of equal volumes of polyisocyanate and polyol on the job for optimum

results.

3.12.2. Single-Package Urethane Systems

Single package polyurethane coating sys- tems are formulated to cure by application of

heat, by oxidation, or by reaction with atmospheric moisture.

3.12.2.1. Heat-cured one-package polyurethane vehicles can be prepared by temporarily

59
blocking the reactivity of the -NCO groups in a polyisocyanate prepolymer by loosely asso- ciating them with a reactant (e.g., phenol)

that is driven off at elevated temperatures.

While the isocyanate groups are blocked off at room temperature, suitable polyol or amine

curing agents may be added vi'hich will not be

effective until the composition is heated to reactivate the isocyanate groups. Such heatcured systems have been particularly useful as wire coatings that permit soldering without the

need for stripping the insulation.

3.12.2.2. Air-cured one-package polyurethane

vehicles may appropriately be termed urethanealkyds, since they are made—like alkyd resins —by cooking together a polyol and a drying

oil, with toluene diisocyanate as the other major

reactant instead of phthalic anhydride. Such

vehicles are sometimes called urethane oils. They dry by oxidation of the carbon doublebonds in the drying oil and hence require the use of ordinary amounts of conventional paint

driers in order to cure within a reasonable

length of time. The performance of such coat- ings is quite similar to that of conventional

alkyds.

3.12.2.3. Moisture-cured one-package polyurethane vehicles utilize the ready reactivity

of isocyanate groups with water to effect cure

of the coating. The functionality and molar

ratios of the reactive ingredients are important factors in the successful formulation of

such vehicles. The reaction between water and

isocyanate first produces an amine and carbon

dioxide gas. The amine reacts with more iso- cyanate and gradually builds up a polymeric

structure, while the carbon dioxide gas diffuses

out of the film. The cured film has properties

approaching those of the two-component sys- tems, but the rate of cure depends on the prevailing weather conditions. In a typical one- package system, the film is laid down from a water-free solution of an isocyanate prepolymer

in organic solvents. The film dries tack-free

in about an hour by solvent evaporation and

then cures by reaction with atmospheric moisture to a crosslinked coating. Recoating is us- ually possible in four to six hours, and optimum

film properties are developed in about a week,

as with the two-package systems. However,

under low humidity conditions (below 30%

R.H.) longer curing times may be required.

3.12.3. Properties and Uses of the

Polyurethanes

It has been stated that a wide variety of

polyurethane coatings for particular applications is possible, depending on the nature of

the reactants employed. For example, by varying the molecular weight of the polyol (and

hence the distance between cross-links), ex- tremely hard or soft flexible polymers can be

obtained. Tough polyurethane coatings combining hardness with flexibility can be prepared which are well-suited as floor varnishes

for severe service, such as on gymnasium floors,

and as porch and deck enamels on wood or

concrete. Their excellent mar and abrasion

resistance is also utilized in furniture finishes.

A polyurethane coating [147] for use on polyester-fiberglass gunstocks over a polyamideepoxy primer [148] is covered by Mil. Spec.

MIL-P-46057. Their water and corrosion re- sistance makes them useful for marine paints

and for metal primers and enamels. Their

weather resistance is utilized in clear varnishes

for boats and exterior wood siding (the polyurethanes do not adhere satisfactorily to oil based wood fillers and stains but adhere well

to alcohol- or water-based stains and fillers)

.

The excellent alkali resistance of the polyurethanes finds use in masonry coatings and swimming pool paints. Although polyurethane coat- ings tend to darken somewhat on exterior

exposure, they have excellent gloss and gloss

retention in applications where prolonged decorative as well as protective qualities are desired. The polyurethanes are still relatively

expensive materials but are becoming cheaper

as their volume of use steadily grows.

3.13. Polyethylene

The outstanding properties of polyethylene are its chemical inertness, solvent resistance,

flexibility, toughness, excellent dielectric and

related properties, and low cost. Although its widest use is in the form of film, sheet, and

molded articles—its coatings applications being limited by its complete insolubility in or- ganic solvents at room temperature—the excellent properties of the resin have been utilized

in the coatings field by employing hot melt or flame-spraying methods (sec. 5.4.3.4), a fluidized resin bed technique (see sec. 5.4.6 on Methods of Application), or water-dispersed

systems to effect coatings application. Though

generally inert, polyethylene is attacked by halogens and concentrated nitric acid and is soluble in hot aromatic hydrocarbons. It is resistant to other mineral acids including hydrofluoric, and is unaffected by strong caustic.

3.13.1. Low- and High-Density Polyethylene

Polyethylene is prepared by the addition polymerization of the ethylene monomer. The

properties of polyethylene, like all polymers,

vary with molecular weight and structure and

60
with molecular weight distribution. In the

case of polyethylene, an additional parameter,

density, is an important factor in determining

the properties of the material. The density of

polyethylene may range from 0.92 (g/cc) in

the older, conventional, low-density type to 0.96 in the newer, linear, high-density type.

Conventional polyethylene is made by a highpressure process, usually by mass polymerization, but sometimes by a "solvent" or slurry

process; the resulting polymer has a branched

structure which reduces the tendency toward

crystallization, the degree of crystallinity being about 60-65 percent. The new, linear type

of polyethylene is virtually free from branching, so that greater alignment and close-packing of polymer chains is possible, and this type

has a degree of crystallinity of 80-85 percent;

it is manufactured by a low-pressure process of solution polymerization employing the

new stereospecific catalysts (see sec. 3.1.4.5 on

Stereoregular Polymers) . The greater crystallinity of linear polyethylene gives it a higher

density, a higher melting point, greater impermeability to moisture and gases, greater

stiffness and tensile strength, and greater sol- vent and chemical resistance than is possessed

by the conventional, low-density type. Molecular weights as high as 2,000,000 can be

achieved in high-density polyethylene, whereas

a molecular weight of about 50,000 is the highest practically obtainable in the conventional

type.

A medium-density polyethylene with properties intermediate between those of the low- and

the high-density types is also being manufactured, by both the high- and the low-pressure

processes with recently-developed modifications. All three types of polyethylene (high-,

medium-, and low-density) are readily melted,

molded, extruded, or heat-sealed. The heatsealing temperature for the linear, high-density

type is about 50 °F (28 °C) higher than that

of the conventional, low-density type.

Since the cost of high-density polyethylene is greater than that of the low-density type, the

choice of type is dictated by the requirements

of the application. For example, where a high

degree of impermeability is desired, a mediumdensity material is greatly superior to the lowdensity type, but the degree of impermeability

is not much increased by further increasing the

density. On the other hand, where maximum

hardness and heat resistance are required, a

high-density polyethylene will give the best results. Uses. The most important application for

polyethylene in the coatings field is in packaging. Extruded in a thin film onto a moving

sheet of paper, aluminum foil, or other packaging material, it provides a clear, strong, flexible, abrasion-resistant, heat-sealable coating of

low permeability that is highly resistant (except at elevated temperatures) to strong acids

(except concentrated nitric) and alkalis, sol- vents, greases, and oils. Polyethylene-coated

aluminum foil is heat-sealable and completely

moisture-proof. Polyethylene-coated cellophane

and polyester film are finding increasing

acceptance. Cardboard cartons for milk and

other liquids are being coated with polyethylene ; plasticizers and other modifiers are unnecessary in polyethylene, and it is odorless,

tasteless, and nontoxic. Polyethylene is ex- truded onto fabrics to make them waterproof.

Nonwoven mats of cellulose fibers coated with

polyethylene are used to make disposable baby

diapers and hospital sheets. Irregularly shaped

metal parts can be coated with polyethylene by

a fluidized-bed technique (see sec. 5.4.6). A

heavy coating of polyethylene can be applied

to metal surfaces by flame-spraying (see sec. 5.4.3.4) ; thus, tough chemical- and abrasionresistant coatings can be applied to chemical

storage tanks, metal bench tops, drums, and

pipe. The excellent electrical properties of

polyethylene are utilized in coatings for wire

and cable.

For elevated temperature or outdoor applications, polyethylene must be stabilized against

oxidation. Effective stabilizers are available.

Carbon black is a particularly effective light

stabilizer for polyethylene subjected to exterior

exposure. Heat stabilization is accomplished

by incorporation of an antioxidant.

3.13.2. Irradiated Polyethylene

Polyethylene can be crosslinked by a few

seconds exposure to sufficiently intense highenergy radiation. By regulating the radiation

dose, the degree of crosslinking can be controlled so that the melting point of the resin

is raised as much as 50 °F (28 °C) above that

of normal polyethylene without greatly alter- ing its basic mechanical properties. Very high

dose levels will render the resin infusible at temperatures up to the decomposition point. It

is interesting to note that the increase in softening temperature induced in conventional

polyethylene by irradiation is about the same

as that achieved in going from low-density to high-density polyethylene. The high-density

material also can be improved by irradiation.

3.13.3. Chlorosulfonated Polyethylene

Chlorosulfonated polyethylene is an elastomeric material produced by the simultaneous

chlorination and sulfonation of already-polymerized relatively high molecular weight lowdensity polyethylene, by treatment with gaseous

chlorine and sulfur dioxide in the presence of a free-radical initiator (e.g., a peroxide) as catalyst. The chlorine atoms reduce the crystallinity (and hence the stiffness) of the polyethylene

61
without greatly changing its desirable properties. The sulfonyl chloride groups provide

reactive sites through which subsequent cross- linking (vulcanization) can take place. By

varying molecular weight, chain branching,

and degree of crystallinity and cross-linking

through control of chlorine and sulfur content, a variety of chlorosulfonated polyethylene derivatives can be prepared. The product offered

commercially is usually derived from a polyethylene having a molecular weight of about

20,000, and contains about 27 percent chlorine

and 1.5 percent sulfur. Curing is effected

through the sulfonyl chloride groups by employing curing systems based on a polybasic

metal oxide or metal salt of a weak acid (e.g.,

litharge or magnesia), an organic acid, and an

organic accelerator. The cured polymer has

excellent resistance to abrasion, heat, weathering, and cracking, and is completely resistant to ozone; it has good resistance to mineral oils and to corrosive chemicals such as oxidizing

acids, alkalis, chrome plating baths, and hydrogen peroxide. The litharge-cured coating sys- tems have good resistance to prolonged water

immersion, but the magnesia-cured systems are

inferior in this respect.

Solution coatings of chlorosulfonated polyethylene can be formulated in a variety of

colors, including white. They can be applied

to virtually all other elastomeric compositions

to provide protection against weathering and

ozone. The coatings may be used without cur- ing if a light stabilizer and detackifying resin are incorporated into the formulation. However, a heat cure is required for maximum re- sistance properties. Coatings that will cure at room temperature can be prepared by using

special accelerators. Unpigmented lacquers

based on chlorosulfonated polyethylene can be

applied to rubber sporting goods and footwear

to provide a durable, lustrous, protective finish. Cotton and synthetic fabrics coated with this

resin exhibit excellent weathering resistance.

Chlorosulfonated polyethylene coatings are proving useful in rugged service applications

such as upholstery fabric, awnings, tarpaulins,

heavy duty gloves, and as industrial mainte- nance coatings in corrosive [141] or abrasive

environments. Coatings of chlorosulfonated

polyethylene have excellent adhesion to chlorinated rubber-primed [140] sandblasted steel. They may also be applied to wood and masonry

surfaces. (See also sec. 3.15.2).

3.14. Fluorocarbon Resins

The fluorocarbon resins are substituted ethyl- ene plymers in which hydrogen atoms have

been replaced with fluorine atoms or both

fluorine and chlorine atoms. The resulting

polymers are characterized by their chemical

inertness, low coefficient of friction, heat re- sistance, and strength and flexibility over a wide temperature range. Of chief interest are

polytetrafluoroethylene and polychlorotrifluoroethylene.

3.14.1. Polytetrafluoroethylene

Polytetrafluoroethylene is a completely fluorinated, linear polymer having a dense, symmetrical, highly crystalline structure that is completely resistant to chemical attack by all known

substances except molten alkali metals, and

fluorine at high pressures. It is the most heatresistant organic polymer known, being capable

of continuous service at temperatures up to 500

°F (260 °C). Although it is thermoplastic and

softens at about 620 °F (327 °C), it does not

melt, and at about 750 °F (399 °C) it decomposes slowly to yield the monomer and lesser

amounts of various gaseous fluorine derivatives. The decomposition products are dangerously irritating and toxic and should not be in- haled. Small amounts of gaseous decomposition

products are given off also at the sintering temperature (650 °F [343 °C]) to which the resin

particles as manufactured must be heated to fuse them into a continuous mass. The resin is available in the form of granules for compression molding, as a powder for extrusion molding, and as an aqueous dispersion for coatings

applications; in each case, fusion of the resin

particles is achieved by sintering as indicated

above.

Polytetrafluoroethylene has a translucent,

white, or grayish color, and a waxy feel. Nothing will stick to the untreated surface ; however,

the surface can be dulled by treatment with a molten sodium-ammonia mixture and thus ren- dered capable of being bonded with various

commercial adhesives. Polytetrafluoroethylene

has a very low coefficient of friction—the lowest of all known solids—and the static coeffi- cient is lower than the dynamic coefficient ; thus,

bushings made of or coated with the resin exhibit smooth, non-binding motion and long wear

without the need for lubrication. Polytetrafluoroethylene retains its good strength and

flexibility over a very wide temperature range,

from -150 to 500°F (-101 to 260°C), and it has very high impact strength. It is nonflammable. Its water absorption is practically zero.

It has a specific gravity of about 2.2. Although

possessing excellent stability, it exhibits some

cold flow (non-elastic, permanent deformation

6^
under stress) at moderately elevated temperatures, the heat distortion temperaure for the

bulk material beging 250 ° F (121 ° C) at 66 psi.

The electrical properties of polytetrafluoroethylene are outstanding. It has a very low

dielectric constant and very low dissipation fac- tor which remain virtually unchanged in the

frequency range from 60 cycles to 10,000 megacycles. Dielectric strength and arc resistance are excellent.

To attain in a coating the maximum performance properties of which polytetrafluoroethylene

is capable, it is necessary that the coating be

free of pinholes. Since the resin does not melt

or flow during the fusion process, pinholes are

likely to be present unless multiple coats (at

least three) are applied.

Despite its high cost, the unique combination

of desirable properties possessed by polytetrafluoroethylene makes its use both practical and

economical for severe applications in which

maximum chemical resistance and heat stability are required. Aside from its uses in molded,

machined, and extruded forms, polytetrafluoroethylene in the form of dispersion coatings may

be applied to metals, glass, ceramics, wire, fabrics, and mats. Sintering at about 650 °F (343

°C) fuses the particles into a continuous, tough,

extremely resistant film that can be used in

such applications as a lining for corrosive-reaction vessels ; as a chemical-resistant coating for

drums and tanks ; as a protective coating on machinery and equipment subjected to heat, fumes,

and spillage; as an anti-adhesion coating on

processing equipment and moving parts subject

to clogging and misfunction as a result of undesired buildup of materials (e.g., buildup of

glue on packaging machines) ; as a low-friction,

wear-resistant coating for machine parts that

cannot be lubricated ; and as an insulating coat- ing for wire and electrical parts for severe or

critical electronic applications.

3.14.2. Polychlorotrifluoroethylene (Kel-F)

Polychlorotrifluoroethylene is a completely

halogenated, linear polymer containing, on the

average, one chlorine atom for every three

fluorine atoms. The fluorine imparts chemical

inertness, thermal stability, and almost zero water absorption to the polymer, while the

chlorine—in part by reducing molecular regularity and crystallinity—contributes a lower

softening temperature, meltability and moldability, reduced cold-flow, and greater trans- parency. The degree of crystallinity depends

on the temperature treatment received during

the manufacturing process ; quick quenching of

the heated polymer yields a relatively amorphous material with much smaller crystallites,

while slow cooling produces a more crystalline

polymer. Molecular weight also can be varied

to produce polymers ranging from low molecular weight oils, greases, and waxes to high

molecular weight (300,000 to 400,000) molding

and dispersion resins.

Although not as extremely inert and thermally stable as polytetrafluoroethylene, polychlorotrifluoroethylene finds many similar uses.

It is resistant to all corrosive chemicals (including fuming oxidizing acids) except molten

alkali metals, fluorine, and highly halogenated

compounds. It is insoluble in all organic liquids at room temperature, but is swelled slightly by

halogenated solvents, toluene, diethyl ether, and

ethyl acetate. At elevated temperatures, it is swelled by still other liquids and can be dissolved by a few, such as highly halogenated

compounds. The softening temperature of the

molding and dispersion resins may vary from

about 350 to about 420 °F (177 to 216 °C), depending on their molecular weight and degree

of crystallinity. Polychlorotrifluoroethylene is capable of continuous service at temperatures

up to 400 °F (204 °C), while retaining an appreciable degree of flexibility at temperatures

around —200 °F (— 129 °C) or lower. It is resistant to weathering and aging, and is nonflammable, strong, tough, and abrasion-resistant. It has greater resistance to cold-flow than

the tetrafluoro polymer. Its electrical properties are excellent; it combines high dielectric

strength and arc resistance with a low dissipation factor (the latter is not as low, however, as

that of polytetrafluoroethylene, polyethylene,

and polystyrene).

For coating purposes, dispersions of finely

divided polychlorotrifluoroethylene in volatile

organic solvents are generally employed. These

resins may be applied by spraying, spreading,

or dip-coating to metals, ceramics, glass, wire,

and certain fabrics and other materials. Special primers are available to increase adhesion

where necessary. After evaporation of the or- ganic solvent the coating is fused by heating to the melting point to form a tough, permanent,

continuous film that has all the properties of

the basic resin. Since the resin actually melts

and flows during the baking process, a pinholefree coating can be obtained. Despite the high

cost of polychlorotrifluoroethylene resins, they

are widely used as corrosion- and chemicalresistant coatings for metals in severe industrial

applications where their exceptional resistance

properties provide long-term savings in maintenance and replacement costs. Typical industrial applications include coatings for troughs,

tanks, vats, hoppers, mixers, paddles, pumps,

and conveyors. Fabrics coated with the resin are rendered waterproof and resistant to heat,

chemicals, fungi, weathering, and abrasion,

while retaining flexibility at very low temperatures.

63
3.15. Synthetic Rubbers

Synthetic rubbers, in addition to their useful- ness as substitutes for natural rubber, also find

important uses as adhesives, and in coatings

where resilience, abrasion resistance, distensibility, and elasticity are of prime importance.

Among the synthetic elastomers available are

a number of materials which combine excellent

heat, chemical, and weathering resistance along

with their desirable elastomeric properties.

Those of particular interest to the coatings field are discussed in the following sections.

3.15.1 Polychloroprene (Neoprene)

Neoprene is a tough, resilient material possessing outstanding resistance to oils, alkalis,

heat, weathering, and abrasion. It will withstand continuous exposure to sulfuric and phosphoric acids and many other inorganic and or- ganic acids, but is decomposed by very strong

oxidizing compounds such as nitric and chromic

acids, sulfur trioxide, and hydrogen peroxide.

Neoprene is swelled and softened by aromatic

and halogenated hydrocarbons, aromatic alcohols, esters, ketones (except acetone), turpentine, and carbon disulfide. It is very resistant

to oils, aliphatic hydrocarbons, aliphatic alcohols, acetone, water, and most salts. Ethers

and aldehydes, in general, have only a mild

swelling effect on the polymer. Neoprene re- tains most of its desirable properties at temperatures as high as 150 °F (66 °C) and is serviceable at temperatures up to 250 °F (121

°C) for some applications. It remains flexible

down to —20 °F (—29 °C), becoming brittle at lower temperatures. Long exposure to sunlight

and weathering has little effect on neoprene ; it tends to become hard and dry after long ex- posure, rather than soft and sticky as does

natural rubber. Neoprene is virtually nonflammable.

Neoprene is prepared by the emulsion polymerization (see sec. 3.1.3.3) of chloroprene in an alakaline medium. The polymerization is halted when about 30 percent complete, while

the polymer is still thermoplastic. The polymer

is recovered by acidifying the emulsion to a

point just short of coagulation and then freezecoagulating the polymer onto a brine-cooled

drum, from which it is stripped, washed, and

dried. Several types of neoprene, differing in

uncured properties and manner of cure, are manufactured. Also, several types are marketed in latex form. The partially polymerized

polymer can be milled and compounded like raw

rubber, and then vulcanized (crosslinked) by

heat and/or catalysts to complete the polymerization. Although heat alone suffices to cure neoprene quite rapidly (in an hour or two) at temperatures around 270 °F (132 °C), accelerators (basic metal oxides, such as magnesium oxide or zinc oxide) are generally employed to shorten the baking time or to lower

the baking temperature. Satisfactory roomtemperature curing neoprene coating formulations also have been developed and are commercially available.

Neoprene coatings are usually supplied as so- lutions of the uncured polymer in aromatic

solvents. Catalysts or accelerators are added

immediately prior to use. Heat-curing types

are usually syrupy liquids that are best applied

by brush or dip-coating, or they may be soft

pastes that can be applied by troweling. Airdrying types may usually be applied by brush,

dip, spray, roller, or flow. Depending on the

nature of the formulation, thicknesses of 3 to 6

mils per coat are readily obtained by the aforementioned methods. Troweling yields coating

thicknesses several times greater per coat. Where maximum protection, durability, and resilience are required (especially on sharply con- toured surfaces) , coatings up to 14 inch in thick- ness can be obtained with multiple trowel coats. An adequate period (several hours) of air drying should precede baking to prevent blister

formation from too rapid an escape of solvent. In general, neoprene coatings are utilized at thicknesses of 10 to 15 mils. A primer is re- quired for maximum adhesion. Chlorinated

rubber-base primers [140] have been very ef- fective for this purpose. Neoprene coatings are generally available only in gray or dark

colors, chiefly black and green.

Uses. The excellent combination of resist- ance and durability properties possessed by

neoprene has been proved in many severe industrial applications since 'its introduction in

1932. Neoprene coatings are widely used to provide lasting protection against corrosion

and abrasion of industrial equipment such as ductwork, pipelines, storage tanks, scrubber

towers, tumbling drums, coal chutes, pump and

blower blades, ship propeller shafts, tool handles, and machinery. Neoprene coatings may

be applied to wood and concrete flooring, ramps,

and steps to provide protection against wear

and chemical splash or spillage ; anti-skid properties can be imparted by incorporating grit or sand in the applied coating. Neoprene finds use

as a chemical- and abrasion-resistant coating

for textiles, rubber, and leather for such items

as industrial gloves, heater and brake hose, belts,

and conveyors. As a coating for ignition wire,

neoprene affords excellent resistance to oil and

heat. Neoprene coatings on military aircraft

have exhibited excellent resistance to abrasion

and erosion, although the v/eight of the thick

coatings employed (about 10 mils) constitutes

64
somewhat of a drawback in aerodynamic applications. Neoprene coatings are widely specified

maintenance finishes in oil refineries, chemical

plants, garages, filling stations, and aircraft

installations.

3.15.2. Ghlorosulfonated Polyethylene

Chlorosulfonated polyethylene, was discussed

previously in section 3.12.3 under Polyethylene.

It may be useful here, however, to point out the

considerable similarity between the properties

and applications of chlorosulfonated polyethyl- ene and those of neoprene, while indicating a

basis for choosing between them. Both are highly suitable for applications in which resist- ance to heat, chemicals, weathering, and abrasion are required. Neoprene is less expensive

than chlorosulfonated polyethylene and is superior in oil resistance. Chlorosulfonated polyethylene, however, possesses superior resistance

to strong oxidizing agents such as nitric [141]

and chromic acids, hypochlorite and chlorine

dioxide solutions, hydrogen peroxide, and ozone —agents which soon deteriorate neoprene. It

is available in a wide range of durable colors,

both light and dark, whereas neoprene is limited to gray or dark colors. Also, chlorosulfonated polyethylene coatings are capable of

service at higher temperatures than neoprene,

the upper limit for continuous service being

about 220 °F (104 °C) as compared with about

150 °F (66 °C) for neoprene. Neoprene, therefore, is the general-purpose material, while

chlorosulfonated polyethylene is used where its specific superior properties justify the higher

cost.

3.15.3, Styrene-Butadiene Rubber (SBR)

Styrene-butadiene copolymers of high butadiene content are soft, elastomeric materials, as distinguished from the harder, high styrene

types usually used in the coatings field, predominantly in latex paints. The high styrene/

low butadiene resins were discussed in section

3.8.2 under Styrene-Butadiene Copolymers.

The styrene-butadiene copolymer elastomers

usually contain about 75 percent butadiene;

however, copolymers ranging all the way from

50 percent butadiene to 100 percent polybutadiene are manufactured, and are available both

as solid polymers and as latexes. They are

vulcanized with sulfur or sulfur-bearing materials. Although the chief use for styrenef butadiene copolymers is for SBR synthetic rubber (formerly called GR-S), they also find some

use in coatings where their elastomeric propj erties are advantageous, as in coatings for

I rug backing, paper, fabrics, and leather. Such

1 coatings are tough, flexible, abrasion-resistant,

' and acid- and alkali-resistant, but are attacked

by aromatic and chlorinated hydrocarbons and

are swelled by oils and greases. Styrene-butadiene rubber is also used in general-purpose adhesives.

3.15.4. Butadiene-Acrylonitrile Rubber

(Nitrile Rubber)

The butadiene-acrylonitrile copolymers (nitrile rubbers) are similar to the butadiene-sty- rene rubbers in general appearance and physical properties, but nitrile rubbers have better

resistance to most solvents (although they are

soluble in ketones), greater resistance to the

deteriorative effects of heat and sunlight, and

excellent resistance to oils, greases, and waxes. The oil resistance increases with increasing

nitrile content. Vulcanization is accomplished

with sulfur or sulfur-bearing materials, as with butadiene-styrene rubbers. Nitrile rubbers have found use in the coatings field as

nonvolatile, non-extractable plasticizers for

phenolic resins and for polyvinyl chloride resins. In latex form, nitrile rubbers have been

used as coatings and impregnants for paper,

fabrics, and leather products.

3.15.5. Isobutylene-Isoprene Copolymer

(Butyl Rubber)

Butyl rubber consists of about 98 percent

isobutylene copolymerized with about 2 percent

isoprene. The unsaturation imparted by the

isoprene renders the copolymer vulcanizable, a property not possessed by 100 percent polyisobutylene. This degree of unsaturation is very

low, however, compared to that found in natural

rubber, and accounts for the chemical inertness

of butyl rubber. Vulcanization, though more

difficult than with diene polymers, can be ac- complished with combinations of sulfur and

thiuram accelerators or with special methylolcontaining resins.

Butyl rubber is highly impermeable to gases

(a property that has led to its wide use for

automobile inner tubes). It has a "dead" feel,

its resilience being strikingly lower than that of

natural rubber.

Butyl rubber is highly resistant to concentrated acids and alkalis, corrosive salts, and

fresh and salt water, and it has found use as

linings for chemical tanks, pumps, and hoses.

Its good electrical insulating properties coupled with heat resistance to about 212 °F (100

°C) and excellent resistance to aging and ozone make it useful as a wire coating. Because of

its paraffinic hydrocarbon nature, it is resistant

to esters, ketones, alcohols, animal fats, and

vegetable oils, but it is attacked by petroleum

hydrocarbons, aromatic hydrocarbons, chlorinated solvents, and carbon disulfide. It remains

flexible at temperatures as low as —400 °F

65
(—240 °C), even after prolonged aging, and is proving useful in cryogenic applications. Its

flexibility has made it useful as a coating for

fabrics. Recently, efforts have been made to

utilize the resistance of butyl rubber to aging,

weathering, sunlight, and ozone in solution-type

industrial maintenance coatings being offered

for metal and masonry surfaces.

3.15.6. Polysufide Rubber

Organic polysulfide polymers result from the

condensation of organic halides with alkali

polysulfides. They range from viscous liquids

to rubber-like solids. The polysulfide rubbers

are processed and handled like natural rubber.

Curing is accomplished with metallic oxides

(usually zinc oxide). Polysulfide rubbers have

excellent resistance to organic solvents, including most aliphatic and aromatic hydrocarbons,

alcohols, ketones, and esters. Coatings based on polysulfide rubber have low permeability to gases, good electrical properties, excellent re- sistance to aging, sunlight, and ozone, and afford good protection against fresh and salt water. Their resistance to acids and alkalis is only fair. Although the tensile strength and

abrasion resistance of polysulfide rubbers is generally lower than that of some other synthetic rubbers, specially formulated coatings

based on polysulfide rubber exhibit excellent

abrasion resistance. Blends of polysulfide rubber with vinyl resins have produced durable and

useful coatings. Polysulfide rubber coatings

have found use as solvent-resistant linings for

chemical tanks and equipment, and as solventresistant coatings for paper, fabrics, leather,

wood, and concrete.

3.15.7. Silicone Rubber

Silicone elastomers are high molecular

weight, linear organosilicon polymers com- pounded with an inorganic filler and a vulcanizing agent (usually a peroxide). Recently,

vinyl curing systems, involving the introduction

of reactive pendant groups into the polymer and

use of a specifically-reactive peroxide, have been

developed. The general chemistry and properties of silicone resins have been discussed in

section 3.9.1. Silicone elastomers are outstanding in their retention of flexibility over a wide

range of temperatures, from —100 to 500 °F

(—73 to 260 °C). Whereas most organic elas- tomers soon soften and deteriorate at temperatures above 200 °F (93 °C), the silicone elas- tomers are virtually unaffected by continuous

service at a temperature of 300 °F (149 °C

and will withstand a temperature of 600 °F

(316 °C) for a brief period. Although very re- sistant to ozone and to hot oils, the silicone elas- tomers are poor in resistance to hydrocarbon

solvents, and they are attacked by acids, strong

alkalis, and steam. Their electrical properties

and their resistance to weathering are excellent.

Silicone rubber coatings are available as solutions, pastes, and water dispersions, for application by spraying, brushing, dip-coating, or spreading. They are applied to fabrics (including glass fabric), ceramics, asbestos paper,

metals, and wire to provide resilient, flexible,

durable coatings that retain their desirable

properties over a very wide temperature range.

3.15.8. Polyurethane Rubber

The general chemistry and properties of the

polyurethanes have been discussed in section

3.12. There it was noted that a wide variety of

products can be made from the wide choice of

polyols available. For the production of polyurethane elastomers, hydroxyl-bearing polyesters or polyethers are employed for reaction

with the diisocyanate (usually toluene diisocyanate) . Because of the wide spacing between

reactive groups in such polyols, a flexible struc- ture is produced. Polyurethane rubber possesses high tensile and tear strength and excellent abrasion resistance superior to that of

the best cold rubber. Resistance to oxygen,

ozone, and ultraviolet is excellent. Resilience

and wear properties are outstanding. Optimum

elastomeric properties, however, are not re- tained over a wide temperature range; polyurethane rubber hardens at moderately low

temperatures (below —20 °F [—29 °C]) and

weakens when subjected to high temperatures,

steam, or hot water. Good resistance to sol- vents, alkalis, and acids can be built into the

polymer. In addition to its uses as a premium

specialty rubber and as a resilient, durable,

foam or sponge rubber, polyurethane rubber is used as a coating to provide wear-resistant ve- neers on tires, soles, heels, belts, rubber and

plastic goods, and flooring. Fabrics coated with

polyurethane rubber are flexible, wear-resistant

and weather-resistant.

3.15.9. Chlorinated Rubber

Chlorinated rubber (or Parlon®, as it is widely known in the trade) is prepared by the

addition and substitution of chlorine atoms into

the polyisoprene molecule to yield a material

containing 63 to 67 percent chlorine. The chlorinated product is a thermoplastic resin rather

than an elastomer and is used principally in

coatings, inks, and adhesives. Chlorinated rubber is outstanding in its resistance to water and

common corrosive chemicals. It possesses a high degree of impermeability to water vapor

and a very low water absorption. It is highly

resistant to strong alkalis and acids, strong

bleaches, corrosive vapors and fumes [140],

soaps and detergents, mineral oils, mold and

mildew. It is deteriorated by heat and ultra66
violet light, but can be stabilized to improve its resistance to these influences. It is odorless,

tasteless, nontoxic, and non-flammable.

Chlorinated rubber is readily soluble in chlorinated aromatic hydrocarbons and has found

important use in corrosion-resistant paints and

primers, industrial maintenance coatings, traffic paints, swimming pool paints [35], and concrete and masonry paints [34] . Other uses are

in flame-retardant coatings, textile coatings,

printing inks, and adhesives. Properly formulated chlorinated rubber coatings have proven

very effective as adherent, corrosion-resistant

primers for metals which are to receive neo- prene topcoats. Paints, enamels, and lacquers

based on chlorinated rubber are very quickdrying compared with most other film-forming

materials.

3.15.10. Polyacrylic Ester Rubber

Polyacrylic ester rubbers are specialty poly- mers that can be cured with special vulcanizing

agents, such as polyfunctional amines, to give

products highly resistant to weathering, discoloration, ozone, and gas diffusion. Although

resistant to many oils, even when hot, they are

not resistant to aromatic solvents nor to glycols or steams. They are not well-suited for low

temperature applications. In both solution and

latex form, the polyacrylic ester rubbers are

finding use in the coatings field.

3.15.11. Gyclized Rubber

Cyclized rubber is made by treating natural

rubber with strongly acidic reagents (e.g., sul- furic or sulfonic acids) which reduce the original unsaturation through the formation of

cyclic structures. The resulting product is hard, resinous, and thermoplastic instead of

rubbery. While remaining soluble, like natural rubber, in hydrocarbon solvents, it is ren- dered much more resistant to water, acids,

and alkalis. Cyclized rubber is supplied in the

form of granules or powders that can be applied

as hot melts or from solution in organic solvents

to give adherent, flexible coatings having low

water penetration and good chemical resist- ance. It finds use in coatings for paper and

metals, in acid- and alkali-resistant paints, in

printing inks, and as a strong metal-to-rubber

adhesive.

3.16. Water-Soluble Resins

3.16.1. Thermoplastic Types

Several classes of water-soluble resins of the

thermoplastic type have been discussed in appropriate sections earlier in this chapter. For

example, methyl cellulose and other soluble

cellulose derivatives were described in section

3.2.3. and polyvinyl alcohol was described under Vinyl Resins in section 3.6.4. Methyl cellu- lose and other soluble cellulose derivatives are

typically employed as thickening, stabilizing,

and emulsifying agents in water-base paints

more often than as coatings in themselves.

Polyvinyl alcohol, while used for the above purposes, also finds use as a film in its own right

because of its excellent resistance to organic

solvents and its high degree of impermeability

to vapors and gases. Moreover, although polyvinyl alcohol itself is a linear thermoplastic

polymer, it can be crosslinked with suitable

chemical agents to insolubilize it, as noted in

section 3.6.4.

3.16.2. Thermosetting Types

The success of water-thinned latex paints in

the consumer field has led to intensive effort on

the part of resin producers and paint manufacturers to develop practical and durable waterbase industrial coatings that will combine the

safety and convenience of latex paints with the

high film integrity of solution-type coatings.

Such coatings are based on a water-soluble

resin that can be converted by heat, oxidation,

and/or chemical agents into a crosslinked, in- soluble, thermoset material. A general discussion of water-solution type coatings, indicating

background, properties, formulation, and use, was presented in section 2.3.6. A more specific

discussion of the resins themselves is given

here.

The thermosetting water-soluble resins generally contain hydroxyl and/or carboxyl groups,

the latter sometimes reacted with a volatile or- ganic base such as morpholine, to render them

water-soluble. These same polar groups impart a chemical reactivity to the polymer molecule which permits it to be crosslinked by reaction with a suitable resin or drying oil, under

the influence of heat, oxygen, and/or catalysts.

Amino resins have been particularly effective

for this purpose.

A water-soluble melamine-acrylic resin for

industrial finishes has been described [123]

which is available as a clear solution in water,

requiring only the addition of pigment to produce a ready-to-spray enamel. When baked,

the acrylic and melamine components become

crosslinked to form a hard, durable, high gloss

finish said to be comparable to the best melamine-alkyds.

Water-soluble baking resins composed of

67
trimellitic anhydride, a glycol, and a dicarboxylic acid have been described [64] ; these are

self-curing vehicles vv^hich, on baking at 350 to 500 °F (177 to 260 °C), are reported to yield

coatings having high gloss, good flexibility and

impact resistance, high hardness, good mar re- sistance, excellent salt-spray resistance, and

good resistance to hot detergents. Vehicles in

which the trimellitic resin is combined with a

water-soluble amino resin can be baked at temperatures as low as 250 °F (121 °C) to yield

coatings having high gloss and good mar resist- ance ; such resin blends are somewhat less flexi- ble and less resistant to detergents than the

self-cured vehicles.

Technological advances from development

work now in progress can be expected to yield an increasing number of water-soluble resins

for industrial applications in the not-distant

future.

3.17. Miscellaneous Resins

3.17.1. Resin Combinations

The scope of this work has permitted consideration of only the main classes of resins and

resin combinations. Obviously, many other

combinations are possible through chemical re- action or physical blending, and have yielded a wide variety of materials for specific coatings

applications. The essential properties of this

at-first-glance bewildering array of materials can often be assessed, for practical purposes,

from a knowledge of the properties of the individual constituents, many of which have been

discussed in this chaper.

3.17.2. Resins Discussed Elsewhere

In some cases, information concerning the

properties and uses of particular materials has

been given elsewhere in the text, where this appeared logical and appropriate; for example,

terpene resiyis and coumarone-indene resins, being primarily varnish resins, were discussed in

section 2.2.2 on Varnish and have been omitted

from the present chapter. In other cases, in- formation relating to a particular class of

resins has been distributed among several

places in the text to make it available where it was most appropriate or needed ; thus, those

aspects of polyvinyl acetate and styrene-butadiene resins which relate directly to latex

paints are discussed in section 2.3.5, while the

general properties of these resins are discussed

in sections 3.6.1 and 3.7.2 of the present chapter. Similarly, water-soluble resins have been

discussed at several places in the text (see sec- tions 2.3.6 and 3.16). In such instances, the

treatment has been selective rather than repetitious, with appropriate cross-references within

the text. The comprehensive Index will provide

a ready guide to all the information relating

to a particular resin or subject within the scope

of this publication.

3.17.3. Natural Resins

These have received only brief treatment

herein (sec. 2.2.2). Aside from their diminishing (though still important) use in the coatings

field because of their replacement by the more

uniform and versatile synthetic resins, natural

resins have been adequately discussed else- where [65, 66].

3.17.4. Furan Resins

These are black, thermosetting resins made

by the polymerization of furfuryl alcohol in

the presence of acidic catalysts. Strong acids,

both mineral and organic, cause the polymerization to occur with explosive violence. Furan

resins are usually supplied as a partially polymerized, neutralized liquid that remains stable on storage, until cured to an insoluble and in- fusible resin by addition of dilute acid at the

time of use. Because of their dark color, the uses of furan resins are primarily functional

rather than decorative. The fully cured resins are resistant to acids (except strong oxidizing

types), alkalis, corrosive fumes, and solvents,

and find use as industrial maintenance coatings,

tank and digester linings, anl laboratory bench

tops. For the latter and similar applications,

the resin may be polymerized in situ by applying alternate coats of furfuryl alcohol and

dilute mineral acid, thereby building up a jetblack, chemically resistant finish.

3.17.5. Polypropylene

The polypropylene of current importance is a recent member of the new family of stereo- regular polymers (see sec. 3.1.4.5) having an

isotactic (highly ordered), highly crystalline structure. (The atactic form is not made

commercially.) Its exceptional properties re- semble, but surpass, those of high-density

polyethylene (see sec. 3.13.1). Although thermoplastic, it possesses unusual heat resistance.

Its melting range is from 325 to 340 °F (163-

171 °C), and it is capable of continuous service at 300 °F (149 °C). It is stronger and harder

than high-density polyethylene, and has greater

resistance to organic solvents. Resistance to

acids, alkalis, and salt solutions is excellent, even at elevated temperatures. Water absorp68
tion is practically nil, and electrical properties are excellent. While likely to find its widest

use in molded articles and film, polypropylene can also be applied as a coating in much the same manner as polyethylene. It can be applied

to metals by dipping the heated object into the

powdered resin (see fluidized resin bed, sec. 5.4.6) and then completing the fusion and flowout process by heating in an oven at about

482 °F (250 °C) . The coating obtained is heatsealable, clear, tough, chemically resistant, and

highly impermeable to many vapors and gases.

Wire coatings, applied by extrusion, exhibit

excellent electrical and mechanical properties,

with high resistance to heat, cracking, and

abrasion. Polypropylene can also be extruded

onto paper and fabrics to provide a flexible,

durable coating, in a variety of colors. Stabilization against sunlight is accomplished, as with

polyethylene, by incorporation of carbon black

or other suitable opacifying agents. Although

still too new to have achieved wide use, polypropylene is among the most promising of the

new plastic materials, and is rapidly becoming more available and lower in cost as commercial

production is stepped up.

3.17.6. Polycarbonate Resins (Lexan)

This is another of the new and unusual

thermoplastic resins that is now commercially

available. It is tough, heat-resistant, dimensionally stable with a low water absorption,

and has good electrical properties. It is unstable in the presence of alkali. Its softening

range is from 420 to 440 °F (216 to 227 °C) , and

it exhibits excellent thermal stability ; it is virtually unaffecetd by an hour of heating at 570°F (299 °C), as might be required during

processing. It is self-extinguishing when

burned (ASTM D 635). Its transparency imparts excellent colorability. Its chief applications, at present, are for molded articles such

as housings for business machines, electrical

apparatus, and portable tools ; parts for appli- ances and heating apparatus, heat-resistant

lenses and instrument windows, and molded

replacements for die-cast metal parts. The

resin can also be extruded into heat-sealable

sheet or film and as a wire coating. Although

polycarbonate resin is still too new to have

found much application in the coating field, its combination of desirable properties coupled

with its thermoplastic nature may lead to coat- ings applications in the future.

Figure 3.1. Roberts jet abrades- for measuring abrasion resistance as described in Fed. Test Method Std. No. lAla, Test Method 6193.

(Photo courtesy of Kameras Instruments.)

69
Figure 3.2. Microknife parallel-groove adhesion tester as used in ASTM Method D

2197-67T.

(Photo courtesy of Gardner Laboratory.)

Figure 3.3. Infrared spec- trophotometer is used to identify hinders, solvents,

and pigments in organic coatings.

70
4. Selection of Coating Systems

Selection of the optimum coating system for a given purpose involves the careful consideration of a number of concurrent and sometimes

conflicting requirements. The final choice often

involves compromises in which moderate shortcomings in secondary properties must be ac- cepted in order to obtain maximum performance

vi^ith respect to properties of primary importance to the intended application. Sometimes

there may be several equally good choices from

a performance standpoint; in such situations

other factors, such as cost or availability, w^ill govern the selection. Seldom does one find the

ideal coating system ; that is, one possessing

every desired performance property along with

low cost, ease of application, and freedom from

fire and toxicity hazards.

Despite the basic importance of the cost fac- tor and the universal desirability of safe and

easy application properties, all of these are secondary to the actual performance properties

of the coating system. Even the cheapest coating material is wastefully expensive if it fails to perform its service function, while an ini- tially expensive material that does the neces- sary job may prove quite economical in the long

run. The old adage about avoiding "pennywise but pound-foolish" practices definitely

applies in the coatings field. Moreover, if the

only coating system that will meet the requirements of an essential service application is one

that is difficult or hazardous to apply, then the

difficulties will have to be met and the risks

taken (with all possible precautions) in order

to achieve the performance properties without

which all else is for naught.

The properties of all important classes of

synthetic resins for coatings were discussed in

detail in Chapter 3. It now remains for us to consider how this information can best be utilized for the selection of suitable materials for

particular applications involving specific types

of substrate and service environment.

4.1. Basis for Selection

When one is confronted with the complex

problem of selecting the best coating system

for a particular application, the basic task is to decide which of the many available resins

will best meet the performance requirements.

It is at this perplexing and critical point in the

planning of an adequate coating system that

the responsible engineer, architect, maintenance

superintendent, or procurement officer most

needs complete, concise, and reliable information to guide his decision, regardless of whether

he himself must specify the particular material

to be used or whether he must merely choose

wisely from among various candidate materials

offered by different manufacturers or suppliers.

Although any resin chosen as the film-forming base can be formulated in a great many

ways with all sorts of modifications to enhance

or diminish certain properties, the fundamental

character of the resin continues to be the basic

factor in determining the properties achieved.

Once the decision as to the basic resin to be

employed has been made, the manufacturer can

generally be relied upon to supply the best, or

at least an adequate, formulation of that type

for the particular application. Implicit here

is the necessity for the procurement official to inform the coatings manufacturer to the fullest

extent possible of the requirements of the application; for only the manufacturer (or his wellinformed suppliers) knows just what goes into

his own formulation and what its detailed performance capabilities are. It is usually impractical and unnecessary for the procurement

official to know or to specify exactly how a given resin is to be formulated, unless a government or other specification is available

which spells out such detailed requirements.

Very few specifications are so restrictive ; many

merely stipulate performance requirements,

making the burden of proper resin selection

and formulation the responsibility of the manufacturer ; others specify the basic type of resin

(and perhaps certain key modifying ingredients) to be employed, and leave to the manufacturer the task of formulating these basic

ingredients into a finished coating material that

will meet performance requirements. A con- venient guide to organic coating materials cov- ered by Federal Specifications is provided by

the guide list and the tabular summary given

in chapter 8 ; however, most of the newer synthetic resin coatings are not yet covered by

Federal Specifications.

Application of the information about synthetic resins given in chapter 3 to the problem

of coating selection will be greatly facilitated

by a logical approach involving:

(1) Consideration of such factors as service

environment, nature of substrate, basic function to be provided, appearance desired, application limitations, and cost—leading to

(2) Designation of various factors or properties as being of either primary or secondary

importance in the intended application.

(3) Scanning of the information on specific

resins given in chapter 3, to select those resins

which possess all (or as many as possible) of

71
the properties of primary importance. Such

scanning may be minimized by limiting it to those classes of resins which are indicated in

subsequent sections of this chapter to be suitable for use on the specific substrate and in the

particular environment of the intended application; it should be recognized, however, that

many less commonly used but potentially applicable materials may be found in the greater

detail of chapter 3. (4) Final selection of the most suitable resin

(or resin blend, if necessary) on the basis of

the number of desirable secondary properties

it possesses in addition to the primary properties that led to its preliminary selection.

The above points will be developed in subsequent discussion.

Where the problem is to determine whether

or not a specific type of coating material being

ofi'ered for a particular application is basically

suitable for that application, the above approach

provides a ready answer, since it is necessary

to refer only to that section of chapter 3 in

which the resin in question is discussed.

Where the task is to specify or select the

most suitable coating for a particular application, the problem is obviously more diffuse and

difficult, since, theoretically, one has the whole

field of synthetic resins to choose from. Even

here, however, the approach suggested above

can provide a solution. Often, to one who has some familiarity with organic coatings, several

candidate materials will immediately suggest

themselves (see section 4.6 on Functional Coatings). It is then necessary only to check the

appropriate sections of chapter 8 against the

check-list of desired properties in order to select

the most suitable of these materials. If the

"most suitable" material thus chosen does not

possess all of the required properties, or if it

is desired to make certain that a better material

has not been overlooked, it will be necessary

to study the information in chapter 3 more

fully. In such cases, and in situations where

a group of potentially suitable materials is not

known, the broad field of organic coatings can

be narrowed by considering the nature of the

substrate, as discussed in subsequent sections

4.2, 4.3, 4.4, and 4.5 of this chapter. These

sections indicate classes of materials that are

suitable for use, respectively, on wood, metal,

masonry, plaster, and wallboard surfaces.

At most, the task of selection will require

a careful reading of all of chapter 3 as well as the present chapter. The reader will thus gain

a knowledge of the properties of synthetic

resins sufficient to enable him to immediately

eliminate some materials as unsuitable, while

pointing to others as potentially suitable materials from among which a final selection can

be made on the basis of a detailed study of their

properties in relation to the needs of the application.

With a general approach to the problem of

coatings selection now in mind, let us consider

in greater detail those factors which constitute a basis for the checklist of primary and secondary properties to be evaluated in conjunction

with the information on synthetic resins given

in chapter 3.

4.1.1. Service Environment

A detailed consideration of the type of service to be encountered will yield many of the

items needed for our checklist of properties.

Is the service environment to be an interior or an exterior one? What are the durability or longevity requirements—is maximum longevity

desired, or is a temporary coating or a strippable coating needed ? What specific endurance

properties are needed—resistance to heat, cold,

sunlight, general weathering? Is resistance to chemicals required—if so, to which types—sol- vents, acids, alkalies, detergents, continual

water immersion? Will the coating be subjected to alternate soaking and drying, in which

case a material of low water absorption and

good dimensional stability will be necessary.

What mechanical properties are needed—should

the coating be hard, flexible, tough, rubbery?

Must it be resistant to abrasion, impact, flex- ing? What are the adhesion requirements?

The answers to questions such as the foregoing

will help to bring the problem into focus.

4.1.2. Nature of Substrate

While seeking a coating having properties

that will enable it to withstand the rigors of

the service environment to which it will be

exposed, one must also consider the nature of

the substrate to be coated. For example, a

coating capable of giving excellent service in a particular environment when applied on a metal surface may fail badly in that same en- vironment if applied on wood, partly because

of the poorer dimensional stability and greater

coating elasticity requirements of wood surfaces. On the other hand, a satisfactory coat- ing for wood may be lacking in adhesion to a nonporous surface such as metal, or it may fail to provide adequate corrosion resistance. Coatings for masonry or plaster must be resistant

to the alkalinity in such surfaces, and must

give good holdout (uniformity of coverage and

appearance) despite the porous and non- uniform nature of such surfaces.

It is evident, therefore, that both the choice

of the film-forming resin and its formulation

into a suitable coating must take into account

the chemical and physical nature and the sur- face topography of the substrate. Smoothness,

porosity, dimensional stability, corrodibility,

dampness, alkalinity, hydrophilic or hydro72
phobic character—all of these attributes of the

surface to be coated affect the choice of a proper coating system. In many instances, the

diverse requirements of substrate and environmental factors necessitate a dual or multiple

coating system in which a primer of specific

composition is used to satisfy substrate adhesion and corrosion-resistance requirements

while a coating of a different composition is used as the topcoat to withstand environmental

conditions. Sometimes an intermediate or tie- coat is used between primer and topcoat to improve the bond between them or to provide

special properties such as filling, hiding, and

bodying.

In general, it may be stated that a coating

material for metal surfaces should at least have

good wetting properties and good specific adhesion, as well as good film integrity to aid

corrosion resistance. Coatings for wood must

have, among other properties, good flexibility

and elasticity and, in many applications, a moderate degree of permeability to water vapor in

order to perform satisfactorily. Coatings for

masonry must be water- and alkali-resistant,

have good sealing properties, and be permeable

to water vapor. Coatings for plaster, in addition to being moisture- and alkali-resistant,

must have a controlled degree of penetration to provide both good adhesion and adequate top- coat holdout.

Specific information on coatings for various

types of substrates is given in sections 4.1

through 4.5 of this chapter.

4.1.3. Basic Function

In selecting a suitable coating for a particular application, it is helpful to keep in mind

the basic function (protective, decorative, or

functional) that the coating is to fulfill, for this can aid considerably in the compilation of an

appropriate checklist of required properties. If the basic purpose of the coating is to protect a

surface from harmful environmental agents

(for example, weathering, fumes, or chemicals), it is obvious that specific resistance to these environmental factors is of primary importance while other factors, such as appearance, are secondary. If the coating is to provide mainly a decorative function, the emphasis

shifts to factors such as color and gloss, and

the retention of these properties in service;

here, the character of the pigment (see sec. 2.3.1) and the pigment volume concentration

(PVC) of the formulation play an important

role along with the resin itself. In cases where the coating is to fulfill one or more definite functional purposes (see sec. 4.6),

certain specific properties can be listed immediately as of primary importance. Thus, a

coating for an industrial smokestack must be

heat-resistant and weather-resistant. A coat- ing for a home space heater must be both heat

stable and color stable at elevated temperatures. Floor coatings must be tough (hard yet

flexible) to withstand the scuffing and abrasion

received in service.

Color and surface texture (gloss) are prime

factors in determining the heat and light reflectivity or absorptivity of a surface ; it is wellknown that light colors and glossy surfaces reflect heat and light, while dark surfaces absorb

such radiation. For example, an outdoor stor- age tank exposed to the heat of the sun will be

maintained at a considerably lower temperature if a light-colored rather than a darkcolored paint is applied to its surface. Heat

build-up in such a tank can also be lessened

(though not to the extent achieved with a white

paint) by employing a metal-pigmented coating

(e.g., an aluminum paint), which possesses a high specular reflectivity. Aluminum-pigmented paints are useful further for their very

good weathering properties.

Other examples of functional coatings are low-modulus or soft materials for acoustical

applications, rubbery coatings to provide re- silence, coatings of high dielectric strength for

electrical insulation, high visibility coatings in- corporating fluorescent pigments, and coatings

with a high degree of imperviousness to vapors

and gases for packaging applications.

Some types of functional coatings depend

strongly upon certain additives in addition to the film-forming resin to achieve their purpose.

For example, despite the importance of the

resin in a high-visibility paint, it is the fluorescent material (see sec. 2.3.1.8) that contributes

the basic functional property. Similarly, fire- retardant coatings (sec. 2.3.8) depend heavily on special types of pigments, fillers, plasticizers,

and flame-retardant additives to achieve their

purpose. Anti-static (explosion-proof) floor coatings for hospital operating rooms, in addition to being based on an abrasion-resistant

resin, are formulated with sufficient carbon to render them adequately conductive. Fungusresistant coatings often contain a fungistat or a fungicide as well as a resistant resin. Socalled sanitary or hygienic coatings incorporate a bacteriostatic substance to keep the surface

"germ-free". Functional coatings such as the

foregoing emphasize the importance of the entire formulation as well as the resin itself in

producing a suitable coating material for a given application.

4.1.4. Appearance

Coating appearance is a consideration in most coatings applications and becomes a factor

of primary importance in cases where the coat- ing serves a decorative function. Aside from

73
the color itself, the appearance of a coating is determined by its degree of transparency,

translucency, or opacity; by its degree of gloss or flatness; and by its surface topography

which may be, for example, textured to simulate fabric or leather, wrinkled or hammertoned to provide special effects, or multicolored

to give interesting and pleasing color effects

while hiding substrate imperfections.

4.1.4.1. Clarity and whiteness.—With the

exception of clear coatings, white coatings, and

a few specialized finishes (e.g., wrinkle and

multicolor finishes), most of the appearance

properties of a coating are determined more by

the manner of formulation and application than

by the nature of the resin itself. Clear, colorless coatings are required where a natural finish

is desired to protect an already attractive sur- face such as beautiful-grained wood or bright

metal. Outstanding clarity and transparency,

together with maximum retention of these

properties on aging, can be obtained with coat- ings based on cellulose acetate butyrate and on

acrylic resins. However, many other resins

which yield clear, colorless films are described

in Chapter 3, including cellulose derivatives,

various vinyl resins, and polystyrene, and cir- cumstances often dictate the choice of one of

these. For example, in an application in which

a clear coating is desired which must also withstand a considerable amount of abrasion, a

clear vinyl coating might be more suitable, be- cause of its inherently greater toughness, than an acrylic coating of better clarity but lesser

abrasion resistance.

White coatings should be based on colorless or nearly colorless resins that do not tend to yellow with age. Excellent retentive white

finishes can be made with resins such as melamine- or urea-formaldehydes, vinyls, acrylics,

selected alkyds, and various cellulose derivatives. Good white finishes can also be made

with resins that have only a pale yellow color,

such as epoxies and drying-oil alkyds. Properly formulated polyurethane coatings give

good results as clear finishes for boats and exterior wood siding, but they tend to darken

somewhat during exterior exposure and hence

do not make the best white finishes. Phenolic

resins also are transparent enough to yield sat- isfactory spar varnishes and floor coatings;

however, their yellow color, which darkens

considerably with age, limits their usefulness

as clear coatings and makes them unsuitable

for white or pastel finishes. In general, even

resins with only a slight yellow or amber cast are unsuitable for applications where maximum

clarity and transparency are required, as in

coatings for artistic paintings, important documents, silver objects, and optical devices.

4.1.4.2. Some resins normally yield TRANSLUCENT COATINGS. Among these are the fluorocarbon resins, polyamides, and conventional

polyethylene (high-density polyethylene is relatively clear). Ordinarily clear coatings can

be made translucent by incorporating small

quantities of a finely divided extending pigment

or filler (see sec. 2.3.1.3), or a somewhat in- compatible resin or plasticizer, to produce a

colloidal suspension that will scatter the trans- mitted light.

4.1.4.3. Opacity is achieved by incorporating

hiding pigments into the formulation. The

opacity or hiding power increases with increasing pigment volume concentration and increasing index of refraction of the pigment (see

sec. 2.3.1.2).

4.1.4.4. A GLOSSY SURFACE is the rule with

clear films laid down from solution in a good

solvent. Glossy surfaces are the result of good

coating flow-out and leveling and good film in- tegrity, resulting in an optically smooth surface. Some resins are inherently capable of

yielding films with exceptionally high gloss;

notable among these are polyurethanes, alkyds,

and epoxy-polyamides. Poor gloss (surface

dulling) of unpigmented coatings may result

from improper formulation or from poor application techniques or conditions. For example,

if the solvent portion of a sprayed coating is too lean in active solvent, or excessively fast in

evaporation rate, the deposited film may be too viscous for good flow-out or too dry for good

coalescence, so that an optically or visually

rough surface is produced. "Blushing" of the

film under high humidity conditions, or "blooming" due to migration of an incompatible constituent to the surface or resulting from an undesirable chemical reaction at the surface, also

may destroy gloss. More information on the

relationships between formulation, application

conditions, and film properties is presented in

section 5.3.2, and in section 7.2. The degree of gloss exhibited by a pigmented

coating depends primarily on the pigment vol- ume concentration (PVC) and, in general, de- creases with increasing pigmentation. Thus,

enam.els are formulated within a PVC range of

15 to 30 percent, with the highest gloss enamels

in the low PVC portion of this range. Gloss

paints are usually in the PVC range of 30 to 45

percent. Semi-gloss paints are in the 40 to 55

percent PVC range. Flat paints are formulated with PVC's generally in the 50 to 65 percent range. Above this range, the critical PVC

(beyond which the paint properties undergo a pronounced and usually undesirable change) is liable to be exceeded, although the actual criti- cal PVC depends on the nature of the materials

(vehicle, pigments, extenders, etc.) involved.

74
Although pigment volume concentration is the

chief factor determining the gloss of pigmented

coatings, other factors such as fineness of

grind, compatability of vehicle components,

vehicle dispersive power, and other formulation variables also affect the degree of gloss obtained. As in the case of clear films, the gloss

of pigmented coatings also depends on the manner of application and on the prevailing application conditions, inasmuch as both of these

affect the structure of the film laid down.

4.1.4.5. Textured coatings having a wrinkled, crackled, pebbled, hammertoned, or other

intentionally "faulty" film structure or finish

find use both for their pleasing appearance and

their serviceability. Such coatings can provide an attractive and durable finish on physically

imperfect surfaces that might require costly

surface preparation if coated with conventional

smooth finishes. They are based on special

resins and formulating ingredients and specially balanced solvent mixtures. The effect

achieved also depends importantly on the manner of application. Various kinds of textured

coatings are described below

:

4.1.4.5.1. Wrinkle finishes are formulated for

fast drying that will form a surface "skin" and

cause partial setting of the film before much

flow and leveling occur. Continued rapid cur- ing and escape of solvent sets up shrinkage

stresses and strains that cause the film to wrinkle. It is apparent that the film must be flexible

if it is to wrinkle rather than crack under the

high drying stresses developed. Wrinkle fin- ishes are usually based on an alkyd or varnish

vehicle in which the modifying oil is of a very

fast drying type, usually tung oil or oiticica oil. Phenolic resin-tung oil combinations have been

notably successful in this type of finish. To

achieve the necessary fast dry and cure, high

proportions (10 to 20 times greater than usual)

of efficient driers such as manganese and cobalt are used in the formulation, and a high proportion of low-boiling fast-evaporating solvents is employed. To further develop the wrinkle

effect, the coating is generally applied in a

single thick coat and is baked to complete the

cure. Fine rather than coarse wrinkle finishes can be obtained by applying the above principles in less drastic fashion.

4.1.4.5.2. Crackles finishes are obtained by

over-coating an ordinary protective base lac- quer with an over-pigmented, brittle coating

that will crack under the stresses developed

during drying. The base coat is usually of a

contrasting color to that of the crackle coat. A clear protective topcoat may be applied over

the crackle coat if desired.

4.1.4.5.3. Pebble finishes are obtained by

spraying a high viscosity coating of high solids

content and fast setting time onto a smooth base

coat in such manner as to produce blobs of coat- ing material that set before flow and leveling can take place. Through control of viscosity,

air and fluid pressure, and speed of travel of

the spray-gun, the pattern of blobs can be

varied to give a variety of textured surfaces,

from a uniformly pebbled finish to one resembling leather grain. Pebble finishes can be

formulated with high quality resins such as

vinyls, epoxies, and alkyds that do not require

further protective overcoating. Simulated

leather finishes having excellent chemical re- sistance, abrasion resistance and general durability can be made with vinyl plastisols (see

sec. 3.6.2) ; these must be given a flash bake to fuse the applied coating into a continuous film.

4.1.4.5.4. Hammertone finishes are generally

one-coat, sprayed coatings formulated with a small amount (1 or 2 ounces per gallon of vehicle) of a fine grade of non-leafing aluminum

pigment plus minimal amounts of such other

colors as may be desired. The solvent mixture

is a combination of fast and slow evaporating

solvents that permits the coating to set fast

enough to prevent running and sagging while

maintaining good fluidity long enough for a vortex action to be set up in the drying and

cooling coating. The swirl of aluminum particles in the convection currents leads to the

typical hammertone configuration in the set

film. After a short period of air-drying, hammertone finishes are baked for final cure. Since

hammertone finishes are largely the result of

the solvent balance and manner of application

of a coating, they may be based on a variety of

different resins selected on the basis of the

protective and durability requii-ements to be

met.

4.1.4.5.5. Multicolor finishes consist essentially of discrete particles of two or more different colored, organic solvent-base materials

suspended in an aqueous solution of a watersoluble polymeric stabilizer. A discussion of

such finishes is given in section 2.3.7. They are being used with particular success as interior

architectural finishes for wood, masonry, plaster, and wallboard—including difficult surfaces

such as basement walls and ceilings of hospitals

and garages as well as walls and furniture in

hotels, industrial plants, public buildings, and

homes. Durable types for use on exterior masonry or metal also are available. In addition

to their generally good durability and pleasing

appearance, multicolor finishes offer easy application with a minimum of surface preparation, good hiding of surface imperfections, uniform masking of areas of different porosity or texture and easy touch-up of damaged areas when necessary.

75
4.1.5. Application Limitations

In selecting a coating material for a given

purpose, it is necessary to take into account

such characteristics as toxicity, flammability,

odor, drying speed, moisture sensitivity, brushability or sprayability, and other inherent properties that affect the manner of application.

For example, toxic or flammable coating materials cannot be used in areas where, for operational or economic reasons, it is not feasible

to shut down the operation. Nor should toxic or flammable materials be used without adequate safety equipment and precautions, particularly in confined areas. A desirable coating

system that requires several days for application and cure would not pose a problem in a new building not yet occupied ; however, in an

occupied hospital or in a busy restaurant, an

odorless fast-drying coating such as a latex

paint might be far more feasible. The ability to dry very quickly is a necessary requisite for

traffic paints—one which excludes many otherwise satisfactory materials.

Other examples of application limitations affecting the broad choice of film-forming resin or type of formulation may be given: When

painting must be done in cold weather (never

desirable but sometimes necessary), a latex

paint might be unsatisfactory because of poor

coalescence (or even freezing) at low temperatures. Plastisols require a flash fusing to form

a continuous film. Heat-convertible coatings

cannot be used unless adequate oven or infrared

baking facilities are available. Oil paints do

not perform well if applied over damp or alkaline surfaces. Latex paints require oil-base

primers before application to wood surfaces.

Polyurethanes adhere better to water-base

stains than to oil-base stains. Phenolic paints

(sec. 2.3.2.2) require thorough surface preparation and limited inter-coat drying periods to give good adhesion. Multicolor finishes must

be applied by spraying—the high shear in- volved in brushing would destroy the emulsion

and discrete character of the particles of col- ored lacquer. Two-component coatings, such

as air-drying epoxies, which must be activated

just before use, require (unless brushing is to be employed) a special type of spray gun that

will meter and mix the components at the gun

at the instant of spraying. And so on (also see

sec. 7.2.4.3).

In short, in choosing a coating material, due

consideration should be given to its application

properties in relation to available facilities and

the environmental conditions under which it will be applied.

4.1.6. Cost

Last but far from least among factors to be

considered in the selection of coating systems

is the inevitable item of cost. Despite the basic

importance of the cost factor, it must always

in any honest appraisal of ultimate economy of

coating operations—be weighed against the performance properties and requirements of the

coating system. A low-cost coating that fails to perform its function is a completely wasted

expense. Considering the relatively high cost of labor involved in applying protective coatings, a short service life of a cheap but inferior

material may make ultimate coating and maintenance costs far higher than if a more expensive but more durable coating had been employed in the first place. This is not to say

that cheap coatings are necessarily inferior, or

that an expensive coating automatically will

perform well. Obviously, it depends on the re- quirements of the application. Where, for ex- ample, a relatively inexpensive rosin-modified

phenolic will perform satisfactorily in an in- terior application, it would be a useless ex- travagance to use the more expensive pure

phenolic type. The latter, on the other hand,

would be more economical, in the long run, for an exterior application. Similarly, an expensive premium material such as a fluorocarbon

resin would be quite inferior as a coating for a masonry wall compared to a much less ex- pensive latex paint.

Aside from the ultimate cost of a coating

material in relation to its suitability and durability in a given application, and apart from

the intrinsic cost of the film-forming resins and

other ingredients that make up the formulation, a number of specific application factors

must be taken into account in arriving at an

over-all cost estimate. Thus, in addition to the

gallon cost of the material as purchased, one must also consider the spreading rate (i.e., the area adequately covered by a unit volume of

coating material, usually expressed as square

feet per gallon), the probable application time

and its equivalent in labor costs, the investment

in regular or special equipment required for

application of the material, and finally the ex- pected useful service life of the coating.

In the absence of actual experience with a

particular material, it is apparent that much of

the information needed for a cost estimate must come from the coatings manufacturer. Information on surface preparation, spreading rates,

application methods, drying time, type of service for which recommended, and an indication

of the order of durability to be expected in the

intended application is usually furnished by

the manufacturer, and is generally reliable if the manufacturer is a reputable one (also see

sec. 7.2.2).

76
4.2. Coating Systems for Wood

Wood is a porous, hydrophilic material of

relatively poor dimensional stability. It swells

and shrinks, expands and contracts, with

changes in moisture content and temperature.

A coating that is to perform successfully on a wood surface must therefore be distensible

and elastic enough to follow the dimensional

changes in the wood without cracking. Good

wetting and controlled penetration of the por- ous wood surface are also requirements for

obtaining good adhesion and adequate holdout

of topcoats. On exterior wood surfaces that are subject to absorption and migration of

moisture from within, as exterior wood siding on a house, the coating m.ust have the additional

property of permeability to water vapor;

otherwise, the entrapped moisture is likely to cause blistering and peeling of the paint film. Even so, excessive amounts of moisture originating inside a structure must be controlled to ensure satisfactory paint performance.

A satisfactory coating system for wood generally requires the use of a primer or undercoater to lightly penetrate and seal the wood,

followed by one or more topcoats of a coating

that will provide the necessary protective properties and desired appearance. For exterior

use, the primer is usually a specially formulated oil or oleoresinous type, as described in

section 2.4.1 and typified by Federal Specification TT-P-25 ; however, self-priming (thinning

with linseed oil) is sometimes resorted to as a matter of convenience, or when a lead-free

coating system resistant to sulfide fumes is re- quired. On interior wood, the first coat is frequently a long oil-alkyd enamel undercoat, such

as that described by Federal Specifications TTE-543 or TT-E-545; alternatively, when con- venience dictates, an appropriate topcoat material may be made into a self-primer by thinning

with one pint per gallon of a suitable thinner

(see sec. 5.3.2). Other federal specifications

for wood primers are TT-P-636 and TT-P-659

(see tabular summary in chapter 8).

The above remarks relating to wood primers

apply primarily to conventional coating sys- tems for wood in which the topcoat is an oil- base or oleoresinous material. The newer synthetic resin coating systems, based on resins

such as cellulose acetate butyrate, polyesters,

and many others may require different types of

primers, most frequently a thinned (selfprimed) version of the topcoat material. The

directions furnished by the manufacturer of

such coatings are the best guide to the type of

primer or sealer required. A discussion of the

types of materials suitable as the main body or

finish coatings for wood is given in the following sections.

4.2.1. Exterior Coatings for Wood

Linseed oil house paints, properly formulated

and pigmented, remain one of the best and

most durable coatings for exterior wood struc- tures in ordinary environments. The linseed

oil film owes its effectiveness to a unique combination of desirable properties (see sec. 2.3.2.1 on Oil Paints), including good adhesion, elasticity, permeability, and weather resistance.

Despite limitations such as relatively poor

water-, chemical-, and abrasion resistance, lin- seed oil paints are excellent in general durability and performance when applied to exterior wood siding on homes and other buildings.

On exterior wood surfaces that are free from

the problem of moisture entrapment and the

related blistering of the paint film, oleoresinous

coatings (see sec. 2.3.2.2) such as oil-modified

alkyds or oil-modified phenolic types may be

used. Long oil-alkyds make excellent architectural paints for exterior trim and trellis applications. Alkyd paints dry faster and

harder than straight oil paints, and have better gloss and color retention along with excellent general durability. Paints based on sili- cone-modified alkyd and phenolic resin vehicles

also are finding use on exterior wood.

Where a clear finish is desired on exterior

wood surfaces, phenolic-tung oil spar varnishes

(and to a lesser extent, alkyd varnishes) are

used. Clear coatings are considerably less durable than pigmented ones (see sec. 2.3.1.1).

Where a good oil or oleoresinous paint may

give four or five years of satisfactory service, a

clear finish based on the same vehicle may fail in a year or two, or even sooner. Woods exposed to marine or immersion conditions, as on boats and over-water structures,

have traditionally been protected with tung oil- phenolic spar varnishes, particularly those

based on 100 percent phenolic resins. Not yet

fully evaluated, but very promising in these

applications, are newer coatings based on epoxy resins, epoxy-polyamide resins, polyesters and polyurethanes.

On new wood surfaces, including exterior

wood siding, latex paints based on acrylic or on

polyvinyl acetate resin emulsions are proving

satisfactory, provided the wood is first treated

with a zinc-free oil-base primer. With some- what greater difficulty, latex paints may also

be applied over previously painted wood surfaces, if loose chalk is first removed and an

oil-base primer applied in accordance with the

manufacturer's instructions.

Wood surfaces, such as porches and decks,

that are subjected to considerable abrasion and

77
wear as well as weathering require coatings

based on tough, durable vehicles. The most

widely used vehicles for this purpose have been

medium or long oil alkyd or phenolic varnishes,

particularly tung oil-phenolic types. A typical

oleoresinous floor and deck enamel is described

by Federal Specification TT-E-487. More re- cently several of the newer synthetic resins,

especially epoxies, polyurethanes, and polyesters, are finding increasing use in floor and

deck finishes ; properly formulated and applied

(sec. 5.3.3.) they are capable of excellent service in such applications, although more expensive than oleoresinous types.

Wood furniture that is to be subjected to out- door exposure can be protected with coatings

based on alkyd or phenolic varnish vehicles,

cellulose acetate butyrate lacquers, polyester

resins, polyurethane finishes, and epoxy resins.

Federal specifications for organic coatings

for exterior wood surfaces are limited chiefly

to a variety of oil, oleoresinous, and alkyd

types. The newer synthetic resins to which ref- erence has been made, while of great potential usefulness in this area, have not yet been

described by formal Federal specifications. Existing Federal specifications for exterior wood

coatings may be readily located by means of

the classified list and summary charts given in

chapter 8. For maximum usefulness, the information

given in this and other sections of this chapter on the suitability of various types of coatings

for use on a specific substrate in a particular

environment should be supplemented by re- ferring to the detailed information about these

resins which is given in chapter 3. For ex- ample, such reference to the information on

phenolic resins will indicate that while modified phenolic resins are used to some extent in

exterior finishes (in cheaper finishes, or in

small proportions to improve the adhesion of a pure phenolic resin), it is the 100 percent

phenolic types which have the best exterior

durability.

4.2.2. Interior Coatings for Wood

Interior coatings for wood, in addition to the

general requirements for good adhesion and

adequate coverage, are usually expected to be

relatively quick-drying, reasonably free from

obnoxious odors and toxic ingredients, attractive in color and appearance, stain-resistant,

washable, and scuff-resistant. Good flexibility

is a necessary property, but elasticity requirements are less severe than for exterior coatings.

Since resistance to sunlight and weather is not

needed, interior coatings may often be formulated with less durable and less expensive

resins, pigments, and additives than are re- quired for exterior finishes.

For interior woodwork, cabinets, and furniture, the most thoroughly proven and widely

used paints are those based on alkyd resins,

particularly medium oil soya-modified types.

Several Federal specifications for interior

alkyd-base paints are included in the summary

given in chapter 8. For the clear finishes generally desired on

furniture, oleoresinous varnishes (see sec. 2.2.2)

and cellulosic lacquers are widely used. Most of

the latter are based on cellulose nitrate modified

with suitable resins and plasticizers. Cellulose

acetate butyrate lacquers also are useful in this

application. Polyester coatings, though rather

difficult to apply, are finding increasing use as premium furniture finishes of great beauty and

durability. Epoxy resins and polyurethanes

may be used as furniture coatings ; their toughness and durability, however, have led to their

wider use as clear finishes for wood surfaces

(e.g., gymnasium floors) that must withstand

severe abrasion and wear.

Clear coatings for interior wood paneling may

be based on the same resins indicated above for

furniture finishes. Bleached shellac varnish

[151] is often used as an attractive, non-darkening finish for wood paneling, although it is more brittle and less durable and chemicalresistant than many other materials. The

shellac may be used as a sealer for the wood,

and then over-coated with a more durable and

resistant finish such as an oleoresinous varnish.

Shellac is used also as a floor sealer, with a

protective overcoat of hard wax. A durable

floor sealer based on a tung oil-phenolic varnish

resin is described by Federal Specification

TT-S-176. Floor sealers are designed to penetrate and seal the wood, leaving only a thin film on the surface which does not show wear as

readily as thicker varnish coatings. Durable

polymeric floor finishes are available based on

a variety of resins or blends of resins such as maleic-modified alkyds, resin-modified phenolics,

cellulose derivatives, and natural resins. A con- ventional floor and trim varnish is covered by

Federal Specification TT-V-71. For maximum

durability with minimum maintenance in severe

service applications such as bowling alleys,

gymnasium floors, and areas of heavy foot traffic, polymeric coatings based on polyurethane or epoxy-polyamide resins are proving highly useful. Where resilience and oil and chemical

resistance are paramount factors, neoprene

coatings may be used on floors. A coating for laboratory bench tops, with

78
excellent resistance to acids, alkalis, and other

corrosive chemicals, can be built up with furan

resins (see sec. 3.17).

Most of the resins mentioned above in con- nection with clear coatings may serve also as

the basis for pigmented paint vehicles.

4.3. Coating Systems for Metals

Although metals are impermeable to moisture

and gases and are relatively stable dimensionally, the common structural metals—particularly iron and steel—are very susceptible to corrosion in a moist atmospheric environment

(above 60 per cent relative humidity at ordinary temperatures), particularly under acidic

conditions such as occur in industrial atmospheres. Corrosion is an electrochemical process

in which chemical changes are produced in the

metal by the flow of current caused by differ- ences in electrical potential arising between dissimilar metals or between dissimilar areas in or around the same metal. The flow of current

dissolves away the anodic (corrodible) material

while affording protection to the cathodic material. Moisture on the metal surface provides

a conductive path through which corrosion cur- rents can flow. The presence of soluble salts or other electrolytes on the metal surface greatly

accelerates the corrosion process and may lead

to severe localized corrosion in the form of pitting. Oxygen or oxygenated areas are in effect cathodic toward iron and steel, so that rusting

(oxidation) takes place rapidly in the presence

of oxygen and moisture. The rust formed is permeable to moisture and is strongly cathodic

to the underlying iron or steel, so the corrosion

proceeds until the metal is destroyed. Mill scale

also is cathodic to iron and steel, and is usually

cracked or loosely bonded; hence the need to remove it, if possible, before painting, especially

if the steel will often be wet or immersed during

service (see sees. 6.3.1 and 7.2.3).

The corrosion of metal which is in continuous

contact with a conducting medium such as moist

soil or water can be suppressed by imposing an

electric current sufl^icient to render the metal

completely cathodic. Such cathodic protection

is particularly applicable to the exterior of

buried pipelines, the interior of water tanks,

and the exterior of ship hulls. In general, however, the most practical and by far the most

widely used method of protecting metal against

corrosion is by the proper application of a suitable organic protective coating system that will

prevent moisture and oxygen from reaching the

surface of the metal and will act as a barrier

against galvanic current (see sec. 7.2.3). All

elements of such a system—surface preparation

and pretreatment, primer, intermediate and fin- ish coats—must be selected with the type of

service in mind, and with care to ensure that

all parts of the system are compatible (see sec.

7.2.2.3). Compatibility implies that each part

of the system will bond properly to other parts

and to the substrate in such manner as to achieve a composite system that will remain

intact and perform its protective function for

the maximum possible time.

The information to be presented represents

established good practice in the selection of

coating systems for metals, particularly steel. Adherence to the suggested practice will ensure

a compatible coating system capable of excellent

service in the intended application. In cases where it is not possible to follow the optimum

procedures indicated, deviations are permissible

provided precautions are taken to avoid possible

difficulties. For example, although it is ordinarily not good practice to apply a phenolic topcoat over an oil-base primer because of the

danger of lifting the undercoat, this may be

done successfully if the oil-base primer is allowed to dry thoroughly before application

of the topcoat. More information of this kind

is presented in section 7.2.2.3. A satisfactory coating system for metal

begins with the application of a suitable cor- rosion-inhibiting primer to the clean (and often

chemically pretreated) surface. The primer,

ideally, should have good wetting properties

(see sec. 7.2.4.1) to provide adequate adhesion

to the nonporous metal surface and to ensure

penetration into crevices and into areas of rust or scale difficult to completely remove (as on

structural steel) . It should also, of course, provide a strong bond with the topcoat. So long

as the primer remains intact, excellent protection against corrosion is afforded the substrate.

An important function of the topcoat (body or

finish coats) , therefore, is to protect the primer

against wear and deterioration so that it can

continue to perform its corrosion-inhibiting

function. Additionally, the topcoat should have

a high degree of imperviousness to moisture,

give the desired appearance, and possess adequate over-all durability in the environment in

which it is to serve. The primary purpose of this section, as with

others in this chapter, is to provide information on the selection of coating systems, by indicating classes of resins or coatings which are particularly well-suited for use on a given surface

under various specified conditions. Important

adjunct information is given at appropriate

places elsewhere in the text, as follows : The

79
formulation and selection of primers for ferrous

metals, galvanized surfaces, aluminum, and

magnesium are discussed in section 2.4.2. Corrosion-inhibiting primers derive their special

protective qualities from the corrosion-inhibiting pigments which they contain ; these pigments and the manner in which they function

are described in section 2.3.1.5. Information on the surface preparation and pretreatment of

metals is covered in section 6.3. In general, once the metal has been adequately prepared

and the proper corrosion-inhibiting primer applied, the topcoat materials that may be used

are similar for most metals. The selection of

topcoat materials possessing the good film

integrity and other attributes necessary for

good performance on metals is discussed below.

Oil-base, alkyd, phenolic, and vinyl types are widely used. Also useful for particular applications on metal surfaces are coatings based on epoxy resins, polyamides, epoxy-polyamides,

chlorinated rubber, neoprene, polysulfides, polyesters, polyurethanes, cellulose esters, styrenebutadiene copolymers, and other synthetic

resins; these often require special primers

which should be applied in accordance with the

instructions of the manufacturer.

4.3.1. Coatings for Structural Steel

The everpresent need for protecting from

corrosion the vast quantities of structural steel in use has led to the development of effective

protective coating systems for these materials.

Chief among such systems from the standpoint

of wide use are the time-tested oil-base and oleoresinous (alkyd and phenolic) types. Of proven

usefulness among newer types of coating materials are systems based on vinyl resins; these

have been particularly efi'ective under severe

corrosive conditions, such as immersion in sea water or exposure to industrial and chemical

environments. Also proving useful under conditions of continual water immersion and severe marine service are the new zinc rich paints (see

sec. 2.3.1.5) based on high percentages of zinc

dust in epoxy, epoxy-polyamide, polyurethane,

and other alkali-resistant vehicles [128].

Where high resistance to strong acids and

alkalis is required, excellent protection is afforded by chlorinated rubber coatings. Chlorinated rubber is also an excellent primer for

metal that is to receive neoprene coatings in

applications requiring exceptional resistance to abrasion, such as structures exposed to winddriven sand. The choice of an optimum coating

system that will provide adequate protection at minimum cost requires consideration especially

of the condition of the surface to be painted and

the nature of the exposure environment.

4.3.1.1. Oil-base and oleoresinous systems. —A common problem in the painting of struc- tural steel surfaces is the presence of harmful

rust and mill scale impractical to remove completely. Such steel requires a linseed oil-base

primer to provide adequate wetting and penetration of pores and crevices. The primer must

also contain corrosion-inhibiting pigments to passivate the steel surface. A typical excellent

primer for this purpose is the straight red-leadhsLse linseed oil paint covered by Federal Specification TT-P-86, Type I. Where faster drying

is necessary and some sacrifice in wetting ability can be tolerated, the type II material of the

above specification may be used instead of type I. The type II primer is based on a red lead-iron

oxide-extender mixed pigment in an alkyd var- nish-raw linseed oil vehicle that will" air dry in

about 16 hours as compared to about 36 hours

for the straight oil base (type I) material.

On smooth, thoroughly clean steel, quickdrying corrosion-inhibiting primers based on

poorer-wetting but more durable alkyd or phenolic varnish vehicles may be used. A typical

red lead-alkyd varnish (linseed oil-modified)

primer that dries in about 6 hours is described

by TT-P-86, Type III. The straight alkyd

varnish vehicle possesses better water and

chemical resistance than the straight oil base

(type I) and oil-fortified alkyd varnish (type

II) vehicles, but it should not be used as a primer on incompletely cleaned steel because of

its limited wetting properties.

Very good durability under conditions of

fresh-water immersion, high humidity and con- densation, or moderately severe chemical atmospheres is obtained with corrosion-inhibiting

primers based on a straight phenolic-tung oil varnish vehicle such as that typified by TT-P-86,

Type IV. The latter contains a red lead-plusextender mixed pigment. While having excellent durability under adverse conditions, its limited wetting ability restricts its use to thoroughly clean steel, preferably steel that has been

blast cleaned or pickled.

An alternative to the red lead type primers

discussed above are primers utilizing a zinc

yellow-iron oxide mixed pigment as the corro- sion-inhibiting base in an oleoresinous vehicle.

The three types described by Federal Specification TT-P-57 are typical ; the pigment portion

contains some zinc oxide and a substantial proportion of siliceous extenders in addition to the

main pigments. The vehicle for type I is a 50/50 blend of a long oil-alkyd resin with raw

linseed oil; its slow drying and good wetting

properties permits its use on steel limited to wire-brush cleaning. Type II is based on a medium oil length alkyd vehicle ; although faster

drying and more durable than type I, its poorer

80
wetting properties permit it to be used only on

smooth, thoroughly cleaned steel ; it is intended

for use as a factory or industrial primer rather

than for structural steel. Type III is based on

a straight phenolic-tung oil vehicle that provides good durability under conditions of severe humidity or fresh water immersion; however,

its limited wetting properties restrict its use

to thoroughly cleaned steel (sandblasted or

pickled), preferably steel that has received a phosphate pretreatment.

A corrosion-inhibiting primer based on the

unique basic lead silico chromate pigment described in section 2.3.1.5 is available under Federal Specification TT-P-615, in four types that are similar in application to the four types of

red lead-base paint described in TT-P-86 and

referred to above. The type I vehicle is a 4:1

blend of raw linseed oil with a long-oil alkyd

resin ; it provides good wetting of metal despite

the presence of small amounts of corrosion

products impractical to remove. The type II vehicle is a 1:1 blend of raw linseed oil with

a medium-oil alkyd resin ; although its wetting

properties are not as good as that of type I, it may be used where faster drying is necessary.

The type III vehicle is an alkyd varnish that

may be used only on thoroughly clean steel be- cause of its fast drying and low wetting properties. Type IV is based on a phenolic-tung oil vehicle that provides good durability under

severe humid conditions or fresh water immersion ; however, its fast drying and limited wetting ability restricts its use to steel that has

been thoroughly cleaned, preferably by sandblast. The basic lead silico chromate pigment

may also be incorporated, in combination with

conventional pigments and extenders, into a

variety of vehicles to produce intermediate coat

and finish coat paints with corrosion-inhibiting

properties.

For the protection of potable water tanks, a zinc dust pigmented primer and paint based on a phenolic varnish vehicle is very suitable.

Such a paint [152] is covered by Military Specification MIL-P-15145 (Formula No. 102).

4.3.1.1.1. Intermediate and finish coats.—

Intermediate coats for application over the

primers that have been discussed may be the same as the primer coat or the same as the finish coat material to be used. Finish coats should

be selected with system compatibility kept in

mind. Thus, as a general rule, oil-base primers

should be over-coated with oil-base topcoats,

alkyd primers with alkyd topcoats, and phenolic

primers with phenolic topcoats. Variations

from the rule are permissible if special precautions are taken to ensure compatibility and

intercoat adhesion, as discussed in section

7.2.2.3. A variety of oil-base and oleoresinous

intermediate and finish coat paints and enamels,

in many different colors, are covered by Federal

specifications. The classified list given in chapter 8 will provide a ready guide to the letter- number designations of these specifications;

composition and performance information for

most of them can be found in the tabular sum- mary of that same chapter.

Among finish coat materials suitable for use, over the aforementioned primers, on structural

steel and other exterior metal surfaces, the fol- lowing are typical : A linseed oil paint incorporating a titanium-lead-zinc type pigmentation,

is covered by Federal Specification TT-P-102;

it is available in white and light tints and has

good resistance to weathering in ordinary en- vironments. A lead-free, fume-resistant linseed

oil paint (white only) is described in TT-P-103.

Black linseed oil paints with very good durability are covered by TT-P-27 and TT-P-61.

A durable red iron oxide paint (so-called "roof

and barn paint") based on a varnish-fortified

linseed oil vehicle is specified in TT-P-31. A

chrome-green long oil-alkyd paint (known as a "trim enamel paint") is covered by TT-P-71.

Where the greater resistance of an alkyd resin

is needed in a coating for relatively smooth

steel that has received an oleoresinous primer, an alkyd gloss enamel such as that described by

TT-E-489 offers excellent durability in ordinary atmosphere; a semigloss alkyd enamel is covered by TT-E-529, and a lustreless alkyd

enamel by TT-E-527. For severe service under

conditions of continuous condensation or fresh

water immersion, in conjunction with phenolic

varnish-base primers, a highly durable (weather- and water-resistant) aluminum phenolic

paint can be prepared by adding two pounds of

aluminum pigment paste (TT-P-320, Type II, Class B) to one gallon of phenolic resin spar

varnish conforming to Federal Specification

TT-V-119; however, aluminum-pigmented vehicles are not suitable for use in strongly alkaline or acidic environments. There, inert pigments are required.

4.3.1.2. Vinyl systems.—For the protection

of structural (and other) steel exposed to severely corrosive conditions in industrial or marine atmospheres, chemical environments,

condensation, and alternate or continuous immersion in fresh or salt water, coating systems

based on vinyl resins give excellent service.

For optimum results the steel should be cleaned

by blast or pickling to remove rust, mill scale,

and all traces of oil or grease. The clean sur- face must be pretreated with a vinyl washcoat

or wash primer such as that specified in Military Specification MIL-C-15328 (Navy Stand81
ard Formula No. 117) before application of the

regular vinyl primer. The wash primer provides a mild phosphating action in addition to depositing a thin vinyl film that provides an

anchor for the corrosion-inhibiting primer

which follows. The most widely used vinyl

priming paint for steel is a red lead type [143,

145] such as that conforming to MIL-P-15929

(Formula No. 119). A zinc chromate vinyl

priming paint [144] such as that conforming to MIL-F-15930 (Formula No. 120) may also be

used with good results.

The intermediate coat may be the same as the priming coat or the same as the finish coat. The most durable and chemical-resistant finish coats are based on straight vinyl resins, typically a vinyl chloride-acetate copolymer resin,

together with inert pigments. Aluminum-pigmented vinyl paint will give excellent service

in marine atmospheres or fresh water immersion but should not be used in strongly alkaline or acidic environments. Straight vinyl resin

paints incorporating suitable inert pigments are

resistant to direct liquid contact with severely

corrosive chemicals such as inorganic acids,

alkalis, and salts ; they are resistant also to ali- phatic liquids, oils, greases, and alcohols ; however, they are attacked by strong oxidizing acids

and by many aromatic organic solvents (see

sec. 3.6.3).

Where the maximum durability and chemical

resistance of straight vinyl paints is not re- quired, excellent service under severe exposure

conditions (as on the boot-topping areas or on

the topsides of marine vessels) may be obtained

with vinyl-alkyd paints such as those described

in a series of Military specifications comprising

the Standard Formula 122 series [68]. These

are a series of paints ranging in shade from

black through gray to white, in either gloss or

dull finish. Their numerical designations in- clude the following: MIL-P-15932 thru 15936,

-16188, -16501, -16502, -16738 [55].

4.3.1.3. Bituminous coatings (also see sec. 2.5) may be used on iron and steel subjected

to severe corrosive atmospheres or underground

burial. For best results the steel should be blast

cleaned or pickled. For very severe exposure

a wash primer pretreatment is helpful before

application of cold-applied asphalt or coal tar

mastics. Coal tar mastic must be over-coated

with coal tar emulsion to prevent checking and

cracking on exposure to sunlight. For underground use in very corrosive soil, best results are obtained with a coal tar primer topcoated

with coal tar enamel (wash primer is not suitable in coal tar enamel systems). In noncorrosive soil, an asphalt varnish such as that cov- ered by Federal Specification TT-V-51 may

suffice. All bituminous coatings are resistant

to water immersion, but not all are suitable for

potable water. (For the latter application, coal

tar enamel has been widely used.) Although

bituminous coatings are resistant to many

chemicals, they are attacked by many organic

solvents and by some chemicals and must be

used with discretion. For example, the coal tars have good resistance to aliphatic liquids

but poor resistance to acids, while the reverse

is true for the asphalts.

4.3.1.4. Galvanized steel may often be

painted without undue difficulty if the galvanized surface is well-weathered but free of dirt

and rust. However, new galvanized steel must

be solvent-cleaned to remove all oil and grease

and for best results (if not factory chromated)

should be pretreated with a basic zinc chromate

washcoat or cold phosphate pretreatment (see

sec. 6.3.3.1). A zinc dust primer such as that

covered by Federal Specification TT-P-641 adheres well and provides good protection for galvanized surfaces ; it will also perform well as the finish coat except in acidic environments.

The TT-P-641 zinc dust paint is available in

three types : Type I, based on a raw linseed oil vehicle, provides the best wetting but is slow to dry. Type II, the type most often used, is based on a long oil, linseed-modified, alkyd resin vehicle that has moderately good wetting properties and is faster drying than the straight oil type. Type III is based on a pure phenolic resin

spar varnish vehicle that is suitable for use under severe moisture conditions or underwater

exposure; however, its limited wetting properties require that the steel be thoroughly cleaned

and given a phosphate pretreatment before

application of the primer. In the case of the

type I and type II materials, pretreatment, while

beneficial, is optional.

Badly rusted galvanized steel should be

cleaned like ordinary rusted steel before painting is attempted. Where it is not possible to remove all corrosion products, a wetting oil (thinned linseed oil) pretreatment may be used

to ensure a bond between the primer and the

steel. The wetting oil treatment permits use of

the faster drying primers (types II and III)

which cannot ordinarily be used on incompletely

cleaned surfaces.

82
4.3.2. Industrial Coatings for Steel

Factory-coated steel products such as rail- road cars, vehicles, machinery, tools, garden

equipment, appliances, metal furniture, etc., may be protected or decorated with a wide

variety of organic coatings. The opportunity

afforded in the factory for thorough surface

preparation and pretreatment, coupled with

selection of a primer and finish coat system

suited to the service environment, leads to excellent service performance. Most factory-applied

coating systems utilize quick-drying, bakingtype primers and finish coats. Baking makes

possible a rapid production line coating operation and results in the development of maximum

adhesion and optimum film properties.

For general use in ordinary environments,

both interior and exterior, medium- to short-oil

alkyd resin vehicles of the baking type are most

widely used. Where exceptional whiteness and

color retention together with very good mar

resistance are required, as on refrigerators,

washing machines, hospital equipment, etc., non-drying oil alkyds modified with urea- or melamine-formaldehyde resins give excellent

service. A detailed discussion of the applications for which the various types of alkyd resins are well-suited is given in sections 3.4.1 and

3.4.2. Baked phenolic resin coatings, particularly

those based on pure phenolic resins of the alcohol-soluble, thermosetting type (see sec. 3.3.2.3)

,

are highly resistant to corrosive atmospheres,

organic solvents, and organic and mineral acids

(except strong oxidizing types) ; they have

proved durable also when submerged in water

or buried in soil. Excellent chemical and abrasion resistance is achieved with baked epoxy resins (see sec. 3.10)

.

Many of the properties of the baked resin are

retained in the amine-catalyzed types which

cure at room temperature. Zinc-rich paints

(see sec. 2.3.1.5) based on epoxy-type vehicles are giving good service as corrosion-resistant

coatings for the undersides of automobiles.

Vinyl coatings, especially vinyl chloride-acetate copolymer types, are widely used for the

protection and decoration of metal, in both

smooth and textured finishes (see sec. 3.6.3, and

the discussion of textured finishes in sec. 4.1.4.5). Plastisols based on polyvinyl chloride

resins (sec. 3.6.2) yield tough, chemical- and

abrasion-resistant coatings for severe service

applications such as plating baths, dish-washer

racks, and chemical tanks.

1 Lacquers based on cellulose nitrate, cellulose

! acetate butyrate, or acrylic resins are often used

I for the protection or decoration of metal. General purpose nitrocellulose lacquers are described in Federal Specifications TT-L-26 and

TT-L-31. The latter is suitable for exterior use

on smooth clean metal—for refinishing or as the

original finish—on vehicles, construction equipment, metal signs, railing, etc. Other industrial coatings for steel include

those based on polyamide and epoxy-polyamide

resins, polyurethanes, polyesters, and neoprene.

Coating systems suitable for steel fuel tanks

and salt water tanks are described [142] in

Mil. Spec. MIL-P-23236.

All the coating systems which have been discussed require proper cleaning and in some

instances pretreatment of the metal surface, as

well as suitable primers to provide corrosioninhibition and topcoat adhesion. The instructions furnished by the manufacturer usually

provide the best guide to their application.

4.3.3. Coatings for Other Metals

Coatings for aluminum, magnesium, brass,

copper, and other metals are, in general, similar once the metal has been properly cleaned, pretreated, and primed. Pretreatments and primers for aluminum and magnesium and their

alloys were discussed in sections 2.4.2.3 and

2.4.2.4. Additional detailed information on

various metals is presented in section 6.3 of

chapter 6 on Surface Preparation and Pretreatment. Copper (section 6.3.6.3), after being

cleaned and perhaps roughened slightly or pretreated, may be primed with an aluminum var- nish such as TT-V-81 or with a zinc dust priming paint such as TT-P-641. Thinned, carbonpigmented oil paints such as TT-P-61 also may

be used as a primer on copper. Terneplate (sec.

6.3.6.6), or "roofing tin," may be painted with

ordinary iron oxide-pigmented paint such as TT-P-31 ; new terne plate must be wiped free

of all oil or grease, using solvents such as tur- pentine or gasoline; the paint should be well

brushed out, since thick coats are prone to crack

later. When coatings based on synthetic resins

(epoxies, polyurethanes, polyesters, neoprenes,

etc.) are selected, the manufacturer's instructions with respect to choice of primer and manner of application should be closely followed to achieve optimum results. The list of finish coat materials that may be

used over suitably primed metal is a long one

that includes many of the synthetic resins discussed in detail in chapter 3. Mention has

already been made, in connection with the discussion of coatings for steel, of oil-base and

oleoresinous coatings, vinyls, epoxies, polyamides, polyurethanes, polyesters, neoprene,

chlorinated rubber, cellulose esters, and other

film-forming resins. The reader is referred to the information given in section 4.1 of this chapter for guidance in a systematic basis for utiliz- ing the comprehensive information on synthetic

resins presented in chapter 3.

83
4.4. Coatings for Concrete and Masonry

The chief characteristics of concrete and

masonry surfaces that must be considered in

the formulation and selection of satisfactory

coating systems are their porous, often rough

texture and the alkaline nature of the portland

cement contained in concrete, stucco, asbestoscement shingle, and in the mortar used in brick*

and cinder block construction. Superimposed

on the inherent porosity of these materials is a surface topography which may vary from the

relative smoothness of cast concrete to the gross

irregularity of stucco. An additional characteristic that must be considered is the hydrophilic nature of concrete and masonry surfaces, as well as the fact that wherever portland

cement is present, some of the water originally

used to prepare it may be present in both hydrated and adsorbed form.

A coating that is to perform satisfactorily on

concrete and masonry surfaces must therefore

be sufficiently free-ftowing to follow and coat

the macroscopic roughness of the surface and

yet not penentrate deeply into the pores ; otherwise, the pores would soak up so much paint

that good hold-out of topcoats would be difficult and costly to obtain. Additionally, the coating

should be insensitive to the moisture and alkalinity likely to be present in such substrates,

particularly those which are unaged.

Before applying paints to concrete and masonry, the surface must be properly cleaned and,

preferably, in the case of open-textured surfaces

such as cinder block, filled with a suitable grout

coat. Surface preparation is discussed in sec- tion 6.4. A primer or primer-sealer is applied

to the prepared surface before application of

finish coats, except when cement-water paints

or whitewash are used. The primer is often

simply a thinned version of the finish paint. A

detailed discussion of primers for concrete and

masonry is given in section 2.4.3. Coating systems for concrete and masonry

may be a cement-water type, an oleoresinous

type, a latex-base (water-thinned) type, or a

synthetic resin type. These are discu.-^sed below

(for whitewash, see sec. 2.3.9.2).

4.4.1. Cement-Water Paints

Cement-water paints, the oldest type, are composed chiefly of portland cement with some

lime (see sec. 2.3.9.1), and are low in cost. When properly applied and damp-cured, they can provide a very durable finish that is perme-

* Painted brick surfaces can be durable only if the brick itself endures. In new construction, the proper grade of brick for the envi- ronment should be specified independently of subsequent painting

considerations. For exterior use, grade SW or MW as defined in ASTM Standard C 62-62 T should be used [160].

able to moisture but unaffected by dampness or

alkalinity. They may therefore be used above

grade outside, and either above or below grade

inside, on aged or unaged concrete and masonry

surfaces. Cement-water paints have good hiding and decorative qualities but become some- what translucent and darker in color when

wetted. Upon drying, the original opacity and

color are regained, but there may be a period

of mottled shading while the drying is taking

place. Cement-water paints will not adhere

well to previously painted or chalky surfaces

and will be weak, brittle, and prone to cracking

if not kept damp during cure, especially during

the first 48 hours after application.

4.4.2. Oleoresinous Paints

Oleoresinous paints, while attractive in appearance, are capable of satisfactory performance only when applied on dry, well-aged (8 to 12 months) concrete or masonry. Their limited

resistance and permeability to moisture does not

permit their use below grade or in other applications where moisture behind the film can

cause it to blister and flake. A suitable oleoresinous paint for application to thoroughly

aged, dry, exterior concrete surfaces is described

by Federal Specification TT-P-24; an aluminum-pigmented paint based on TT-V-81 mixing varnish also may be used. Oleoresinous

paints suitable as both primer and finish coat on aged, dry, interior concrete and masonry sur- faces are covered by Federal Specifications

TT-P-30 and TT-P-47 as well to TT-P-24.

Alternatively, for interior masonry, if the sur- face is sealed with an oil-base primer-sealer,

such as that described by Federal Specification

TT-P-56, and given a suitable enamel undercoat (such as TT-E-543), then almost any con- ventional oleoresinous interior paint may be

applied as the finish coat. (Additional information on oleoresinous paints is given in sec. 2.3.2.2.)

4.4.3. Latex Paints

Latex paints, because of their good durability

and relative insensitivity to dampness and alkalinity along with ease of application and

clean-up, have become the most widely used

coatings for both exterior and interior concrete

and masonry surfaces. (A comprehensive discussion of latex paints is given in sec. 2.3.5.)

All three main types of latex paint—acrylic,

polyvinyl acetate, and styrene-butadiene—may

be used on either exterior or interior concrete

or masonry; however, best results on exterior

exposure are obtained with the acrylic and the

84
polyvinyl acetate types, while the unmodified

styrene-butadiene type is generally restricted to interior use. The finish coat material, when

thinned, may serve also as the primer-sealer.

As with other masonry paints, it is important

that the surface be adequately prepared (clean,

chalk-free, and filled if necessary) before paint

is applied. Since the paint itself is water-based,

dampness of the surface is not a problem. The

good resistance of latex paints to alkalinity permits them to be used on unaged (but not on

fresh, uncured) concrete and masonry. Federal

Specifications for exterior latex paints include

TT-P-19 (acrylic), TT-P-55 (polyvinyl acetate), and TT-P-99 (styrene-butadiene). An

interior latex paint of unspecified resin type is covered by TT-P-29.

4.4.4. Synthetic Resin Coatings

Synthetic resin coatings, other than latex

paints, which are being used successfully for severe service on concrete and masonry include

epoxy resins, polyesters, polyurethanes, and

various synthetic rubbers. Filled epoxy resins

and filled or glass-fibre reinforced polyester

resins can be applied in thick mastic-like coats

to yield hard, glossy, tile-like coatings having

great mechanical strength, high chemical resist- ance, and great beauty. Polyurethane resins

produce tough, durable coatings for concrete

floors; they are also used in swimming pool

paints because of their good water and alkali

resistance. Of proven durability and widest use among swimming pool paints are those based on a chlorinated rubber vehicle; such a paint

is covered by Federal Specification TT-P-95.

Solution-type styrene-butadiene coatings also

are used for this purpose. A synthetic rubberbase paint for interior concrete floors exposed

to dampness is described by TT-P-91. Also

useful for chemical- and abrasion-resistant

service on concrete and masonry are coatings

based on synthetic rubbers such as neoprene,

chlorosulfonated polyethylene, and polysulfides.

Traffic paints for concrete (and bituminous)

surfaces on streets, highways, and airfields are formulated with a variety of quick-drying resins

and solvents, including short-oil alkyd and phenolic varnishes, styrenated-alkyds, chlorinated

rubber, and solution-type styrene-butadiene

resins, together with suitable pigments and

plasticizers to produce a tough, adherent,

weather-resistant coating. Federal Specifications for traflic paints include TT-P-85, TT-P87, and TT-P-115.

Multicolor coatings (see sec. 2.3.7) can provide an attractive and durable finish for concrete and masonry surfaces. They may be

formulated for either interior or exterior use. A multicolor lacquer based on an alkyd-modified

cellulose nitrate vehicle is covered by Federal

Specification TT-L-45; type I is for interior use and type II for exterior.

The suggested uses for the various synthetic

resins indicated above are not mutually exclusive ; many of the resins may be used with good

results for applications for which others have

been mentioned as outstanding. Reference

should be made to the detailed information given

for each resin in chapter 3 and to the guidelines given in section 4.1 of this chapter to provide a sound basis for the selection of an optimum coating material for the intended

application.

4.5. Coatings for Plaster and Wallboard

The key to successful painting of plaster and

wallboard is in the selection and proper application of a suitable primer-sealer (see sec. 2.4.4) . Primer-sealers are specially formulated

materials designed to properly seal the porous

surface, bond well to the topcoat, and provide

good topcoat hold-out. They are available either as oleoresinous or latex-base types. The oleoresinous types offer the good brushability, level- ing, and adhesion typical of oil-base paints, and

provide better wetting and adhesion on chalky

surfaces than is obtained with the latex types.

They should be applied only on dry, well-aged

(60 to 90 days) plaster. The latex-base primersealers are characterized by ease of application,

fast drying, and good water and alkali resist- ance. The latter permits their use, if necessary, even on damp or only briefly aged (but not

freshly applied) plaster, provided topcoating is deferred until the primed plaster has dried adequately. Whenever possible, new plaster should

be allowed to dry for at least two weeks before

applying even a primer-sealer. Latex-base

primer-sealers, such as that covered by Federal

Specification TT-P-650, are particularly wellsuited for application on gypsum wallboard

("sheet rock") because of their nonsensitivity

to the alkalinity in the plastered tape joints employed and because they do not raise the nap of

the outer paper layer as do oleoresinous types.

A phenolic emulsion-type dry wall primer also

is available for use on gypsum wallboard.

Composition-board (fiberboard) and other

non-alkaline wallboards (see sec. 6.5.2) may be

sealed with either the latex or oleoresinous type

of primer-sealer with good results. (Primersealers are also discussed in sec. 2.4.4.)

Either the latex-base or oleoresinous type of

primer-sealer may be over-coated with virtually

85
any conventional type of finish coat—latex, oil- base, or alkyd. A latex paint suitable for in- terior use both as primer-sealer and finish coat

is described by Federal Specification TT-P-29

;

"self-priming" is accomplished by thinning with

water. An oleoresinous primer-sealer is provided by TT-P-56, and an alkyd flat paint suitable for use both as primer-sealer and finish

coat is covered by TT-P-30. A wide variety of

oil-base and alkyd-base coatings suitable for

application on properly primed plaster and wallboard are covered by Federal specifications and

may be readily identified through the quick

guide list and the tabular summary presented

in chapter 8. In a three-coat oleoresinous system, the middle coat may be an enamel undercoater such as

that conforming to Federal Specification TTE-543 or TT-E-545 (odorless). A one-coat

oleoresinous finish is provided by TT-P-47;

however, such a coating should not be expected

to provide the appearance and durability obtainable with multiple-coat systems.

Multicolor finishes (see sec. 2.3.7) may be

applied on plaster and wallboard with attractive

results. They are particularly effective in

masking surface imperfections and obliterating

differences between dissimilar substrates. A

multicolor lacquer based on an alkyd-modified

cellulose nitrate vehicle is covered by Federal

Specification TT-L-45.

In short, in coating plaster and wallt)oard,

apply the right primer-sealer to the properly

aged and prepared substrate, and almost any

conventional type of finish coat may be applied over it without difficulty.

4.6. Functional Coatings

The preceding sections of this chapter have

dealt with the problem of selection of coating

systems chiefly from the standpoint of the type

of substrate involved and whether the service

environment was an exterior or interior one. In general, this is a practical and effective

approach to the problem of coatings selection.

There are times, however, when the importance

of one or more functional properties over- shadows ordinary environmental and substrate

considerations—when the problem of selection can best be narrowed by first examining those

materials which are capable of providing one

or more all-important functional properties.

This is particularly true when the functional

requirement is a very rigorous one. For example, if complete inertness to all known chemicals

at temperatures up to 500 °F (260 °C) is re- quired, then the quest for a suitable material can

at once be narrowed to polytetrafluoroethylene —provided that one knows that this material

possesses such resistance.

As part of the philosophy and systematic

approach to the problem of coatings selection

offered in section 4.1 of this chapter, it was suggested that when the requirements of the application are considered by one who has some

familiarity with organic coatings, several can- didate materials will often immediately suggest

themselves, thus providing a practical starting

point for a detailed check of properties among

a limited group of materials. For those who

may not yet have this familiarity, the information which follows should provide a useful step

toward acquiring it. Our treatment of this

aspect of coatings selection is not exhaustive

and is desirably limited to indicating only those

materials which are generally recognized as out- standing with respect to the particular property

under consideration. In this way, subjective

considerations are minimized and excessive

overlapping of candidate materials is avoided.

In making use of the information given, one

should guard against the temptation to "overselect"; that is, against choosing a premium

material where an ordinary coating would suffice. Over-selection unnecessarily restricts the

number of potentially suitable materials, thereby

lessening the opportunity for obtaining a coat- ing that meets the minor as well as the major

requirements of the application. The fact that a particular resin appears repeatedly in the

functional listings characterizes it as a material

of exceptionally good all-round properties but

does not necessarily make it the best material

for every job or even for most jobs. Often,

those materials which are outstanding in a number of important functional properties may have

disadvantages in other respects, such as high

cost, difficult application properties, limited pot

life, the need for baking or fusing, etc. An

objective appraisal of the properties needed for a given application, along the lines suggested

in section 4.1, is essential to ensure a satisfactory coating system at minimum cost. The ability to associate certain properties

with certain materials can be very helpful in

narrowing and solving the problem of coatings

selection. The following discussion seeks to provide this facility with respect' to a selected

group of functional properties.

4.6.1. Chemical Resistance

While some organic coating materials may

appropriately be called chemical-resistant be- cause of their relative inertness to a wide spectrum of corrosive agents, the term "chemicalresistant" is more meaningful and useful when

the types of chemicals to which the coating is

86
resistant are specified. For example, baked

(thermoset) phenolic resin coatings are generally considered to have excellent chemical

resistance, and they do—to most acids, fats,

oils, organic solvents, and water—but they may

be quickly destroyed by strong oxidizing acids or strong alkalis. Generally good chemical

resistance may be the rule for a particular

material; yet the exception can be costly and

dangerous.

The only organic material that is completely

inert to attack by all known chemicals (except

molten alkali metals, and fluorine at high pressures) is polytetrafluoroethylene. All others,

however resistant, have an "Achilles heel" of

which the potential user should be aware—by

conducting his own tests if necessary. Limitations of knowledge and space preclude a pinpointing of all such weaknesses here. However,

the considerable information offered will simplify and expedite the task of coatings selection.

4.6.1.1. Acid resistance.—Excellent resist- ance to dilute and concentrated acids (except

strong oxidizing types) is provided by coatings

of neoprene, polyvinyl chloride, polyvinylidene

chloride-vinyl chloride copolymer, polypropylene, polyethylene, and butadiene-acrylonitrile

copolymer (nitrile rubber) . Excellent resist- ance even to strong oxidizing acids is provided

by polytetrafluoroethylene, polychlorotrifluoroethylene, chlorosulfonated polyethylene [141],

isobutylene-isoprene copolymer (butyl rubber),

and chlorinated rubber [140]. Good resistance

to non-oxidizing acids is also afforded by polystyrene, furan resins, solution-type styrenebutadiene copolymers, silicones, cyclized rubber, polyesters, and acrylic resins; also, by

baked thermoset coatings of pure phenolic

resins, unmodified epoxy resins, and amino

resins.

4.6.1.2. Alkali resistance.—Excellent re- sistance to dilute and concentrated alkalis is provided by coatings of polytetrafluoroethylene,

polychlorotrifluoroethylene, polyethylene, polypropylene, neoprene, baked epoxies (unmodified), polyamides, butyl rubber (isobutyleneisoprene copoylmer), polyvinyl formal, polyvinyl chloride, vinyl chloride-vinylidene chloride

copolymer, and chlorinated rubber. Good re- sistance to alkalis is also afforded by polystyrene, furan resins, amino resins, ethyl cellulose,

chlorosulfonated polyethylene, styrene-butadi- ene resins, butadiene-acrylonitrile copolymer

(nitrile rubber), polyurethanes, acrylic resins,

cyclized rubber, and coumarone resins.

4.6.1.3. Oxidation resistance.—In addition

to the materials listed as resistant to strong

oxidizing acids (sec. 4.6.1.1), good resistance

to oxidation under conditions of exposure to sunlight and weathering is obtained with sili- cone resins, polysulfied rubber, cellulose acetate

butyrate, acrylic resins, polyurethane rubber,

polyacrylic ester rubber, polyvinyl acetate,

selected alkyds (e.g., melamine- or silicone-alkyds) , vinyl chloride-acetate resins (stabilized)

,

epoxy resins, cyclized rubber, furan resins, pure

phenolic resins, polyesters, and polyurethanes.

4.6.1.4. Resistance to organic solvents.—

Complete resistance to all known organic sol- vents is provided by polytetrafluoroethylene.

Excellent resistance to a wide spectrum of

organic solvents is obtained with water-soluble

resins such as polyvinyl alcohol and methyl

cellulose. Polyethylene is highly resistant to

all organic solvents except hot aromatic and

chlorinated hydrocarbons. Polypropylene is even more resistant than polyethylene. Excellent resistance to most organic solvents is provided by baked pure phenolic resins, baked

epoxies, and furan resins. Polychlorotrifluoroethylene is insoluble in all organic solvents at room temperature but is swelled by halogenated

solvents, toluene, diethyl ether, and ethyl acetate; at elevated temperatures it is swelled by

still other materials and will dissolve in a few

of them. Very good resistance to a wide variety of organic solvents can be obtained with

polysulfide resins. Good resistance to many

organic solvents is also obtainable with properly formulated polyurethanes, polyesters, vi- nylidene chloride copolymers with vinyl chloride or acrylonitrile, and epoxy-polyamide

resins.

Excellent resistance to aliphatic hydrocarbons, fats, oils, greases, and waxes is obtainable

with a wide variety of coating resins, notably

the fluorocarbons, polyethylene and polypropylene, neoprene and chlorosulfonated polyethylene, polyvinyl formal and polyvinyl alcohol,

cellulose esters, acrylic resins, and most thermoset resins such as epoxies, melamines, polyurethanes, polyesters, etc. The wide variations in solvent resistance that

may result from differences in resin composition and formulation, together with the wide

variety of organic solvents that are available,

make it necessary to interpret and use data on

solvent resistance with discretion backed up

by laboratory testing as may be required. It

is evident, nevertheless, that the guidelines provided above can minimize the work required

by permitting a judicious preliminary selection

of potentially suitable materials.

4.6.1.5. Water resistance.—Outstanding re- sistance to water and very low water absorption with excellent dimensional stability are provided by the fluorocarbon resins, polyethylene

and polypropylene, polyvinylidene chloride co- polymers (saran), polystyrene, chlorinated rub87
ber, and certain silicone resins. Good resist- ance to water is also provided by cellulose

nitrate, cellulose acetate-butyrate, ethyl cellulose, phenolic resins, vinyl chloride and vinyl

chloride-acetate resins, acrylic resins, epoxies

and polyamides, polyurethanes, and polyesters.

4.6.2. Heat Resistance

The need for heat resistance immediately

brings to mind polytetrafluoroethylene, silicone

resins, and polycarbonate resin as the outstanding heat-stable resins. Very good heat resist- ance compared to most resins is provided by

coatings of neoprene, polychlorotrifluoroethyl- ene , baked phenolic resins, chlorosulfonated

polyethylene, polyvinyl formal, cellulose acetate, ethyl cellulose, isobutylene-isoprene co- polymer (butyl rubber), polypropylene, amino

resins, furan resins, polyurethanes, epoxies,

and styrene-butadiene resins. Polymers w^ith

low softening temperatures but good resistance

to discoloration or degradation by heat within

their working range include acrylic resins, cel- lulose acetate butyrate, and polystyrene.

4.6.3. Abrasion and Mar Resistance

To be resistant to abrasion a coating material must either be tough and resilient enough

to "ride with the punch," or else hard enough

to be immune to the destructive stresses to which

it is subjected. The resilient type of abrasion

resistance is provided by many of the synthetic

rubbers (see sec. 3.15), notably neoprene,

chlorosulfonated polyethylene, and polyurethane rubber. Outstanding toughness and

abrasion resistance also is exhibited by polyvinyl formal and by polytetrafluoroethylene.

Hard coatings with excellent mar resistance

include baked epoxy resins, amino resins (especially melamines), polyurethanes, baked phenolic resins, and polycarbonate resin. Tough

abrasion-resistant coatings are also obtained

with polyvinyl chloride plastisols, polypropylene, polyethylene, polychlorotrifluoroethylene,

epoxy-polyamides, ethyl cellulose, and various

vinyl and styrene-butadiene resins.

4.6.4. Electrical Properties

Excellent electrical properties, including high

dielectric strength and arc resistance and

low dissipation factor are exhibited by polytetrafluoroethylene, polyethylene, polypropylene, polystyrene, polyvinyl formal, and polyvinyl chloride. Also very useful for electrical

applications are neoprene, polychlorotrifluoroethylene, silicone resins, epoxy resins, polyamides, ethyl cellulose, isobutylene-isoprene rubber (butyl rubber), polysulfide rubber, and

polycarbonate resin.

4.6.5. Impermeability

Outstanding impermeability to gases and

vapors is exhibited by vinyl chloride-vinylidene chloride copolymers, polytetrafluoroethylene, isobutylene-isoprene copolymer (butyl

rubber), polyvinyl alcohol, polypropylene,

chlorinated rubber, polyethylene, polysulfide

rubber, and polyacrylic ester rubber.

4.6.6. Transparency

Outstanding clarity, transparency, and nonyellowing properties are exhibited by acrylic

resins, polystyrene, cellulose acetate butyrate,

polyvinyl butyral, polyesters, and polycarbonate resin.

4.6.7. Fire Resistance

Among nonflammable resins with self-extinguishing properties are the fluorocarbons,

chlorinated rubber, polycarbonate resin, polyvinyl chloride, polyvinyl chloride-acetate, and

certain silicone resins.

4.6.8. Radiation Resistance

A definitive statement on the high energy

radiation resistance of organic coating materials is difficult because of the limited, often

empirical, and sometimes contradictory nature

of the available experimental and performance

data. Moreover, most of the information con- cerning organic coatings must be extrapolated

or inferred from data obtained on much thicker

sheet or bulk plastics. To the extent that oxygen (and perhaps other environmental agents)

may affect the behavior of the material under

irradiation, such extrapolation may be misleading because the thickness of the material, to- gether with its permeability, determines the

quantity of oxygen that can diffuse into it and

be available for reaction where radiation ef- fects are being induced below the surface.

Thus, a thin film (i.e., a coating) of an oxygensensitive polymer—even a relatively impermeable one—may be quickly destroyed by irradiation because of diffusion of oxygen throughout

the material whereas a thick sheet or block of

the same material may suffer only surface

degradation. Nevertheless, a few basic facts

about the effects of high energy radiation on

polymeric materials can be stated here, and

certain polymers may be broadly classified on

the basis of their resistance to such radiation.

For detailed information, the reader is re- ferred to the literature [155, 156].

It is known that high-energy ionizing radiation (x-ray, y-ray, /3-particles, etc.) induces

either crosslinking, chain scission, chain stripping (removal of chain substituents), or a

88
combination of these processes in high polymeric materials into which the radiation is absorbed. These effects occur regardless of the source of the high energy radiation—whether

generated by high voltage electrical machines

such as a Van de Graaff generator or a linear

accelerator, or emanating from radioactive iso- topes such as cobalt-60 obtained by exposing

the metal to pile radiation in a nuclear reactor, or from the atomic pile itself. Nor are the

effects significantly different whether the high

energy radiation is electromagnetic (y-rays) in

nature or in the form of high speed electrons

(/3-particles) . Crosslinking, as explained in

section 3.1.1, is a basically favorable process

which may improve many of the physical and

chemical properties of the polymer, such as imparting greater hardness, better solvent re- sistance, and a higher softening point. Eventually, however, under prolonged irradiation even the most resistant polymers finally embrittle and deteriorate. Chain scission or stripping, since it degrades the polymer, is detrimental from the start. Whether a given

polymer is improved or degraded by moderate

doses of high energy radiation depends on whether the cross-linking or the scission (and/

or stripping) reaction predominates, as determined by the chemical structure of the

polymer; that is, by the nature and bond

strengths of the main chain and side groups,

steric factors, and the shielding effect of substituent groups. For example, it has been

found that pendant benzene rings, perhaps be- cause of their efficient dissipation of energy,

exert a shielding effect that protects the main

chain from scission and promotes crosslinking

rather than degradation. Such is the case in

polystyrene which exhibits the greatest resist- ance to high energy radiation of any of the

common plastics. On the other hand, polyvinyl chloride is quickly degraded even by low

doses of high energy radiation because of its high susceptibility to chain stripping.

The radiation resistance of high polymers is of interest both from the point of view of

obtaining improved (crosslinked) materials

through manmade irradiation and from the

standpoint of the use of polymeric materials

in environments where they must withstand

exposure to moderate amounts of radiation, as

in a space capsule or satellite. From both

points of view, crosslinking rather than chain

scission is the desirable effect. Polymers that

tend primarily to crosslink under high energy

radiation and may therefore be classed as relatively radiation-resistant include polystyrene,

polyethylene, polypropylene, linear polyamides

(nylon resins), polyesters, and silicones (particularly phenyl substituted types). Polymers

that may be considered as moderately resistant to radiation include styrene-acrylonitrile

and styrene-butadiene-acrylonitrile copolymers, polyacrylic acid and esters, polyacrylamide, polyurethanes, diallyl phthalates,

polychloroprene (neoprene) , chlorosulfonated

polyethylene, and SBR rubber (formerly called

GR-S) . Natural rubber, too, can be vulcanized

(crosslinked) with high energy radiation. In the case of all the polymers named above, it should be remembered that in the presence of

unlimited amounts of oxygen the crosslinking

reaction may be offset or even outweighed by

increased chain scission.

Finally, it may be helpful to point out those

classes of polymers that are known to undergo

chain scission and/or chain stripping rather

than crosslinking when exposed to high energy

radiation and that consequently soon deterio- rate under irradiation. Such "negative" in- formation may be particularly useful when

applied to polymers that are noted for their

outstanding chemical inertness and might

therefore have been expected to be radiationresistant also. The fluorocarbon resins, in- cluding polytetrafluoroethylene, polychlorotrifluoroethylene, and polyvinyl fluoride, all are degraded by high energy radiation, especially

in the presence of oxygen. Polymethyl methacrylate and cellulose acetate butyrate, though

noted for their excellent resistance to weathering and ultraviolet radiation, suffer severe degradation when exposed to high energy

radiation. Similarly, polyisobutylene, despite

its chemical inertness, undergoes rapid chain

scission under high energy irradiation, even

in the absence of oxygen. Other common poly- mers that have shown [157] poor resistance

to high energy radiation include polyvinyl

chloride homopolymer and copolymers, polyvinylidene chloride, styrene-butadiene copolymer, polyethylene terephthalate, polycarbonate

resin, cellulose esters and ethers, and amino

resins and phenolic resins unless mineral filled. Mineral filled epoxy resins also have withstood high energy radiation fairly well. It is apparent that reinforcement with inorganic

fillers may permit the use of some polymers

under radiation conditions that would quickly

destroy the unmodified polymer. It is possible

also that even the less radiation-resistant materials may be suitable in certain applications

where the degradation suffered is not critical

(for example, when irradiation results in the

loss of elongation or tensile strength in an

application where neither property is important because the material needs only to remain

in place without physical strain).

89
4.7. Summary

Briefly, this chapter has indicated both a philosophy and a systematic approach to the

problem of coatings selection, together with

specific recommendations regarding coating

systems for various substrates and environments. The guidelines provided, in combination with the detailed information on the properties of coating materials presented in chapter

3, should furnish a sound basis for the selection of a suitable material for a given application in the great majority of cases. As an

added feature, to provide the potential user with a facility for associating certain resins

with certain functional properties, section 4.6

has presented a listing of typical coating properties and the resins which are outstanding in

providing these properties.

Figure 4.1. Xenon-arc accelerated weathering machine.

Operating conditions are described in ASTM Method E

240-64T.

Figure 4.2. Interior of carbon-arc accelerated weathing machine.

Principles and operating conditions are given in ASTM Method E 188-63T.

90
5. Storage, Safety, and Application Methods

The proper storage and handling of organic

coating materials and the usual methods for

applying them are adequately discussed in a number of Departmental and Bureau paint

manuals (see references [40] and [41], and

General Bibliography in chapter 9). It is not a purpose of this publication to duplicate these

detailed treatments of the subject. It is desirable, however, within the comprehensive yet

concise scope sought in this volume, to point

out the basic precautions that should always

5.1. Storage of

While different coating materials vary greatly

in their storage stability—some remaining

useable for years while others deteriorate in a few months—all organic coating materials

should be stored in tightly closed containers in a covered, well-ventilated area where they will

not be exposed to excessive heat, fumes, sparks,

flame, or direct sunlight. Heat accelerates the

thickening or hardening of reactive vehicles

during storage and increases the fire hazard

normally present with coating materials con- taining volatile, flammable solvents. Freezing

may break the emulsion and destroy the use- fulness of latex paints, although some may

withstand a few freeze-thaw cycles; waterbase coatings should be protected against freezing. Low temperatures down to 0 °F (—18 °C)

thicken but do not ordinarily harm solvent-base

coatings; however, the material must be

warmed before use to restore a normal viscosity for proper application.

Pigmented materials tend to settle in the

container and may form a cake that is difficult to redisperse. Red lead paints are notably

5.2. Safety

The combustible and toxic nature of most

organic solvent-base coating materials should

be kept in mind whenever such materials are

stored, handled, or applied. Containers should

be opened carefully in a direction away from

oneself or others, since volatiles or unexpected

gaseous reaction products may sometimes build

up pressure inside the container. Open flames

should be kept away from the area where the

paint is being used, and sparks should be

avoided. Adequate ventilation should be provided indoors to prevent the build-up of toxic or explosive concentrations of solvent vapors.

Special precautions are necessary when spraying, whether indoors or outside, to avoid in- halation of toxic fumes and dusts.

5.2.1. Toxicity

Because of its importance to the health of

the worker and to those responsible for his

be observed in dealing with potentially flammable, toxic, and sometimes "perishable" materials such as paints and other organic

coatings. It is also appropriate to present

sufficient information about the various methods of applying coatings—particularly the less commonly used or newer methods such as

fluidized bed and airless spray—to enable the

reader to understand the principle of operation and the chief advantages and disadvantages of each method.

Coating Materials

troublesome in this respect. Such difficulties can be minimized by inverting containers at

intervals of a month or two.

Maintenance of proper identification of paints

in storage will save time and avoid costly

mistakes. Containers should be relabeled

when necessary to ensure legibility. Previously opened containers, in particular, should

be checked for tight closure and legibility of

identification.

Paint solvents and thinners should be stored

with the same or greater care than the paints

themselves.

Application equipment should be cleaned

promptly and thoroughly with a suitable sol- vent after each use, and stored in a clean place.

Brushes that receive daily use may be kept in

good condition by suspending them in a suitable

solvent between periods of use; however, this

practice may create a fire hazard when volatile

solvents are employed unless care is taken to keep the containers well-covered with foil or

film or stored in a special cabinet.

Precautions

safety, the subject of toxicity is discussed here

in somewhat greater detail than is found in

most paint manuals; however, the treatment

is necessarily limited. Detailed authoritative

information on the toxicity of specific solvents,

pigments, resins, and other constituents of or- ganic coatings is available from the U. S. Public

Health Service (Department of Health, Education, and Welfare, Washington, D.C.), in the

Chemical Safety Data Sheets issued by the

Manufacturing Chemists Association (Washington, D.C.), and in a number of comprehensive reference works [69, 70], Government

and industrial pamphlets [71, 72], and manufacturers' technical bulletins.

Even common solvents that are ordinarily

considered innocuous may be harmful if inhaled

in high concentrations or for prolonged periods.

For example, turpentine can cause narcotic

poisoning and kidney damage when excessive

amounts are inhaled. Systemic damage may

91
also result from an acute dose (sufficient to produce unconsciousness), or from repeated

excessive inhalation of such common solvents as the acetate esters (methyl-, ethyl-, butyl-,

and amyl acetate), amyl alcohol, methyl iso- butyl ketone, and possibly methyl ethyl ketone.

Solvents such as toluene, xylene, ethyl, ether,

cyclohexanone, butyl alcohol, and the nitroparaffins have a relatively low chronic toxicity,

but their narcotic effect in high concentrations can lead to unconsciousness followed by respiratory failure and death unless the victim is promptly restored to fresh air. Even the least

toxic of the common paint and lacquer sol- vents—including mineral spirits, petroleum

naphtha, gasoline, acetone, isopropyl alcohol,

and ethyl alcohol—although well-tolerated in

relatively large concentrations and having little or no chronic ill effects, can be dangerous in

very high concentrations because of their narcotic effect, excepting the alcohols which are

irritants (to the eyes and upper respiratory

tract) rather than toxicants.

Some common solvents—usually not used

directly in paints but often encountered in

paint removers and cleaning liquids—are dangerously toxic even at relatively low concentrations; for example, methyl alcohol when in- haled or ingested repeatedly or to the point

of unconsciousness may cause death or permanent blindness. Benzene is a dangerous, cumulative poison that causes severe damage to the

liver and blood-forming tissues of the body.

Many chlorinated solvents, particularly chlorinated aliphatic hydrocarbons, are toxic;

chloroform, for example, has poisonous aftereffects, while carbon tetrachloride and nowseldom-used tetrachloroethane are dangerously

toxic with severe chronic, cumulative effects on the kidneys, liver, and lungs. Since solventtype paint removers frequently contain highly

toxic solvents, they should be used only with

adequate ventilation or a respirator.

The toxic systemic effects of organic solvents can be produced by absorption through the

skin as well as by inhalation. Acute or chronic

dermatitis or systemic poisoning may result

from repeated or prolonged contact of toxic

solvents with the skin. Sometimes, the vehicle

resin itself may be irritating or toxic to the

skin while the resin is in the uncured state. The catalysts or hardening agents (e.g., organic

amines) used in two-component coating sys- tems are frequently volatile and toxic materials

until they have been incorporated into the

resin.

Some of the pigments used in organic coat- ing materials are toxic. Lead compounds and

chromates are particularly dangerous, whether

ingested or inhaled. Lead fumes generated

while burning off old paint are very poisonous.

Inhalation of the dust and debris from sanding

and brushing operations is a health hazard.

Sandblasting, in particular, requires a suitable

mask, goggles, and protective clothing. Some

paint additives, such as the mercurial com- pounds used to impart fungicidal properties,

may be toxic if ingested.

The toxicity hazards associated with organic

coating materials have been pointed out not

to discourage their use or arouse fear but to emphasize the importance of adequate safety measures to prevent accident and injury. With

adequate precautions based on knov/ledge of

the properties of these materials, they may

be handled confidently and safely. The basic

precautions to be observed are simple and

practical

:

(1) Avoid unnecessary or prolonged in- halation of fumes and dusts; always provide

adequate ventilation, or use a suitable mask

or respirator (remember that gas mask canisters will absorb only limited amounts of

noxious vapor and must be renewed at frequent

intervals)

.

(2) Avoid unnecessary or prolonged contact with the skin; the use of protective clothing

and skin creams will eliminate the need for

scrubbing the skin with toxic solvents.

(3) Avoid ingestion of toxic materials;

wash hands carefully before eating or smoking;

change clothing that has soaked up toxic

materials.

(4) Know the material with which you

are working; read and heed the precautions on

the label—it will tell you when an unusual

hazard exists and how to avoid it. (5) Make safety "second nature"; don't sneer at safety regulations or at the fellow

who observes them; be alert to possible hazards, avoid them, and stay healthy.

Water-base paints are free from the toxicity

hazards associated with organic solvents and

are generally based on non-toxic resins. They

may contain pigments or additives, however,

which may be harmful, so that ingestion should

be avoided. The manufacturer's label is a

useful guide in such matters.

5.2.2. Fire

Most organic coating materials are combustible (water-base coatings excepted), primarily because of the flammable solvents and

thinners which they contain. They are safe

while stored in tightly closed containers away

from heat and flame. When containers are opened and while the coating is being applied,

open flames and heated objects (lighted cigarettes, for example) should be kept out of the

area. When applying organic-solvent base coat- ings indoors, adequate ventilation should be

provided to prevent the build-up of explosive

concentrations of vapor. Nevertheless, flame

and spark should be avoided. Smoking should

92
be prohibited. Blow-torches should not be

used even in other parts of the building into

which solvent vapors may diffuse. Care should

be taken to extinguish all pilot flames on heaters and stoves. Sparking motors should not

be in operation. Even an electric light switch

or the spark from a metal tool can be a source

of danger if explosive concentrations of vapor

are present—hence the need for adequate

ventilation.

Rags used to wipe up linseed oil or other

drying oils or oxidizable liquids are a frequent

source of spontaneous combustion when left lying around. They should be disposed of by

controlled burning, or by drenching them with

water and putting them in a covered metal

container.

iVhen burning old paint off wood, the torch

must be handled with care to avoid setting fire to the substrate. A water hose should be kept

ready for use if needed.

Paint and solvent fires usually cannot be

fought with water. Water is immiscible with

most paint materials; where large quantities

of flammable liquids are involved the water

will only further spread the flame because the

lighter liquids float on the water surface.

Such fires are most effectively fought with

carbon dioxide, with foam, or with dry-chemical

extinguishers, which smother them. This

should be remembered if there is ever occasion

to use a hand fire extinguisher on burning

solvents while waiting for the fire department

to arrive. It should be remembered also that

fire in a paint storage area may be quickly

followed by explosion as containers become

over-heated, and that many burning paints and

solvents give off toxic fumes. Chlorinated

hydrocarbons, for example, while usually non- flammable or less flammable than most other

solvents, give off deadly fumes when heated

to decomposition. While it may be worthwhile

to fight a small incipient fire with a hand ex- tinguisher while awaiting the fire department,

it should be realized that a small fire can develop into a very large fire in a few seconds,

with consequent risk to life.

5.2.3. Falls and Spillage

A large proportion of the injuries suffered

by painters is the result of falls from ladders

or scaffolds, frequently involving paint spillage as an accompanying hazard. Such accidents are usually the result of carelessness in the

rigging, maintenance, or inspection of scaffolds

and accessory equipment, or in the placement of

ladders. Even a short fall can be painful and

dangerous. One should be certain that ladders,

scaffolds, ropes, and moorings are in proper

condition, and that mooring hooks are firmly

attached at a location amply strong enough to support the weight. Although these precautions appear self-evident, accidents continue to occur because they are ignored or overlooked.

In the event of a major spillage of volatile

flammable liquid, don't wait for a fire—call your disaster control unit or the fire department immediately.

5.3. Mixing, Thinning, and Activation

Most of the coatings in use today are readymixed materials—ready for use as supplied by

the manufacturer except for the need for:

(1) stirring or mixing pigmented materials to uniformly redisperse the solids in the vehicle,

(2) appropriate thinning when necessary for

priming purposes or spray or other application,

and (3) proper activation in the case of twocomponent reactive systems.

In a few instances, the pigment and vehicle

of a pigmented coating system are furnished

in separate containers for mixing just prior to use in order to avoid undesirable settling,

caking, flocculation, or deteriorative changes

during storage. For example, aluminum paints

[73] are sometimes prepared by mixing the

aluminum paste or powder into the appropriate vehicle just before the paint is applied.

This procedure is designed to overcome loss

of leafing properties of the aluminum pigment

and gassing in the container during storage;

both of these undesirable characteristics have

been encountered with improperly formulated

aluminum paints. Gassing (hydrogen formation) may result in the blowing off of friction

top lids from cans with the creation of a fire hazard. Zinc dust paints also are supplied

with pigment and vehicle in separate packages

because of the tendency toward gassing when

mixed with the vehicle and stored.

Before mixing or thinning paint for use, it should be brought, if necessary, to the proper

application temperature. Cold paint is most

conveniently and safely warmed by letting it stand in a warm room. Where more rapid

heating is required, a hot water bath or electric

blanket may be employed, but the warming

jacket should not be above 90 °F (32 °C).

Organic coating materials should never be

heated over an open flame because of the great

danger of localized overheating, decomposition,

explosion, and fire.

5.3.1. Mixing of Paints

Ready-mixed paints are usually furnished in cans of five gallons or less, to permit convenient mixing and to avoid the deterioration that

may occur in large containers or drums which

are used up more slowly and acquire a large

93
air space as the contents dwindle. Thorough

mixing of pigmented coating materials just

before use is necessary to redisperse settled or caked pigment and ensure a smooth, uniform

mixture that incorporates all the original pigment into the vehicle. Hand-mixing methods,

power stirring, or automatic shaking or vibration may be employed.

Automatic shaking, where available, is con- venient and rapid; usually, from three to fif- teen minutes is required. The violent agitation, however, produces many fine bubbles in

the paint which may be objectionable, particularly in enamels. It may be necessary to let the paint stand for some time to let the bubbles

escape and then give it a final mixing by

hand before using it. Final hand-mixing is desirable in any event to ascertain that no undispersed material remains on the bottom or

sides of the container.

Ready-mixed paint in the usual size containers (up to five gallons) may be hand-mixed

conveniently and effectively in the following

manner: Pour off the thin supernatant material into a clean container; if only light set- tling has occurred, pour off about one-third to one-half of the paint. Stir the remaining portion with a paddle or power stirrer until a uniform consistency is obtained, making sure that

all material settled on the bottom or sides of

the container is re-incorporated. If necessary,

stir the poured-off portion also. Slowly, with

continual stirring, return the poured-off portion to the original container. Finally, "box"

the paint by pouring it back and forth between

the two containers several times. While the

paint is being used, it should be stirred at frequent intervals to maintain a uniform mixture

and to prevent skinning.

When a previously opened container is used

again, the same mixing procedure indicated

above is used, after first carefully removing

any skin that may have formed. If pieces of

the skinned material fall into the paint, or if a few small clumps of hard-to-disperse material

are present in the paint, these can be removed

by straining through clean cheesecloth or fine

wire screen. Paint containing a large number

of such clumps, or a considerable quantity of

settled, caked material that cannot be redispersed, is not suitable for use and should be

discarded.

When tinting is to be done, the base paint

should be thoroughly mixed before adding the

tinting liquid. The latter should be added

slowly with continual stirring, and the mixing should be continued until a completely uniform distribution of the tinting color is obtained.

Clear coatings, such as varnishes or clear

lacquers, do not ordinarily require stirring.

However, if the material near the bottom of

the can is thicker than the rest, it should be

stirred gently to restore a uniform mixture.

Vigorous stirring, particularly shaking, should

be avoided, since it introduces air bubbles that

will impair the appearance and serviceability

of the deposited film. If air bubbles are in- advertently introduced, the coating material

should be allowed to stand before use until the

bubbles have escaped.

5.3.2. Use of Thinners

The most common reason for thinning paints

and other organic coating materials is to re- duce the viscosity—either for the purpose of

rendering the material suitable for use as a priming coat, or to render it suitable for application by a means different from that for

which it was initially formulated. For ex- ample, oil-base paints are usually suitable for

brush application as furnished; however, they

may be thinned further (self-primed) for use

as the first coat on bare wood—or they may be

thinned to permit their application by spraying.

Lacquers, on the other hand, are generally furnished in a viscosity ready for spray application, since this is the preferred method of application. Brushing lacquers also are available

but they must be applied with awareness of

their quick-setting characteristics which limit

their brushability to only three or four strokes*

immediately after application.

Quick-drying coating materials such as lac- quers, which undergo considerable cooling due

to the rapid evaporation of solvent from the

applied film, may require the addition of some

blush-retardant thinner when applied under

adverse conditions of high humidity. "Blush"

is a whitish discoloration of the film caused by

the condensing of moisture from the air into

the drying, cooling film. The visual appearance

is the result of: (1) the presence of fine droplets of water incompatible with the organic

solvents in the coating, and (2) the actual precipitation, in very fine form, of some of the

film-forming material by the nonsolvent water.

Although the deposited water will eventually

evaporate, the precipitated film material will

remain as a discoloration and discontinuity

which impairs the usefulness of the coating.

Blush can be avoided if sufficient high-boiling

solvent is present in the final stages of the drying process to redissolve the precipitated material. Blush-retardant thinners contain a high

proportion of high-boiling active solvent which

not only lessens the blushing tendency by reducing the drying rate and the resultant cooling of

the applied film but also provides additional active solvent for redissolving any precipitated

film material during the final stages of drying.

Blush retarders must be used sparingly and

with discretion because too much can upset the

original solvent balance of the formulation and

94
create application difficulties (e.g., running,

sagging, prolonged tackiness) while also affecting, sometimes adversely, the structure and

properties of the film that is laid down.

Yet another reason for thinning an organic

coating material is to replenish solvent lost through evaporation from the container while

the material is in use or between usage. Normally, such adjustment is best made with a thinner the composition of which is essentially the same as that employed in the original

solvent formulation. It should be recognized,

however, that in the case of a composite thinner

covering a range of boiling points, the bulk

of the solvent loss will be in the lower-boiling

constituents. Therefore, if the solvent loss has

been excessive—as indicated by a pronounced

increase in the viscosity of the paint in the

absence of chemical reactivity—it is advisable

that the restoring thinner be richer than nor- mal in low boiling constituents. The problem

of proper thinning is most difficult with criti- cally balanced complex solvent formulations

such as those used in lacquers and dopes; it is least critical with paints that utilize single

solvents or simple blends of similar solvents,

such as the turpentine and/or mineral spirits

which constitute the volatile portion of oil- base or oleoresinous paints.

5.3.2.1, Principles of proper thinning.—

Whether the purpose of thinning be to reduce

viscosity, retard blush, replenish lost solvent, or some other function, certain basic principles

should be kept in mind to ensure satisfactory results. Every organic coating material is a specially formulated material designed for a particular type or area of service. The volatile

portion of the vehicle has a major role in

determining the solids content and application

viscosity of the material, its drying rate, the

structure and adhesion of the deposited film,

and the resultant effects on appearance, mechanical properties, and durability. When the

original material must be thinned, for whatever

reason, the essential solvent balance must be

maintained in order to obtain the desired film

properties for which it was designed.

The principles and importance of proper sol- vent balance in determining the properties and

performance of the coating may be seen from

consideration of a typical lacquer formulation

(the general characteristics of a lacquer are

discussed in section 2.2.4). The nonvolatile

portion of a lacquer typically may consist of

the film-forming base, a modifying resin, a

plasticizer, and possibly a stabilizer or other

additive—plus suitable pigments where color or opacity are desired. The volatile vehicle is a mixture of active solvents (including latent

solvents) and diluents, in a range of boiling

points and associated evaporation rates. The

active solvents are needed to dissolve the filmformer and produce a solution of high solids

content at the proper application viscosity.

The diluents, although nonsolvents, are welltolerated and serve to extend the solvents for

economy while contributing to the adjustment

of viscosity, drying rate, and gel properties.

The amount of volatile liquid contained at the

gel stage is an important factor in determining

the structure of the film that is laid down as the residual liquid evaporates. In general, the

low-boiling constituents of the volatile vehicle

contribute solvent power and fast setting, the

medium-boiling components control the drying

rate to allow good leveling, the diluents (usually

medium-boiling liquids) regulate the on-set of

the gel stage, and the presence of some mediumto high-boiling active solvent imparts blush

resistance and ensures solubility of the filmformer in the final stages of the drying. To

meet the concurrent and often" conflicting re- quirements of (1) high solids content for good

film build per coat at a workable viscosity, (2)

good flow and leveling without runs or sags, and (3) fast drying without the development

of film faults such as blush or cratering, it is essential to employ a suitably formulated mixture of active solvents and diluents carefully

balanced for proper solvent power, evaporation

rate, and gel stage composition.

To ensure that the performance characteristics so painstakingly built into the original

formulation will not be nullified by improper

thinning, the following principles and precautions should be observed

:

(1) Use thinners only when specified or necessary, and in the minimum amount that will

do the job. Even an ideal thinner may adversely affect the application properties,

spreading rate, and ultimate durability of the

coating if used where it is not needed.

(2) Whenever possible, use only that thin- ner or type of thinner which is specifically

recommended by the manufacturer of the coat- ing material.

(3) When the manufacturer's thinner, or a known substitute, is not available, make sure

that the thinner to be used is completely compatible by adding it to a test quantity of the

original coating material and noting if any

flocculation, separation, or other abnormality

occurs in the thinned paint and in a film prepared from it. (4) In choosing or preparing one's own

thinner, use a composition approximating that

of the volatile vehicle of the original coating

material. Compositional information, if not

available from the label, may usually be obtained from the covering specification. Federal specification thinners for various types of

coatings are listed in section 8.1.9. (5) If the composition of the volatile vehicle

is not known and its nature cannot be inferred

from its odor, prepare a thinner on the basis

95
of known solvents and nonsolvents for the

film-forming resin, keeping in mind the formulating principles previously discussed. (Information on the solubility of resins is available in chapter 3), If the identity of the resin

base also is not known, then trial and error

testing of small batches with various classes

of solvents may reveal a compatible solvent for

emergency use. (6) Before thinning, thoroughly mix the

original coating material to a uniform consistency as described earlier, add the necessary

amount of thinner, and again stir to a uniform

consistency.

(7) When using thinners, observe all due

precautions against fire and toxicity hazards as described in section 5.2.

5.3.3. Activation of Two-Package Systems

Two-package coating systems are those in

which the film-forming resin and the curing or hardening agent are furnished in separate con- tainers for mixing just prior to use. Typical

two-package systems include amine-cured

epoxy resins, styrene-plus-peroxide-cured polyesters, amine- or polyol-cured polyurethanes,

and metallic oxide-cured neoprenes and chlorosulfonated polyethylenes. Of these, the epoxies,

polyesters, and polyurethanes cure at room

temperature whereas the last two are usually

heated after the catalyst or accelerator has been

added.

Activation of two-package systems poses no

special problems if the following precautions

are taken

:

(1) Many of the catalysts or curing agents

employed are toxic prior to incorporation into

the resin; in such cases, inhalation, skin contact, or ingestion should be avoided.

(2) It is important that the two components

be uniformly mixed in the proportions recom- mended by the manufacturer to achieve optimum film properties.

(3) The activated mixture has a limited pot

life—usually from one to eight hours, depending on the amount of catalyst used, the

temperature, and the nature of the materials

involved. Follow the manufacturer's instructions.

(4) The curing reaction frequently is ex- othermic; hence the coating may seem quite

thin or be too thin for some applications shortly

after being activated, because of the viscosity

reduction resulting both from the addition of

liquid reagent and from the temperature rise. In the case of epoxy resins, for example, an

induction period of about 45 minutes after addition of the catalyst is recommended before

using the coating material.

(5) In some instances, the thickness of the

deposited film has a marked effect on the rate and extent of the cure achieved. Polyester coatings, for example, may not cure satisfactorily

in thicknesses of less than six mils (0.006

inch) because of excessive loss of volatile catalyst, air inhibition, and reduced reaction temperature because of loss of the heat generated

by the exothermic reaction.

(6) As a rule, follow the manufacturer's

mixing and application instructions and take

the recommended precautions.

5.4. Application Methods and Equipment

The choice of an appropriate application

method from among the many available for

applying organic coatings depends on a variety

of factors, including the nature of the coating

material, the type and texture of the surface on which it is to be applied, the shape and size of

the object or area to be coated, environmental

considerations, the available eauipment and facilities, the skills available, fire and toxicity

hazards, the time factor, and—last but not

least—labor costs. Obviously, the choice of method also depends

upon an awareness of what methods are available and upon a knowledge of the technique

involved and the advantages afforded by each.

Detailed information about the commonly used

methods of brushing, spraying, dipping, and

spreading is readily available in many published books, manuals, pamphlets, and specifications [81, 96, 122, 1541. Similarly, detailed

instructions for use of the newer or less wellknown methods, such as airless spray and fluid- ized bed, are provided by the manufacturers or

suppliers of such equipment. Therefore, in the

interests of adequate yet concise coverage, the

objective in the sections which follow is to pre- sent the essential details of each method and

to provide a sound basis for the selection of an

appropriate application method. Toward this

end, there is given for each method a description of the principle involved, the basic techniques and tools or equipment required, the

main advantages afforded, and the chief limitations.

In general, application in the field is limited

to brushing, rolling, and conventional cold or hot spraying methods, while factory coating

operations are generally accomplished with more elaborate types of spraying (electrostatic, airless, etc.) or by special techniques

such as fluidized bed.

5.4.1. Brush Application

Applying paint by brush is an ancient technique that still is used for a substantial portion

96
of the painting done in the field. While con- siderably slower than spraying, it gives excellent results when done with skill using the

proper brush and technique. Brushing works

the paint into pores and crevices in a way that

spraying does not accomplish. At the same

time, the brushing works surface dust and

foreign matter into the paint, in effect cleaning away surface contaminants and thereby

promoting good adhesion. For this reason, brushing is generally the preferred method of

applying primers, especially on rough or imperfectly cleaned surfaces.

A high-quality brush of the proper size and

shape is essential for good work. In the long

run, the best brush—though initially expensive—is the most economical. The best brushes

of today are made of specially selected Chinese

hog bristles, sorted and bundled by skilled

craftsmen to produce a quality tool. The bristles

of such brushes have split or flag ends which

are indispensable for holding an adequate quantity of paint and distributing it smoothly over

the surface; the bristles continue to split as they wear down, thus maintaining their effi- ciency. Good nylon brushes with bristles having artificially flagged ends are also available;

while they do not have the lasting paint-holding

and brushing quality of the best bristle brushes,

they will outwear bristle about four-to-one and

are particularly useful for painting rough sur- faces such as brick and cinder block. Nylon

brushes are useful also with water-thinned

paints because they are unaffected by water

which tends to make bristle brushes soggy and

less workable ; however, they are not suitable for

applying lacquers because the synthetic bristles

may be attacked by the strong solvents used

in these formulations.

A high-quality paint brush is an expensive

tool that deserves proper care to maintain its usefulness. A new brush should be broken in

before use by shaking and combing it free of

loose hairs and dust, soaking it in linseed oil for a day or two, and finally rinsing it in paint

thinner. The brush should be suspended so that

the bristles do not rest on the bottom of the

container during soaking. When painting, the

brush should not be used sideways or forced

into corners or tight places; such abuse will

ruin its shape and destroy its efficiency.

Brushes are available in a wide variety of sizes

and shapes and textures for almost any paint

job. After each use, brushes should be cleaned

thoroughly with a suitable thinner and stored

in a clean place ; if stored in oil or thinner between usage, the brush should be suspended as mentioned above (also see section 5.1).

Brush application of paint is a skill that must

be acquired by practice and experience ; nevertheless, certain basic principles should be fol- lowed. The paint should be brushed on in one

direction, then cross-brushed. Oil-base paints

should be brushed out thoroughly. Quick-drying paints should be brushed only enough to spread them evenly. Very quick-setting coat- ings such as lacquers must be applied quickly

in only two or three strokes to avoid disturbing

the gelled film—for this reason, lacquers are

usually applied by spraying or dipping rather

than by brushing. Near the end of each stroke,

the brush should be gradually lifted to prevent

roughness and brush marks. The paint should

be stirred frequently during use to avoid settling. Since brushing "works" the paint, it is essential—to avoid lifting—that an applied

coat be thoroughly dry before brushing on

another coat.

5.4.2. Roller Application

The roller applicator is a fast and convenient

(and therefore economical) means for applying various types of pigmented coatings to

large, flat surfaces that are reasonably free of

interference from architectural protrusions

and mounted fixtures. It gives excellent results on porous or textured surfaces such as brick,

cinder block, and plaster. And it is one of the

best methods available for painting wire fence

and grillwork. While it does not lend itself to the fine workmanship that can be achieved with

skilled brush application, the roller is an excellent and proven tool which is capable of good

results in the hands of either the skilled painter

or the do-it-yourself-er. The roller applicator

may be used with oil paints, alkyd enamels,

latex paints, and various other synthetic resinbase coatings that do not set up too quickly.

Quick-setting materials such as lacquers cannot

be applied by roller. The usual roller consists of a metal cylinder

or cylindical frame that rotates on an axle

supported at the ends by a frame to which a handle of any convenient length is attached.

The cylinder is covered with a replaceable

"sweater" of a suitable napped material

usually wool, mohair, nylon, or acrylic fiber. Covers for virtually any type of job are available. For use with water-base paints, a type

of roller cover is available in which the napped

material is bonded to a stiff hollow core of a

suitable plastic or a resin-impregnated cardboard which can be easily slipped on and off the roller frame. The length and texture of the

nap or pile are of prime importance in determining the application properties of the

tool. Rough surfaces require long-nap roller covers. The longer the nap, the more the

amount of paint that can be held, and' the

greater the stippling or "orange peel" effect. Wool covers hold more paint than other kinds

and produce an attractive stipple that makes

them preferred for most interior painting of

walls and ceilings with oil- or alkyd-base

97
paints. Lamb's wool covers with naps of

various lengths are used for both smooth and

rough surfaces—a %-inch nap gives good re- sults on light stucco or rough plaster. Mohair

covers are used where a particularly smooth

finish is desired, as in the application of gloss or satin enamel. Covers of nylon and acrylic

pile are useful with latex paints because they

do not mat or wilt when water-soaked; they

also wear well when used on rough-textured

surfaces such as masonry. Care should be

taken not to use synthetic resin covers in strong

solvents that may attack them. Polyester

resins can be efficiently applied with roller covers of wool frieze (having a shaggy pile).

Types of rollers are available which carry

a supply of paint inside the cylinder, the paint

feeding through the napped cover as it is used

;

however, it has been difficult to achieve a uniform flow of paint by this method, and it has

not found wide favor. Pressure feed also has

been used—with the paint fed from source to

roller on a continuous basis through flexible

hose; this method also has found only limited

application, mainly for coating very large

areas. In the conventional or dipping method of

roller application, the coating material is placed

in a tray having a sloping bottom, and the

roller is dipped into the tray and rolled back

and forth until it is loaded with the paint.

Excess paint is rolled out on the grid of the

dip tray, and the paint is then applied to a

strip of surface by rolling back and forth

slotvly until complete coverage is obtained.

Rolling should not be continued too long, lest the paint set up and be ruined. During roller

application, adjacent strips should be lapped as

in other methods of painting. Since the roller

will not fit into corners, the edges of walls and

ceilings must be painted ("cut in") by brush

before beginning the roller application. The

roller should follow the brushing promptly to avoid lap marks.

To keep rollers and covers in good condition,

it is essential that they be thoroughly cleaned

after each use by squeezing out excess material

and washing in a suitable solvent. Latex

paints can be washed out with soap and water,

with thorough rinsing. Roller covers should

be dried on their rollers to avoid shrinkage.

Wool covers may be combed while damp to minimize matting.

5.4.3. Spray Application

More paint is applied by spraying than by

any other method because of its general applicability to most painting situations, and be- cause of its speed and resultant savings in

time and labor costs. Spraying is particularly

convenient for overhead work, for large areas, and for reaching into areas inaccessible to

brushing. When skillfully done, it produces

a coating of uniform thickness and appearance

unobtainable by brushing. Since the paint is merely laid on without being worked into the

substrate, it is essential that the surface be

clean to obtain good adhesion.

Successful spray painting is dependent on

skill developed through knowledge and experience. Although a variety of different spray

methods are available, all require clean paint,

clean equipment, and a basically similar move- ment of the gun. The gun should be held perpendicular to the surface at all times, at a

distance of six to ten inches, and should be

moved in a straight line at uniform speed, with

proper "triggering" at the beginning and end

of each stroke. Proper triggering consists of

starting the stroke before the trigger is pulled

and releasing the trigger just before the end

of the stroke to prevent the build-up of excessive amounts of paint at the beginning and

end of each pass. Care should be exercised

not to tilt the gun or move it through an

arc ; both these faults will result in a distorted

spray pattern and a non-uniform coating.

Arcing of the gun can be avoided by proper

wrist action and body motion. Correct lapping

is very important in achieving a uniform thick- ness and appearance. The lapping should be

done in such manner that the center of the

spray strikes the bottom of the previous pass. Corners should be sprayed by directing the

gun toward the corner at a 45-degree angle to each side so that both sides of the corner are

equally coated at the same time as the gun is moved along the corner (on vertical corners, the gun must be turned on its side—the idea

is to utilize the long dimension of the elliptical

spray pattern for accomplishing the doubleedged coating operation)

.

The chief disadvantages of conventional

spraying methods are those associated with

the overspray caused by air and paint rebound

from the surface being painted. The overspray

wastes paint, necessitates the careful masking

of adjacent areas, may contaminate surrounding objects (at a considerable distance when

wind-borne), and may build up dangerous con- centrations of toxic or fiammable vapors and

mists (see section 5.2 for Safety Precautions).

To minimize these drawbacks, new methods

such as electrostatic and airless spraying have

been developed and are coming into widespread

use, particularly for factory and industrial

painting operations.

Both conventional and newer methods of

spray application are described in the subsections which follow.

5.4.3.1. Conventional cold spray is the

application method most widely used in the

field. While possessing the disadvantages

98
associated with overspray (see above), the

method is fast and economical compared to brushing, especially on large jobs or rough

surfaces. The equipment required is relatively

simple and inexpensive compared to that of more elaborate methods. A typical system consists of an air compressor capable of providing

up to about 90 lb/ in- at the gun, an air trans- former to regulate the pressure to the gun

while filtering out dust and dirt and trapping

out oil and water, flexible hosing to carry air

and paint to the spray gun and its attached

reservoir, and the spray gun itself. The latter

is provided with an on-off trigger and with means for .adjusting the flow of air and fluid to the spray head to obtain a desirable flow

rate, proper atomization, and the desired spray

pattern. The spray head is a removable as- sembly that includes a fluid-controlling needle, a hollow fluid tip, and an air cap. The fluid

tip and air cap are chosen to suit the viscosity and solvent balance of the coating to be

sprayed; it is essential always to use the type

of fluid tip and air cap specifically recom- mended for the material to be sprayed.

Before spraying, the coating material should

be thinned in accordance with the manufacturer's instructions, and the air and fluid

pressures at the gun should be set and balanced to give an adequate flow of material with

proper atomization and a normal spray pattern. If the ratio of air pressure to fluid

pressure is too low, the spray pattern may be

too heavy at the center or splotchy due to in- sufl^cient atomization. Too high an air-to-fluid

pressure ratio results in over-atomization with

the production of excessive fog and mist during

the spraying operation. ' Misting can also be

caused by holding the gun too far from the

surface being sprayed, with the result that the

paint particles become too dry enroute and

are blown off the surface as a powder instead

of adhering to it and coalescing. The thinner

the liquid, the lower are the air and fluid

pressures required for spraying; however, a

; material should never be thinned more than is

I recommended merely to make spraying easier

excessive thinning can produce its own quota

of serious film faults, such as runs, pinholing,

cratering, and blushing. Overspray can be re- duced by using the lowest atomization pressure

that will do the job satisfactorily. As a general guide, the common paints and enamels

I are usually sprayed satisfactorily at air presI sures of 60 to 70 lbs/ in- and fluid pressures of

j 15 to 20 lb/in-, while lacquers and other lowI solids liquids may be sprayed at air pressures

I of 40 to 50 lb/in- and fluid pressures of 10 to

I 15 lb/in2.

The techniques to be used in spray application have been discussed in the introductory

portion of section 5.4.3.

5.4.3.2. Airless spray is one of the recent

developments in methods of spray application.

Hydraulic pressure rather than air is used to accomplish the atomization of the coating material, in much the same way that a water

spray is produced at the nozzle of a garden

hose. High pressures—around 2500 lb/in-

are required to force liquids of paint viscosity

through the very small, precision orifice employed. Nozzles, valves, hoses, and other parts

must be designed to withstand high pressure

and erosion; hoses are usually lined with

Teflon and wrapped with wire; nozzles are

often made of tungsten carbide. A spray pat- tern similar to that obtained in air atomization

is produced by a precisely milled slot on the

face of the orifice. The high pressure required

is usually generated by an air-powered fluid

pump having a high fluid-to-air pressure

ratio that permits hydraulic pressures of 2500

lb/in- to be obtained with only about 100

lb/ in- of air pressure at the input. The compressed air serves only as a safe and convenient

source of power for driving the fluid pump

motor; air does not contact or become incorporated into the coating material.

The coating material is carried to the gun

by a single hose; the flow is controlled by an

ordinary high pressure valve. The gun itself does not have a trigger and has no means for

varying the spray pattern; however, a variety

of spray caps for use with materials of different viscosities are available. Once the air

pressure to the fluid pump has been adjusted,

the flow of coating material to the gun is controlled only by the valve, which must be turned on and off at the beginning and end of what is essentially a continuous spraying process. Although the problem of balancing air and fluid

pressures is not present, the lack of fine control over the spraying process reportedly re- sults in a rougher film than that obtainable

by conventional spray. Airless spraying is therefore better suited for applications where

protection rather than appearance is the paramount concern, and it is most feasible where

relatively large surfaces and objects or a con- tinuous production line is involved.

Both portable and fixed types of airless spray

equipment are available. Also, airless spray

may be combined with hot spray techniques

(see section 5.4.3.3) to permit spraying at lower pressures through a triggered gun. With

hot airless spray, the paint must either be re- circulated between gun and reservoir or the

feed hose must be hot-water jacketed all the

way to the gun in order to maintain the re- quired temperature and viscosity. (At least one manufacturer of hot airless spray equipment identifies his process by the term "airless

spray" alone.)

The chief advantage of airless spray over

99
conventional spray lies in the great reduction

in overspray achieved by elimination of the

atomizing air. Since most of the sprayed paint

stays on the surface to which it is directed,

better coverage is obtained, paint v^aste is minimized, fog and mist are virtually eliminated,

and the hazards of toxicity and fire are con- siderably reduced. With less solvent lost betw^een the spray gun and the surface, the coat- ing reaches the surface in a more fluid state,

and most coating materials can be sprayed at package viscosity v^ithout the time and expense

of thinning. Highly viscous materials may be

difficult to atomize by airless spray. Also,

coatings containing particles too large to pass

through the orifice of the gun (e.g., fibercontaining paints and most multicolor lacquers)

cannot be sprayed by the airless method. However, with most materials, a high rate of application and high film build per coat are

possible with airless spray.

Because of the operational convenience, speed,

and economy of airless spray, it is finding wide

use in industrial finishing and maintenance

painting.

5.4.3.3. Hot spray, in which the coating

material to be sprayed is first heated to a temperature in the vicinity of 150-175 °F,

(66-79 °C), is applicable to both airless and

conventional spraying techniques. The paint

is safely and rapidly heated by passage through

a hot water heat exchanger supplied from a

directly heated boiler and is carried to the

gun by a heated hose. Heating reduces the

viscosity of the material, lessens the amount of

thinner required, and permits spraying at a higher solids content. The result is heavier

film build per coat, with attendant economy

of time and labor. Hot spray is accomplished

at lower pressures than conventional spray,

with a considerable reduction in the amount

of overspray and its accompanying evils. The

higher spraying temperature and lesser amount

of thinner to be evaporated virtually eliminate

the tendency toward blushing, even under ad- verse humidity conditions, so that year-round

spraying is possible.

Coatings for hot spray application must be

specially formulated for proper solvent bal- ance and viscosity; otherwise, the coating may

suffer from poor coalescence, dusting, pinholing, and other defects. With proper formulation (e.g., elimination of fast-evaporating sol- vents), hot spray coatings are less prone than

conventionally sprayed coatings to such defects

as running, sagging, orange peel surface, etc.

5.4.3.4. Flame spraying is a technique that

permits thermoplastic materials that are in- soluble in organic solvents to be applied to metal and other surfaces. The coating material in the form of a powder is almost

instantly liquefied as the resin particles, sus- pended and propelled by compressed air,

emerge from the flame-ringed nozzle of a specially constructed gun. Upon striking the surface, the liquefied resin particles immediately

"freeze" or congeal into a continuous coating.

Although the flame-spraying technique has

been used chiefly for applying polyethylene

coatings to metal surfaces, the technique is applicable to other thermoplastic resins (e.g., flu- orocarbons and polysulfides) which are chemically stable at their melting point for the brief

second during which the resin is in the liquid

state. Metal surfaces permit the option of

heating the surface also, when practicable, to achieve better coalescence and a smoother film. Coatings applied by flame-spraying to clean,

properly prepared surfaces adhere quite well

and provide a means for utilizing as coatings

a number of highly chemical-resistant resins

which cannot be conveniently applied by solution or dispersion techniques (the latter re- quires fusing, which may not be feasible in some applications).

5.4.3.5. Electrostatic spray utilizes the at- traction between electrically charged materials

of opposite polarity as a basis for the rapid

application of uniform coatings, even on complex shapes, with a minimum of overspray.

The coating particles are charged to a high

negative potential (up to 100,000 volts) as they

contact or pass through a suitably shaped elec- trode. The object or surface to be coated is grounded, so that it is effectively at a high

positive potential with respect to the negatively

charged coating particles. The atomized particles are therefore drawn swiftly to the sur- face and captured, with very little overshoot even on objects with an open structure. The

method requires that the coating material be

essentially nonconductive so that its charge will

not be dissipated by current flow between

particles, and that the surface to be coated be

electrically conductive enough to remain at ground potential so that its attraction for the

negatively charged particles can be maintained.

Several different methods of electrostatic

spraying are available. While all utilize the

basic principle of electrostatic attraction by

giving the coating particles a high negative

charge while maintaining the surface at ground

potential, they differ in the manner in which

the coating material is atomized and directed.

Also, the original use of electrostatic spray

for applying liquid coating materials has re- cently been supplemented by an electrostatic

method for applying dry powder coatings.

5.4.3.5.1. Liquid electrostatic spray may be

of the air-atomized type or the airless type. A

conventional spray gun is used in the air100
atomized type but is operated at the lowest

pressure (about 12 Ib/in^) that will atomize

the paint and start it from the gun. The atomized coating material is sprayed into a negatively charged field where the particles too become negatively charged and are drawn to the grounded (effectively positive) object to be coated. The conventional spray gun permits

triggered control of the spraying process.

5.4.3.5.2. Airless electrostatic spray uses ei- ther centrifugal methods or hydraulic pressure

to atomize the coating material, and the material is applied in a continuous rather than a triggered operation. In a typical centrifugal

method, the paint is pumped from a small hole

in the center of a negatively charged spinning

disc that imparts a similar charge to the paint.

The paint becomes atomized as it is thrown

from the edges of the fast spinning disc. The

charged particles are then drawn to the

grounded objects which are positioned in a

circle around the spinning disc. The objects

may be slowly rotated while the disc is moved

up and down to provide complete coverage on

all sides. In another centrifugal method, the

paint is fed into a negatively charged spinning

bell that gives the paint a negative charge and

directs the atomized particles driven from its rim toward the grounded object to be coated.

The centrifugal methods are useful chiefly for

industrial finishing, where the necessary spray

equipment and automatic positioning and con- veyor facilities can be set up on a permanent

basis.

The most widely applicable method of airless electrostatic spray combines the normal

airless spray technique discussed in section

5.4.3.2 with the electrostatic principle described

in this section. Here, the negatively charged

coating material is atomized by hydraulic pres- sure as it leaves the orifice of the airless spray

gun and the charged particles are drawn to the grounded (positive) surface to be coated

by electrostatic attraction as before. This

method is adaptable for field application as

well as for industrial finishing.

All wet electrostatic spray methods afford

similar advantages of minimum overspray with

a great reduction in the hazards, waste, and

inconvenience normally associated with over- spray, together with substantial savings in

time, material, and labor costs compared to conventional methods of application.

5.4.3.5.3. Dry electrostatic spray is a comparatively recent development [741 in which

the coating material is sprayed in the form of a dry powder that goes on as a one-hundredpercent-solids coating. The powder is charged

to a high negative potential (about 90,000

volts) and then fed at very low air pressure

(about 1 lb/in-) through a spray nozzle. The

charged particles leaving the nozzle are at- tracted to any electrically grounded object in their general path and will adhere to the sur- face long enough to permit a heat cure that

fuses the particles into a continuous film. Uniform coatings up to 20-mils thick can be applied in a single pass of the spray gun, even

on complex shapes. Objects with open structures, such as wire mesh and fan guards, can

be coated on both sides at the same time by

using an ungrounded surface on the far side as an electrostatic reflector to repel particles

back to the object. An application rate equivalent to laying down a three-mil-thick coating

at the rate of one square foot per second is possible.

Since the coating is all solids, the fire and

toxicity hazards and the expense of volatile sol- vents are completely eliminated. Spray booths

are not needed, and the method is well suited

for either field or factory application. Despite

the high voltage employed, the method is completely safe because of the very low current

involved (only 0.2 milliampere) . Any type of

powder that is essentially nonconducting can be

sprayed onto any surface that is at least mildly

electrically conductive. Thermoplastic (heatfusible) materials such as polyvinyl chloride,

cellulose derivatives, fluorocarbons, polyethylene, and polypropylene are readily applied by

this method ; however, the method is applicable

to thermosetting resins such as epoxies only if a curing agent inactive at room temperature

but effective when heated can be incorporated

into the material. Inorganic fusible powders

such as talc, vitreous frit, and metallic oxides

also can be applied by dry electrostatic spray.

Surfaces that have been successfully coated in- clude metals, glass, heated plastics and ceramics, and moist paper and cloth.

It may be noted that the dry electrostatic

spray process is most useful with many of the same resins employed in the fluidized bed technique (see section 5.4.6). However, large objects (e.g., automobile bodies) are more easily

coated by electrostatic spraying, and thin coat- ings can be applied with a greater degree of

uniformity.

5.4.3.6. Dual-gun spraying provides a practical means for spraying two-component coat- ing systems that must be activated at the time

of use and have only a limited pot life after

activation. Epoxies, polyesters, and polyurethanes are typical of the coating materials that are being successfully applied by the dual-gun

technique. Such materials cannot be applied

by conventional spray gun because their continually changing viscosity makes them difficult to control, and the gun would be capable of

only limited operation before it would have to be thoroughly cleaned to avoid permanent clogging with hardened resin. Specially designed

101
dual spray guns are now available [63] that are capable of continuously metering and mixing the two components (resin and catalyst) in

the correct proportion at the instant of spraying. Resin and catalyst are fed to the spray

gun through separate hoses, one or both of

which may be heated if necessary to reduce the

viscosity of the material. Built-in metering

devices of almost any needed precision deliver

the two materials to a chamber in the spray

head, where they are intimately mixed in the

exact proportions required for proper cure and

then sprayed from the orifice under sufficient

hydraulic pressure to produce atomization (cf., airless spray, section 5.4.3.2).

5.4.4. Dipping and Flow Coating

5.4.4.1. Dipping is a fast and economical

method of industrial finishing—one that lends

itself to a continuous conveyorized operation

from initial cleaning of the surface to final curing of the coating. Despite the seeming

simplicity and ease of dip coating, it is actually a difficult process because of the many variables

that must be closely controlled to obtain a satisfactory finish. Because of these difficulties,

combined with the need for a relatively large

volume of material in the dip tank, dipping is not usually a convenient or economical way of

coating objects in the field, especially on an

intermittent or sporadic basis.

Of basic importance in determining the thick- ness and uniformity of the coating are the

composition and viscosity of the coating material, the rate of withdrawal of the object,

and the effectiveness of the drainage in preventing drips and fat edges in visible areas. Control of viscosity requires control of both

temperature and solvent evaporation in the dip

tank. Loss of solvent does not merely raise

the viscosity ; through the differential evaporation of lower-boiling constituents, the solvent

balance also is changed—and along with it the

drying and drainage behavior of the coating.

Adding more of the original thinner can re- store the proper viscosity but does not restore the original solvent balance. In industrial

finishing, a practical remedy for the evaporation problem is to maintain a nearly solventsaturated atmosphere in the dipping room;

however, this greatly increases the risk of fire and requires every possible precaution against

sparks and flame. Water-thinned coatings are

free from the problems of differential evaporation and fire hazard but may introduce cover- age and drainage problems of their own. The shape and surface texture (e.g., porosity)

of the object and the rate of its withdrawal

from the dip tank have a direct effect on drainage and on the thickness and uniformity of

the coating. The faster the object is withdrawn, the greater is the thickness (up to a

certain point), and the less uniform the coat- ing tends to be. In general, best results are obtained by withdrawing the object at a slow,

uniform speed that permits most of the drainage to take place as the withdrawal proceeds.

However, when a second coat is to be applied on a material that is quickly attacked by the

solvents in the formulation (as happens with

many lacquers) , it may be ncessary to perform

the dipping quickly to avoid redissolving some

or all of the previous coat. The dip tank should be no larger than is necessary to accommodate the object and should

have a minimum cross-sectional area to minimize solvent evaporation and coating deterioration. An agitator should be provided for

the continual stirring of pigmented materials.

Clear coatings should be stirred occasionally

and whenever the material in the tank is re- plenished. The tank should be kept covered

when not in use to reduce the fire hazard.

5.4.4.2. Flow coating is a process in which

the coating material is flowed onto the object

from a hose, after which the excess material

is allowed to drain off into a shallow tank or drip pan and recovered for reuse. Flow coat- ing is very similar to dipping in its requirements for close control of the coating composition, viscosity, drying rate, and drainage

characteristics—but it has the advantage of

not requiring a large dip tank. The coating material is supplied from a central reservoir

which can feed several hoses at a time if desired, while permitting convenient sampling

and control of the coating composition at its source. In both the dipping and the flow coating

processes, the manner in which the object is hung (particularly if it is of complex shape)

is of the utmost importance in achieving a uniform coating without unsightly drip beads

or fat edges.

5.4.5. Knife and Roller Coating

Knife coating and roller coating are highspeed industrial coating methods for applying

a wide variety of coating materials to flat sur- faces on a continuous production line basis.

Both are essentially spreading methods.

5.4.5.1. Knife coating is similar in principle to the doctor-blade technique used in the

laboratory for the application of uniform films.

It can be used for coating rigid, flat sheet material or for materials that can be held flat on

a rigid substrate while being coated. Only one

side can be coated at a time. The coating material is flowed on and spread evenly over the

surface as the latter is drawn beneath the

knife or doctor-blade. The coating thickness

102
will vary with nonuniformity in the thickness

of the sheet stock.

5.4.5.2. Roller coating utilizes hard rubber

or steel rolls to apply the coating to flexible,

flat sheet stock of metal, cloth, paper, and thin

composition board. The coating material is applied and spread evenly over the surface of

the material by a roller rotating in a direction

opposite to that of the strip of material being

coated, while the strip is held against the coat- ing roller by tension furnished by the driving

rolls. The coating roller receives its paint from

a feed roll that dips into a paint reservoir as

it rotates and then offsets the paint onto the

coating roller. Coating speeds of fifty feet per

minute are readily achieved. If desired, both

sides of the strip can be coated at the same

time. The method is not applicable to thick

stock that will not bend over the rolls.

5.4.6. Fluidized Bed Coating

Fluidized bed coating is one of the recent

innovations in the coatings field. It is a rapid,

convenient, and economical method for applying solventless (100 percent solids) coatings on

objects of metal, glass, ceramic, plastics, and

other materials capable of withstanding the

heating and fluxing temperatures involved in

the process. It also makes possible the utilization of insoluble resins that cannot be readily

applied as a coating by ordinary methods. The

technique is particularly suitable for coating

small objects and for applying thick, uniform

coatings even on complex or undercut shapes.

The method is applicable to both clear and

pigmented coatings.

In the fluidized bed process, the coating material in the form of a fine (50-150 mesh) dry

powder is "fluidized" by suspending the coat- ing particles in a controlled upward stream of

gas (usually compressed air or nitrogen)

.

This is accomplished in a vessel that is divided

into an upper and lower chamber by a porous

membrane that retains the powder while permitting passage of the gas. The coating particles must be spherical in shape to present a uniform surface resistance in the gas stream

and to avoid the agglomeration that tends to

occur with irregularly shaped particles. Both

powder and gas must be clean and dry to en- sure free mobility of the particles. Their

mobility is enhanced by a small electrostatic

charge which keeps the particles apart. The

fluidized powder occupies three to four times

the volume of the original powder. A properly fluidized bed looks and behaves like a liquid

and offers negligible resistance to the movement

of an object immersed in it. The article to be coated is preheated to a temperature above the melting point of the

resin and then dipped into the fluidized bed,

where it acquires a uniform coating of the

material as the particles melt onto it. (Irregularly shaped or undercut objects should be

moved through the bed to obtain complete

coverage.) Upon withdrawal, the coating congeals into a more or less continuous film which

may be further annealed in an oven to ensure good leveling and a pinhole-free coating and

to complete the curing of a thermosetting resin.

The coating thickness obtained depends on the

preheat temperature, the shape of the object,

and the duration of the dip. Thin-walled sec- tions may present some diflflculties because of

quick cooling while in transit from preheat to

dip. Since the object must be suspended while

being dipped into the fluidized bed, an uncoated

area is left at the point of suspension which

can be touched up by filling it with powder and

then fusing it with a blast from a hot-air gun.

While the fluidized bed method is most widely

applicable to thermoplastic resins which are

readily fused into a continuous film, it can also

be used with thermosetting resins that can be

liquefied by heat prior to being finally cured

by after-heating; epoxy resins are a notable

example. Thermoplastic resins which have

been successfully applied by the fluidized bed

technique include virtually insoluble resins such

as polyethylene, polypropylene, nylon, and the

fluorocarbons, as well as soluble resins such

as polystyrene, vinyls, acrylics, and cellulosics.

When using the fluidized bed method, it should be borne in mind that the powdered

material presents health and safety hazards

unless adequate facilities are available for

hooding, exhaust, and dust collection.

103
Figure 5.1. Roller-coated sheet metal stock ("coil coating") passes through final inspection station to recoiling

equipment.

(Photo courtesy of Industrial Finishing.)

Figure 5.2. Electrostatic spray gun paints chain link fence from one side with veTij

little overspray.

(Photo courtesy of Ransburg Elec- tro-Coating Corporation.)

104
6. Surface Preparation and Pretreatment

The common denominator in all surface preparation is thorough cleaning of the surface.

To the clean surface, pretreatments may be

applied to enhance adhesion and (in the case

of metals) to provide a barrier against the

spread of corrosion. Whereas the purpose of

the basic surface preparation is to achieve a

physically clean surface, the pretreatment is a chemical process, the functions of v^hich are

:

(1) to change the original surface to one that

possesses a greater surface area and/ or a

greater specific affinity for the primer to be

applied, and (2) in the case of metals, to passivate the surface and retard corrosion.

The choice of a practical method of surface

preparation depends on the nature and condition of the surface, its size and shape, the

equipment available, the type of coating to be

applied, and the service environment. Methods

range from light brushing or a simple solvent

wash to heavy sandblasting. Dirty or corroded

surfaces, coatings of limited v\^etting ability,

and severe service environments require vigorous, thorough cleaning methods; for example,

rusty steel that is to receive a phenolic coating

system to withstand marine exposure must be

thoroughly cleaned by sandblasting before the

coating is applied if good service performance

is to be achieved. On the other hand, a solvent

wash may suffice for an aluminum surface that

6.1. Importance of Surface

The importance of proper surface preparation to the durability of any coating system

cannot be overemphasized. Without proper

surface preparation, the finest paint—applied

with the greatest skill—will fall short of its maximum performance capabilities or may even

fail miserably. Considering the proportionately high cost of the labor involved in applying

protective coatings as compared to the cost of

materials, it would appear to be axiomatic that

only the most durable materials applied in a manner that achieves maximum service should

ever be used. Yet there are some who, while

recognizing the value of high-quality materials

and workmanship, fail to grasp the fundamental importance of surface preparation as

the foundation upon which all else depends.

There seems to exist in some cases a subconscious willingness to take a chance when it comes to the hard work of surface preparation.

But while an attitude of "the old surface isn't going to show, so why should I bother with it" may be understandable in the unversed do-ityourselfer, such misconception has no place in

is to receive a wash primer pretreatment; and

new wood siding that is to receive an oil paint

may need no cleaning at all (although knotsealing and puttying of nail-holes may be

necessary) . The important point is that whatever method of surface preparation is selected

must be suited to the job and thoroughly performed ; skimping in either the selection or the

execution of a method of surface preparation

is an invitation to trouble.

A variety of chemical pretreatments, many

of them of a proprietary nature, are available

for use on specified types of surfaces. Pretreatments for metal find the widest use; they

are based on various chemical agents, including

phosphating, chromating, and oxidizing agents,

formulated for maximum effectiveness on the

particular metal to be treated. Pretreatments—usually in the form of acidic agents or

resinous surface conditioners—are also useful on concrete and masonry surfaces. Plastics,

particularly those with low energy (non-adherent) surfaces such as polyethylene and the

fluorocarbons, may be treated with strong acid,

caustic,' or oxidizing agents to etch the surface

for better coating adhesion.

In the sections that follow, the basic importance of proper surface preparation is stressed,

and specific information on the preparation and

pretreatment of various kinds of surfaces is presented.

Preparation and Pretreatment

the workmanship of the professional painter

or responsible coatings official. A coating can provide its function only so long as it remains intact and firmly bonded

to the surface it is to protect or decorate. Good

adhesion requires that the coating material be

brought into intimate contact with the surface

itself (or a firmly bonded chemical pretreatment layer) , without intervening corrosion

products or contaminants, to permit the interaction of molcular forces of attraction directly

between the coating and the substrate material.

Thus, a properly prepared surface is essentially a clean surface, but not necessarily the ultimate

in cleanness. It is frequently impractical, for

economic reasons or because of equipment limitations, to achieve a perfectly clean surface; nor is this usually necessary. However, adequate surface preparation can and must be

achieved in all cases if the coating is to adhere

firmly and durably. An adequately prepared

surface not only provides a good anchor for

the coating but also ensures a surface free of

corrosion products or contaminants which

105
might shorten the life of the film by spreading

along the coating-substrate interface and destroying adhesion, or by actually breaking

through the coating.

The value of chemical pretreatment, particularly for metals, lies in the thin, tightly

adherent, ideally roughened, paint-receptive

6.2. Preparation

6.2.1. Bare Wood

6.2.1.1. General. — Wood surfaces to be

painted should be clean, free of cracks and

splinters, and properly seasoned (moisture content below 15 percent for exterior wood, and

less than 10 percent for interior wood). Excessive moisture in the wood is accompanied by

large dimensional changes as it dries, which

may cause cracking of the paint film; also, the

entrapped moisture may cause blistering of the

film, with early flaking or peeling. Green wood

should be allowed to cure for at least a week

in clear, warm weather before painting is at- tempted. Seasoned wood that has become wet

should be allowed to dry until it is dry to the

touch before painting.

Cracks and nailholes should be filled with

putty or plastic wood. Puttying must be preceded by application of a primer to prevent

loss of oil from the putty by absorption into

the wood. The putty should be allowed to dry

for at least 48 hours in the case of shallow

holes or cracks, and for a period of from one to three weeks for deep fillings. Plastic wood

although more expensive and more difficult to handle than putty—may be used directly on the

bare wood and hardens in a few hours. When

dry, the filled areas should be lightly sanded

before painting.

Loose dust, dirt, and sandings should be

brushed away. Scraping or wire brushing may

be used to dislodge mortar scale, pitch streaks,

and other tightly adherent foreign substances.

Grease and oil can be removed by washing with

mineral spirits or white gasoline (caution).

Shop markings may be removed by sanding or

a solvent wash. Smoothing may be done

with sandpaper, steel wool, or power sanding

machines.

6.2.1.2. Exterior wood should be properly

seasoned, clean, and dry before painting. Although a smooth surface is not necessary, the

wood should be brushed, scraped, washed, or sanded as may be required to produce a surface

free of dust, dirt, oil, grime, sap streaks,

splinters, projections, and very rough areas. Knots require special treatment to minimize

the possibility of discoloration, cracking, and

premature failure of the paint applied over them. As much sap as possible should be re- moved from the knot by scraping and by washing with turpentine. When the area is dry a

film that is formed on the metal surface. Not

only does such a film provide an excellent base

for coatings adhesion, but it also resists the

spread of corrosion beneath it, so that damage

to the substrate from a break in the film is not as likely to progress beyond the area of

immediate damage.

of Wood Surfaces

good knot sealer should be brushed on, such

as the WP-578 knot sealer developed by the

Western Pine Association and covered by Military Specification MIL-S-12935 [149]. This

knot sealer consists of a ten-to-one blend of a phenolic resin and polyvinyl butyral in denatured alcohol solution. It has proven more

effective than shellac or aluminum paint for

this purpose.

The puttying of nailholes and cracks is an

important part of the preparation of exterior

wood surfaces for painting; however, it should

be done after the priming coat has been applied and dried, as indicated in section 6.2.1.1,

The putty should be allowed to dry thoroughly

before the finish coats are applied,

6.2.1.3, Interior wood should be well sea- soned, clean, and dry when painted. It is essential that the surface be sanded to a smooth

texture before painting to obtain a good fin- ished appearance. The type and amount of

sanding required depends on the initial roughness of the surface. To avoid unnecessary

scratching or gouging of the wood with a re- sultant increase in the amount of labor ulti- mately required to achieve a smooth surface,

sandpaper of the minimum coarseness that will

do the job should be used. Where an initially

rough surface is involved, sanding should start with a more or less coarse grade of sandpaper

and proceed through intermediate grades to a

fine grade for finishing. The sanding should

be done in the direction of the wood grain.

Sanding of woodwork and cabinet work may

be accomplished by starting with a medium

grade sandpaper (No. 1) and finishing with

fine sandpaper (No. 0 or finer). On furniture, or where the finest finish is desired, the final sanding should be done with a very fine grade

of sandpaper (No. 4/0).

Fine steel wool may be used instead of sandpaper and is particularly useful on rounded or irregularly shaped surfaces.

Floors are best sanded by machine, working

in the direction of the wood grain. For the

first rough sanding, coarse sandpaper is used

(No. 21/2 for hardwood and No. 3 for softwood

floors), followed by No. li/o sandpaper, and

finishing with a fine grade (No. 0) for hardwood and a medium grade (No. 1) for soft- wood (fine sandpaper becomes gummed by soft

woods under power sanding conditions).

Wherever nails are used, they should be

106
countersunk and the nailholes filled with plastic

wood or putty (the surface must be primed or sealed before puttying, as indicated in sec. 6.2.1.1) . Plastic wood—although more difficult to handle than putty—is often preferred for

its more natural appearance and quick-drying

properties. As previously noted, putty may re- quire days or weeks of drying before it can be

painted satisfactorily. When thoroughly dry,

the fillings should be sanded smooth before

painting.

Where stain is to be used, the stain is applied directly to the bare wood surface. The

stain, when dry, is followed by a wash coat (frequently one pint of shellac varnish thinned

with four to seven parts of denatured alcohol)

and given a further fine sanding. At this point,

open-grain woods, such as oak and walnut, are given a coat of paste wood filler thinned to brushing consistency. The filler is allowed to

set for about twenty minutes, and the excess

is then removed by wiping across the grain.

After the wood filler has dried, any remaining

excess is sanded away, leaving the stained,

filled wood surface ready for the clear finish coats.

6.2.2. Previously Painted Wood

The repainting of wood surfaces is most

easily accomplished when it is done at the

proper stage in the life of the coating—that

is, when the coating is well-worn but still basically sound. At this stage, little if any

surface preparation is required other than the

brushing or washing away of loose chalk, dust,

dirt, or grime. On the other hand, if the old

paint is allowed to deteriorate to the point

where large areas of the paint are flaking and

peeling, or the paint is cracked and the edges

curled, then it will have to be removed completely to prepare the surface properly for re- painting. Complete removal of the old paint

is a laborious, expensive task that must be

accomplished with torches or paint removers,

accompanied and followed by thorough scrapj ing and sanding of the surface. Obviously, it

I pays to repaint at the proper time. This is best determined through periodic close inspection of the surface.

; The preceding remarks are particularly ap1 plicable to exterior painted surfaces. Most

white outside paints, and tinted paints to a

lesser extent, are formulated to give controlled

chalking of the coating which assists in keeping

it clean. The ideal time to repaint such sur- faces is when the paint is worn thin to the

f point where bare wood begins to show, or when

1 minute checking and crumbling begin. Such

; surfaces take paint well and need little surface

preparation. If chalking has kept the paint

I clean, brushing with a painter's duster may

il be all the preparation required. If chalking is

excessive or if the old paint is soiled, washing

with detergent followed by a thorough hosing

down with water may suffice. In some in- stances, a final light sanding of the old paint

may be desirable. Small deteriorated areas may be scraped, wire-brushed or sanded down

to bare wood and spot-primed before over-all

repainting. In such cases, the boundary of

intact paint should be sanded to a feather edge.

If deterioration of the paint has progressed

to the point where flaking and peeling, or widespread cracking and edge curling, have taken

place, then the old paint will have to be entirely

removed. On exterior surfaces, this is probably best accomplished by burning off the paint,

with due precautions against the hazards of

fire and toxic fumes (see sec. 5.2). The burning off should be done by an experienced

worker. The paint is heated sufficiently to soften it and cause it to blister so that it can

be removed with a scraper. Care must be

taken not to char the wood surface. Finally,

the wood will require sanding to complete its preparation for painting.

Where burning off is not practicable, scraping and wire brushing may be used to remove

the old paint, although the process may be

long and tedious. Liquid paint removers (such

as those covered by Federal Specification TTR-251 or Military Specification MIL-R-46073)

may be used effectively; however, they are

relatively expensive and difficult to handle, and

repeated applications and scraping may be re- quired. Liquid paint removers are more ef- fectively utilized indoors, where burning off

is not feasible, and where a smooth final sur- face is desired. They give best results when

applied on horizontal surfaces; vertical sur- faces require a viscous remover of the semipaste type. The wax film that forms on the

surface of the paint remover should not be

disturbed, since its function is to seal in the

volatile active solvents until they have done

their job of softening the paint. The directions

of the manufacturer should be carefully fol- lowed when using liquid paint removers. Since

they nearly always contain toxic volatile sol- vents, adequate ventilation should always be

provided during their use. When removal of

the paint has been effected, the residual wax

must be removed from the surface by washing

with mineral spirits, followed by a detergent

wash and thorough rinsing with water. (Waxfree solvent-type paint removers are available

but are generally not as effective as the waxcontaining types.)

After the basic surface preparation has been

accomplished, nailholes, cracks, and other defects should be puttied (after priming) as in

the case of initially bare surfaces.

Interior paint coatings, unless worn thin by

abrasion or repeated washing, are usually re- painted because they have become dull, dis107
colored, or dirty—or because a change of color

is desired for decorative rather than functional

reasons. Such surfaces are relatively easy to prepare for repainting. The most important

requirement is that the surface be free of loose

dirt and grease. Indoor surfaces often become

coated with a film of grease or oil from cooking

vapors and from contact with people or objects.

The new paint will not adhere long to a greasy

surface. All such contaminants should be re- moved by thorough washing, first with mineral

spirits, then with a detergent, followed by thorough rinsing with water. If the old paint is glossy, it should be lightly roughened with

sandpaper or steel wool to improve paint

adhesion.

In all repainting, care should be taken not

to build up too great a thickness by applying

too many coats or by repainting too frequently.

A thick paint coat is prone to cracking and

chipping off because it becomes too rigid to follow the normal dimensional changes in the

wood.

6.3. Preparation and Pretreatment of Metal Surfaces

The basic requirement in the preparation of

metal surfaces for painting, as with other surfaces, is cleanness. However, the degree of

cleanness which is practicable or necessary

particularly in the case of steel surfaces

depends on the condition of the surface, the

type of coating system that is to be applied,

the nature of the service environment, and the

cost factor in relation to the improved durability to be expected from more thorough sur- face preparation. The choice of a suitable

method of surface preparation also depends on

the location, size, and shape of the surface to be painted, and on the cleaning equipment and

facilities that are available. Structural steel often must be painted in the field, under conditions involving certain limitations of surface

preparation and coating application. Industrial steel products coated at the factory can

be prepared for painting by thorough cleaning and pretreatment processes that yield optimum results.

Before considering the detailed information

given in the following sections on the preparation of specific types of surfaces, the reader

should consult the general information in the

introductory sections of this chapter on the

basic importance and purpose of adequate sui'- face preparation and pretreatment.

Because of the enormous tonnage of steel used in structural applications and manufactured articles, and because of the continual

need for protecting this strong but corrosionsusceptible metal from damage under a wide

variety of environmental conditions, the methods of surface preparation employed for steel encompass virtually the entire spectrum of

available methods. We shall therefore describe

the various basic methods of surface preparation in the discussion of steel surfaces which

follows, and shall merely make reference to these methods as needed in subsequent sections

dealing with other metals.

6.3.1. Gleaning of Steel (and Iron) Surfaces

Although sandblasting to "white" metal

(often followed by chemical pretreatment)

provides the optimum surface for painting

structural steel, the labor and high cost of this

type of cleaning is ordinarily justified only

when maximum performance under severe

service conditions is required. Maximum performance in difficult environments usually in- volves synthetic resin-base coatings of high

durability but limited wetting power; such

coatings require thoroughly clean (and often

pretreated) surfaces to develop adequate and

lasting adhesion. Examples of such coatings are

the phenolic and vinyl paints recommended in

section 4.3.1 for severe marine service and

immersion conditions. Oil paints, on the other

hand, although suitable only for mild exposure

in ordinary atmospheric environments, possess

inherently good wetting properties that permit

their use even on incompletely cleaned steel from which it has not been practical to remove

all surface rust and corrosion products.

While the wetting ability of the primer to be

used is the immediate factor determining the

degree of surface preparation required, the

choice of primer in turn depends to a large

extent on the requirements for compatibility

and good adhesion of the topcoat. Therefore,

the surface preparation actually depends on

consideration of the coating system as a whole.

As part of the information presented in chapter 4 on Selection of Coating Systems, the general type of surface preparation required or considered adequate for particular types of

coating systems and service environments was

indicated. In the present section, the details

about the various methods of surface preparation and pretreatment are given.

6.3.1.1. Solvent cleaning with organic sol- vents is a very effective method for removing

oils, greases, waxes, shop markings, drawing

and cutting compounds, and other solventsoluble foreign matter from the surface to be

painted. The removal of such contaminants is usually prerequisite to the effective use of mechanical methods for removing rust, mill scale,

and heavy dirt encrustations that cannot be

removed by solvent cleaning. Mechanical

cleaning methods, short of sandblasting to

108
"white" metal, do not completely remove such

oily contaminants and may spread them over

the surface in a film that can seriously impair

the adhesion and durability of the applied

coating.

6.3.1.1.1. Wiping, dipping, or spraying.—

Since organic solvents remove oils, greases, and

w^axes by dissolving them, the cleaning liquid soon becomes contaminated with the removed

products and must be purified or renewed to avoid redepositing an oily residue on the work

when the solvent evaporates. This is particularly true when wiping, dipping, or spraying

methods are employed; in these cases, it is essential that the final rinsing be done with

clean solvent. ' The rather high cost of organic

solvents, the large quantities required for sol- vent cleaning, the expense of solvent purification, and the need for precautions against fire and toxicity hazards combine to make organic

solvent cleaning a relatively expensive though

effective method of removing oily and waxy

contaminants.

6.3.1.1.2. Vapor degreasing is the most ef- fective and economical type of solvent cleaning; it is also the least hazardous. In the

vapor degreasing process, the article to be

cleaned is suspended in the solvent vapor above

a boiling cleaning liquid (usually stabilized

trichloroethylene or perchloroethylene, because

of their virtual nonflammability and their relatively low toxicity as compared to other chlorinated hydrocarbons). The hot, distilled sol- vent vapors condense upon the relatively cool

object, dissolving oils and greases and carrying

them away as the condensed droplets fall back

into the parent liquid. Cold water pipes or

coils are often placed in the upper part of the

degreasing tank or chamber to effect further

condensation of vapor and to provide a vapor

ceiling (above which vapors will not rise).

The unique advantage of vapor degreasing over

other organic solvent cleaning methods is that

the article being cleaned is continuously washed

with clean, distilled solvent, while the con- taminants accumulate in the liquid below.

Eventually, of course, the parent solvent must

be purified by distillation, preferably in an

auxiliary still, after first removing any undissolved water by a separatory process. To

maintain an efficient degreasing operation,

purification of the solvent should take place

hefore the concentration of contaminants

reaches about 25 percent. An incidental but

important benefit of vapor degreasing is that

the degreased work emerges warm and dry

and ready for any additional cleaning, pretreating, or finishing operations that are to follow.

Organic solvents do not remove corrosive

salts, soaps, and other harmful water-soluble

contaminants from the surface. These may

be washed away with water, steam, detergents, emulsions, or alkaline cleaners.

6.3.1.2. Steam cleaning in combination

with detergents, while comparable in cost to solvent cleaning, is an effective means for re- moving heavy soil as well as greases and oils from metal surfaces ; it does not remove rust and

mill scale. If a strongly alkaline cleaner is used, the action of the caustic together with

the high temperature and high velocity of the

steam will remove old paint as well as surface

soil and grime. Steam and/or hot water at a temperature of about 300 °F (149 °C) and a pressure in the vicinity of 200 lb/ in-, and con- taining the desired detergent, is directed

against the work from a distance of about six

inches, with the nozzle tilted slightly in the

direction of travel. Speed of travel, number

of passes, and time between initial wetting

and subsequent cleaning should be adjusted to the amount and nature of the soil to be re- moved. The time between initial passes and

later return allows the detergent to act upon

and loosen foreign matter to permit its complete removal during the final passes. Any

alkaline residue remaining on the surface after

the cleaning operation must be removed by

thorough rinsing with fresh water. Steam

cleaning is particularly well-suited for cleaning

in the field, where the location, size, and shape

of objects and structures frequently do not

lend themselves to efficient cleaning by other

methods. Workers engaged in steam cleaning

operations should be protected against burns

and chemical injury to the eyes and skin.

6.3.1.3. Emulsion cleaning with emulsifiable solvents combines some of the cleaning

action of both water and solvents. The emulsifiable solvents used are composed essentially

of an organic grease solvent, an emulsifying

agent, blending agents, and a petroleum thin- ner such as kerosene or mineral spirits. The

work is immersed in the emulsifiable solvent

(or the solvent may be brushed or sprayed onto

the work) and allowed to remain for several

minutes to dissolve oil and grease and to loosen

dirt. The dissolved and loosened oil and soil are then emulsified and fiushed away by a pressurized hot-water rinse. A small residue of

emulsion usually remains on the surface and

will leave a thin film of oil if not removed.

Removal of this residue is readily accomplished

by washing with steam, hot water detergents, or solvents. Cleaning with emulsifiable sol- vents permits removal of heavy oil and grease

contamination at lower cost than with organic

solvents alone ; also, corrosive salts and other

water-soluble contaminants are washed away

during the pressurized rinse. Since most of

the surface soil is emulsified and washed away

109
during the rinsing process, the emulsifiable

solvent itself becomes contaminated only slowly

and requires only occasional renewal. It should be kept in mind that the hydrocarbon

solvents contained in these cleaners present

the usual fire hazard associated with the use

of organic solvents.

6.3.1.4, Alkali cleaning provides an eco- nomical and effective means for removing oil, grease, wax, corrosive salts, and other soil (except rust and mill scale) from metal surfaces. The strength of the cleaner can be

varied from mild to caustic by varying the

type and concentration of alkali employed.

Among commonly used alkaline cleaners are trisodium phosphate, sodium tetraborate (borax), sodium meta- and ortho-silicate, sodium

carbonate (soda ash), and—where maximum

saponifying strength is needed—sodium hydroxide (caustic soda). Most proprietary al- kaline cleaners employ blends of these materials to obtain a more versatile and more

effective cleaning action ; surface active (wetting) agents and emulsified hydrocarbon sol- vents are sometimes added to increase the

grease-removing action of the cleaner.

Alkaline cleaners must be used hot (about

175 °F [79 °C]) for maximum effectiveness.

They accomplish their cleaning action by sa- ponifying oils and greases to form soluble

soaps, and by loosening other types of soil by

a detergent action that displaces the soil from

the surface. The saponified oils and the loosened and suspended soil are readily flushed

away by thorough rinsing with hot water.

Strong alkaline cleaners based largely on so- dium hydroxide are effective paint strippers

for removing old paint from steel surfaces.

Alkaline cleaners may be utilized by immersing the article to be cleaned in a soak tank of

hot cleaner, by pressure-spraying the cleaner over the object, or by an electrolytic process

which generates gas bubbles that aid greatly

in dislodging soil. Following the use of aklaline cleaners, care must be taken to rinse all alkaline material from the surface; even small

traces of alkali, soap, or detergent left on the

surface are detrimental to coating adhesion

and durability. To ensure neutralization of

any alkaline residue, a passivating dichromate

or dilute chromic acid final rinse may be used,

unless a phosphating or other suitable pretreatment is to follow.

While alkali cleaning is free from the fire and toxicity hazards of organic solvent cleaning (unless emulsified solvents have been in- corporated), the corrosive effects of alkaline

materials on the skin and on ordinary clothing

must be guarded against. Caustic soda, in

particular, can cause serious burns to the skin

and eyes and is extremely irritating to the

nasal and bronchial membranes if inhaled.

Workers should wear splash goggles, protective

rubber gloves and garments, and respirators

when preparing or using caustic solutions.

In case of accidental contact of caustic soda

solution with the eyes or skin, the affected

parts should be flushed immediately with large

amounts of water, and a physician should be

consulted promptly.

6.3.1.5. Acid cleaning or pickling is a very effective method for completely removing

all rust and mill scale from steel surfaces by

chemical and/or electrolytic action. The acids

commonly employed are phosphoric, sulfuric,

and hydrochloric—either singly or in combination. Occasionally, in special cases involving

stainless or other alloy steels, hydrofluoric or

nitric acid also may be used for brightening

the surface or for scale removal.

6.3.1.5.1. Phosphoric acid cleaning is an excellent time-tested method for removing oil, grease, soil, corrosive salts, and light rust from

metal surfaces. The phosphoric acid solution

is combined with detergents and wetting agents

to assist in soil removal and contains organic

solvents such as alcohols and ketones to aid in

the removal of oils and greases. Where heavy

grease or oil contamination is present, it is desirable first to remove most of this by cleaning

with organic solvents in order to avoid rapid

fouling of the phosphoric acid cleaner. Although it is possible to remove even heavy rust and mill scale from steel surfaces by using hot,

strong phosphoric acid pickling solutions (15-

50% phosphoric acid) for a long period of

time, it is preferable to employ a sulfuric acid

pickling method (as described in the following

section) for the eflJicient chemical removal of

heavy corrosion products.

Cleaning with phosphoric acid solutions can

be accomplished by dipping the work into a tank of the hot or cold cleaner, or by flowing

or brushing the cleaner onto the work. After

removal of soil and rust, the surface should be

washed off with clean water (or wiped off with clean, damp rags if rinsing is not practical because of the size or location of the

structure. Military Specification MIL-M10578 describes two types of phosphoric acid

metal cleaner, both of which must be diluted

with three times their volume of water before use [78]. The type I cleaner, before dilution,

contains about 47 percent by volume of phosphoric acid (85%), and is used where the sur- face can be washed off with water after the

cleaning process. The type II cleaner, before

dilution with water, contains about 14 percent

by volume of phosphoric acid (85%), and may

be used where washing with water is not practicable and the surface is merely to be wiped

off with rags. Unlike alkali cleaning or strong acid pickling,

110
phosphoric acid cleaning does not leave a harmful residue on the washed or wiped surface. Instead, the phosphoric acid gently

etches the surface and reacts chemically with

it to produce a minute layer of iron phosphate

which provides both a mechanically and chemically receptive surface for subsequent coating.

(The thin phosphate layer formed on the sur- face during phosphoric acid cleaning or pickling is not a substitute for—and should not be

confused with—the heavy phosphating pretreatments which are described in sec. 6.3.2.)

6.3.1.5.2. Pickling with strong acids—usually

sulfuric, but sometimes hydrochloric (muriatic) or other acids as already noted—is accomplished in dilute solution (usually at a

concentration of 5 to 10 percent by weight)

and at a temperature of 140 °F (60 °C) or

above. Rust is readily dissolved by this treat- ment, forming ferrous salts which remain in

solution. Mill scale is attacked more slowly;

removal of mill scale is dependent largely on

penetration of cracks and crevices by the acid

which thus works its way beneath the scale

where hydrogen is evolved and "blasts" the

scale from the metal. The rate of rust and

scale removal increases with increasing temperature, and with increasing acid concentration up to a certain point (about 50 percent

by weight for sulfuric acid, and about 30 percent for hydrochloric acid) . Adequate agitation must be provided to maintain a normal

pickling rate. Inhibitors must be present in

the acid bath to prevent excessive attack upon

the metal itself in cleaned areas while rust and

scale removal is still taking place in other

areas; inhibitors also minimize surface bubbling and "hydrogen embrittlement" of the

steel by controlling the amount of hydrogen

evolved. The buildup of ferrous salts in the

pickling solution slows down the rate of rust

and scale removal; when this can no longer

be compensated by increasing the temperature

and acid concentration, the pickling solution

must be discarded and a fresh bath prepared.

Before pickling is begun, oily contaminants

(grease, wax, etc.) should be removed from

the surface by solvent cleaning, alkaline cleaning, or other suitable degreasing method (see

preceding subsections) . After the pickling is completed, the work should be promptly and

thoroughly rinsed with clean cold water; any

acid residue will cause rapid rusting of the steel surface. The cold water rinse should be fol- lowed by a dilute dichromate, chromic acid, or phosphoric acid rinse to passivate the sur- face (chromium compounds should not be used

if phosphate pretreatments are to follow ; however, prompt treatment with dilute phosphoric

acid is highly desirable in any event)

.

While pickling is an effective method for

removing all rust and mill scale from steel, it

is limited to objects that will fit into the available pickling baths. It is therefore largely a factory technique that finds its widest use on

structural steel leaving the mill. Workers en- gaged in pickling activities must be carefully

protected against acid splash or inhalation by

wearing goggles, respirators, and suitable rubber clothing—including boots, gloves, and

aprons.

Electrolytic pickling is more rapid than or- dinary chemical pickling for removing mill

scale because of the greater evolution of hydrogen and the resultant greater agitation and

"blasting" action. There is little difference in

acid consumption between the electrolytic and

chemical processes, provided the latter is properly inhibited to avoid unnecessary solution of

clean metal.

6.3.1.6. Hand cleaning methods are suitable for removing loose rust, loose mill scale,

dirt incrustations, and loose paint from metal

surfaces that are to receive paints of good wetting ability (e.g., oil paints) that are destined

for mild, noncorrosive atmospheric exposure.

Hand cleaning usually suffices also for interior

surfaces and for the spot cleaning and priming

involved in most maintenance painting where

the surface has not been allowed to deteriorate

badly. Hand cleaning methods do not remove

heavy or tightly bound rust and mill scale and

therefore are not satisfactory where coatings

of limited wetting ability (e.g., phenolic varnish paints) are to be used or where severe atmospheric exposure or water immersion conditions are encountered. Oil and grease, if present, must be removed by solvent cleaning

(sec. 6.3.1.1) before commencing hand cleaning operations, since the latter may further

spread oily contaminants over the surface.

Hand cleaning is accomplished with a variety

of hand tools, including wire and stiff bristle

brushes, scrapers (often with special shapes

for reaching into difficult areas), impact and

chipping tools such as hammer and chisel, and

abrasive materials (emery cloth, sandpaper,

and steel wool). Normally, chipping and

scraping precede wire brushing. Finally, all loosened matter should be brushed or swept off and the surface blown clean with compressed

air (vacuum cleaning also is suitable). Workers should be protected with goggles and dust

respirators. If cleaning must be done in areas where fiammable vapors may be present, non- sparking tools must be used to avoid the danger

of explosion.

Since hand cleaning is slow and laborious

(hence expensive) and of only limited efficacy,

it is used primarily for small or noncritical

jobs, for jobs that are inaccessible to power

tools, where power tools are uneconomic or

unavailable, where the surface is not very dirty or corroded (as in maintenance painting done

Ul
at the right time) , and where the coating sys- tem to be applied can tolerate or overcome the

residues remaining after hand cleaning.

6.3.1.7. Power tool cleaning removes little more dirt, rust, and mill scale than hand tool

cleaning; however, it is less laborious and

much more rapid. Like hand cleaning, it re- moves only loose mill scale and rust. (Blast

cleaning—as described in the following subsection—is required to remove heavy rust and

tight mill scale.) Since not all corrosion

products are removed from the surface by

power cleaning, the method is limited to coat- ing systems that have good wetting ability for

incompletely cleaned surfaces, or to coating

systems that will not be subjected to severe en- vironments or immersion conditions. Power

cleaning is adequate also in most maintenance

painting situations where the surface is in rea- sonably good condition and only spot cleaning

to the bare substrate is necessary.

The tools required may be electric or pneumatic powered. They include large rotary

power wire brushes, power grinding wheels

and sanding machines, and power-driven ro- tary or piston type hammers and scalers for

chipping and descaling. Of these, power wire

brushes in a variety of sizes, textures, and

shapes are the most widely used; cup shapes,

radial types and disc shapes are available.

Oil and grease should be removed from the

surface by solvent cleaning (sec. 6.3.1.1) before beginning power tool cleaning, to avoid

spreading such contaminants over the surface. Chipping and scaling operations, if needed, should precede wire brushing. Great

care is required, when using power impact tools, to avoid gouging sound metal. Excessive wire

brushing of unremovable mill scale will burnish the surface and greatly reduce the adhesion of the applied coating. Areas not accessible to power tools should be cleaned by hand

methods. After completion of the mechanical

cleaning, all lessened material should be swept

from the surface, and the surface should be

blown clean with compressed air or cleaned

by vacuum. Goggles and dust respirators

should be worn for protection. If power cleaning must be done in areas where flammable

vapors may be present, nonsparking motors

and tools should be used to avoid the danger

of explosion.

6.3.1.8. Sandblasting is a form of blast

cleaning in which sand is employed as the abrasive which is propelled by compressed air

through a nozzle directed against the sur- face to be cleaned. Other forms of blast cleaning utilize crushed iron grit (grit blasting) or

iron shot (shot blasting) as the abrasive;

these may use either compressed air or centrifugal wheels to propel the abrasive. Grit

blasting and shot blasting are very costly methods, the use of which is largely restricted to shop work or vacuum blasting where the ex- pensive abrasive materials can be recovered

for reuse; moreover, these methods usually

yield a greater surface roughness (higher an- chor pattern) than is desirable for most coat- ing applications. The present section will be

limited to a discussion of sandblasting as a most effective method of preparing steel and

other surfaces for painting.

The height of the anchor pattern, or the

maximum height of profile of a blasted surface, is the distance from the bottoms of the

lowest pits to the tops of the highest peaks.

The larger the size of the abrasive employed,

the higher is the anchor pattern or the greater

is the surface roughness produced. For good

durability, the overall thickness of the dry coat- ing must be great enough to cover even the

peaks with an adequately thick protective film. Thus, surfaces that are to receive a thin coat- ing require a shallower anchor pattern and

hence must be cleaned with a finer abrasive

than surfaces that are to receive a thick coating. Thicker coatings will not only tolerate a deeper anchor pattern but may benefit in adhesion and durability from the greater sur- face contact and better "tooth" provided by

the rougher surface. There is, of course, a

practical limit to the surface roughness that can be tolerated by films of practical thickness

;

peaks that are too thinly coated become focal

points for wear and corrosion and premature

coating failure. The effectiveness of sandblasting as a means of preparing steel surfaces

for painting lies in the ready availability and

relatively low cost of sand having a particle

size conducive to production of an excellent

anchor pattern on the cleaned surface.

6.3.1.8.1. Sandblasting to "white" metal, while

not always feasible or economical, is the most

thoroughly effective method available for preparing steel surfaces for painting. It removes

all rust, mill scale, corrosive salts, dirt, grease,

oil, wax, and other foreign matter from the

metal—no matter how heavy the corrosion or contamination—leaving a completely clean uniform surface with an ideally roughened texture

or profile (anchor pattern) for maximum coat- ing adhesion and durability under the most

severe service conditions. Although sandblasting is a relatively expensive method of surface

preparation, the greater coating durability

obtainable under adverse service conditions will

often make it the most economical method in

the long run. This is especially true in severe exposure environments, such as chemical atmospheres or water immersion. Thorough sandblasting is, in fact, the only method of surface

preparation that will ensure maximum coating

performance under such rigorous conditions.

112
There may be occasional situations, however, in

which the pattern of corrosion would not justify

complete sandblasting to white metal even for severe service exposure. For example, to re- move the last traces of rust from a deeply pitted

surface may be impracticably expensive because

of the extra labor involved and the excessive

removal of sound metal required to reduce the

entire surface to the level of the pits. In general, sandblasting can advantageously be employed wherever the greater coating durability

obtainable by this method justifies the extra

time and cost.

6.3.1.8.2. In less severe service environments,

where maximum coating durability is not re- quired, lesser degrees of sandblasting than

cleaning to white metal are usually satisfactory.

So-called commercial blast cleaning envisions a good but not perfect cleaning job in which

nearly all rust, mill scale, and contaminants are removed from the surface, but not necessarily

to the point where a surface of uniform color,

texture, and cleanness is obtained. This degree

of cleaning is significantly lower in cost than

blast cleaning to white metal, and is generally

adequate for all but the most rigorous types

of service exposure.

6.3.1.8.3. To meet certain situations in cor- rosive environments where commercial blast

cleaning is inadequate but white metal blast

cleaning is prohibitively expensive, an intermediate grade called "near-white" blast cleaning, permitting a small specified amount of

streaking and shadowing, has recently been

established by the Steel Structures Painting

Council [153] after successful use in industrial,

state, and federal trials.

6.3.1.8.4. So-called "brush-off" blast cleaning is a rapid type of superficial sandblasting intended

to remove all loose rust, loose mill scale, and

loose paint from the surface, leaving only tightly

adhering, intact mill scale and rust. This degree of surface preparation is comparable or superior to the best that can be accomplished

by hand or power tool cleaning methods, and

is adequate for coating systems with good wetting ability which are to withstand only mild

exposure.

6.3.1.8.5. Wet sandblasting employs a slurry of

sand and water instead of dry sand as the abrasive. Heavy rust and tight mill scale are effec- tively removed by this method, and the dust

problems and hazards of dry sandblasting are

largely eliminated. However, the clean, wet

surface begins to rust again almost immediately

unless corrosion inhibitors are incorporated in

the blast water. Even when inhibitors are

used, some doubt exists as to the efficacy of the

passivated surface in promoting maximum adhesion and durability of the applied coating system. Wet sandblasting is not widely used now

as a method of preparing steel surfaces for

painting.

6.3.1.8.6. Dry sandblasting equipment and

techniques.—An efficient sandblasting operation

requires the right combination of nozzle type,

hose size, abrasive size, air pressure and flow,

and sand handling capacity—also, a competent

nozzleman. Nozzles of widely different durability and cost may be made of ceramics, cast iron,

steel alloys, tungsten-carbide, and other suitable

materials. Nozzle size is limited by the requirement that the hose be 3 or 4 times larger in

diameter than the nozzle and yet be reasonably

flexible and manageable. For general sandblasting work, a %-inch nozzle, requiring a

114, -inch hose, is the largest size that is practical. However, nozzles up to %-inch diameter

can be used on large flat surfaces where their

rigidity and unwieldiness do not seriously affect the overall production rate. A %-inch

nozzle operated at an air pressure of 90 lb/in ^ requires about 210 ft ^/min of air and will pass

about 1500 pounds of sand per hour. Both sand

and air must be dry and oil-free. An optimum

anchor pattern for painting is obtained with an

abrasive size of 20 to 50-mesh. During blasting the nozzle is usually held at a slight angle

from the perpendicular at a distance of 12 to 18 inches from the surface; however, an angle

of about 45 degrees may be more effective for

dislodging loose corrosion products or loose

paint.

6.3.1.8.7. Vacuum blast cleaning systems are

available for specialized use where dust is not

permissible or when the abrasive is to be re- covered. The blast nozzle is enclosed in a hollow cone having a brush at one end to trap the

abrasive and cleaning debris while the vacuum

exhaust is applied through the side of the cone.

6.3.1.8.8. Cost and Production Rate.—The

overall cost per unit area of sandblast cleaning

is determined primarily by the production rate,

the cost of labor, and the cost of the abrasive.

The production rate (surface area cleaned per

unit time) will vary greatly with the nature

and amount of corrosion products to be re- moved, with the degree of surface cleanness to be achieved, with the type of equipment and

blasting conditions employed, and with the skill of the nozzleman. The available figures for

production rates obtainable by the various degrees of sandblasting (viz, white metal, commercial, and brushoff) are not in good agreement; however, as a rough guide-line, under

ordinary conditions, sandblasting to white metal may proceed at the rate of 50 to 150 square

feet per hour, while the production rate for

commercial blast cleaning may be about 3 times

greater, and the production rate for brushoflf

113
blast cleaning may be 6 to 8 times greater.

Obviously, cleaning to white metal is considerably more expensive than commercial blast

cleaning and is many times more costly than

brushofF blast cleaning. The degree of cleaning to be done should be adequate for the particular coating system and service environment

involved. Cleaning beyond this point is un- necessary and uneconomic, while less than the

required degree of cleaning ultimately results

in higher costs by shortening the service life of the coating.

6.3.1.8.9. Safety.—Sandblasting is a hazardous operation unless adequate precautions are taken to protect personnel and property against

harm from dust and flying chips and particles.

The nozzleman should be equipped with a U.S.

Bureau of Mines approved helmet connected to a

source of clean, compressed air. Other workers

in the area should wear goggles and filter- type air respirators. Sandblasting is a con- stant menace to unprotected eyes, and inhalation of the dusts may cause silicosis. Blasting

should not be done in areas where explosive

concentrations of volatile vapors may be present. Blast hoses should be grounded to dissipate static charges. Nearby property should

be protected by covering, masking, or wrapping.

Ropes used for scaffolding should be heavily

wrapped to prevent damage. Wet sandblasting,

while free of dust hazards, presents a hazard

from the toxic, soluble chromium compounds

(corrosion inhibiting agents) contained in the

mist. For detailed information on safe practice in sandblasting, reference may be made to the codes or manuals of various national associations concerned with this problem [75,76,77]

.

6.3.1.8.10. After sandblasting is completed,

any traces of residual dust, debris, or abrasive

should be completely removed by brushing with

clean brushes, blowing with clean compressed

air, or vacuuming. The fresh, sandblasted,

bare metal surface is very susceptible to corrosion and will rust quickly if wet or exposed

to high humidity. The clean surface must be

treated or primed promptly, before rusting or contamination can occur. Even in ordinary

mild atmospheres, it is highly desirable that

the surface be primed or chemically treated

within eight hours after sandblasting.

6.3.1.9. Flame cleaning, depending on the

manner in which it is employed, can be used

either to remove all loose mill scale and rust

from unpainted steel or to remove old paint

from metal surfaces. For the removal of mill

scale and rust from unpainted steel, an intensely

hot, high-velocity, oxy-acetylene flame is passed over the metal surface at the rate of about 15

to 35 feet per minute. The rapid heating and

differential expansion of the mill scale and base

metal, aided by the "explosive" action of water

vapor generated beneath the scale, breaks it loose from the surface so that it can be removed

by scraping and wire brushing. Rust is con- verted to a black powder that may then be

brushed from the surface. Heavily rusted and

heavily scaled steel may require a slow traverse

or several passes of the flame to accomplish

the removal. Tightly bonded mill scale is not

removed by flame cleaning.

Immediately following the flame treatment,

the surface should be power wire brushed or scraped to remove all loosened rust, scale, and

foreign matter, and then swept or brushed free

of any residual dust and debris. Promptly

after the flame treatment and cleaning—while

the metal is still warm and well above the temperature of the surrounding atmosphere—the

prime coat should be applied. Unless the prime

coat is applied promptly to the warm metal,

before any moisture becomes absorbed on the

surface, most of the benefits of the flame cleaning will be lost and the results will be little if any better than power tool cleaning alone. The

requirement that priming follow so closely upon

the heels of the flame cleaning and wire brushing creates both scheduling problems and a possible fire hazard, since the primers employed

usually contain volatile, flammable solvents.

The priming must be conducted in a manner

that will not result in the buildup of explosive

concentrations of solvent vapor in the area of

the flame.

If oil or grease are present on the surface,

they should be removed by solvent cleaning

and the solvent allowed to evaporate—before

using the flame cleaning method. Grease and

oil may char under the rapidly moving flame,

leaving an objectionable carbon deposit on the

surface.

When properly done, flame cleaning is con- sidered to give results better than that obtainable by power tool cleaning alone, but it is not

a substitute for sandblasting to a "commercial"

surface or to "white" metal.

Flame cleaning is very useful in maintenance

painting where it is necessary to remove all the old paint from the metal. For this purpose,

slow traverse of the flame head and multiple

passes are usually required. While the usual

oxy-acetylene flame cleaning equipment is suitable, other types of torches utilizing propane,

gasoline, or other fuels capable of hot flame

may be used. Coatings that soften with heat can be removed with a scraper immediately

following the flame. For coatings that are not

softened appreciably by heat, the flame must

be held at one place until the coating is burned

away, after which the ash and any carbonaceous residue must be removed by scraping or wire brushing. The considerable labor involved

in the removal of such residues can be reduced

114
by the use of special flame cleaning equipment

in which the oxy-acetylene flame is supplemented by a jet of oxygen which assists the

burning and at the same time blows the residue

from the surface as it forms.

6.3.2 Chemical Pretreatment of Steel

(and Iron) Surfaces

As indicated previously, metal surfaces are given chemical pretreatments to form a corrosion-inhibiting, paint-receptive coating on the

surface through a chemical reaction with the

metal. In practice, such conversion coatings

for steel and iron are nearly always insoluble

metal phosphates, although chromate and oxide

coatings find limited application. Phosphate

coatings, though insoluble in water, are capable

of absorljing small amounts of corrosion inhibi- tors (notably, chromic acid) from dilute solution to enhance corrosion resistance and further

increase the life of the applied paint. It is for

this reason that most phosphatizing processes

specify a final rinse in dilute chromic acid or a dilute chromic-phosphoric acid mixture.

In addition to the direct phosphating types of

inorganic pretreatment, wide and effective use

is made of the wash primer or washcoat type

of organic pretreatment coating. The latter

provides a mild phosphatizing action on the

surface while laying down a thin, very adherent

organic film. Organic pretreatment coatings

should not be used over surfaces that have al- ready received a phosphate pretreatment.

The chief types of inorganic phosphate pretreatment and the organic coating (wash primer) type of pretreatment are described in the

following subsections.

6.3.2.1. Cold phosphate pretreatment

(PHOSPHORIC ACID TYPE) : This type of pretreatment utilizes relatively dilute phosphoric

acid solutions to etch the surface lightly and

convert a very thin layer of the metal to an

insoluble phosphate coating weighing about 8

milligrams per square foot of surface. The

solutions employed are similar to those used in

the phosphoric acid cleaning of metals as described in section 6.3.1.5.1; however, when

these materials are used as a pretreatment, the

emphasis is upon chemical conversion of the

metal rather than on cleaning. The surface

should previously have been cleaned free of all

oil, grease, dirt, mill scale, and rust—except for

very light rust which may be converted during

the treatment. If used over mill scale, the

scale may later loosen and lift the paint. Unlike acid cleaning processes in which the solutions may be used hot and profusely and for

relatively long periods of time, the technique

in pretreating is to apply only the necessary

amount of conditioner and to leave it on only

for a brief period—until the acid has reacted

with the metal to form the desired gray-white

phosphate coating—before washing or wiping

off the excess acid and any loose residue.

Suitable cold phosphating solutions containing phosphoric acid, an oil solvent, a wetting

agent, and water are described in Military

Specification MII^M-10578 [78]. This specification covers two types of metal conditioning

compounds, both of which must be diluted with

three times their volume of water before use. The two types differ primarily in phosphoric

acid concentration. Type I, after dilution for

use, contains about 10 percent by weight of

phosphoric acid ; type II contains about 3 percent by weight of phosphoric acid after dilution.

The stronger, type I solution is used where some light rust is present and where the sur- face can be washed off to remove all reacted

phosphoric acid. Free phosphoric acid on the

surface is detrimental to paint life. After

the treated surface has been thoroughly

washed, it should be giver, a final rinse with a

dilute chromic acid or dilute chromic-phosphoric acid mixture to passivate the surface.

A chromic acid concentration of about 0.02 to 0.05 percent by weight is satisfactory.

The weaker, type II conditioner is used

where it is not practicable to rinse off the

treated surface. The conditioner, after proper

dilution (with three times its volume of water,

or more if necessary) , should be applied sparingly to the clean metal surface and allowed

to react with the surface and dry upon it to form a grayish-white deposit of insoluble phosphate. If the proper strength and amount of

solution are used, all the phosphoric acid will

be reacted and it will not be necessary to wipe

off or further treat the surface. However, if any unreacted phosphoric acid remains on the

surface (as evidenced by a dark, sticky residue) , it must be wiped off with damp rags or with rags wet with an alcohol-water mixture

to remove all traces of stickiness, and the sur- face again allowed to dry. The dried surface

should have a white appearance; however, any

excessive or loose powdery deposit should be

brushed off with dry brushes or dry rags, or removed with clean compressed air or by

vacuuming.

The treated surface should be given a priming coat of paint as soon as practicable, before

any rusting or contamination of the surface

occurs. Cold phosphate pretreaments are not in- tended for use in maintenance painting where

the acids employed may contact and damage

existing paint. The solutions should not be

stored or mixed in uncoated steel or galvanized

steel containers; glass or earthenware vessels are satisfactory.

Precautions are necessary to protect workmen from accidental splashes of the acid solutions. The solutions are particularly danger115
ous to the eyes; splashproof eye shields or goggles should be worn to prevent painful,

serious injury. Acid-resistant rubber gloves,

aprons, and boots should be provided where

needed.

6.3.2.2. Cold phosphate pretreatment

(ZINC phosphate type) : Proprietary zinc

phosphatizing solutions containing zinc phosphate, phosphoric acid, and an activating agent

are available which can be applied unheated

to steel or galvanized surfaces, using ordinary application methods such as brush, spray,

dip, or flow coating. The surface to be treated

should first be cleaned by an appropriate

method (see sec. 6.3.1) to remove any oil, grease, dirt, rust, and mill scale. The dilute,

unheated phosphatizing solution is then applied in accordance with the manufacturer's

instructions and allowed to remain on the

surface until the zinc phosphate coating is formed (usually for two to five minutes).

The surface is then thoroughly washed with

clean water. A final rinse in dilute chromic

acid or a dilute chromic-phosphoric acid mixture is usually specified in order to achieve max- imum corrosion resistance and paint life. The

priming paint should be applied to the phosphated surface as soon as practicable after it is dry.

6.3.2.3. Hot phosphate pretreatment

(zinc phosphate type) : These are proprietary

processes based on specially formulated, balanced solutions of phosphoric acid saturated

or supersaturated with acid zinc phosphate

salts (plus accelerating agents) that deposit a

heavy, visibly crystalline coating of zinc phosphate (weighing about 150 to 400 milligrams

per square foot) on the surface of the metal.

By providing substantial protection against the

spread of corrosion beneath the paint film as

well as a physically receptive surface for excellent paint adhesion, the heavy phosphate

coating significantly increases the durability

of the applied paint, particularly under severe

service conditions involving corrosive atmospheres or water immersion. (For mild exposure conditions, where it is neither economic

nor necessary to thoroughly clean the metal

surface by sandblasting, the benefits of hot

phosphatizing may not be sufficient to justify

the additional cost of such treatment.) Hot

zinc phosphate pretreatments are largely limited to objects of a size and shape that can be

treated in dip tanks or by spraying in appropriate enclosures. They are therefore particularly suitable for the pretreatment of fabricated steel on the production line. There have

been some developments in special thioxotropic

or gelled phosphate solutions for field application, but these have not been widely used. Hot

phosphate treatments are not used in mainte- nance painting.

Hot phosphate pretreatments require a completely clean bare surface, preferably one that

has been sandblasted if heavy rust or mill

scale was present. In any event, the surface

should be free of oil, grease, dirt, paint, rust,

and mill scale. Federal Specification TT-C490 [79] specifies that the clean metal should

be exposed to the hot phosphating solution for

at least three minutes if a dip method is used

or for at least 1 minute if spraying is employed.

The equipment used should be constructed of

materials resistant to the action of the phosphating solution and should not contain copper alloy fittings or brazing where they may

come in contact with the solution. Fog sprays

should be provided on both dip tanks and spray

equipment to prevent the solution from drying on the work surface before the necessary subsequent washing with water. Hot phosphate

residues are detrimental to paint life and must

be removed by thorough washing with hot or

cold clean water, followed by a final rinse with

a dilute chromic acid or chromic-phosphoric

acid mixture to enhance corrosion resistance

and paint life. The work should remain in the

chromic rinse for a minimum of 1 minute when

dipped or for at least 30 seconds in spray processes. TT-C-490 specifies that the chromic

acid rinse solution be maintained at a temperature of 140 to 210 °F (60 to 99 °C) and a pH of 2 to 4. Industry recommendations do

not necessarily coincide with those given in

TT-C-490, and the instructions of the manufacturer or supplier should be consulted in the

case of specific proprietary materials. Industry recommendations frequently call for lower

temperatures (room temperature to about

160 °F [71 °C]) and a higher, more stringently controlled pH (4 to 5) than those given

in TT-C-490.

The zinc phosphate coating should be contin- uous, uniform in texture, evenly deposited, and

gray to black in color. It should not be mottled

in appearance nor show any smut, powder, cor- rosion products, or white stains from dried

phosphating solutions. Light brown or orange

stains from the chromic acid rinse do no harm

;

however, deep brown stains may be evidence of

heavy concentrations of chromic acid which

might cause blistering of some paint films. The

minimum weight of the phosphate coating

should be 150 milligrams per square foot for

spray processes and 300 milligrams per square

foot for dip processes.

Care should be taken to prevent burns from

the hot solutions or toxic effects from the

chromic acid rinse. Splash-proof eye shields

should be worn to avoid painful damage to the

eyes. The phosphated surface should receive a priming coat of paint as soon as practicable,

before any contamination or deterioration of

the surface occurs.

116
6.3.2.4. Hot phosphate pretreatment

(iron phosphate type) : These are proprietary processes based on balanced aqueous solutions of phosphoric acid and acid phosphate

salts, with or without accelerating agents,

which build up a coating of insoluble amorphous

iron phosphate on the surface of the metal.

The iron phosphate coatings thus obtained are

not as heavy as those yielded by the hot zinc

phosphate processes. A minimum coating

weight of 40 milligrams per square foot is usually specified, and the coating weight that can

be built up usually does not exceed 100 milli- grams per square foot. The thinner, amorphous

coating of iron phosphate is more flexible than

the crystalline zinc phosphate type and hence

is j)referred in applications where the metal is to oe formed after painting. Like zinc phosphate coatings, the iron phosphate coatings

provide substantial protection against the

spread of corrosion beneath the paint film as

well as a physically receptive surface for excellent paint adhesion, thus contributing significally to increased paint life, especially

under severe service conditions such as cor- rosive atmospheres or water immersion.

Hot iron phosphate pretreatments are limited to objects of a size and shape that can be

treated in dip tanks or in specially designed

spray enclosures. They are particularly suitable for the factory pretreatment of production line items. As with other hot phosphate

pretreatments, the surface to be treated must

be completely clean—free of oil, grease, dirt,

paint, rust, and mill scale. The treatment should

be continued until a phosphate coating is produced which is insoluble in water and has a

color ranging from a golden yellow to purple.

Federal Specification TT-C-490 [79] specifies

that the items be exposed to the phosphating

solution for at least three minutes in dip processes or for at least 1 minute in spray processes.

The article should then be rinsed in clean

water, followed by a final rinse with a dilute

chromic acid (or dilute chromic-phosphoric

acid) solution similar to that used in zinc phosphate processes (see sec. 6.3.2.3). The hot

phosphate residues are detrimental to paint life and should not be allowed to dry upon the sur- face prior to removal by washing.

The insoluble, golden yellow to purple iron

phosphate coating should be continuous, uniform in texture, and evenly deposited. There

should be no smut, power, corrosion products,

or white stains due to dried phosphating solutions on the surface.

Precautions should be taken to prevent burns

from the hot solutions or toxic effects from the

chromic acid rinse. Splashproof eyeshields

should be worn to prevent painful, serious damage to the eyes. The phosphated surface should receive a coat

of priming paint as soon as practicable, before

any contamination or deterioration of the sur- face occurs.

6.3.2.5. Organic pretreatment coating

(WASH primer) : The wash primer or wash

coat type of pretreatment provides a mild phosphatizing action on the metal surface while

laying down a thin, highly adherent organic film

which provides an excellent base for most or- ganic coating systems. The wash primer type

of pretreatment is particularly useful where

the size and shape of the work preclude the use

of hot phosphate solutions or where mixed

metal assemblies are to be treated. The wash

primer pretreatment is of maximum effective- ness and outstanding benefit when used on

thoroughly cleaned steel (sandblasted or

pickled) which will be exposed to severely cor- rosive atmospheric conditions or to water immersion. It also improves the durability of

conventional paints on hand cleaned steel ex- posed to corrosive atmospheres or water immersion. For hand-cleaned steel to be ex- posed in mild, ordinary atmospheres, the wash

primer pretreatment—while beneficial—may

not be economically justifiable. Wash primer

should not be used on phosphated steel or over

other pretreatments.

6.3.2.5.1. Two-component wash primer.—

The most widely used and proven type of wash

primer organic pretreatment coating is the

two-component type comprising (1) a base

material containing an alcohol solution of polyvinyl butyral resin pigmented with insoluble

basic zinc chromate, and (2) an activating

diluent in the form of an alcohol solution of

phosphoric acid. This is the type of pretreatment commonly known as WP-1, or as Bureau

of Ships Formula No. 117 as covered by Military Specification MII^C-15328 [30] . A similar composition providing a smoother finish for

aircraft use is covered by MIL-C-8514 (Aer)

[80] . Application methods for this type of pretreatment are given in MIL-C-8507 [81].

The resin base and the acid diluent must be

mixed just before use and should be applied

within 4 hours after mixing. The fresher the

mixture, the better are the results obtained.

Since the phosphoric acid reacts with the vinyl

resin and the pigment, as well as with the

metal surface, the activated mixture may gel, or, in any event, become useless for its chief

function of promoting good adhesion if it stands

for more than about 8 hours. It is essential

that the base material be thoroughly mixed by

vigorous mechanical shaking or stirring to re- incorporate all settled pigment or caked material before adding the activating diluent (the

latter also should be shaken before use).

For use, one part by volume of the phosphoric

acid activator-diluent should be added sloivly,

ivith continuous stirring to prevent localized

117
gelling, to four parts of the resin base. The

resin base should never be added to the diluent.

The activated mixture should be thinned to spraying consistency with alcohol ; generally, a volume of thinner equal to that of the added

diluent will suffice, but up to double this volume

is permissible if necessary to obtain a thin, wet,

uniform film. The added alcohol should nor- mally be either ethyl alcohol or isopropyl alcohol; however, under high humidity conditions, some butyl alcohol may be substituted for a

portion of the lower boiling alcohols to an ex- tent sufficient to prevent blushing. The dry

film thickness should be 0.2 to 0.3 mil for optimum results and should not exceed 0.5 mil in

any case. At the correct thickness, the film

will have a greenish-yellow translucent appearance through which uneven coloring in the

substrate will show. Wash primer should never be applied so thick as to hide the substrate. It should be emphasized that the

washcoat must be laid down wet to obtain a continuous film; otherwise, the coating will be

powdery and its adhesion seriously impaired.

Brush application, while generally less satisfactory than spray, may be used on rough sur- faces or for touching up small areas. When

spot treating previously painted surfaces, the

wash primer should ordinarily be applied only

to bare areas, although overlap of the surrounding paint is not harmful provided the washcoat

adheres well to the old paint and does not lift it. The wash primer pretreatment should not be

thought of as a coat of paint. While it may

afford temporary protection for several weeks

under mild, dry conditions (more than is afforded by an inorganic phosphate coating), it is not intended for use as a shop coat or to protect against exposure. For optimum adhesion

and best results, the wash primer should re- ceive a coat of priming paint as soon as it is completely dry—usually in from one to four

hours after the pretreatment. Intercoat adhesion will be impaired either by premature application of the primer or by unnecessarily long

aging of the washcoat. Virtually any type of

primer and many types of coatings can be used

over the wash primer pretreatment with good

results; certain lacquers and vinyl paints are among the exceptions that require special

primers or intermediate coats.

6.3.2.5.2. Single-package tvash primers based on lead chromate pigment or on chromic-phosphate pigments have recently become available.

A wash primer of this type for use on steel,

aluminum, and magnesium is covered by Federal Specification MIL-P-14504 [82]. These

are proprietary materials, and the directions

of the manufacturer should be followed in their

use. While the convenience of a single-package

wash primer is obvious, these materials do not

yet have the long history of successful use that

is enjoyed by the two-component wash primers,

and the one-package wash primers have not

found wide use.

6.3.3. Galvanized Surfaces

Serious difficulty has often been experienced

in obtaining good adhesion of conventional

paints to galvanized steel, especially to new galvanized surfaces. All too often, the paint peels or flakes off after a short period of exposure.

A chief cause of such early adhesional failure,

apart from improper cleaning of the surface,

is thought to be the reactivity of new zinc with

many paint vehicles. While the nature or mechanism of this reaction is not well understood,

practical means for dealing with the problem

are available. Good adhesion and durability of

paints applied on galvanized surfaces can be

ensured by: (1) thorough cleaning of the surface, and (2) adequate weathering and /or

chemical pretreatment of the surface to form a

paint-receptive barrier layer that insulates the

paint film from the reactive zinc while shielding

the zinc from further attack by atmospheric

corrosive agents.

6.3.3.1. New galvanized surfaces frequently are coated with factory processing compounds, temporary protective oils, and other

contaminants harmful to paint adhesion unless

removed prior to pretreatment or painting. In

addition, most of the galvanized sheet and strip

made today is given a chromate treatment of one kind or another at the mill to protect against

white stain, particularly if the metal is destined

for warehouse storage. Surfaces thus treated

may not be receptive to some types of paints

and cannot be further pretreated by the methods usually prescribed for galvanized steel. Where it is desired to use a particular pretreatment (e.g., hot phosphating) on galvanized

steel, the problem is best avoided by specifying "no treatment" on the mill order. When

the metal is obtained from a warehouse and

the nature of the factory treatment is unknown,

probably the best practice is to solvent-clean

the surface (using a spectrum of solvents if necessary) and then apply a zinc dust primer

such as that covered by Fed. Spec. TT-P-641

[29], followed by the desired finish. The problem has been considered by the American Iron

and Steel Institute [83] and by the American

Society for Testing and Materials [84].

Zinc dust primers generally give good results on clean galvanized surfaces with or without

pretreatment. However, pretreatment is beneficial and is recommended where the maximum

possible service life is sought, provided that the

galvanized surface has not previously been

given an interfering factory chromate treat- ment.

118
For optimum results, new galvanized sur- faces should be thoroughly solvent-cleaned by

one of the methods described in section 6.3.1.1 to produce a surface completely free of oil, grease, processing compounds, temporary protective substances and other foreign matter.

The clean surface, if not factory chromated as noted above, should then be given one of the

following pretreatments

:

(1) Hot zinc phosphate pretreatment similar

to that described in section 6.3.2.3 but specially

formulated for galvanized surfaces, provided

that the item is of a size and shape that can

be treated in hot dip tanks or in special spray

enclosures. A pretreatment of this type is covered by specification MIL-T-12879, Type I, Class 1 [85].

(2) Cold zinc phosphate pretreatment of the- brush-on or wash type described in section

6.3.2.2. (3) Wash primer type of organic pretreatment coating as described in section 6.3.2.5. (4) Cold phosphate pretreatment of the dilute phosphoric acid type as described in sec- tion 6.3.2.1. (5) Chromate pretreatment of the general

nature described in section 6.3.6.1 and as cov- ered by specification MIL-T-12879, Type I, Class 2 [85].

The best choice among the above recommended

pretreatments is largely a matter of convenience as determined by the size and shape of the

object, its location, and the application equipment and facilities available. The most generally applicable and most widely used pretreatment is the wash primer type; it gives

excellent results in virtually all situations. The

proprietary zinc phosphate pretreatments,

either hot or cold, also give excellent results;

where the hot zinc phosphate pretreatment is feasible—as in factory coating operations—it is probably the best pretreatment available.

The dilute phosphoric acid type of cold phosphate pretreatment, while less effective than

the others, may be the expedient choice in sit- uations where the treated surface cannot be

rinsed off or where other application limitations exist.

Chromate treatments are more often used for

the protection of zinc surfaces against white

stain rather than as a paint base; to ensure

the paintability of a chromate-treated surface,

it is essential that a prepaint type of treatment

be specified [85]

.

"Home cure" treatments such as washing the

galvanized surface with vinegar, cider, acetic

acid, muriatic (hydrochloric) acid, or copper

sulfate solution have been shown to be useless or even harmful and should be avoided.

6.3.3.2. Old galvanized surfaces that have

weathered for six months or more—so that oils.

processing compounds, and protective treat- ments (including factory chromate treatments) —have been largely removed from the surface

may frequently be painted satisfactorily with

conventional paint systems provided the sur- face is first cleaned with solvent to remove

any residual oil or soil. For reliable results,

however, chemical pretreatment (if no residual

chromate coating is present) and/or the use

of zinc dust primer is highly recommended.

If the surface is too soiled for adequate cleaning with solvents alone, it may be brushed

lightly with steel wool to assist in removal of

the soil and then brushed, blown, vacuumed,

or again solvent washed. Rusted areas on

galvanized steel should be hand cleaned or wire

brushed in the same manner as rusty steel. If large areas of the galvanized surface are badly rusted, it will be necessary to clean the

entire surface by the same vigorous methods

(e.g., acid cleaning or sandblasting) that are used for rusted steel (see sec. 6.3.1).

6.3.3.3. Cadmium surfaces may be cleaned,

pretreated, and successfully painted by the same methods that are employed for galvanized

(zinc-coated) surfaces (also see sec. 6.3.6.2).

6.3.4. Aluminum and its Alloys

Aluminum and its alloys should be solventcleaned by one of the methods discussed in

section 6.3.1.1 to remove all oil, grease, soil,

and other foreign matter. The use of mechanical cleaning methods is damaging to the surface, particularly where a thin cladding of

aluminum is present on a substrate of a harder

alloy. Where the surface is too soiled or deteriorated to be adequately cleaned with sol- vents alone, the minimum amount of brushing

with stiff bristle or aluminum wool that will

dislodge the soil should be used.* All debris

from the mechanical cleaning- operations should

be swept from the surface or removed with

clean, compressed air or by vacuuming, and

any residual oily contaminants should be re- moved by another solvent wash.

The clean surface should be pretreated immediately by one of the following methods:

(1) Wash primer type of organic coating

pretreatment described in section 6.3.2.5 and

covered by MIL-C-15328 [30].

(2) Chemical coating pretreatment covered

by Military Specification MIL-C-5541 [86].

(3) Anodic coating pretreatment by the

chromic acid process or the sulfuric acid process, both of which are covered by MIL-A-8625

as Type I and T^pe II, respectively [87].

steel wool or wire brushing should be avoided, since it may

result in the embedment of fine steel particles in the soft aluminum

surface that are difficult to dislodge and may cause serious galvanic

rusting and pittine.

119
Of the above methods of pretreatment, the

wash primer type is best suited for use in the

field. It gives excellent results and is widely

used in both field and industrial applications.

For best results, the wash primer should re- ceive a coat of priming paint within about 1 to 4 hours—after it is adequately dry but before

it ages unnecessarily. A zinc yellow primer

such as that covered by Fed. Spec. TT-P-666

[88] is highly recommended for use over the

wash primer on aluminum surfaces. Other

primers in which zinc yellow constitutes a major portion of the pigment, such as that covered

by TT-P-645 [89], may also be used.

The chemical "films" covered by MIL-C-5541

are intended to be chemical pretreatment coat- ings designed to improve paint adhesion and

to inhibit underfilm corrosion. Products meeting these requirements^ usually produce a chromate-type coating on the surface of the

aluminum, either by chemical reaction with

the surface or by deposition of the coating

from solution or by a combination of the two

processes. Despite the wide and effective use

of these chromate-type coatings, their exact

chemical composition is not reliably established.

They have been variously described as chromium chromates, chromate-chromic oxides, and

chrome-aluminum oxide complexes ; however,

all of them are derived from acidic hexavalent

chromium solutions of the general nature described in section 6.3.6.1, and they all produce

thin amorphous corrosion-inhibiting films having excellent paint-base qualities on aluminum

surfaces. In an older, less widely used in- dustrial pretreatment process, coatings consisting essentially of zinc and aluminum phosphates are formed on the aluminum surface by

treating it with zinc phosphate solutions con- taining fluoboric acid; these are crystalline

phosphate coatings similar to those formed on

steel or zinc by other proprietary processes (see

sees. 6.3.2.2 and 6.3.2.3). While many of the

chemical pretreatments which have been mentioned are designed for industrial use, others

may be used in the field; the instructions of

the manufacturer should be followed in their

selection and use. The anodic coating type of pretreatment

(covered by MIL-A-8625), using chromic acid

or sulfuric acid as the electrolyte in an anodizing bath, is limited to industrial applications.

The oxide coatings produced by anodic treat- ments are denser and less porous than most

chemical conversion coatings, but they may still be improved by sealing with a hot water treat- ment containing dilute amounts of chromate or dichromate salts.

1 See Qualified Products List QPL-5541 established for Military

Specification MIL-C-5541 by the Bureau of Naval Weapons, Depart- ment of the Navy, copies of which can be obtained by writing the Commanding Officer, Naval Supply Depot, 5801 Tabor Avenue,

Philadelphia, Pennsylvania 19120.

6.3.5. Magnesium and its Alloys

The corrosion resistance and the paintability

of magnesium and magnesium-base alloys can

be greatly increased by application of a suitable

chemical or electrochemical pretreatment prior

to application of a suitable primer. (Primers

for use on magnesium surfaces are usually of

the zinc chromate type. They must be carefully

selected to provide adequate adhesion (see sec. 2.4.2.4)). Most of the paint-base treatments

are of the chromate type. Before chemical pretreatment, the magnesium surface should be

thoroughly cleaned to remove all oil, grease,

welding flux, soil, oxides, or other foreign

matter.

6.3.5.1. Cleaning.—Since magnesium, unlike aluminum, is not attacked by caustic solutions, highly alkaline cleaners similar to those

used for the heavy duty cleaning of steel (see

sec. 6.3.1.4) may be employed, and this is the

preferred method for cleaning ordinary grease

and soil from magnesium-base surfaces. The

alkaline cleaner should be maintained at a above 11 for general cleaning and at a pH

above 13 for the removal of graphite lubricants ; the latter is accomplished by soaking the

part for 10-20 minutes in the hot caustic (about

200 °F [93 °C]), followed by thorough rinsing

in cold water and immersion in a chromicnitrate pickle to complete removal of burned-on

lubricant. Old paint coatings may be removed

with alkali paint removers such as that covered

by Federal Specification TT-R-230 (see sum- mary chart in sec. 8.2).

6.3.5.1.1. Electrochemical cleaning considerably shortens the cleaning time. Using an

alkaline cleaning bath at a temperature of

about 200 °F (93 °C) and a pU above 11, the

parts are made cathodes in the bath employing

a current density of 10 to 40 amperes per

square foot at 6 volts for 3 to 10 minutes.

6.3.5.1.2. Alkaline cleaning of any type must

always he folloived by thorough rinsing in cold

ivater. Alkaline residues are very detrimental

to paint life. Even when acid pickling is to be

used subsequent to alkaline cleaning, it is essential to rinse away all alkaline or soapy residues,

since soaps carried over into the acid bath will

form an oily layer on the surface of the liquid

which will contaminate the work as it is immersed or withdrawn.

Where large amounts of oily or greasy con- taminants are present on the magnesium surface, they should be removed by solvent cleaning (see sec. 6.3.1.1) prior to alkaline cleaning

to avoid premature fouling of the alkaline

cleaner.

120
6.3.5.1.3. Acid pickling is used to remove

oxide layers, old chemical finishes, burned-on

drawing and forming lubricants, and other

water-insoluble or non-emulsifiable substances

from magnesium-base surfaces. Chromic acid

pickling is preferred for general cleaning and

for the removal of surface oxidation and cor- rosion products; this type of pickle does not

result in dimensional loss and may be used on

parts with tolerance limits. A chromic-nitrate

pickle is used primarily for the removal of

burned-on graphite lubricants. Sulfuric acid

pickling is used on magnesium sand castings

to remove the effects of blasting operations;

alternatively, a nitric-sulfuric acid pickle may

be used for this purpose. A chromic-nitrichydrofluoric acid pickle may be used for the

pickling of castings, particularly die castings.

A strong phosphoric acid pickle also may be

used for cleaning all types of die castings. For

the removal of mill scale, particularly from

sheet material, and to ensure maximum cor- rosion protection, an acetic-nitrate pickle is employed; this pickle normally removes from

0.0005 to 0.001 inch of metal from the surface.

Acid cleaning must always be folloived by thorough rinsing in cold running tvater. Further details about the aforementioned

acid pickling and alkaline cleaning materials

are given in Military Specification MIL-C-3171

[90] ; this specification also describes methods

for the chemical pretreatment of magnesium

surfaces. Useful information on the cleaning

and pretreatment of magnesium surfaces may

also be found in the publication "Magnesium

Finishing" [90a] and in the Metal Finishing

Guidebook [91].

6.3.5.1.4. Mechanical cleaning methods (see

sees. 6.3.1.6, 6.3.1.7, and 6.3.1.8), including

sanding, grinding, wire brushing, and grit-,

shot-, or sand-blasting, may be used on rough

unmachined castings that are discolored or cor- roded. Blasting methods may result in surface

contamination which will greatly increase the

initial corrosion rate of magnesium. Hence,

blast cleaning should always be followed by

pickling in a sulfuric acid or sulfuric-nitric

acid pickle until 0.002 inch of surface has been

etched away.

6.3.52. Chemical pretreatments for in- creasing the corrosion resistance of magnesium

alloys and for producing a paint-receptive sur- face are covered in detail in Military Specification MIL-M-3171 [90] and elsewhere [90a,

91]. Since virtually all the processes now in use for the pretreatment of magnesium sur- faces (except the HAE process) were developed by a single manufacturer [90a], these

processes have been identified for the sake of

clarity by their commonly known proprietary

designations in the discussion which follows.

Before application of chemical treatments, the

surface should be thoroughly cleaned by the

methods indicated in the preceding section.

Five types of chemical pretreatment are described in MIL-M-3171 ; these are summarized

below

:

6.3.5.2.1. Type I Chrome-Pickle Treatment

(Dow No. 1) is of general applicability and

is the most widely used chemical treatment for

magnesium surfaces. In addition to providing

a suitable base for paint, it is used for the

temporary protection of parts during storage

and shipment, and for touching up previously

treated work. The treatment is applicable to

all magnesium alloys (including those containing steel or brass inserts) when close toler- ances are not required; the treatment removes

about 0.0006 inch from the surface of the metal.

The treatment utilizes a sodium dichromatenitric acid solution for wrought magnesium,

with the addition of a small amount of sodium,

potassium, or ammonium acid fluoride to the

bath for treating molded or cast magnesium.

Dipping is the preferred method of application, but brushing may be employed on work

too large for immersion. Close control of

immersion time (normally 30 seconds to 2

minutes) in relation to bath composition is essential. After immersion, the part should be

allowed to drain for a few seconds before being

washed thoroughly in cold running water fol- lowed by a hot water dip to facilitate drying.

In brush application, fresh solution should be

generously applied and remain on the surface

for at least one minute before being thoroughly

washed off with cold running water. Excessive

immersion, or failure to keep the surface wet

with solution during brush application, will

produce a powdery coating that is unsatisfactory as a paint base. When properly applied,

the Type I treatment produces a matte gray

to yellow-red, iridescent coating that exhibits a pebbled etch finish under magnification. A

bright, brassy, relatively smooth surface (the

result of excessive nitric acid or nitrate salt build-up in the bath) is not suitable as a base for paint, although it is satisfactory

for the protection of parts during shipping and

storage.

6.3.5.2.2. Type II Sealed Chrome-Pickle

Treatment (Dow No. 10), while less often

specified than Type III, may be used for the

general long term protection of all magnesiumbase alloys when close dimensional tolerances are not required. It provides a good paint base

in addition to its protective qualities. While

most commonly used for wrought products, it may also be used on castings. The treatment

is a combination of chrome pickle and dichromate boil. The chrome pickle treatment is

121
applied as described under Type I, except that

the parts need not be dried after rinsing. The

freshly chrome pickled parts are then immersed

for 30 minutes in a bath of boiling sodium

dichromate solution saturated with calcium or magnesium fluoride and maintained at a pH

of 4.0 to 5.5. After the immersion, the parts

should be thoroughly rinsed in cold running

water, followed by a hot water dip to facilitate

drying. (The dichromate treatment must not

be applied to old chrome pickled surfaces ; the

old chrome pickle film should have been re- moved by chromic acid pickling and/or heavy

duty alkaline cleaners as described under Cleaning in sec. 6.3.5.1). The coating produced has

the brown color of the dichromate superposed on the varying matte gray to yellow-red iri- descence of the chrome pickle.

6.3.5.2.3. Type III Dichromate Treatment

(Dow No. 7) is suitable for the long term

protection of all magnesium-base alloys except

those containing manganese or cerium, including work for which close dimensional tolerances are required. It provides a very good combination of paint base and protective qualities. The

treatment does not appreciably affect dimensions and normally is applied after machining.

Parts containing bearings and inserts of brass,

bronze, cadmium plate, and steel are not affected

by the treatment, but aluminum is rapidly at- tacked during the hydrofluoric acid dip which

is an important step in this treatment. Where

aluminum inserts or rivets are used, an acid

fluoride dip is employed instead of the hydrofluoric acid dip. The hydrofluoric acid or acid

fluoride dip both cleans and activates the magnesium surface (however, the surface should

have previously been cleaned by the methods

indicated in sec. 6.3.5.1). The hydrofluoric

acid bath must be maintained between 10 and

20 percent HF; if allowed to become weaker

than 10 percent HF, the magnesium will be

severely attacked. It is recommended that

wrought alloy containing 3 percent aluminum

and 1 percent zince be immersed for 30 seconds

and that all other wrought alloys and castings

be immersed for 5 minutes at a temperature

of 70-90 °F (21-32 °C). After immersion, the

parts should be thoroughly rinsed in cold run- ning water to minimize fluoride carry-over into

the dichromate bath which would render it in- operative. After the fluoride treatment, the

parts are immersed for 30 minutes in a boiling

sodium dichromate solution saturated with cal- cium or magnesium fluoride and maintained

at a pH of 4.0 to 5.5 (as described for the

type II treatment) . Properly applied coatings

vary from light to dark brown depending on the

-alloy. After the dichromate treatment, the

parts should be given a thorough rinsing in cold

running water, followed by a hot water dip to

facilitate drying.

6.3.5.2.4. Type IV Galvanic Anodizing Treatment (Dow No. 9) may be applied to all magnesium-base alloys for long term protection

and to provide a good base for painting. The

treatment causes no dimensional change and

may be applied after machining operations.

The parts are first cleaned as indicated in sec- tion 6.3.5.1 and then given the hydrofluoric acid

or acid fluoride dip specified in the type III treatment. The work is then galvanically anodized for 10 to 30 minutes in a solution containing 4 oz. of ammonium sulfate, 4 oz. of sodium

dichromate, and 1/3 fl. oz. of ammonium hydroxide per gallon of solution. The pH of the

bath is maintained between 5.3 and 6.0 by additions of a chromic-sulfuric acid solution con- taining 5 percent by weight of each. The

anodizing is carried out at a bath temperature

of 120-140 °F (49-60 °C) and a current density

of 2-10 amperes per square foot. The tank, if steel, is made the cathode; if the tank is nonmetallic, steel cathode plates must be provided.

The work is electrically connected to the cathode

through an external circuit that includes an ammeter and rheostat; the work must not be

allowed to contact the cathode directly. Ordinarily, 70 to 150 ampere-minutes per square

foot of work is sufficient to produce a uniform

coating. A properly applied type IV coating

is usually uniformly black or dark brown in

color. Gray and nonuniform coatings indicate

that articles were not properly cleaned before

treatment or that the anodizing solution was

depleted. After immersion, the parts should

be thoroughly rinsed in cold running water, fol- lowed by a hot water dip or exposure to heated

air to facilitate drying. The specified paint

coating (usually a zinc chromate primer)

should be applied as soon as practicable after

the treated parts are thoroughly dry.

6.3.5.2.5. Type V Caustic Anodizing Treatment, while of some benefit for the protection

of magnesium-base alloys, is no longer recom- mended because of the availability of newer

and more satisfactory anodic treatments, such

as those described in the following section.

6.3.5.3. Anodic or electrochemical treatments are generally considered to provide the

maximum in corrosion resistance and paint

adhesion. Hard coatings with good abrasion

resistance also can be obtained. Anodic processes produce relatively thick, dense, adherent

protective coatings on magnesium alloys. It

is essential that the work be clean and that

good electrical contacts be established to ensure

a uniform coating.

6.3.5.3.1. The HAE process [92, 93], developed at Frankford Arsenal, is an electrochemical process for producing inorganic coatings

having excellent corrosion resistance and paint

122
base qualities on all magnesium alloys. The

processing bath contains potassium hydroxide,

potassium fluoride, trisodium phosphate, aluminum hydroxide, and potassium manganate in

aqueous solution. The bath can be maintained

indefinitely by replenishing chemicals. The

solution is strongly oxidizing and must not be

allowed to come in contact with organic materials. The processing tank is made of black

iron or unlined steel, with provision for

circulating and cooling the electrolyte. For

the bifluoride-dichromate post-treatment often

specified, a tank lined with polyethylene, a

vinyl, or other inert material is required.

The magnesium alloy should be suitably

cleaned (see subsec. 6.3.5.1) before chemical

pretreatment. Heavy oxide films or corrosion

products should be removed by a chromic acid

pickle (other acid pickles should be avoided).

Otherwise, a hot alkaline cleaning and cold

water rinse is usually adequate. Light chromate films left on the magnesium surface do

not interfere with formation of the heavy

(Type II) HAE coating; however, such films

should be removed from surfaces that are to receive the light (Type I) HAE coating. The

process utilizes closely controlled alternating

current. Electrical connections to the work and

to bus bars must be firmly made with magnesium connectors and leads ; the portion of the

leads at the liquid-air interface should be protected with vinyl-type electroplater's tape.

Parts to be processed are divided into two

groups of approximately equal area, each group

constituting one of the electrodes. Coatings of

different hardness and thickness, and of characteristic color, can be produced from the same

chemical bath by varying the operating conditions (current, time, and/or temperature).

The current density must be closely controlled

throughout the process by the use of suitable

control circuits which increase the voltage as required to maintain a constant current as the

electrical resistance increases with the build-up

of the anodic coating.

Three classes and various grades of HAE

coatings, comprising light and heavy types, are covered by specification MIL-M-45202

[93]. Type I, Class A is a light, smooth, tan

coating having a thickness of about 0.0002 inch,

produced by treatment for 6 to 8 minutes in a

bath at room temperature employing a current

density of 18 to 20 amperes per square foot

maintained over a voltage range from 0 to 80

volts. The anodic coating is then given a hot

sodium chromate (1% solution) post-treatment

and dried without rinsing. This type of coat- ing is particularly useful for providing a good

paint base and good corrosion protection where

tolerance requirements, time or cost limitations, or lack of substrate rigidity preclude use of

the full, hard brown coating. Alternatively, if a clear coating is to be applied, the chromate

post-treatment may be omitted and the anodic

coating instead rinsed thoroughly in cold then

hot water followed immediately by dyeing with

special solutions. Type I, Class B is the so- called "low voltage coating" produced by treat- ment for 15 to 20 minutes in a 140 to 150 °F

(60 to 66 °C) bath at 9 volts and a current

density of about 40 amperes per square foot. The treatment is applicable only to aluminumbearing magnesium alloys. The low voltage

coating is smooth, olive-drab in color, and

builds up to about 0.0004 inch but is somewhat

softer and more porous than the Class A coating. It is given a hot sodium dichromate

(0.3% solution) post-treatment and dried without rinsing.

Type II, Class A is the heavy, hard, brown

coating obtained by a full HAE treatment in

which the work is immersed for 60 to 90 minutes at room temperature at a current density

of 15 to 20 amperes per square foot maintained

over a voltage range from 0 to 85 volts. The

coating builds up to about one mil (0.001

inch) in thickness and provides the maximum

in corrosion resistance and abrasion resistance

as well as an excellent paint base. Corrosion

resistance is enhanced by post-treatment in a

hot sodium chromate solution (1%) or, to an

even greater degree, by post-treatment in an ammonium bifluoride-sodium dichromate bath

(10% bifluoride, 2% dichromate, room temperature) ; the post-treatment is followed by

drying without rinsing. An additional degree

of corrosion resistance to salt spray is imparted

if the post-treatment includes artificial aging

for several hours in a humidified enclosure

heated to about 185 °F (85 °C).

Among the HAE coatings described above,

the heavy coating with bifluoride-dichromate

post-treatment (Type II, Class A, Grade 3 of

MIL-M-45202) is the one most frequently

specified and the treatment considered to be

the most economical and effective as a base for

organic finishes.

6.3.5.3.2. Acid Chromate Anodic Process

(Dow No. 17).—This is a proprietary process

[94] which is applicable to all magnesium al- loys. Several classes of these coatings are covered by specification MIL-M-45202 [93].

The process produces coatings that have excellent paint base and corrosion protection

qualities, along with good hardness and abrasion resistance. The process utilizes a hot

aqueous electrolytic bath containing ammonium

bifluoride, sodium dichromate, and phosphoric

acid. Either alternating or direct current may

be employed, at voltages up to 110 volts. Treatment time depends on the current density (5

to 50 amp /ft-) and may vary from a minute

to an hour. Three types of coating may be

produced, depending on the voltage employed;

these are: (1) W-volt clear coating used as a

123
base for transparent lacquers or varnishes to impart a final metallic appearance similar to

clear anodizing on aluminum; (2) 60 to 75-volt

low voltage thin coating (light green, about

0.0003 inch in thickness) for a good combination of paint base and corrosion protective

qualities; and (3) 75 to 90-volt regular full coating (dark green and about 2 mils in thickness) for the best combination of abrasion re- sistance, corrosion protection, and paint base

qualities. Before treatment, parts should be

cleaned in an alkaline cleaner (see subsection

6.3.5.3.1) and thoroughly rinsed with cold

water. After anodizing, the parts are rinsed

in cold water, followed by a hot water dip or

rinse to facilitate drying. (If the anodized

parts are to he left unpainted, they should be

sealed by immersion for 15 minutes in an

aqueous sodium silicate bath maintained at 200-212 °F (93-100 °C) followed by rinsing

in cold water then hot water and drying)

.

6.3.6. Other Metals

The surface preparation and pretreatment of some of the less frequently painted metals are

briefly considered in the following subsections.

6.3.6.1. General.—Metals that are difficult to paint or pretreat chemically should be thoroughly cleaned and, in many cases, roughened

before painting.

6.3.6.1.1. The cleaning process should remove

all oil, grease, soil, dirt, and other foreign

matter from the surface. Solvent cleaning

(see sec. 6.3.1.1) is of general applicability,

although other cleaning methods (section 6.3.1)

may be used where appropriate or necessary.

Sandblasting accomplishes both cleaning and

roughening; where sandblasting is unfeasible

or too severe, the use of emery paper can

often provide the degree of roughening needed

for adequate paint adhesion.

6.3.6.1.2. The wash 'primer type of pretreatment described in section 6.3.2.5, in addition to

its wide and effective use on steel, zinc, and

aluminum, is also applicable to other metals,

including cadmium, copper and its alloys, stainless steel, nickel, chromium, titanium, lead, and

terneplate—but not necessarily under all types

of topcoats.

6.3.6.1.3. Proprietary chrornate treatments

are available [91a, 95] that provide excellent

corrosion protection and impart good paint re- ceptivity to surfaces of zinc, cadmium, copper,

brass, bronze, silver, aluminum, and magnesium

(chromate treatments for the latter were cov- ered in sec. 6.3.5.2). These chromate treat- ments employ acidic solutions containing hexavalent chromium ions (derived from chromic

acid, dichromates, chromates, or a combination

of these), a mineral acid (usually sulfuric),

and sometimes an organic acid also. The

chromate coatings are formed non-electrolytically by simple immersion of the parts in the

chromating bath at room temperature or

slightly higher, for periods ranging from a few seconds to a few minutes depending on the

type of treatment. Spraying or brushing techniques may also be used in some instances

where immersion is not feasible. The chromate

coatings produced by these treatments are very

thin (0.00002 inch or less) amorphous films

with excellent corrosion-inhibiting properties

that enhance paint life while protecting the

metal.

A number of systems for the preparation

and painting of various metal surfaces are

specified in MIL-STD-171 (ORD) [96].

6.3.6.2. Zinc and cadmium.—These surfaces

should be thoroughly solvent-cleaned to produce

a surface completely free of oil, grease, dirt,

processing compounds, and temporary protective compounds. Vapor degreasing is preferred. The clean surface, if not previously

chromated at the factory, should then be given one of the five pretreatments indicated in the

discussion of galvanized surfaces in section

6.3.3. (Sec. 6.3.3 also deals with the problems

involved in the painting of factory-chromated

galvanized surfaces.)

6.3.6.3. Copper, brass, and bronze.—The

surface should be cleaned free of all oil, grease,

dirt, and other foreign matter and then roughened by sanding, light sandblasting, or phosphoric acid etching (sec. 6.3.1.5). Alternatively, the clean surface may be given a wash

primer pretreatment as described in section

6.3.2.5, or a chromate treatment such as that

described in section 6.3.6.1. (Also see sec, 4.3.3).

6.3.6.4. Silver may be protected against

tarnishing by application of a suitable chromate treatment or a protective lacquer or both.

The silver should first be cleaned free of oil, grease, fingerprints, tarnish, polishing compounds, and other foreign matter. A proprietary treatment which produces a clear

chromate coating on silver is available as indicated in section 6.3.6.1. The chromate coating

affords a good base for a clear lacquer if desired.

6.3.6.5. Tin should be solvent-cleaned to rej move all oil, grease, dirt, and other foreign | matter from the surface. For hot dip tin no further pretreatment is necessary. Electrodeposited tin (deposited from an alkaline stan- nate bath) should be treated with a hot chromic

acid-phosphoric acid aqueous solution (contain124
ing equal weights of each acid and maintained

at a pR of 2.0 to 3.0 at 160 to 180 °F [71 to 82 °C]) before application of a protective coating. Baked lacquers make effective coatings.

6.3.6.6. Terneplate should be cleaned (usually by solvent cleaning) free of oil, grease,

dirt, and other foreign matter. If feasible, an

oxalate or chromate base pretreatment coating

should be applied. Otherwise, a wash primer

pretreatment coating (see sec. 6.3.2.5) may be

applied to the clean surface. Alternatively, a

clear varnish size coat is sometimes applied

to the clean surface before application of a

finish coat. Adhesion is improved if the sur- face is roughened before application of organic

coatings. A conventional iron oxide "roof and

barn paint," such as that described by Federal

Specification TT-P-31, provides an economical

and reasonably durable coating for "roofing

tin" (as terneplate roofing is commonly called)

,

6.3.6.7. Lead, while needing little protection,

is frequently painted to obtain a more pleasing

appearance. The surface should be cleaned and

roughened before receiving paint. A wash

primer pretreatment (sec. 6.3.2.5) may be used

to obtain increased adhesion.

6.3.6.8. Titanium and titanium alloys that are to receive organic coatings should be sol- vent cleaned to remove oil, grease, and other

foreign matter and then sandblasted. If sandblasting is not feasible, the surface should be

given a wash primer pretreatment (as described in sec. 6.3.2.5).

6.3.6.9. Stainless steel, nickel, and chromium, because of their noncorrosive nature and

intrinsically pleasing appearance, are seldom

painted. When an organic coating is desired,

the surface should be thoroughly cleaned

(preferably by solvent-cleaning) to remove all

oil, grease, dirt, and other foreign matter and

perhaps roughened slightly with emery paper

to promote adhesion. The wash primer type

of pretreatment (sec. 6.3.2.5) may also be used on these metals to obtain improved adhesion.

6.4. Preparation of Concrete and Masonry Surfaces

As with other surfaces, a basic requirement

in the preparation of concrete and masonry for

painting is that the surface be clean: free of

dust, dirt, oil, grease, efflorescence, chalk, and

loose material. Additional problems in the

painting of concrete and masonry surfaces, particularly when new or insuflliciently aged, arise

from: (1) unbound moisture within the concrete (and in the mortar joints of other

masonry) remaining from the original mixing

with water, (2) the presence within the material of soluble alkaline substances that are brought to the surface by the outward move- ment of moisture and deposited as efflorescence,

(3) possible contamination with form oil or

concrete curing compounds, (4) glazed areas

resulting from casting against a smooth, non- absorbent form.

Depending on the condition of the surface

and the nature of the coating system to be

applied, the proper preparation of concrete and

masonry surfaces for painting may involve one

or more of the following steps: (1) aging or weathering, (2) cleaning, (3) patching, grouting, and filling, (4) roughening, (5) pretreatment, (6) surface conditioning. These factors are discussed in the following subsections, and

the extent to which they must be considered in

applying various types of coating systems is indicated.

6.4.1. Aging and Weathering

Where time permits, aging is probably the

most convenient and effective single method of

preparing concrete and other masonry surfaces

for painting. In some cases, as with glazed

asbestos-cement shingle, weathering for several years is virtually the only practical way to obtain a surface suitable for painting. Aging

permits the concrete to dry out, neutralizes the

surface alkalinity, and reduces the inner, free

alkalinity through continuing cure and by

moisture-borne migration of alkaline substances

to the surface where they are gradually neutralized by reaction with the carbon dioxide

in the atmosphere.

With the escape of moisture and the reduction in the amount of soluble alkaline substances within the material, efflorescence de- creases and eventually ceases for practical

purposes. However, efflorescence may recur even after extensive aging if the concrete

should again become wet internally.

During weathering, oily contaminants are

gradually washed and worn away, and the

surface acquires a weathered roughness that

provides a good mechanical bond for paint.

While all this is well and good, many months

or even years may be required for the practical

completion of this aging process, depending on

the thickness and porosity of the material and

the nature of the service environment. It is frequently impractical to delay painting for

such long periods of time. Fortunately, al- though long aging is necessary for the safe

application of oil-base paints, there is now

available a variety of water- and alkali-resistant

types of coatings which require relatively little aging of the substrate.

125
6.4.1.1. Latex (water emulsion) paints

(see sec. 2.3.5), because of their relative in- sensitivity to water and alkalinity, may be

applied on damp, only briefly aged (but not

fresh, uncured) masonry surfaces. As a general rule, at least 4 weeks aging is desirable

whenever possible. Although the surface may

be damp, it should not be wet when the latex

paint is applied.

6.4.1.2. Cement-water paints (see sec. 2.3.9.1) are completely resistant to the moisture

and alkalinity in concrete and may be applied

virtually as soon as the concrete has set. Little or no aging is required.

6.4.1.3. Alkali-resistant synthetic resin

COATINGS (e.g., epoxies, polyesters, polyurethanes, and various synthetic rubbers) may be

applied without long aging of the surface in

most instances ; however, it is usually essential

that the surface be dry. The instructions of

the manufacturer usually provide the best

guide to the use of such materials.

6.4.1.4. Oil-base and oleoresinous paints

may be safely applied only on dry, thoroughly

aged concrete or masonry. Where such paints

must be applied to concrete that has aged less than a year, it has been suggested [97] that a pretreatment consisting of an aqueous solution

containing 2 percent zinc chloride and 3 percent

phosphoric acid be applied to the surface and

allowed to dry upon it without rinsing. After

drying, loose salts should be removed by light

dusting, but the adherent encrustation of in- soluble salts should be left on the surface. The

application of dilute zinc sulfate solution to concrete to reduce the surface alkalinity, al- though a popular practice in the past, is no

longer considered to be an effective treatment

because it may leave soluble salts on the sur- face that may increase the tendency toward

blistering of the paint film.

6.4.2. Gleaning

The cleaning methods used should remove

dirt, dust, oil, grease, efflorescence, loose chalk,

and other loose material or foreign matter

from the surface (see applicable parts of sec. 6.3.1). Latex paints have good resistance to moisture and alkalinity, but because of their

nonpenetrating nature and poorer wetting

properties they require more thorough surface

preparation than solution-type coatings, particularly with respect to removal of all loose

chalk. Even a moderately chalky condition is not a satisfactory base for latex paint; a suitable surface conditioner (see sec. 6.4.6) that

will wet and penetrate the chalk must be used

to obtain satisfactory adhesion of the latex

paint. A moderate degree of chalking is not

a problem with oil-base paints, since these

effectively penetrate and bind the chalk.

6.4.2.1. Dust, dirt, and loose material frequently can be removed from the surface merely

by bristle-brushing or by hosing down with

clean water. When necessary, scraping, wirebrushing, or sandblasting may be employed to dislodge foreign matter ; such mechanical methods should be followed by hosing down with

water.

6.4.2.2. Form oil, grease, and other oily con- taminants are eventually removed by weathering, although many months may be required,

depending on the degree of contamination and

the severity of the weather. In the absence of

adequate weathering, oily contaminants should

be removed either by (1) scrubbing with a solution of trisodium phosphate (4 oz. per gallon of

water) followed by thorough rinsing of the

surface with clean water, or (2) etching the

surface with a 5-10 percent solution of muriatic

(hydrochloric) acid in the manner described

below for the removal of efflorescence. When

the latter (acid) treatment is used, it must

be followed either by thorough rinsing with

clean water or by neutralization with trisodium

phosphate solution or ammonia water (10%)

followed by thorough rinsing with plain water.

The choice as to whether an acid or an

alkaline cleaning method should be used, and

whether—in the case of acid cleaning—to use

only a clean water rinse or to neutralize before

rinsing, depends on the nature of the coating

system to be applied. Since many latex emulsions are alkaline in nature and therefore

acid-sensitive (such is the case with styrenebutadiene latexes and with acrylic latexes),

alkaline cleaning or neutralization of acid with

trisodium phosphate solution is usually recom- mended as a precaution against breaking of

the emulsion when it is applied to the surface.

On the other hand, this may not be a critical

factor with polyvinyl acetate latexes which are normally slightly acidic. With oil-base or oleoresinous coatings, which are alkali-sensitive,

alkaline washes are best avoided. Where specialized synthetic resin coatings are to be used,

the instructions of the manufacturer should

be followed.

Oily contaminants can be removed from the

surface by mechanical methods if desired.

Traces of oil can be removed by rubbing the

surface with steel brushes or coarse abrasive

stones. However, if the surface is generally

contaminated, it is much more effective to

lightly sandblast the area to be painted, or

else to postpone painting until the oil has been

removed by the action of the weather. Mechanical cleaning methods should be followed

126
by hosing down the surface with water to wash

away all loose debris.

6.4.2.3. Efflorescence, when present, must

be completely removed before painting is attempted; otherwise, it will spoil both

the appearance of the paint job and its performance. It has already been indicated that

efflorescence is caused by the moisture-borne

migration of soluble alkaline salts from the

interior of the material to the surface where

the salts are deposited as a whitish bloom or

encrustation. The removal of efflorescence is best accomplished by sandblasting, if possible;

otherwise, abrasive stones or wire brushing

may be employed, followed by treatment with

dilute muriatic (hydrochloric) acid. An acid

concentration of 5 to 10 percent is usually employed, but concentrations as high as 20 percent

are sometimes used. The surface is first wet

with water, then the acid is applied liberally

and allowed to remain on the surface for about

five minutes. The surface is then scrubbed

with a stiff brush and thoroughly rinsed with

clean water to remove all traces of acid. If necessary, the treatment is repeated, until all efflorescence has been removed. A neutralizing

rinse with trisodium phosphate solution or ammonia water is often recommended when

certain types of latex paint are to be applied, as indicated in the preceding subsection. The

neutralizing solution, in turn, should be thoroughly rinsed away with clean water.

6.4.2.4. Mildew or mold if present (usually

only on previously painted or organically con- taminated surfaces) should be removed by

scrubbing with trisodium phosphate solution

(4 oz. per gallon of water) or with a 5 percent

solution of sodium hypochlorite such as the

common household chlorine bleach, followed in

either case by thorough rinsing with clean

water.

6.4.3. Patching, Grouting, and Filling

Large cracks, holes, and other damaged areas should be repaired by patching with a suitable

material such as the commercially available

cement-sand mixtures, latex-concrete mixtures,

stucco patch, or other cementitious materials

that are formulated for this purpose. Adequate time for drying or aging of repairs should

be allowed before painting. Open-textured

masonry surfaces such as cinder block should

be given a grout coat or a fill coat, depending

on the type of coating that is to be applied. A

grout coat consisting of a 50:50 cement-sand

mixture provides an excellent base for cement

water paints. The grout coat, like cement

water paint, must be kept damp while curing

for the first 48 hours to ensure proper hydration

of the cement and freedom from cracking or

powdering. When a latex paint is to be used,

it is preferable to employ a special fill coat made

by combining white portland cement, washed

silica sand, and water with a mixing liquid

containing the same latex polymer that is used

in the latex paint to be applied. In some cases, some of the latex paint itself is added to the

fill coat mixture. Formulas for fill coats are given in the applicable Federal Specifications

for latex paints [5, 6, 7]. Such fill coats do not

require damp curing; however, they should be

allowed to dry for 24 hours before painting.

Since both fill coats and grout coats such as

those described above are alkaline in nature,

it is unsafe to use them under oil-base or oleoresinous paints unless adequate time for aging

(at least 90 days) can be allowed (see preceding sec. 6.4.1)

.

6.4.4. Roughening

Some concrete surfaces are so dense, or glazed from having been cast against a smooth

or nonabsorbent form, that good paint adhesion

is difficult to obtain. Such surfaces should be

roughened before painting. This is best accomplished by light sandblasting where feasible.

Alternatively, the surface may be etched with

a 5 to 10 percent solution of muriatic (hydrochloric) acid. The surface should be wet down

with water before applying the acid. The acid

etching process should be continued until the

surface acquires the texture of a fine sandpaper. The surface should then be rinsed thoroughly with clean water. Where specified for

the particular type of paint to be used, the sur- face may be given a neutralizing alkaline rinse

with dilute trisodium phosphate solution or ammonia water, as discussed in the preceding

subsection 6.4.2.2 on cleaning, before rinsing

thoroughly with clean water.

When neither light sandblasting nor acid

etching are feasible, the glazed or dense surface

may be roughened by rubbing it with coarse abrasive stones.

6.4.5. Pretreatment

Chemical pretreatment of concrete and masonry surfaces, other than treatment with

hydrochloric acid as already mentioned for

etching the surface or for the removal of

efflorescence, is largely limited to situations in

which oil-base or oleoresinous paints are to be

applied to surfaces which have not been adequately aged. In such cases, the pretreatment

with zinc chloride-phosphoric acid solution described above under Aging (sec. 6.4.1) is thought to be beneficial on the basis of available evidence [97]. This pretreatment, however, has not had wide use because only a

limited amount of oil-base or oleoresinous paint

is used on masonry surfaces today, now that

127
the water- and alkali-resistant latex paints are

available. At best, pretreatment must be considered as less effective than thorough aging

in producing a paintable surface.

6.4.6. Conditioning of Chalky Surfaces

The nonpenetrating character of latex paints

which gives them their excellent sealing properties on porous surfaces of concrete and

masonry is associated with low wetting properties that prevent the development of good adhesion to chalky surfaces. Previously painted

concrete or masonry surfaces that exhibit

chalking not practicable to remove by sandblasting should be brushed as free as possible

of loose chalk and then treated with an alkaliresistant, penetrating type of surface conditioner before application of latex paint. A

typical surface conditioner suitable for this

purpose may be a relatively low viscosity coat- ing based on a tung-oil phenolic resin or a

soya- or linseed-oil alkyd of long oil-length in an

organic solvent thinner, possibly together with

a small proportion of pigments and extenders

(also see sec. 2.4.3). A number of proprietary

products for this purpose are available and

are usually included as part of the manufacturer's line of latex coating systems. The

surface conditioner penetrates and binds the

chalk and provides an adherent base for the

Litex paint. Even the use of a surface conditioner is not adequate when the masonry sur- face is in poor condition with heavy chalking

or multiple layers of weak cement-type paints;

in such cases, sandblasting is the only effective

method of preparing the surface (also see sec. 6.6.4.1).

6.4.7. Summary

The nature and degree of surface preparation

required for concrete and masonry before painting depends to a large extent on the nature of

the coating system that is to be applied. In

all cases the surface should be sound, free of

dirt, oil, efflorescence, and loose material, and

rough enough to provide for good mechanical

bonding of the coating. Where practicable,

light sandblasting provides the best surface

preparation; it is not, of course, a substitute

for aging where the latter is required. Opentextured masonry surfaces should receive a grout or fill coat before painting. Cementwater paints may be applied on damp, unaged

concrete and masonry and on surfaces previously painted with cement-water paint, even

if chalking or dusting is present, so long as the

old coating is firmly adherent. Latex paints

may be applied on damp, relatively briefly aged

surfaces that are clean and free of chalk ; when

all loose chalk cannot be removed, a penetrating

liquid surface conditioner should precede application of the latex paint. Oil-base and oleoresinous paints may be safely applied only on

dry, well-aged concrete and masonry surfaces

that are not likely to become internally wet

again during service after painting.

6.5. Preparation of Plaster and Wallboard Surfaces

Plaster and wallboard surfaces require relatively little preparation for painting; however,

as in any painting, it is essential that the

surface be sound and clean. Since plaster and

wallboard are porous materials, the clean sur- face must be properly sealed to obtain good

hold-out and uniform appearance of topcoats.

Latex primer-sealers are particularly suitable

for this purpose because of their nonpenetrating nature, but their lower wetting ability necessitates a cleaner surface than is required

for solution-type coatings. Both latex-type and

oleoresinous primer-sealers are discussed in

sections 2.4.4 and 4.5. Application of the

proper primer-sealer allows the use of virtually

any type of topcoat.

6.5.1. Plaster

To achieve a good finished appearance, it is necessary that cracks, holes, indentations, and

other defects be repaired before painting. This can be accomplished by filling the damaged

areas with spackling compound or patching

plaster. To obtain a good anchor for the patch

where large cracks or holes are involved, it is desirable to undercut the edges of the area to be filled and to wet the edges with water to prevent excessive loss of water from the patching

material into the adjacent plaster. (Very

large holes or areas of loose plaster should, of

course, be replastered before painting.) After

the repaired areas have dried for at least three

days, they should be sanded smooth with fine

sandpaper.

6.5.1.1. New plaster, because of its high

moisture content and alkalinity, should be

allowed to age for 60 to 90 days Isefore application of oleoresinous paints. Where latex

paints are to be applied, at least two weeks of

aging is highly desirable, and a month or more

should be allowed whenever practicable. Apart

from aging, new plaster frequently needs only

to be dusted to prepare it for painting.

6.5.1.2. Old plaster or plaster that has be- come soiled should be washed with clean water.

If a latex primer-sealer is to be applied, it is permissible to use a mild detergent such as

128
trisodium phosphate solution as a cleaning aid,

followed by a clean water rinse. Excessive

wetting or soaking of the plaster should be

avoided.

Oily contaminants, if present, must be re- moved to obtain good paint adhesion. Although

this can be accomplished by washing the sur- face with a detergent solution, it is usually preferable, especially where an oleoresinous coat- ing system is to be applied, to "dry clean" the

surface by wiping or washing it with a suitable

solvent such as mineral spirits. Wet surfaces

must be allowed to dry thoroughly before application of oleoresinous paints. Latex paints

may be applied while the surface is still damp,

but not wet. Painting with latex coatings may,

therefore, advantageously begin soon after

washing of the surface is completed, thus minimizing the possibility of recontamination.

6.5.2. Wallboard

A number of different kinds of wallboard

(including prefinished wallboard) are available

under a somewhat confusing variety of generic

and proprietary names. For practical painting

purposes, these may be considered within two

broad categories : ( 1 ) Those which are essentially paper-covered inorganic plasters, frequently alkaline in nature, or which are joined

or finished with alkaline plasters, as typified

by the common gypsum wall board or "dry

wall" construction (also known as "sheet rock"

or plasterboard), and (2) non-alkaline, organic

composition boards composed of compressed

and/or resin-bound wood or cellulose fibers in a variety of physical forms, and known by such

designations as beaverboard, fiberboard, hardboard, Masonite, pressed wood, etc.

6.5.2.1. Gypsum wallboard consists of a

core of gypsum plaster sandwiched between exterior skins of heavy, felted paper. In dry

wall construction, the sheets of wallboard are

butted together and nailed to the studs. The

nails should be countersunk and covered with

joint cement. All joints should be filled with

joint cement, taped, and plastered. When dry,

the assembly should be sanded smooth leaving

feathered edges. Care in sanding is required

to avoid roughing up the paper surface. After

sanding, the surface should be wiped free of

dust, preferably with a damp sponge ; it is then

ready to receive an emulsion-type primersealer. Solution-type oleoresinous primer-sealers are not satisfactory for use on gypsum

wallboard because they tend to raise the nap

of the outer paper layer.

6.5.2.2. Fiberboard, hardboard, and other

nonalkaline wallboards based on wood or cellu- lose fibers should have all nails countersunk,

the holes filled with spackling compound, and

sanded smooth. The surface^hould be brushed

or vacuumed to remove all dust and is then

ready to receive either a latex primer-sealer or an oleoresinous solution-type primer-sealer.

6.6. Preparation of Previously Painted Surfaces

The amount of surface preparation required

for a previously painted surface depends on the

condition of the old paint, the nature and condition of the substrate, and the kind of paint

to be applied in repainting. Surface preparation is an integral part of the general problem

of maintenance painting discussed below.

6.6.1. Maintenance Painting

All organic coatings eventually deteriorate or wear away and therefore require periodic

renewal. Since the service life of the coating

will vary widely with the type of coating and

the nature of the service environment, no fixed

interval between repainting can be stated.

However, when accurate records of paint jobs are kept, the history can provide a good esti- mate of when repainting may be needed, for

purposes of advance planning. The actual

time to repaint should be determined by periodic examination of the surface, since the cost of repainting is closely tied to the condition of

the coating and substrate.

The essential elements of an effective and

economical maintenance painting program are

:

(1) Periodic careful inspection of the painted

surface, with touch-up painting where needed

for damaged or exceptionally worn areas; (2)

repainting at the right time, when the old paint

is well-worn but not so badly deteriorated as

to require extensive or complete removal, and

before the substrate itself is exposed to deteriorative influences; (3) adequate surface

prepartion which will remove all loose paint

and foreign matter, leaving a clean, sound sur- face for the new paint; and (4) selection of a

suitable recoating system that will bond properly to the old paint, or to the substrate where

it is exposed. Provided that the old paint sys- tem gave satisfactory service the choice of a

similar system will assure compatibility.

Repainting at the right time minimizes the

labor and expense of surface preparation.

Paint Avorn thin but still firmly bonded to the

substrate need only be dusted or washed to ensure a clean surface before repainting. On

the other hand, in situations where the old paint

has been permitted to deteriorate to the point

of extensive cracking, curling, or peeling—or where the substrate has been laid bare and

damaged or corroded—it may be necessary to

129
expend a great deal of effort and time in completely removing the old paint and in cleaning

dirt or corrosion products from the substrate. In the latter case, the cost of surface preparation may be considerably greater than the cost

of paint and painting.

Painting too often or too soon also should be

avoided. Not only is this uneconomical but it ultimately results in the build-up of an excessively thick coating that may be brittle and

prone to cracking and chipping, particularly on

dimensionally unstable substrates such as wood

or thin metal.

Mildew or mold may form on coatings in

warm, damp climates or locations. Before

repainting is done, the fungus should be re- moved by scrubbing the surface with a solution of trisodium phosphate (4 oz. per gallon

of water) or with a 5 percent sodium hypochlorite solution such as the common household

chlorine bleach, followed in either case by

thorough rinsing with clean water. It is desirable to remove mildew or mold whenever it is noticed during the periodic maintenance in- spections, since if allowed to remain it will cause progressive destruction of the coating.

The criteria which determine the right time

for repainting, and the problems encountered

and procedures to be followed in maintenance

painting, are somewhat different for different

types of substrates in various environments.

Surface preparation for the maintenance painting of the various types of substrate is discussed in the following sections.

6.6.2. Previously Painted Wood Surfaces

The preparation of previously painted wood

surfaces for repainting has been fully discussed

in section 6.2.2.

6.6.3. Previously Painted Metal Surfaces

Preventive maintenance in the form of periodic touch-up painting of localized areas that

show excessive wear, damage, or rusting is an

important factor in obtaining maximum service

life from coatings on metal surfaces. The lo- calized bad spots can be cleaned by wire brushing, then spot-primed and repainted with very

little difficulty or expense.

At the stage in the life of the coating when

it is well-worn but still performing its protective function in all but a few localized areas, the protection can be prolonged and the labor

and expense of extensive surface preparation can be avoided by application of a single overall coat of paint. In such cases, localized bad

spots should be wire-brushed and spot-primed,

and the entire surface should be cleaned free

of dust, dirt, oil, grease, and foreign matter by

washing with a mild detergent solution or

where considerable oil or grease is present

by washing with solvents as described in section

6.3.1.1. The clean and spot-primed surface is then ready to receive a full coat of paint, preferably of the same type as before to assure

compatibility with the old paint.

When the coating failure is general, as evidenced by numerous and widely distributed

rust spots, or by widespread peeling, flaking,

blistering, or exposure of bare substrate, it is necessary to remove the old paint completely

and to clean the metal surface properly before

repainting can be done successfully. Removal

of the old paint and cleaning of the surface can

be accomplished simultaneously by the methods

of sandblasting, flame cleaning, or alkali cleaning described in section 6.3.1 (subsecs. 6.3.1.8,

6.3.1.9, and 6.3.1.4, respectively). Alkali cleaning does not remove rust. Of the several

methods of paint removal and surface preparation mentioned, sandblasting is by far the most

effective where feasible.

6.6.4. Previously Painted Concrete and

Masonry Surfaces

6.6.4,1. Latex (emulsion) paint.—If the

old paint is well-worn but in basically good

condition—that is, free of excessive chalking,

flaking, blistering, or peeling—the necessary

surface preparation before repainting with

latex paint may involve only a thorough washing with clean water, or with a detergent such

as trisodium phosphate solution followed by a thorough clean water rinse, to remove dust,

dirt, oily contaminants, and loose material

from the surface.

Loose or heavily chalked surfaces, particularly of cement-type paints, cannot be satisfactorily repainted. Not only is paint adhesion

to such surfaces poor but failure will occur within the chalk layer itself through crumbling

and distintegration. Lightly chalked surfaces

may sometimes be satisfactorily prepared by

removing as much chalk as possible by brushing

and washing followed by treatment of the sur- face with a penetrating type surface conditioner

as described in section 6.4.6. However, the

only completely effective way to prepare a

heavily chalked surface or a surface with multiple layers of weak cement paint for satisfactory repainting is to remove the chalk or cement paint thoroughly by sandblasting.

Where sandblasting is not feasible, power wire

brushing methods may be employed, but with

greater labor and less satisfactory results.

Complete removal of the old paint in the area

of failure is also indicated when the paint is blistering, flaking, or peeling. Such failure,

particularly if accompanied by efflore'^cence or

discoloration, is usually caused by excessive

moisture coming from within the concrete or masonry surface. Where the source of the

130
moisture is from outside the masonry (e.g.,

from the ground on the exterior of a wall below grade) then the source will have to be

eliminated through better drainage or by exterior waterproofing before successful repainting can be accomplished.

6.6.4.2. Solvent-thinned paints of the oil- base or oleoresinous type, while less moistureand alkali-resistant than latex paints, have

better wetting and penetrating properties and

therefore require less stringent surface preparation. Moderately chalked areas of firmly

attached paint (including old but sound ce- ment-water paint) ordinarily require only

brushing to remove loose chalk and soil from

the surface. If oily contaminants are present,

they can be removed by washing with mineral

spirits or other suitable solvent. The surface

must be thoroughly dry when repainting.

Blistering and peeling or other evidence of

moisture failure preclude the application of

oil-base or oleoresinous paints unless the source

of the moisture can be completely eliminated.

Loose paint in localized areas should be re- moved by brushing or scraping. Where the

paint is in generally poor condition, with widespread flaking or peeling, complete removal by

sandblasting or power wire-brushing is in- dicated.

When repainting is to be done with solventthinned or 100-percent-solids coatings based on specialized synthetic resins, the instructions

of the manufacturer concerning surface preparation should be closely followed.

6.6.4.3. Cement-water paints (see sec. 2.3.9.1) may be applied directly over firmly

adherent cement-type paints, even if chalking

or dusting is present; brushing with a stiff brush is usually the only surface preparation

required. However, if the old cement paint is crumbling, flaking, or scaling, it should be

completely removed by sandblasting, or by

treatment with hydrochloric acid in the manner

described for the removal of efflorescence (sec.

6.4.2). Old coatings of whitewash (see sec. 2.3.9.2) also should be completely removed with

hydrochloric acid before repainting. Cementwater paints will not ordinarily adhere to or- ganic coatings; the latter must be completely

removed, preferably by sandblasting, before

repainting.

6.6.5. Previously Painted Plaster and

Wallboard Surfaces

The surface to be repainted should be clean

and sound. Damaged areas should be repaired

by filling with spackling compound or patching

plaster and sanding smooth. All loose or blis- tered paint should be removed by scraping and

the surrounding area sanded to a feather edge.

Dust, dirt, and moderate amounts of oily con- taminants may be removed by washing with a detergent such as trisodium phosphate solution

(4 oz. per gallon of water) . Where considerable oily contamination is present, it is preferable to solvent-clean the surface (see sec. 6.3.1.1) with a grease solvent such as mineral

spirits. "Dry cleaning" with solvent is especially desirable if oleoresinous paint is to be

applied, since it eliminates the need to allow

time for thorough drying. If all chalk cannot

be removed, an oleoresinous paint may provide

better adhesion than the latex type. The latter, however, may be applied while the surface

is still damp (but not wet) from washing.

If blistering and efflorescence are in evidence,

the presence of excessive moisture in the wall

is indicated, and the source of the moisture

should be located and eliminated, if possible,

before repainting.

If the paint is in poor condition as evidenced

by general scaling, cracking, blistering, peeling,

etc., then the old paint should be entirely re- moved with a solvent-type paint remover such

as that covered by Fed. Spec. TT-R-251 or

Mil. Spec. MIL-R-46073. Such paint removers

contain toxic volatile solvents which may also

be flammable, and they must be used with caution and adequate ventilation. If the paint re- mover contains wax, as is usually the case, the

stripped surface must be washed off with mineral spirits before proceeding with sealing and

painting.

131
6.7. Summary

The foregoing chapter has emphasized the

fundamental importance of proper surface

preparation in obtaining maximum service life from the applied coating and has presented

detailed instructions for the cleaning and pretreatment (if necessary) of virtually all types

of surfaces likely to be painted. Cleaning

methods have been described in detail primarily

in the discussion of steel surfaces (sec. 6.3.1)

where they find their v^^idest spectrum of applicability; thereafter, to conserve space, these

cleaning methods have been referred to as needed to indicate their applicability to other

types of surfaces. The pretreatment of metal

surfaces has been comprehensively covered and

includes not only steel but also surfaces of

aluminum, magnesium, zinc, copper, and other

metals.

While the bulk of the presentation has dealt

with the preparation of new or previously unpainted surfaces, the preparation of previously

painted surfaces has also been considered, in

relation to the general problem of maintenance

painting. When the old paint is completely

removed, the principles of cleaning and pretreating the bare surface are similar to those

for previously unpainted surfaces.

As a general rule to be adhered to v^henever

possible, surface preparation should be fol- lowed as soon as practicable by application of

the priming paint, before the clean surface can

again become soiled or corroded.

The detailed information that has here been

presented, in conjunction with the general in- formation given in chapter 4 on the basic types

of surface preparation required or considered

adequate for particular types of substrates and

environments, should provide a practical basis

for preparing virtually all surfaces for painting.

Figure 6.1. Glossmeter measures specular reflectance of surfaces and coatings as de- scribed in ASTM Method D 523-66T.

132
Figure 6.2. Portable impacttype slipperiness tester for measurement of relative slipperiness of different walkway

surfaces for various types of footwear, as described in ASTM Method E 303-66T. A mechanical heel of leather or rub- ber forms the lower part of the pendulum and sweeps over the sur- face when the pendulum is released. Height of swing beyond surface in- dicates "antislip coeflScient."

7. Consolidation of Coatings Information

7.1. A Glance Backward and Forward

Previous chapters of this publication have

dealt with major areas of the organic coatings

field, including: A description of the types of

coatings that are available (chapter 2) ; a detailed discussion of the properties of synthetic

resins on v^^hich most modern coatings are based (chapter 3) ; presentation of a basis for

the selection of coating systems for specific

purposes (chapter 4) ; instructions for the

storage, safe handling, and efficient application of organic coating materials (chapter 5) ; and detailed information on the preparation

and pretreatment of surfaces that are to receive

organic coatings (chapter 6).

Subsequent chapters present a tabular sum- mary of Federal Specifications for organic

coating materials (chapter 8), and a selected

bibliography of v^orks in the coatings field (chapter 9). Finally, a comprehensive Index

is provided. The aforementioned tabular summary is preceded by a quick guide list that provides a key to the information in the tables;

together, they constitute a comprehensive

source of information relating to Federal Specifications for organic coating materials.

Considerable information relating to the nature and arrangement of the subject matter

presented in this publication, and guidance in

its effective utilization, is given in the Preface

and in Chapter 1. A careful reading of these

sections will prove very worthwhile. In sec- tion 1.3 is given a description, with examples,

of the manner in which the Index has been or- ganized to provide maximum coverage and

utility with a minimum number of entries.

The present chapter rounds out the material

presented by providing important supplementary information and by furnishing examples

of use of the monograph in solving typical coat- ing problems.

7.2. Supplementary Information

The following sections present general information, important details, and helpful hints

not covered elsewhere in the text.

7.2.1. Painting Conditions

The ideal time for painting is during warm,

dry weather. In any case, the painting should

be done under favorable atmospheric conditions

that will promote proper drying and avoid con- densation of moisture on the uncured film. As

a rule, exterior painting should be done only

in clear, dry weather at moderate temperatures

(45 to 95 °F [7 to 85 °C] are the nominal limits usually specified) . When the surface and the

paint are cold, there is a greater possibility of

condensation, drying is slower, the tendency

to run or sag is greater, and the extra thinner

required to reduce the paint viscosity for proper

application may unbalance the formulation

with adverse ejffects on spreading rate and film

properties (see sec. 5.3.2 for a discussion of the

principles of Iproper thinning) . Under high

humidity conditions, even in warm weather,

again drying is slowed, condensation is very

likely to occur, and the addition of blush-retardant thinner may adversely effect the film

properties. Outside painting should never be

done in rain or snow, or when dew or frost is present on the surface. On days when a sharp

drop in temperature of 20 degrees Fahrenheit

or more is expected, it is inadvisable to do any

painting later than mid-afternoon. Such a temperature drop will frequently chill the air

below the dew point and result in the deposition

of dew or frost upon the uncured paint film.

While latex paints are not sensitive to moisture in the same sense that organic solventtype paints are, they will not dry well under

humid conditions and will not withstand rainfall occurring during or shortly after their application. Although they may be applied without difficulty on damp or lightly wet surfaces,

they should not be applied to materials that are water-soaked or to surfaces covered with heavy

dew or with frost. (Cement-water paints preferably are applied to surfaces that have been

thoroughly dampened with water, but they can- not be used in freezing weather.) Latex paints

may suffer from poor coalesence at temperatures below 50 °F (10°C), and their use at near-freezing temperatures is completely precluded.

The upper temperature limit for satisfactory

painting, while nominally given as 95 °F

(35°C), is actually determined by the nature

of the coating material. Neither the atmospheric temperature nor the temperature of the

surface to be painted should be so high as to cause blistering or pinholing of the paint film

through too-rapid evaporation of solvent, or poor leveling and faulty surface appearance be- cause of abnormally fast setting of the film. In

practice, this seldom happens except with fastdrying lacquers or when paints are applied to surfaces exposed to the hot sun ; such surfaces,

particularly if metal, may be considerably hotter than the ambient atmosphere. It is for this

135
reason that painting in the hot summer sun is not considered to be good practice.

Interior painting can often be done when

weather conditions preclude outdoor painting;

however, it is generally desirable to do interior

painting during warm, clear weather when

windows and doors can be left open to provide

the ventilation needed to prevent dangerous

build-up of toxic or flammable vapors. In the

latter respect, latex paints with their freedom

from fire and toxicity hazards are more suitable for year-round painting.

In general, painting should begin as soon as

possible after surface preparation (including

adequate time for drying) has been completed —before the clean surface can again become

soiled or corroded. In multiple-coat systems,

it is desirable that subsequent coats be applied

without prolonged delays which might contaminate the surface or adversely affect intercoat

adhesion.

7.2.2. General Coating Requirements

7.2.2.1. Thickness.—The number of coats

of paint required for a particular application

is determined by the film thickness needed to accomplish the desired protective or decorative

function and by the film build per coat. The

latter in turn depends on the spreading rate and

solids content of the applied coating.

7.2.2.1.1. Spreading rate is the area covered

by a unit volume of coating material, usually

expressed in terms of square feet per gallon.

Spreading rate is directly related to wet film

thickness by the equation

231.0 "~

144.0 xS

where Tw is the wet film thickness in inches,

and S is the spreading rate in square feet per

gallon. Tables and graphs giving the computed relationship are readily available in

various paint manuals and handbooks [40, 98]

.

For convenience, a few representative values

are given in the table below

:

Wet film thickness

(mils)

Calculated spreading

rate (ftygal)

Spreading

rate (ftygal)

Calculated wet film thickness

(mils)

1.0 1604 1600 1.00

1.5 1069 1300 1.23

2.0 802 1000 1.60

2.5 642 700 2.29

3.0 535 650 2.47

3.5 458 600 2.67

4.0 401 550 2.92

4.5 356 500 3.21

5.0 321 450 3.56

6.0 267 400 4.01

7.0 229 350 4.58

8.0 201 300 5.35

9.0 178 250 6.42 10.0 160 200 8.02 150 10.69

The dry film thickness can be calculated by

multiplying the wet film thickness by the volume fraction of nonvolatile vehicle plus pigment in the whole paint.

In general, the thicker the coating is, the

greater are the durability and protection provided, up to the point where the coating be- comes so thick and inflexible that it is prone

to crack and chip away from the substrate.

Also, several thin coats provide better insur- ance against pinholes and holidays than a single thick coat because of the improbability that

such flaws would occur at exactly the same

point in successive layers of paint. Coatings

that serve chiefly a decorative function in mild

service environments need not be as thick as

coatings that must withstand rigorous service

in corrosive or abrasive environments. For

optimum service performance, it is important

that the coating be applied at the spreading

rate recommended by the manufacturer or in

the applicable specification, so that the proper

final film thickness will be obtained. Recommended spreading rates normally take into ac- count the nature of the surface and differences

in coverage requirements with succeeding coats, as well as the composition and function of the

coating material and the nature of the service

environment. While spreading rates therefore

may vary widely with different materials and

applications, the following discussion of typical surfaces and coating materials may serve as a guide while emphasizing the importance

of adhering to recommended spreading rates. For adequate film build on rough, porous

surfaces such as concrete and masonry, a con- siderably lower (heavier) spreading rate is re- quired for the first or primer coat than for

subsequent coats, to allow for absorption and

for the filling of pores and crevices. For example, the priming paint (perhaps a properly

thinned latex paint) might be applied at a spreading rate of only 20 square feet per gallon,

while the unthinned finish paint might be

spread at the rate of about 600 square feet per

gallon.

Wood, also, is a porous material which re- quires a lower spreading rate for the priming

paint than for subsequent coats. For com- monly used paints on exterior wood surfaces,

the optimum total dry film thickness is 4 to 5

mils (0.004 to 0.005 inch). This is readily

achieved with a three-coat system comprising

one primer coat and two finish coats. The

primer is normally applied at a spreading rate

of about 450 square feet per gallon, and each

coat of finish paint at about 650 square feet

per gallon. Two-coat systems (one coat of

primer and one finish coat) capable of building up a 4-mil dry coating thickness also are

available. Such systems utilize specially formulated paints having a relatively high solids

136
content and applied at a low (heavy) spreading rate. In practice, heavy spreading rates without the development of runs, sags, or other

faults are difficult to achieve except by skilled

painters working under favorable temperature and humidity conditions.

The spreading rates of paints commonly applied on plaster and wallboard surfaces are, in

general, similar to the requirements for wood

surfaces.

On smooth nonporous surfaces such as metal,

optimum spreading rates may be very different

for different primers and finish coats, depending on their composition and function and on

the over-all thickness requirements of the application. Thus, a zinc chromate primer on aluminum is most effective when applied in a very thin coat (0.4 to 0.6 mil dry film thickness)

at a spreading rate of about 1600 square feet

per gallon, while a zinc yellow-iron oxide

primer on steel may be spread at the rate of

about 650 square feet per gallon, and the primer

coat in a vinyl resin coating system may be

applied at only 250 square feet per gallon. Finish coats in an oleoresinous paint system typically may be applied at a spreading rate of

about 550 square feet per gallon, while body

and finish coats in a heavy duty vinyl coating

system may be applied at only 150 to 200 square

feet per gallon. Dry film thickness requirements may range from 3 mils in a two-coat

paint system for mild exposure, to 5 mils or more in a typical four-coat oleoresinous paint

system for severe atmospheric exposure, and

to about 12 mils in the six-coat vinyl resin coat- ing system for steel subjected to severe exposure under continuous immersion in salt water.

With so many variables affecting spreading

rate, the value of having and of following a specification or the manufacturer's recommendations is quite apparent.

When repainting, the thickness of new paint

applied should, ideally, merely replace the

paint that has been worn away, to restore the

original coating thickness. As already noted,

the build-up of excessive film thickness can result in stresses within the coating and at the

coating-substrate interface that may cause premature coating failure by cracking and chipping. Excessive thickness is most likely to cause failure on dimensionally unstable surfaces, such as wood. When doing interior re- painting, it should be remembered that indoor

coatings do not wear away like coatings ex- posed to the weather, and repeated repainting

may eventually build up a very thick unsatisfactory film. It is a good rule, when repainting

little-worn interior surfaces, to limit the thick- ness of the applied paint to the minimum needed

to hide the old surface properly.

In all painting, care should be taken to apply the coating at as uniform a thickness as

possible. Spreading rates have little meaning

in terms of coating system durability and appearance if the paint is not evenly spread. In

critical coating applications or where doubt

exists, it is advisable to measure the wet film

thickness at intervals during the painting process; also, if feasible, the dry film thickness

should be measured in a representative number

of locations to provide a final check on the

quality of the job.

7.2.2.1.2. Thickness measurement.—Some of

the instruments that are useful for the meas- urement of coating thickness are described

briefly in the following pages. Detailed discussions of such instruments may be found in the

specific references cited and in references [98]

and [161].

Wet film thickness can be measured readily

and nondestructively on any reasonably smooth,

flat, rigid surface by standard methods [99]

employing readily available test instruments,

such as the Interchemical or Pfund gages. The

Interchemical Wet Film Thickness Gage is es- sentially a triple wheel, having an eccentric

recessed inner wheel concentric with and affixed to circular outer rolling wheels ; when

the gage is rolled over the wet surface, the wet

film thickness is indicated by the point at which

the paint first contacts the eccentric wheel. The

Pfund Gage utilizes a convex lens of calibrated

curvature, which is pushed through the wet film

to the substrate; from the known radius of

curvature and the diameter of the paint spot

produced on the convex lens, the wet film thick- ness can be calculated.

Dry film thickness on non-metallic substrates

cannot be measured nondestructively by presently available methods. On sheet or panel

material amenable to measurement by dial gage

or micrometer [100, 101], the nominal coating

thickness can be computed as the difference between the thickness of the coated and the uncoated (or stripped) material. Other devices,

such as the Gardner Scratch (Saw-tooth)

Thickness Gage, employ calibrated scratching

or penetrating wheels or tools as a means of

measurement. A simple, pencil-like "go-no go"

gage [103] equipped with three fine teeth

scaled to scratch the paint surface to specified

depths has been proposed for use in the field by the Federal Housing Administration. A re- cently-introduced Paint Inspection Gage [104]

combines a precision V-groove cutting tool with

a 50-power gaging microscope that permits observation of film thickness and other visual

characteristics of the applied film. While these

instruments (dial gage and micrometer ex- cepted) for measuring dry film thickness on

nonconductive substrates have not yet been

adopted as standard methods, they represent

useful approaches to a basically difficult problem.

On metallic or conductive substrates the meas137
urement situation is much better. Measurements of dry film thickness on such surfaces can

be made non-destructively, with ease and speed,

by standard methods employing readily available instruments that operate on magnetic or

electronic principles, in conjunction with refer- ence standards of known coating thickness. On

magnetic (ferrous) substrates, an instrument

such as the Aminco-Brenner Magne-Gage, operating on the magnetic principle, can be used

[102, 105]. In essence, this instrument when

calibrated measures the force required to pull a permanent bar magnet from the coated surface. Since the magnetic attractive force is a

function of the distance between the bar magnet and the magnetic substrate, it is similarly

related to the thickness of the intervening non- magnetic coating material, as given by a cali- bration curve obtained from measurements on standard panels of known coating thickness.

While the Magne-Gage is capable of a high

order of precision, even on very thin coatings,

it is basically a laboratory instrument which

requires a level, vibratinn-free location that is free of stray magnetic fields. For easy use in the field on magnetic substrates, a variety of less precise but convenient

hand-held magnetic-type instruments are available. Typical of these is the Elcometer, which

incorporates a strong permanent magnet and

utilizes the magnetic flux across an air gap in

the instrument as a measure of the proximity

of its probe feet to a ferrous surface. A pointer

attached to an armature in the air gap responds

to the changes in the magnetic flux with changing distance, thus indicating the thickness of

the interposed coating on a direct-reading cali- brated scale. On nonmagnetic metal substrates, electronic

instruments operating on the "eddy current"

principle can be used to measure dry coating

thickness. Such instruments depend on the

effect which the conductive base metal exerts on the inductance of a probe coil connected into a suitable electronic sensing circuit. The magnitude of the inductive effect depends on the

distance between the coil and the metal and is therefore also related to the thickness of the

intervening coating, as given by a calibration curve generated from measurements on standard panels of similar substrate material and

known coating thickness. An early instrument

of this type, the Filmeter [106], senses the

inductive effect on the probe coil as a change

in the frequency of an associated oscillator circuit, as measured by restoring the original frequency through "zero-beating" against a fixed

reference oscillator. A dial reading corre- sponding to the frequency change is converted

to film thickness by use of a calibration chart

furnished with the instrument together with

reference panels of known thickness.

Im.proved instruments based on the eddy current principle are now available which greatly

simplify the above type of measurement while

extending the range, speed, and precision of

the measurements. Typical of these are the

Boonton, Dermitron, Elcotector, and Permascope instruments. They indicate the magnitude of the inductive effect related to the coat- ing thickness in terms of an immediate meter

reading which is readily converted to film

thickness by the use of known thickness standards and a calibration chart; or, the meter

scale may be calibrated in terms of known

thickness standards to give direct readings of

coating thickness. While these modern instruments have not yet been incorporated into the

body of standard test methods, evaluative work

now in progress points to their early adoption

in this respect.

It should be noted that although the eddy

current-type instruments were designed especially to meet the need for a nondestructive

method for measuring the thickness of organic

coatings on nonmagnetic metal substrates, this

does not preclude their use on magnetic substrates or any other type of electrically con- ductive substrate.

7.2.2.2. Drying time.—The time required^

for a freshly applied coating to dry sufficiently to permit satisfactory recoating and placement

into service varies widely with the type of coat- ing and with the prevailing atmospheric conditions. It depends also on the thickness and number of coats in the coating system and on the

severity of the intended service environment.

For example, under favorable atmospheric

conditions, the drying time between coats may

vary from one hour for a latex paint to 48 hours

for an oil-base paint; however, considerably

longer drying times may be required for these same materials under conditions of high humidity, low temperature, or poor air circulation. A five-coat paint system would normally

require a longer drying time between coats than

a three-coat system. Coatings destined for severe exposure in corrosive or abrasive en- vironments should be more thoroughly cured

before being placed into service than coatings

that are to serve under mild conditions. In

cases—contrary to usual practice—where quickdrying coatings containing strong solvents are

to be applied over slow-curing coatings (e.g.,

when a phenolic topcoat is to be applied over an oil-base primer), it is essential that the

undercoat be allowed enough additional drying time to assure a thorough cure in order to avoid wrinkling or lifting by the topcoat.

While it is apparent frcm the foregoing that

absolute requirements for drying time cannot

be laid down, general guidance is usually provided in the applicable paint specifications, on manufacturers' labels, and by the criteria for

138
stages of drying (dry-hard, dry-through, etc.)

defined in Method 4061 of Federal Test Method

Standard No. 141 and elsewhere. The nominal

minimum times for recoating specified in the

Federal Specifications for various organic coat- ing materials may be seen from the tabular

summary presented in chapter 8. While an adequate drying period between

successive coats of paint is essential, excessively long periods of drying between coats of a multicoat system are undesirable because the

exposed surface may become contaminated or

the coating may cure to a very hard film with

adverse effects upon intercoat adhesion.

7.2.2.3, Coating system compatibility.—

A compatible coating system is one in which

each applied coat contributes to the over-all

performance capability of the system by bonding properly to the substrate or coating beneath

it without the development of adverse effects such as lifting, wrinkling, mottling, bleeding,

etc. Compatibility, in this sense, begins with

consideration of the nature and condition of

the substrate and the selection of a suitable

primer coating that will have good adhesion to both the substrate and to the topcoat while providing the particular functional properties

needed on the given substrate (e.g., sealing of

wood or masonry surfaces, wetting and cor- rosion-inhibition of metal surfaces, and so on —as described in detail in those sections of this

volume dealing with coating systems for each

important type of substrate (chapter 4) . In- termediate and finish coats should be of a type

that will wet (and perhaps slightly attack) the

preceding coats to ensure good adhesion without the development of the aforementioned film

defects.

As a general rule, compatibility of a multicoat system can be assured by using basically

the same chemical type of coating material

throughout when practicable. For example,

if the nature and condition of the surface have

dictated the choice of an oil-base primer (as

might be the case on hand-cleaned steel), then

it would be good practice to use oil-base intermediate and finish coats, provided the latter

would meet the exposure requirements. On

clean, sandblasted steel permitting the use of a fast-drying oleoresinous primer, compatibility could be assured by using oleoresinous-type

intermediate and finish coats. An all-latex

paint system would be a compatible coating sys- tem for masonry. The principle involved in all these cases is to achieve approximately the same kind of drying behavior, solvent action,

solvent resistance, and chemical nature in each

part of the coating system to minimize the

risk of incompatibility. Other examples may

be given : Lacquers normally should be applied

only over other lacquers, since they contain

strong solvents that would severely attack oilbase, oleoresinous, and various other types of

coatings. Many coating systems based on synthetic resins, such as vinyls, neoprenes, epoxies,

polyesters, and others, depend upon special

primers for the development of good adhesion

to the substrate and require close adherence to the manufacturer's recommendations regarding

materials and procedures to obtain a compatible coating system capable of maximum performance.

Under some conditions it may not be practicable or desirable to adhere to the general

rule of using basically similar materials

throughout the coating system. Such might be

the case, for example, when incompletely

cleaned steel requiring an oil-base primer must

serve in a severe environment involving high

humidity and water immersion conditions for

which an oil-base topcoat would not be suitable.

In such cases, the broad principle of compatibility outlined above can still be applied with

successful results by selecting a coating sys- tem in which there is a progressive transition

in properties from inner to outer coats. In the

above example of conflicting requirements between primer and finish coats, satisfactory

results could be achieved by following the

oil-base primer with an alkyd varnish base

intermediate coat and finishing with a phenolic varnish-base topcoat, provided sufficient

additional drying time for thorough curing were allowed between each applied coat. An exception to the general rule of choosing

chemically similar materials for multicoat sys- tems occurs in cases where a dark undercoat

may attack or be attacked by a chemically similar light-colored topcoat, resulting in bleeding

of dark material into the light coating. In such

cases, the remedy is to use a topcoat sufficiently

different in chemical nature that it will not

interact with the undercoat. For example, an

asphaltic coating will bleed into an oleoresinous

topcoat (both being hydrocarbon in nature)

when the asphalt is attacked by the hydrocarbon solvents in the oleoresinous coating at the

time it is applied; later, the asphalt may continue to attack and slowly migrate (bleed) into

the oleoresinous topcoat. On the other hand, a

shellac varnish based on an alcohol solvent may

be applied over asphalt without danger of lift- ing or bleeding eith,er at the time of application or later. When employing different types

of materials in the same system, care must be

taken to ensure adequate intercoat adhesion.

Materials too widely different in chemical nature will not bond properly to each other, and

the coating system may delaminate when

placed into service.

A mixed coating system is desirable also in cases where the substrate contains extractable

matter which may bleed into the applied paint.

For example, it is not feasible to apply some

latex paints directly on redwood because the

139
water in the paint may extract soluble colored

matter from the wood and cause staining of

light topcoats. If an oil-base primer is first applied to the redwood, however, to provide a

barrier against water-extractable stains, then

latex-type finish paints may be used satisfactorily. In short, compatibility is generally favored

by employing chemically similar vehicles in

each part of the coating system ; however, cer- tain types of different coatings may be intermixed when required, if precautions are taken

to ensure compatibility.

7.2.2.4. Workmanship and general appearance.—A coating system that has been

carefully selected to meet the needs of a particular application deserves the careful workmanship necessary to ensure its full performance

capability. Shoddy workmanship can spoil

both the appearance and the service performance of even the finest coating materials.

Adequate time, skill, and care must be given

to every aspect of coatings application, from

preparation of the surface to application and

cure of the final finish coat. Workmen should

not only be skilled and conscientious but should

also be experienced in the use of the particular

coating materials which they are applying.

Even basically skilled personnel need instruction and training in the use of new or unfamiliar materials requiring unusual techniques or precautions before they can be expected to do

a good job in the field. Good workmanship begins with the realization that high quality in materials is not a substitute for high quality in workmanship, and

vice versa. Nor does skill in using the tools of

his trade give the workman the knowledge or

right to alter a carefully designed paint formulation by thinning or mixing on the job in any

manner except that recommended in the applicable specification or by the coatings manufacturer. Indiscriminate thinning or mixing of

materials on the job may later result in a poor

appearance or impaired performance even when no outright incompatibility is noticed at the time of the thinning or mixing.

A properly applied coating should cover the

substrate uniformly and present a neat, uniform appearance, free from runs, sags, ridges,

lap marks, unnecessary brush marks, or other

evidences of faulty workmanship. There

should be no pinholes, holidays, mottling,

blush, cratering, dusting, orange peel, or other

signs of unfavorable application conditions,

improper thinning, or incorrect formulation.

The finished paint job should show no gouges,

scratches, thumb or finger prints, embedded

dirt, or other marks of carelessness.

Good workmanship is fostered by neat work

habits and a concientious attitude which as- sure the proper care of tools and equipment,

avoidance of the waste and unsightliness of

drips and spillage, and protection of adjacent

or nearby surfaces and objects by the use of

drop cloths or masking tape as needed.

Finally, the essence of good workmanship

in painting may be epitomized in the instruction to mix thoroughly, thin properly, and apply uniformly at the recommended spreading

rate.

7.2.3. Application of Gorrosion-Inhibitive

Primers

Information concerning corrosion-inhibiting

pigments and primers has been given at several places in the text. The pigments themselves and the manner in which they function were described in section 2.3.1.5. Their formulation into primers for various metals was

discussed in section 2.4.2, and their role in

the composite coating system was indicated in

the discussion of Coating Systems for Metals

given in section 4.3 and its various subsections.

The theory of corrosion was considered in the

introductory portion of section 4.3, wherein the

galvanic nature of the corrosion process was

pointed out. It remains now to point out cer- tain important principles which must be borne

in mind in the application of corrosion-inhibitive primers.

When applying corrosion-inhibitive primers

to metal surfaces, it is essential that the entire

surface area be coated. Otherwise, the cor- rosion at small, localized uncoated areas (pinholes and holidays) may be intensified far beyond that which would have occurred without

any coating at all. The reason for this lies in

the galvanic or electrochemical nature of the

corrosion process. Differences in electrical

potential arising between dissimilar metals or

dissimilar areas in or around the same metal

cause a flow of electric current that dissolves

away the anodic (corrodible) material while affording protection to the cathodic material. If the anodic areas are very small compared to the cathodic area, all the current which flows

will be concentrated at these small anodic areas and they will rapidly become deeply pitted or perforated.

As an example, consider the case of a brass

valve connected between lengths of steel pipe.

The steel pipe, being anodic to the brass, will

corrode while protecting the brass. If one

should be tempted to apply a protective coating

only to the corroding steel pipe, a situation

could arise in which a pinhole in the pipe coat- ing would become the anode for all the galvanic

current furnished by the much larger brass

cathode, and perforation of the pipe would soon

result. The best way to avoid such a potentially

disastrous corrosion situation is to paint both

the anodic and the cathodic surfaces.

While the above principles have been pre140
sented in terms of a coating system employing

a corrosion-inhibiting primer because of the

general use of such primers on metal exposed

to corrosive environments, the principles are

equally applicable to any barrier coating applied to a metal substrate.

12A. Helpful Hints

The following is a collection of miscellaneous

helpful hints and useful facts not given or not

emphasized elsewhere in this publication.

7.2.4.1. Wetting properties of common

COATINGS.—Among the oil-base, oleoresinous,

and vinyl coatings that have proved widely

useful for the protection of steel (see sec. 4.3.1)

there is a progressive increase in resistance to corrosive environments (and, happily, in drying speed) as one progresses from straight oil- base vehicles to alkyd-base vehicles to phenolicbase vehicles to vinyls. Unforunately, this

progressive, advantageous increase in corrosion

resistance is offset by a progressive decrease in

the surface-wetting ability of these vehicles.

Thus, the oil-base vehicles which are the least

resistant to corrosive environments are the

slowest drying and have the best wetting properties, whereas the vinyl vehicles which are

the most corrosion-resistant and most rapid

drying have the poorest wetting ability. The

lower the wetting ability of the paint, the more

thorough must be the surface preparation to ensure adequate adhesion and durability.

While one should always select a coating sys- tem which will have adequate durability in the

environment to which it is to be exposed, it is obviously uneconomical to apply the more

corrosion-resistant coating systems in mild en- vironments where the expense of extra surface

preparation necessitated by their lower wetting

ability cannot be justified on the basis of

increased service life.

7.2.4.2, Sources of specifications.—Some

uncertainty and confusion exist regarding procurement sources of Government and other

specifications. The following is presented as a

general guide.

7.2.4.2.1. Federal Specifications are prepared

under the direction of the Administrator of

General Services by appropriate technical

agencies of the Government and are promulgated by the Administrator. All Federal Specifications, including interim specifications, are

listed in the Index of Federal Specifications,

Standards, and Handbooks [67] which is prepared by the General Services Administration

of the Federal Supply Service. Government

activities should obtain copies of this Index and

the periodic cumulative supplements through

departmental channels where established, or

by forwarding a purchase order to the Specifications Activity, Printed Materials Supply

Division, Building 197, Naval Weapons Plant,

Washington, D.C. 20407. Non-government activities may purchase copies of the Index from

the Superintendent of Documents, U. S. Government Printing Office, Washington, D.C.

20402. The specifications and other documents

listed in the Index must be obtained, however,

from the Specifications Activity identified

above. Single copies of product specifications

required for bidding purposes may be obtained

without charge from GSA Business Service

Centers serving regional areas in various parts

of the country, as identified in the Index of

Federal Specifications. A quick guide to and

summary of Federal Specifications for organic

coating materials are given in chapter 8.

7.2.4.2.2, Military Specifications are listed in

the Department of Defense Index of Specifications and Standards [68] which is for sale by

the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

The specifications and other documents listed

in the Index are available to qualified activities on request from the Commanding Officer, Naval

Supply Depot, 5801 Tabor Avenue, Philadelphia, Pennsylvania, Attn: Code CDS. Specific

references to a selected group of Military Specifications for organic coating materials are given in the bibliography of Chapter 9.

7.2.4.2.3, Federal Test Methods for organic

coating materials are compiled in Federal Test

Method Standard No. HI for Paint, Varnish,

Lacquer, and Related Materials; Methods of

Inspection, Sampling, and Testing which may

be purchased from General Services Administration, Business Service Center, Washington,

D.C. 20405, for approximately $3.00 each.

7.2.4.2.4, ASTM Test Methods may be purchased by Government activities through the

Federal Supply Schedule (FSC Group 76, Part

I, Item No. 36-1). Nongovernment activities

may purchase them directly from the American

Society for Testing and Materials, 1916 Race

Street, Philadelphia, Pennsylvania 19103.

7.2.4.2.5, ASA Standards* may be purchased

by both Government and nongovernment activities directly from the American Standards

Association, Inc., 10 East 40th Street, New

York, New York 10016.

7.2.4.3. Application hints (also see sec, 4.1.5).

7.2.4.3.1. When applying multicoat paint sys- tems, the use of a contrasting shade or color for

* Now called USASI Standards, since the name of the American

Standards Association was recently changed to United States of America Standards Institute.

141
successive coats will provide visual help in

assuring the uniform and adequate application

of each coat.

7.2.4.3.2. When painting metal, care must be

exercised to avoid leaving any exposed metal or holidays which could become focal points for

the start and spread of corrosion. Inadvertent

exposure of bare metal or inadequate paint

thickness is most likely to occur in difficult paint-holding areas such as sharp edges, rough

or unclean welds, sharp prominences, and pits or crevices in the surface. In such cases, careful brush application—particularly of the first or primer coat—generally gives better results

than other application methods because it provides an opportunity to work the paint into or onto the irregular surface (see sec. 5.4.1)

.

7.2.4.3.3. When applying latex paints, it should be remembered that while these may be

applied without difficulty on damp or lightly

wet surfaces, they should not be applied to water-soaked surfaces or to substrates containing large amounts of internal moisture, such

as fresh concrete or fresh plaster. Normally,

at least 4 weeks of drying is desirable before

painting the latter surfaces, to allow for a re- duction of the free moisture and excessive

alkalinity. Under exceptional circumstances,

this drying time may be reduced to only two

weeks before applying a first coat of latex paint

or primer-sealer, provided that application of

the topcoat is deferred until the primed plaster or concrete has dried sufficiently by diffusion of

moisture through the still-permeable first coat. The application of latex paints has been discussed at several places in the text, including

sections 2.3.5.3, 2.4.3, 2.4.4, 4.4.3, 4.5, 6.4, 6.5.1,

and 7.2.1.

7.2.4.3.4. Vinyl coating systems for the protection of steel under severely corrosive conditions in industrial or marine environments depend upon special primers for the development

of maximum adhesion and corrosion resistance.

The most widely used primer for this purpose

is a vinyl-red lead type such as that described

by Military Specification MIL-P-15929, A zinc

chromate-vinyl type primer such as that described by Military Specification MIL^P-15930

also may be used. It must be emphasized that

both of these primers require the use of a vinyl

wash primer or washcoat pretreatment such as that described by Military Specification MILC-15328 (see sees. 4.3.1.2 and 6.3.2.5).

7.2.5 Test Equipment and Test Methods

The subjects of test equipment and test

METHODS cover a broad field which (with the

exception noted below) is beyond the scope

of the present volume. For information on

these subjects reference should be made to standard works such as the Gardner Paint Test

Manual [98], the Book of ASTM Standards,

Federal Test Method Standard No. 141, and

the works of Mattiello. Information is given

also in the books by Payne, by von Fischer, by

Burns and Bradley, and by others listed in the

General Bibliography of chapter 9. This manual does, however, present information on methods for measuring thickness, as an integral part of the discussion of coating thickness

requirements given in section 7.2.2.1.

7.2.6. Paint Film Defects and Failures

Paint film defects and failures are beyond

the scope of this volume. This subject has

been covered in depth by Hess [107] and in

the work of the Research Committee of the

Federation of Paint, Varnish, and Production

Clubs [108]. The subject also has been con- sidered by Chatfield [109] and others, and in a number of articles in the literature. Definitions and pictorial examples of specific types

of failure (e.g., blistering) may be found among

the Standards of the American Society for

Testing and Materials (see General Bibliography) . Good pictorial examples of paint fail- ures on wood surfaces are given in the paint

manual of the Corps of Engineers [41] and

in the older National Bureau of Standards

Paint Manual [110] which is superseded by

the present publication.

7.2.7. Basic Information

As part of the consolidation of information

which is a prime objective of the present chapter, it may be useful and appropriate to listapart from the Index or the Table of Contents

specific sections in the text where important

fundamental or basic information is given:

Subject Section NATURE of: Organic coatings 2.1 Drying oils 2.2.1 Primers 2.4 FORMULATION of: Strippable coatings 2.2.7 Latex paints 2.3.5.4 Multicolor finishes 2.3.7 Fire-retardant coatings 2.3.8.1, .2, .3 Textured coatings 4.1.4.5 FUNCTION of: Pigments in paint 2.3.1.1, .3, .5 Functional coatings. 4.6 POLYMERS:

Polymeric nature of synthetic resins 3.1 Relation between polymer structure and

properties 3.1.1 The functionality concept in polymerization 3.1.2 Methods of polymerization 3.1.3 Types of polymers according to structure. 3.1.4 Classification of synthetic resins 3.1.5

142
THEORY:

Formation of a latex film 2.3.5.5 Theory of corrosion 4.3, 7.2.3 Principles of proper thinning 5.3.2 MISCELLANEOUS:

Basic objectives and arrangements 1.2 Coating system compatibility 7.2.2.3 Cost of painting 4.1.6

Effects of high energy radiation 4.6.8 Factors determining gloss of a coating 4.1.4.4 Importance of surface preparation and

pretreatment 6.1 Structure and use of index 1.3 Toxicity of coating materials 5.2.1 Quick guide to Federal Specifications for organic coatings 8.1, 8.2

73. Use of the Text to Solve Specific Coating Problems

The following sections are intended to illus- trate the usefulness of the monograph in solving coating problems by indicating suitable

coating systems for a variety of typical applications, together with specific cross-references

to the places in the text where this information can be found. The examples given serve

the dual purpose of furnishing specific solutions

to typical coating problems while providing an

insight into effective utilization of the information contained in the text.

It should be emphasized that the recommendations made, while assuring a suitable coating

system for the intended application, do not

preclude appropriate modifications or the use

of other types of coating systems which may

be equally suitable. The examples given are

typical and illustrative rather than unique solutions to the problem at hand.

Before considering specific solutions, it will

be helpful to review the general approach to the problem of coatings selection which is given

in the introduction and section 4.1 of chapter 4. The examples to be given represent the practical application of those basic principles.

In using the text, the information on selec- tion of coating systems will be found in chapter

4, application methods are described in chapter

5, and surface preparation is covered in chapter

6, In all painting operations, the safety precautions emphasized in section 5.2 should be

kept in mind and adhered to. A quick guide

to and summary of Federal Specifications re- ferred to is given in chapter 8. In the examples which follow, all the information given may be found in the text, as indicated by appropriate cross-references to specific sections in the text.

7.3.1. Example No, 1: Selection and Application of a Suitable Coating

System for New Exterior Wood Siding on a Home in an Ordinary Urban Environment

Wood surfaces require the use of a primer

or undercoater for best results (4.2) . For

exterior use, a specially formulated oil or oleoresinous type such as that described in section

2.4.1 and typified by Federal Specification

TT-P-25 is highly desirable. The undercoat

should be followed by one or preferably two

topcoats of a linseed oil paint (4.2.1). Reference to the Quick Guide to x^'ederal Specifications (sec. 8.1.3) reveals a half dozen oil-base

paints. Further reference to the summary

chart of Federal Specifications (sec. 8.2) and

a glance down the "for use on wood" column

leads to three exterior oil paints for wood,

among which TT-P-102 is seen to be appropriate for general use in ordinary environments.

The wood surface should be clean and dry

when the paint is applied; prepare the surface

in the manner described in section 6.2.1, Application by brush (5.4.1) is preferred (see

"application method" column in summary

charts of section 8.2). The painting should

be done in favorable weather (7.2.1). The

undercoat should be applied at a spreading rate

of about 450 square feet per gallon and the

topcoats at about 650 square feet per gallon

to yield an over-all dry film thickness of 4 to 5

mils (7.2.2.1). A drying time of 48 hours

should be allowed between each coat, as mentioned in section 7.2.2.2 and as specifically indicated in the summary chart (8.2) for Fed. Spec.

TT-P-102. The paint should be applied in a workmanlike manner that produces a good final appearance (7.2.2.4).

Note: To avoid unnecessary repetition, the copious

cross-references provided in this first example are not repeated in such detail in subsequent examples.

7.3.2. Example No. 2: Selection and Application of a Coating System

for Structural Steel to be Exposed in a Moderate

Industrial Environment

A satisfactory coating system for steel begins

with the application of a suitable corrosioninhibiting primer to the clean metal (4.3).

Exposure in a moderate industrial environment

suggests the use of an alkyd resin-base system,

which is intermediate in durability between a

straight oil-base system (for mild exposure)

and a phenolic resin system (for severe ex- posure under conditions of condensation or fresh water immersion). To ensure a compatible system (7.2.2.3), it is normally desirable that the alkyd-base topcoat be applied over an alkyd varnish-base primer. The latter re- quires smooth, thoroughly clean steel for the

development of adequate adhesion, as would be

obtained by sandblasting to "white metal"

(6.3.1.8.1). Assuming that only "brush-off"

blast cleaning (6.3.1.8.4) or power tool cleaning (6.3.1.7) is feasible in this instance, the

143
choice of a suitable primer is narrowed to either an oil-base type or a raw linseed oil- fortified, long oil-alkyd type, with the latter

preferred because of its better resistance and

its better compatibility with the alkyd topcoat

to be employed. All the foregoing information

is contained in section 4.3.1.1, which also indi- cates that suitable primers having this general

composition are covered by Federal Specifications TT-P-86, Type II or TT-P-57, Type I. Reference to the summary chart of section 8.2

also indicates the composition of these primers,

and further reveals that the TT-P-86, Type II primer is somewhat faster-drying than the

TT-P-57, Type I primer. Assuming that the

TT-P-86 primer is selected for its faster drying and because of the proven performance of

red lead-type primers, it is desirable that the

16-hour nominal drying time given in the chart

be increased to at least 24 hours to ensure compatibility with the alkyd-base topcoat to be applied. A high quality alkyd gloss enamel

suitable for the topcoat is described by Federal

Specification TT-E-489, as indicated under

"Intermediate and Finish Coats" in section

4.3.1.1.1.

7.3.3. Example No. 3: Selection and Application of a Coating System

for Galvanized Steel to be Exposed in an

Ordinary Environment

Galvanized steel that is well-weathered and

free of rust, oil, or dirt can be coated with

conventional paints without undue difficulty,

but the use of a zinc dust primer such as that

covered by Federal Specification TT-P-641 is recommended (4.3.1.4). In this instance the

Type II material, based on a long-oil, linseedmodified, alkyd resin vehicle provides a good

combination of wetting properties, drying time

(nominally 18 hours as indicated in the sum- mary chart for TT-P-641 in section 8.2), and

durability. New galvanized steel must be thoroughly washed with solvent (6.3.1.1) to remove

processing contaminants from the surface. If

it is known that the galvanized steel has not re- ceived a factory chromate treatment (6.3.3.1),

best results will be obtained by use of a cold

zinc phosphate pretreatment of the brush-on

or wash type (6.3.2.2) or by applying a wash

primer type pretreatment coating (6.3.2.5) , before applying the zinc dust primer. The type

TT-P-641 material may also be used as the

topcoat in this system.

7.3.4. Example No. 4: Selection of a Coating System for the Inside of a Potable-Water Tank

For the protection of potable water tanks,

section 4.3.1.1 recommends the use of a zinc

dust-pigmented primer and paint based on a

suitable (non-toxic) phenolic varnish vehicle.

Such a paint is covered by Military Specification MIL-P-15145 (Formula No. 102).

7.3.5. Example No. 5: Selection and Application of a Coating for a Cinder-Block Basement Wall

Select a cement-water paint of the desired

color (sees. 2.3.9.1 and 4.4), The surface

should not previously have been painted with an organic coating and should be free of chalky

or crumbling, old cement paint (6.6.4.3) ; otherwise, it must be prepared by power wirebrushing to remove all the old coating.

Dampen the surface before applying the

cement-water paint, and keep the surface damp

while the coating cures, especially for the first 48 hours (4.4.1). A cement water-paint, properly-applied and cured, provides a very durable

finish that is permeable to moisture but unaffected by dampness or alkalinity. Federal

Specification TT-P-21 covers a cement-water

paint (located by reference to "quick guide"

in section 8.1.3), with further details in sum- mary chart of 8.2)

.

7.3.6. Example No. 6: Selection and Application of a Coating System

for Exterior Concrete

A latex paint system is suitable for application to a properly prepared exterior concrete

surface (4.4). Either an acrylic type such as

that covered by Federal Specification TT-P-19

(see summary chart, sec. 8.2) or a polyvinyl

acetate type such as that conforming to Federal

Specification TT-P-55 may be used. The concrete should be allowed to age as long as practicable, but not less than 30 days, before painting (6.4.1). The surface should be cleaned

free of dirt, curing compounds, loose chalk, and

other foreign matter by the methods described

in section 6.4.2. Efflorescence, if present,

must be completely removed—by sandblasting

(6.3.1.8) when feasible; otherwise, by treat- ment with dilute muriatic (hydrochloric) acid.

A primer coat (usually simply a thinned version of the topcoat) should be applied to the

clean surface, followed by one or two coats

of the unthinned topcoat material (2.4.3 and

4.4.3). It is usur.lly advantageous to dampen

the concrete surface before applying the primer

or primer-sealer (2.4.3, 7.2.1, and 7.2.4.3.3).

Drying time between coats need be only an hour or two in clear, dry weather (summary

chart, 8.2),

7.3.7. Example No. 7: Coating System for Gypsum Wallboard

(Dry Wall)

Section 4.5 indicates that a latex system is suitable for use on gypsum wallboard and

144
recommends use of a latex primer-sealer such as

that covered by Federal Specification TT-P-650

to avoid raising the nap of the outer paper

layer. The primer-sealer may be follov^ed vv^ith a topcoat of latex paint conforming to Fed.

Spec. TT-P-29. Alternatively, oil-base or alkyd-base paints may be applied over the latex

primer-sealer.

7.3.8. Example No. 8: Selection and Application of a Coating System

for Aluminum Exposed to a Moderately

Corrosive Atmosphere

Solvent-clean the surface to remove all oil, grease, and soil (6.3.1.1). Pretreat the sur- face vi^ith vi^ash primer such as that conforming

to Military Specification MIL-C-15328 in the

manner prescribed in section 6.3.2.5. Follov^

with a zinc yellow primer (6.3.4) such as that

covered by Fed. Spec. TT-P-666. The zinc

chromate primer may be over-coated with any

one of a wide variety of coatings, including

oil-base or oleoresinous types, vinyls, epoxies,

cellulose esters, etc., as indicated in section 4.3.3 and as described in chapter 3.

7.3.9. Example No. 9: Coating for Wooden Gymnasium Floor

A polymeric finish such as a polyurethane or an epoxy coating will provide excellent durability in a severe service application such as a

7.4. Summary

Preceding chapters of this publication have

described its objectives and arrangement, discussed the early history of paint and varnish,

described the types of coatings that are available, given the properties and uses of all the

important synthetic resins available today, provided a basis for their selection, and given

information on application methods, surface

preparation, and pretreatment.

The present chapter consolidates the information given and presents important supplementary information.

gymnasium floor, bowling alley, or areas of

heavy foot traffic (4.2.2). Epoxy-polyamide

coatings also provide good durability in such

applications. A conventional floor varnish of

moderate durability is covered by Fed. Spec.

TT-V-71 (see summary chart, sec. 8.2).

7.3.10. Example No. 10: Acid-Resistant Coating

Consult section 4.6 which indicates outstanding materials for specific functional purposes.

Acid-resistant coatings are named in section

4.6.1.1. Possible choices are (1) polyvinyl

chloride coating for resistance to all except

strong oxidizing acids, and (2) chlorinated

rubber coating for resistance even to strong

oxidizing acids. If required, even greater acid

resistance could be obtained with coatings such

as polytetrafluoroethylene or cholorotrifluoroethylene.

7.3.11. Example No. 11: Fire-Retardant Coating System

The function, performance capability, and

classification of fire-retardant coatings are discussed in section 2.3.8. A basis for their selec- tion is given in section 2.3.8.5, wherein refer- ence is made to a number of Federal and

Miliary Specifications for fire-retardant coat- ings (see Bibliography, references 19 through

24).

and Conclusion

The two remaining chapters present a quick

guide and a summary of Federal Specifications

for organic coating materials, and a selected

bibliography of works in the coatings field. While the monograph has endeavored to be

a comprehensive guide rather than a specific

directive for the use of organic coatings, much

of the information presented may be the basis

for such instructions, particularly where the newer materials are to be utilized.

It is hoped that the publication will serve as a comprehensive, unifying treatise in the field of organic coatings.

145
Figure 7.1. Eddy-current thickness gage

for nondestructive measurement of thick- ness of nonconductive coatings on metal

substrates, as described in ASTM Method

D H00-67T. (See text, p. 138.)

Figure 7.2. Magne-Gage measures thickness of norv- magnetic coatings on magnetic (ferrous) substrates

(see text, p. 138) as described in ASTM Method D

1186-53.

(Photo courtesy of American Instrument Co.)

146
Figure 7.3. Interchemical wet film thickness gage, with holder; when gage is rolled over wet surface, thickness

is indicated by point of first contact of paint with recessed, eccentric, calibrated inner wheel, as described in ASTM

Method D 1212-5U (also, see text, p. 137).

(Photo courtesy of Gardner Laboratory.)

Figure 7.4. Balanced-beam scrape-adhesion tester determines load required to remove coating with V-shaped

stylus, as described in ASTM Method D 2197-63T.

147
Figure 7.6. Sward hardness rocker; the harder the surface, the greater the number of swings

obtained before the rocking amplitude decays to a specified level.

( Photo courtesy of Gardner Laboratory.

)
8. Summary of Federal Specifications for Organic Coatings

This chapter presents a concise summary of

important Federal specifications for organic

coating materials in the form of two complementary tables. The first is a list of letter-number

designations of Federal specifications classified

according to generic type and use for quick,

convenient reference. While this table is complete, at this writing, with respect to organic

coatings, the listing of accessory materials is limited to a few of the more important thinners, paint removers, and other auxiliary items.

In addition to being more convenient and in- formative than a straight alphabetic or numerical listing, the classified list provides an

entree to the more comprehensive tabular

summary which follows it. The second table offers a compendium of the

essential technical information contained in a

selected list of important Federal specifications

for organic coatings. The concise tabular ar- rangement provides a rapid and convenient means for obtaining at a glance the key in- formation in a given specification, or for finding a Federal specification for a coating material

to meet the needs of a particular application.

8.1. Quick Guide to Federal Specifications for Organic Coating Materials

The following list gives only letter-number

designations^ of Federal specifications, but it groups them under generic headings that facili- tate the finding of a particular type of material. The full titles of these specifications

may be found in the Index of Federal Specifications [67] . More comprehensive information

about many of these materials can be found

in the tabular summary of important Federal

specifications which follows the classified list.

8.1.1. Enamels

ALKYD—Gloss: E-489, E-491, E-505, E-560

Semi-gloss: E-485, E-509, E-529

Lustreless: E-527

Rust-inhibiting: E-485

Heat-resistant: E-496

Undercoat: E-543, E-545

Floor and deck: E-487

8.1.2. Lacquers

Acid-resistant: L-54

Cellulose nitrate: L-26, 1^31, 1^40

Cellulose nitrate-Acrylic: L-50 (aerosol type)

Multicolor: L-45

Rubbing: L-57

Spraying (general purpose) : L-58

8.1.3. Paints

Alkyd, flat: P-30

Alkyd resin emulsion: P-18

Cement-water: P-21

1 For the sake of brevity, the prefix "TT—", which applies to all specifications listed, has been omitted.

Cold water: P-22, P-23

Fire-retardant: P-26, P-34

Heat-resisting: P-28

Iron oxide type: P-31

LATEX—Acrylic: P-19

Polyvinyl acetate: P-55

Styrene-butadiene: P-0099

Type unspecified: P-29

Oil-base (no resin): P-20, P-27, P-61,

P-102, P-103, P-104

Oil-alkyd: P-53, P-59, P-71, P-81

Oil-base or Oleoresinous: P-14, P-47, P-51

Oleoresinous emulsion: P-18, P-88

Phosphorescent: P-54

Red-lead-base: P-86

Rubber-base: P-91, P-95

Stencil: P-98

Traffic: P-85, P-0087, P-115

8.1.4. Primers

For Wood: P-25, P-636, P-659

For Ferrous Metals: P-57, P-86, P-615,

P-636, P-645, P-659, P-662, P-664

For Aluminum and Magnesium and their Alloys:

P-645, P-666

For Galvanized Metal: P-641

For Masonry, Plaster, and Wallboard: P-29,

P-30, P-56, P-650

149
8.1.5. Sealers

Varnish type: S-176

Lacquer type: S-171, S-190

Sealer and lubricant (for wood) : S-180

8.1.6. Stains

Opaque, exterior: S-706

Semi-transparent, exterior: S-708

Interior (oil type): S-711

8.1.7. Varnishes

Asphalt: V-51

Damar: V-61

Mixing (for aluminum paint) : V-81

Oleoresinous: V-71

Rubbing: V-86

Shellac: S-300

Spar:V-109, V-119, V-121

8.1.8. Miscellaneous Coatings

Calcimine: C-96

Rust-inhibitive compound: C-530

Strippable: C-517

Surfacer: S-810

Vehicle underbody: C-520

8.1.9. Accessory Materials

Aluminum-pigment powder and paste: P-320

Caulking compound (plastic) : C-598

Driers: D-643, D-651

Fillers: F-320, F-336, F-340

Paint Removers: R-230, R-243, R-251

Putty: P-781, P-791

Rubbing Compound: R-771

THINNERS—Dope and lacquer: T-266

Mineral spirits (petroleum): T-291

Mineral spirits (odorless): T-295

Synthetic enamel: T-306

Turpentine: T-801

Notes: (1) For full titles of specifications, or for a straight alphabetic or numerical listing, consult

the published Index of Federal Specifications [67].

(2) For more comprehensive information on a selected group of important organic coating

materials from the above list, see the tabular sum

mary which follows in Section 8.2 of this chapter.

Figure 8.1. Improved dip-coater prevents test panel

from turning and scraping against sides of narrow

dip tank. (Roberts, A. G., & R. S. Pizer, Improved

dip-coater, Anal. Chem. 26, 790, Apr. 195i).

150
8.2. Tabular Summary of Important Federal Specifications for Organic Coatings

8.2.1. Description and Use

The tabular summary lists the specifications

in sequence by letter and number, so that similar materials are generally grouped together.

It will be noted here and in the Index of Federal

Specifications [67] that Federal specifications

for organic coating materials (e.g., TT-L-31)

bear the general group designation "TT", fol- lowed by a subgroup letter denoting the general

type of coating (for example, E for enamel,

L for lacquer, P for paint or primer, etc.) and

finally an identifying number. For each specification, there is tabulated a brief description

of the material and its basic purpose, the nature

of the coating vehicle, information on which

surfaces and under what conditions the material is used, the method of application and

cure, the drying time, outstanding properties

of the material, and notes on its intended use. Basically, as stated in the introduction to

this chapter, the tables may be used either to obtain at a glance the key information contained in a particular specification, or to find a Federal specification for a coating material

to meet the requirements of a particular application. More specifically, among the ways in

which the tables may be used are the following

:

(1) To identify the material described by a

specification for which only the letter-number

designation is known, and to give the essential

properties and applications of the material.

For example, the table quickly indicates that

TT-L-31 describes a cellulose nitrate gloss

lacquer, alkyd-resin modified, for use as a

weather-resistant protective coating on smooth,

clean, exterior metal; also, that the lacquer is applied by spraying and air-dries suffici- ently for recoating in six hours, plus other

information.

(2) To find a specification or a type of coat- ing suitable for interior or for exterior use (or both) on a particular type of substrate. For

example, if one needs a coating for interior

wallboard, a glance down the "wallboard"

column will indicate immediately, by code letter "1", a number of potentially suitable materials

from among which a further selection may be

made on the basis of the additional information

given as to type of coating, nature of vehicle,

properties, and use. If, for instance, TT-E-543

is considered, the table indicates further that

this is an enamel-type undercoat and that the

wallboard must oe sealed with a suitable

primer-sealer before application of the undercoat.(3) To find a specification for a coating intended for a certain type of use, such as an

oil-base paint (house paint), a floor and deck

enamel, a spar varnish, a latex paint, a primer

for aluminum, a traffic paint, a swimming pool

paint, etc. Such coatings can be found readily

by scanning the material description blocks in

the first column.

(4) To locate a coating based on a particular

type of vehicle, such as a drying oil, an oleoresin, an alkyd resin, cellulose nirate, polyvinyl

acetate, rubber, etc. As in (3), this is readily

accomplished by scanning the material description blocks in the first column. These

blocks usually list the basic type of vehicle even when this is not given in the formal specification title or in conventional title indexes.

Alternatively, one may locate specifications

relating to the purposes of (3) and (4) above

by referring to the classified list of specifications given in Section 8.1 of this chapter.

The tabular summary charts are on the fol- lowing pages.

Figure 8.2. Modem outdoor exposure racks can be set for any angle in a single longitudinal plane. Hinged cross bars are adjustable to retain specimens of different lengths.

151
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Figure 8.3. Cinder-block test house for evaluating durability of coatings for masonry surfaces.

Wall specimens for paint exposure tests are shown in foreground.

Figure 8.4. A typical masonry-wall specimen used to conduct paint exposure tests.

168
9. Selected Bibliography

The following is a selected bibliography of

important reference works, specifications, and

articles found useful in preparing this monograph or considered to be valuable sources of

further information in the field of organic

coatings. While not in itself an exhaustive

list, it provides basic references which, in many

instances, are good sources of further refer- ences to the more detailed literature.

The Bibliography is divided into two main

sections. The first is a list to which specific

reference has been made in the text. These

specific references indicate immediate sources

of further information about the material

or topic under discussion. Includprl are

references to a number of Military ("MIL")

specifications of particular usefulness. Federal

("TT") specifications for organic coatings,

while referenced to some extent in this bibli9.1. Specific

(1) Sabin, A. H., Technology of paint and varnish, 2d ed. (John Wiley & Sons, New York, N.Y., 10016,

1916).

(2) Raw materials index, National Paint, Varnish and

Lacquer Association, 1500 Rhode Island Ave., N.W., Washington, D.C. 20005.

(3) Higgins, E. J., Stainless steel metal protective paint

for corrosive areas. Am. Paint J., 44. No. 1, 27

(Sept. 14, 1959).

(4) Cowling, J. S., and F. M. Noonan, Fluorescent high

visibility paints for aircraft. Official Digest, 31. No. 415, pt. 2 (Aug. 19 9). (4a) Military Specifi^-ation MIL-P-21563, Paint system,

fluorescent, for aircraft application.

(5) Federal Specification TT-P-99, Paint, styrene-butadiene emulsion, exterior.

(6) Federal Specification TT-P-55, Paint, polyvinyl

acetate emulsion, exterior.

(7) Federal Specification TT-P-19, Paint, acrylic emulsion, exterior.

(8) Zola, J. C, Coating composition and preparation

thereof, U.S. Patent No. 2,591,904, April 8, 1952.

(9) Tremain, A., and W. Morris, Single-coat multicolor

finishes. Paint Technology, 21. No. 234, 83 (March

1957).

(10) Federal Specification TT-L-45, Lacquer, multicolor, aqueous, dispersion type (for spray application).

(11) Test method for fire hazard classification of building

materials. Underwriters Laboratory Standard UL723, Chicago,. 111. (12) Method of test for surface burning characteristics of building materials, ASTM Designation E84-61,

1965 Book of ASTM Standards, pt. 14 (American

Society for Testing and Materials, Philadelphia, Pa. 19103).

(13) Robertson, A. F., D. Gross, and J. J. Loftus, A

method for measuring surface flammability of materials using a radiant energy source. Proceedings, Am. Soc. Testing Mats., 56. 1437 (1956).

(14) Federal Standard No. 136, Flame spread properties

of materials (NBS radiant panel method); also, ASTM E162-62T. Method of Test for Surface Flammability of Materials Using a Radiant Heat

Energy Source, 1965 Book of ASTM Standards,

pt. 14 (American Society for Testing and Materials,

Philadelphia, Pa. 19103).

ography, are covered more comprehensively in the tabular summary of Federal specifications

presented in Chapter 8. Since Govnrr-ment

specifications are in a continuous state of

change which precludes up-to-date subletter

listings except in current indexes [67, 68],

reference in the Bibliography is to the basic

specification only, with the understanding that

the latest revision in eifect is the one normally

applicable.

The second section of the Bibliography lists works of general usefulness in the field of

organic coatings, including some volumes dealing with the theoretical aspects of polymer

chemistry and the properties of organic coating

materials. These general references are ar- ranged alphabetically by author (except where

the title is of over-riding significance) and are not numbered.

References

(15) Federal Specification SS-A-118, Acoustical units; prefabricated (Test for flame resistance. Sect.

4.3.3). (16) Method of test for fire retardance of paints, ASTM

Designation D 1360-58 (Cabinet method), 1965 Book of ASTM Standards, pt. 21 (American

Society for Testing and Materials, Philadelphia, Pa. 19103).

(1 ' Method of test for fire retardancy of paints, ASTM

Designation D 1361-58 (Stick-and-wick method),

1965 Book of ASTM Standards, pt. 21 (American

Society for Testing and Materials, Philadelphia, Pa. 19103).

(18) Duggan, J. J., Fire protective coatings. Paper pre- sented at the twentv-sixth annual fire department

instru tors conference at Memphis, Tenn., Feb.

23-26, 1954.

(19) Gross, D., and J. J. Loftus, Surface ammabili y of fire-retardant and conventional paint assemblies. Fire Research Abstracts and Reviews, Vol. 3, No.

3, 1961 (NAS-NRC).

(20) Military Specification MIL-P-17970, Paint, nonflaming (dry), chlor nated alkyd resin, soft white,

semigloss, formula No. 124/58.

(21) Military Specification MIL-P-17971, Paint, nonflaming (dry) , chlorinated alkyd resin, pastel green, semigloss, formula No. 125/58.

(22) Military Specification MIL-P-17972, Paint, nonflaming (dry), chlorinated alkyd resin, bulkhead,

gray, semigloss, formula No. 126/58.

(23) Military Specification MIL^P-17973, Paint, fire retardant, alkyd-chlorinated rubber, green gray,

semigloss, formula No. 127.

(24) Military Specification MIL-P-17974, Paint, fire retardant, alkyd-chlorinated rubber, yellow gray,

semigloss, formula No. 128.

(25) Federal Specification TT-P-26, Paint, interior, white

and tints, fire retardant.

(26) Federal Specification TT-P-34, Paint, exterior, fire- retardant, white and light tints. (27) Federal Specification TT-P-86, Paint: red-lead-base,

ready-mixed.

(28) Federal Specification TT-P-57, Paint, zinc yellowiron oxide base, ready-mixed.

(29) Federal Specification TT-P-641, Primer, paint; zinc dust-zinc oxide (for galvanized surfaces).

169
Military Specification MIL-C-15328, Coating, pre- (64 treatment (Form No. 117 for metals).

Alsebrook, W. E., The coating of magnesium alloys, Corrosion Technology, 2. 113 (April 1955). (65 Federal Specification TT-P-21, Paint, cement-water,

powder, white and tints (for interior and exterior

use). (66 Federal Specification TT-P-24, Paint, oil, concrete and masonry, exterior, eggshell finish, ready-mixed, (67 white and tints. Federal Specification TT-P-91, Paint, rubber-base,

for concrete floors. Federal Specification TT-P-95, Paint, rubber-base (68

(for swimming pools). Federal Specification TT-P-18, Paint, alkyd resin- emulsion, exterior, paste, tints and white. Federal Specification TT-P-56, Primer coating (69

(primer-sealer), pigmented oil, plaster and wall- board. Federal Specification TT-P-29, Paint, latex base, (70

interior, flat white and tints. Abraham, H., Asphalts and allied substances, 5th ed., vols. 1 and 2 (D. Van Nostrand Company, Inc., (71 Princeton, N.J., 1945). Paint Manual, U.S. Dept. of Interior, Bur. of Recla- mation, Supt. of Documents, U.S. Gov't Printing

Office, Washington D.C. 20402, 1961). (72 Paint Manual—new construction and maintenance

(Engineering manual, civil works and military construction, pt. CXXXIV), Dept. of Army, Corps of (73 Engineers, Washington, D.C. 20310, (July 1956). Coal tar enamel protective coatings for steel water

pipes of sizes 30 inches and over. Am. Water Works (74 Assoc. Specification C203-57 (American Water Works Association, 2 Park Avenue, New York 16, (74a

N.Y.).

Military Specification MIL-C-18480, Coating com- (75 pound, bituminous, solvent, coal tar base (cold- applied). (76 Bracconot, H., Ann. Chem. Phys., 52. 290 (1833). Military Specification MIL-N-5538, Nitrocellulose,

technical, for use in organic coatings. (77 Military Specification MIL-D-5553, Dope, cellulose

nitrate, clear. Military Specification MIL-D-5549, Dope, cellulose- (78 acetate-butyrate, clear, for aircraft use. Military Specification MIL-D-5550, Dope, cellulose- (79 acetate-butyrate, pigmented, camouflage.

Military Specification MIL-D-5551, Dope, cellulose- acetate-butyrate, pigmented, gloss. (80 Roberts, A. G., and S. G. Weissberg, Film-forming

properties of cellulose acetate propionates, Ind. (81 Eng. Chem., 43. 2088-2098 (Sept. 1951). Military Specification MIL-E-7910, Ethyl-cellulose.

Military Specification MIL-L-6805, Lacquer, Cam- (82 ouflage.

Baekeland, L. H., Ind. Eng. Chem., 1. 149 (1909). Military Specification MIL-P-8585, Primer coating, zinc chromate, low-moisture-sensitivity.

Military Specification MIL-P-15935, Paint, outside, gray. No. 11 (vinyl-alkyd) (Form No. 122-11). . Military Specification MIL-L-7178, Lacquer, cellu- ^ lose nitrate, gloss, for aircraft use. PHtt, K. F., Weather-resistant polyvinyl alcohol

coatings. Paint and Varnish Production, 49. 27

(Aug. 1959). /05 Military Specification MIL-P-14276, Paint, heat ^ resisting, silicone, aluminum.

Federal Specification TT-P-28, Paint, aluminum, /gg heat resisting (1,200°F.). Military Specification MIL-P-20087, Paint, heat (87

resisting. Military Specification MIL-P-14105, Paint, heat (88

resisting, olive drab (for steel surfaces). Fortener, R. G., Improved epoxy performance, Official (89 Digest, 31. 127 (Jan. 1959). Gusco Process Equipment, A. Gusmer, Inc., Wood- (90 bridge, N.J.

Boltori, B. A., and R. E. Van Strien, Water-soluble baking resins from trimellitic anhydride, Amer.

Paint J., 44. (41), 71 (June 20, 1960). Mantell, C. L., C. W. Kopf, J. L. Curtis, and E. M.

Rogers, The technology of natural resins (John

Wiley & Sons, Inc., New York, N.Y. 10016, 1944). Natural Resins Handbook, Amer. Gum Importers

Assoc. Inc., 360 Furman St., Brooklyn, N.Y., 1939. Index of Federal Specifications, Standards and Handbooks, General Services Administration, Federal Supply Service (Supt. of Documents, U.S. Govern- ment Printing Office, Washington, D.C. 20402). Index of Specifications and Standards of the Dept. of Defense, Defense Supply Agency (Supt. of Documents, U.S. Government Printing Office, Washington, D.C. 20402).

Sax, N. I., Dangerous properties of industrial materials (Reinhold Publ. Corn., New York, N.Y.

10022, 1957). Scheflan, L., and M. B. Jacobs, The handbook of solvents (D. Van Nostrand Co., New York, N.Y.,

1953). Humphrey, H. B., and Genevieve Morgis, Safety

with solvents. Bureau of Mines Information Circular 7757 (U.S. Dept. of the Interior, Washington, D.C. 20240, Aug. 1956). Lehmann, K. B., and F. Flurry, Toxicology and

hygiene of industrial solvents (Williams and Wilkins Co., Baltimore, Md., 1938). Edwards, J. D., and R. I. Wray, Aluminum paint and

powder (Reinhold Publ. Corp., New York, N.Y.

10022, 1955). Electrostatic coater sprays dry powders, C&EN, 40. 42 (June 4, 1962). Military Specification MIL-S-12935, Sealer, surface, knot. Safety in sandblasting. National Industrial Sand

Association, Washington, D.C. Code of recommended good practice for metal clean- ing sanitation, American Foundrymen's Assoc., Golf and Wolf Road, Des Plaines, 111. ASA Standard Z37.7-1943, Allowable concentrations

of chromic acid in chromates (American Standards

Assoc., 10 East 40th St., New York, N.Y. 10016).

Military Specification MIL-M-10578, Metal conditioner and rust remover (phosphoric acid base). Federal Specification TT-C-490, Cleaning methods

and pretreatment of ferrous surfaces for organic

coatings.

Military Specification MIL-C-8514(Aer), Coating

compound, metal pretreatment, resin-acid. Military Specification MIL-C-8507(Aer), Coating,

wash primer (pretreatment), for metals; application of. Military Specification MIL-P-14504, Coating, pre- treatment, one-package wash primer (for steel, aluminum, and magnesium).

Finishes for metals—paintability of galvanized steel, Publication No. 1005, Building Research Institute, 1725 De Sales St., N.W., Washington, D.C. 20036

(1963). Tentative methods for preparation of zinc-coated

steel surfaces for painting, ASTM Designation

D2092-62T, 1966 Book of ASTM Standards, pt. 21, p. 452 (American Society for Testing and

Materials, Philadelphia, Pa. 19103).

Military Specification MIL-T-12879, Treatments,

chemical, prepaint and corrosion-inhibitive, for zinc surfaces. Military Specification MIL-C-5541, Chemical films

for aluminum and aluminum alloys. Military Specification MIL-A-8625, Anodic coatings

for aluminum and aluminum alloys. Federal Specification TT-P-666, Primer coating, zinc yellow, for aluminum and magnesium surfaces. Federal Specification TT-P-645, Primer, paint, zinc- chromate, alkyd type. Military Specification MIL-M-3171, Magnesium

alloy, processes for corrosion protection of.

170
Magnesium finishing, Dow Chemical Co., Midland,

Mich.

Metal Finishing Guidebook, 30th edition, 1962, pp. 488-505 (Metals and Plastics Publications, Inc., 381 Broadway, Westwood, N.J. 07675).

Ibid., pp. 506-514.

Evangelides, H. A., HAE coatings for magnesium.

Product Finishing, 7. 54-60 (Oct. 1954). Military Specification MIL-M-45202, Magnesium

alloys, anodic treatment of. Dow No. 17, Chemical treatment for magnesium,

(Dow Chemical Company, Midland, Mich.).

Chromate treatments for metals. The New Jersey Zinc Co., 160 Front St., New York, N.Y. 10038).

Military Standard MIL-STD-171(0RD), Systems

for preparation, painting, and finishing metal and

wood surfaces (Department of the Army, Ordnance

Corps, Washington, D.C. 20310). Burnett, G. E., and A. L. Fowler, Painting exterior concrete surfaces—with special reference to pre- treatment, J. Am. Concrete Inst., 43. 1077 (June

1947). Gardner, H. A., and G. G. Sward, Paint Testing

Manual, Physical and chemical examination of paints, varnishes, lacquers, colors, 12th ed. (Henry

A. Gardner Laboratory, Inc., Bethesda, Md.,

1962). Methods for measurement of wet film thickness of paint, varnish, lacquer, and related products,

ASTM Designation D 1212-54, 1966 Book of ASTM Standards, pt. 21, p. 238 (American Society

for Testing and Materials, Philadelphia, Pa. 19103).

Method for measurement of dry film thickness of paint, varnish, lacquer, and related products,

ASTM Designation D 1005-51, ibid., p. 193. Dry film thickness (mechanical type gage). Method

6183, Federal Test Method Standard No. 141, "Paint, Varnish, Lacquer, and Related Materials; Methods of Inspection, Sampling, and Testing"

(General Services Administration, Business Service Center, Washington, D.C. 20405).

Dry film thickness (magnetic type gage). Federal Test Method 6181, ibid. FHA Paint Gauge, Wilks Precision Instrument Company, Bethesda, Maryland.

Tooke, Jr., W. Raymond, A paint inspection gage.

Official Digest, 35. 691-698 (July 1963). Method for measurement of dry film thickness of nonmagnetic coatings of paint, varnish, lacquer,

and related products applied on a magnetic base,

ASTM Designation D 1186-53, 1966 Book of ASTM Standards, pt. 21, p. 215 (American Society

for Testing and Materials, Philadelphia, Pa. 19103).

Methods for measurement of dry film thickness of nonmetallic coatings of paint, varnish, lacquer, and

related products applied on a nonmagnetic metal

base, ASTM Designation D 1400-58, ibid., p. 276. Hess, Manfred, Paint film defects (Reinhold Publishing Corp., New York, N.Y. 10022, 1951). Film formation, film properties, and film deterioration, C. R. Bragdon, Ed., Study by Research

Committee of Federation of Paint and Varnish

Production Clubs (Interscience Publishers, New

York, N.Y. 10016, 1958).

Chatfield, H. W., Ed., The science of surface coatings, chap. 16 (D. van Nostrand Company, Inc., New York, N.Y., 1962). Walker, P. H., and E. F. Hickson, Paint Manual—

with particular reference to Federal Specifications,

Building Materials & Structures Report BMS105,

National Bureau of Standards, U.S. Department of Commerce (Supt. of Documents, U.S. Government

Printing Office, Washington, D.C. 20402, October

1945). Federal Specification TT-P-25, Primer, paint, exterior (undercoat for wood, ready-mixed, white and

tints). Federal Specification SS-A-694, Asphalt, petroleum,

roof coating, brushing consistency.

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

Military Specification MIL-R-3472, Roof-coating;

asphalt-base emulsion. Standard specifications for asphalt-base emulsions for use as protective coatings for metal, ASTM Desig- nation D 1187-66, 1966 Book of ASTM Standards,

pt. 21, p. 217 (American Society for Testing and

Materials, Philadelphia, Pa. 19103). Standard definitions of terms relating to paint, varnish, lacquer, and related products, ASTM Designation D 16-64), ibid., p. 20. MiHtary Specification MIL-C-15203, Coating compound, bituminous, emulsion type, coal tar base. Pass, A. and J. F. Meason, Metallic pigments in protective coatings. Official Digest, 33. 507 (April 1961). Chandler, R. H., Zinc dust paint—chemical or elec- trochemical protection? Corrosion Tech., 8. 165

(No. 6, 1961). Federal Specification TT-R-271, Resin, phenolformaldehyde, paraphenyl.

Federal Specification TT-R-266, Resin, alkyd; solutions. Farstad, D. K., A new water soluble linseed oil, Amer. Paint J., 45. 44 (May 22, 1961). Military Specification MIL-P-6808 (ASG), Primer

coating, zinc chromate, for aircraft and missile applications, application of. Hensley, W. H., and H. J. West, A melamine-acrylic water soluble resin for industrial finishes. Paint and

Varnish Production, 50. 41 (July 1960). Petropoulos, J. C, Charles Frazier, and L. E. Cadwell, OflScial Digest, 33. 719-36 (June 1961). Military Specification JAN-E-480, Enamel, baking,

phenol- or urea-formaldehyde.

Whitewash and cold water paints. Bulletin No.

304-G, National Lime Association, Washington, D.C. (1955). Federal Specification TT-P-650, Primer coating,

latex base, interior, white (for gypsum wallboard).

Military Specification MIL-P-21035, Paint, high

zinc dust content, galvanized repair.

Military Specification MIL-P-23377 (Wep), Primer

coating, epoxy-polyamide, chemical and solvent

resistant, for weapons systems.

Military Specification MIL-C-22750 (Wep), Coating, epoxy-polyamide, chemical and solvent re- sistant; for weapons systems.

Military Specification MIL-C-22751 (Wep), Coating

system, epoxy-polyamide, chemical and solvent

resistant; process for application of. Military Specification MIL-P-27316A (USAF),

Primer coating, epoxy, for metal surfaces. Military Specification MIL-P-149, Plastic coating

compound, strippable (hot dipping).

Military Specification MIL-L-20209, Lacquer, strippable, spray or dip (for packaging).

Military Specification MIL-V-13750, Varnish,

phenol-formaldehyde, clear, and aluminum pig- mented.

Military Specification MIL-C-18467 (NOrd), Coating, phenolic resin base, baking.

Military Specification MIL^C-18468 (Wep), Coating, phenolic resin base, air-drying.

Military Specification MIL-L-19537 (ASG), Lacquer, acrylic-nitrocellulose, gloss (for aircraft use). Military Specification MIL-P-21698 (Wep), Paint,

fluorescent, for aircraft, process for application of. Military Specification MIL-P-22636, Primer coating, for red fuming nitric acid resistant paint.

Military Specification MIL-P-14458, Paint, rubber, red fuming nitric acid resistant). Military Specification MIL-P-23236 (Ships), Paint

coating systems, steel ship tank, fuel and salt water ballast. Military Specification MIL-P-15929, Primer coating, shipboard, vinyl-red lead (Formula 119—for hot spray).

Military Specification MIL-P-15930, Primer, vinyl- zinc chromate type (Formula 120 for hot spray).

171
(145) Military Specification MIL-P-23281 (CG), Primer,

vinyl-red lead (for brush or spray).

(146) Military Specification MIL-C-27227 (USAF), Coating, polyurethane, thermal resistant, for aircraft application.

(147) Military Specification MIL-P-46057 (MR), Coating,

polyurethane.

(148) Military Specification MILr-P-46056 (MR), Primer

coating, polyamide-epoxy.

(149) Military Specification MIL-S-12935, Sealer, surface, knot.

(150) Military Specification MIL-L-18389, Lacquer, vinyl- idene resin, water and fuel resistant, white and

orange. Formula No. 113/54.

(151) Federal Specification TT-S-300, Shellac, cut. (152) Military Specification MIL-P-15145, Paint, zinc dust pigmented, fresh water tank protective,

Formula No. 102. (153) Steel Structures Painting Bulletin, Vol. 5, No. 1 (Fall, 1963), Steel Structures Painting Council, 4400 Fifth Avenue, Pittsburgh, Pa. 15213.

(154) Military Specification MIL-F-007179B (ASG), Finishes and coatings: general specification for protection of aerospace weapons systems, structures, and

parts. (155) Bovey, F. A., The effects of ionizing radiation on

natural and synthetic high polymers (Interscience

Publishers, New York, N.Y. 10016, 1958).

(156) Charlesby, A., Atomic radiation and polymers

(Pergamon Press, Inc., New York, N.Y., 1960).

(157) Pascale, J. V., D. B. Herrmann, and R. J. Miner,

High-energy electron radiation resistance of plastics. Modern Plastics, 41. 239 (Oct. 1963).

(158) Malm, C. J., and H. L. Smith, Gel lacquer technique

for protective coatings, Ind. Eng. Chem., 38. 937

(1946). (159) Murray, B. G. and A. J. Andren, How to work with

gel coats, Modern Plastics, 39. 119 (June 1962).

(160) Specifications for building brick (solid masonry units made from clay or shale), ASTM Designation

C 62-62T, 1965 Book of ASTM Standards, pt. 12 (American Society for Testing and Materials,

Philadelphia, Pa. 19103).

(161) Keinath, George, The measurement of thickness,

NBS Circular 585 (Supt. of Documents, U.S. Government Printing Office, Washington, D.C.

20402).

9.2. General References

Adhesion and adhesives, fundamentals and practice (Papers read at symposium at Case Inst, of Tech. and at London

Soc. of Chem. Ind., 1954).

Alfrey, Turner, Jr., Mechanical behavior of high polymers

(Interscience Publishers, New York, N.Y. 10016, 1948).

Alfrey, T., Jr., J. J. Bohrer, and H. Mark, Copolymerization

(Interscience Publishers, New York, N.Y. 10016, 1952). American Society for Testing and Materials, 1965 Book of ASTM Standards, pt. 20, Paint—Materials specifications

and tests; naval stores; aromatic hydrocarbons. (Phila- delphia, Pa. 19103).

American Society for Testing and Materials, 1965 Book of ASTM Standards, pt. 21, Paint—tests for formulated

products and applied coatings (Philadelphia, Pa. 19103).

Battista, 0. A., Fundamentals of high polymers (Reinhold

PubHshing Corp., New York, N.Y. 10022, 1958). Bawn, C. E. H., The chemistry of high polymers (Inter- science Publishers, New York, N.Y. 10016, 1948). Billmeyer, F. W., Jr., Textbook of polymer chemistry

(Interscience Publishers, New York, N.Y. 10016, 1957). Blout, E. R., W. P. Hohenstein, and H. Mark, Monomers

(Interscience Publishers, New York, N.Y. 10016, 1949-

1951). Bovey, F. A., Emulsion polymerization (Interscience Publishers, New York, N.Y. 10016, 1958). Bovey, F. A., The effects of ionizing radiation on natural and

synthetic high polymers (Interscience Publishers, New

York, N.Y. 10016, 1958).

Bureau of Ships Manual, Chapter 19—Preservation of ships

in service. Navy Department (Supt. of Documents, U.S. Government Printing Office, Washington, D.C. 20402). Bureau of Yards and Docks Technical PubUcation TP-Pw30, Maintenance and operation of public works and public

utilities, pt. I

Painting (Navy Department, Washington,

D.C. 20350).

Burns, R. M., and W. W. Bradley, Protective coatings for metals, 2nd ed. (Reinhold Publ. Corp., New York, N.Y.

10022, 1955).

D'Alelio, G. F., Fundamental principles of polymerization;

rubbers, plastics and fibers (John Wiley & Sons, New

York, N.Y. 10016, 1952). Delmonte, John, The technology of adhesives (Reinhold

PubHshing Corp., New York, N.Y. 10022, 1947).

Doolittle, A. K., The technology of solvents and plasticizers

(John Wiley & Sons, New York, N.Y. 10016, 1954).

Eirich, F. R., Branched molecules. New York Academy of Sciences Annals, Vol. 57, art. 4 (1953). Epstein, George, Adhesive bonding of metals (Reinhold

Publ. Corp., New York, N.Y. 10022, 1954). Fancutt, F., and J. C. Hudson, Protective painting of structural steel (The MacMillan Company, New York,

N.Y., 1957). Federal Test Method Standard No. 141, "Paint, Varnish,

Lacquer, and Related Materials; Methods of Inspection,

Sampling, and Testing" (General Services Administration, Business Service Center, Washington, D.C. 20405).

Film formation, film properties, and film deterioration, C. R. Bragdon, Ed., Study by Research Committee of Federation of Paint and Varnish Production Clubs (Interscience

Publishers, New York, N.Y. 10016, 1958). Flory, P. J., Principles of polymer chemistry (Cornell University Press, Ithaca, N.Y., 1953).

Frith, E. M., and R. F. Tuckett, Linear polymers (Long- mans, Green and Co., London-New York, 1951).

Fischer, E. K., Colloidal dispersions (.John Wiley & Sons, New York, N.Y. 10016, 1950).

Gailer, J. W., and E. J. Vaughan, Protective coatings for metals (Charles Griffin & Company, Ltd., London, 1950). Gardner, H. A., and G. G. Sward, Paint Testing Manual,

Physical and chemical examination of paints, varnishes,

lacquers, colors, 12th ed. (Henry A. Gardner Laboratory,

Inc., Bethesda, Md., 1962). Gaylord, N. G., and H. F. Mark, Linear and stereoregular addition polymers: polymerization with controlled propagation (Interscience Publishers, New York, N.Y. 10016,

1959). Golding, Brage, Polymers and resins (D. Van Nostrand

Company, Inc., Princeton, N.J., 1959). Hess, Manfred, Paint film defects (Reinhold Publishing

Corp., New York, N.Y. 10022, 1951). Houwink, R., Ed., Elastomers and plastomers, vol. 1 (Elsevier PubHshing Co., Amsterdam (distributed by D. Van Nostrand Company, Inc., Princeton, N.J. (1950). Houwink, R., Fundamentals of synthetic polymer tech- nology in its chemical and physical aspects (Elsevier PubHshing Co., Amsterdam (distributed by D. Van Nostrand

Company, Inc., Princeton, N.J., 1949). Huggins, M. L., Physical chemistry of high polymers (John

Wiley & Sons, New York, N.Y. 10016, 1958). Kline, Gordon M., Ed., Analytical chemistry of polymers. Part I. Analysis of monomers and polymeric materials,

plastics-resins-rubbers-fibers (High Polymers, Vol. XII,

Interscience PubHshers, Inc., New York, N.\ . 10016,

1959). Part II. Analysis of molecular structure and chemical groups (Interscience PubHshers, Division of John Wiley

& Sons, Inc., New York, N.Y. 10016, 1962). Part III. Identification procedures and chemical analysis

(ibid., 1962). Mark, H., and A. V. Tobolsky, Physical chemistry of high

polmeric systems, 2nd ed. (Interscience Publishers, New

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(John Wiley & Sons, New York, N.Y. 10016, 1941-1946).

Meyer, K. H., Natural and synthetic high polymers; a text- book and reference book for chemists and biologists, 2d ed. (Interscience Publishers, New York, N.Y. 10016, 1950).

172
Modern Plastics, Annual Encyclopedia Issue (McGrawHill, Inc., New York, N.Y. 10021).

Payne, H. F., Organic coating technology, vol. I (John

Wiley & Sons, New York, N.Y. 10016, 1954). Payne, H. F., Organic coating technology, vol. II (John

Wiley & Sons, New York, N.Y. 10016, 1961). Perry, H. A., Adhesive bonding of reinforced plastics

(McGraw-Hill Book Co., New York, N.Y. 10036, 1959). Perry, T. D., Modern wood adhesives (Pittman Publishing

Co., New York—Chicago, 1944). Raw materials index, National Paint, Varnish and Lacquer

Association, Washington, D.C. 20005. Schildknecht, C. E., Polymer processes; chemical technology

of plastics, resins, rubbers, adhesives, and fibers (Inter- science Publishers, New York, N.Y. 10016, 1956). Schmidt, A. X., and C. A. Marlies, Principles of high polymer

theory and practice (McGraw-Hill Book Co., New York, N.Y. 10036, 1948).

Singer, Elias, Fundamentals of paint, varnish and lacquer

technology (The American Paint Journal Company, St. Louis, Mo. 63103, 1957).

Steel Structures Painting Manual, vol. 1—Good painting

practice, Joseph Bigos, Ed. (Steel Structures Painting

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(Reinhold Publishing Corp., New York, N.Y. 10022,

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coatings (Reinhold Publishing Corp., New York, N.Y.

10022, 1953).

173

Index

A

Accelerators (see Curing agents)

Acid cleaning of steel, 110-111 phosphoric acid method, 110-111

beneficial effects of. 111

for light rust removal, 110

Military specifications for, 110

pickling. 111 bath control, 111 by use of strong acids in dilute solution, 111

electrolytic type. 111

rust and mill scale removal by. 111 surface passivation after. 111

solvent precle^ning, need for, 110, 111 Acrylic resins, 3, 52-53

clarity, 52 compatibility with other resins, 52

composition, 52

in latex paints, 19

properties, 52 thermosetting types, 19, 53

uses, 52-53

vi^ater-reducible type, 53 Adhesion

dependence on wetting of surface by coating, 5, 141 importance of adequate surface preparation for, 105- 106

of coatings on galvanized surfaces, 118

of coatings on plastics, 105

of phenolic paints, 15

of polyester coatings, 45 requirements for, 105 Aging (see also Weathering)

of concrete and masonry surfaces before painting, 28,

84, 125-126

of new plaster before painting, 29, 85, 128

Airless spray, 99-100 equipment design, 99

reduction of overspray in, 100

Alkali cleaners, 110

cleaning action of, 110 hazards in use of, 110 Alkyd paints, 15 Alkyd resin emulsions, 16-17, 20

film hardening process, 17 Alkyd resins, 3, 43-45 (see also Polyesters)

amino-alkyds, 46 composition and properties, 43-44

effect of oil-modification, 44

in coatings for steel, 80, 81, 82, 83 manufacture of, 43-44

resin-modified, 45

silicone-modified, 45

styrenated, 45, 54

uses, 44-45, 83 Aluminum

anodic pretreatment for, 119, 120 chromate treatments for, 120, 124

coating systems for, 83, 145

pretreatment of, 119-120, 124 primers for, 27, 120 avoidance of lead-containing pigments, 27

surface preparation for painting, 119-120 wash primer pretreatment for, 119-120 Aluminum paints, 13, 55 Aminoplasts (see Amino resins)

Amino resins, 46-47 blended with alkyd resins, 46 formation, 46

properties, 46 urea and melamine resins, compared, 47

uses, 46-47 Anodic pretreatments (see also Surface pretreatments)

for aluminum, 119, 120

for magnesium, 122-124 Appearance of coatings, 73-75

clarity and whiteness, 74

factors in, 73

gloss, 74-75

opacity, 74

shade, 12 surface texture, 75

translucency, 74 workmanship important in, 140

Application methods and equipment, 96-103 (see also Paint application, and under specific application

method)

basis for selection, 96

brush, 96-97

dipping, 102

flow coating, 102

fluidized bed, 103

knife coating, 102-103

limitations, 76

roller applicator, 97-98

roller coating, industrial, 103

spraying, 98-102

airless, 99-100

conventional, 98-99 dual-gun, 101-102

electrostatic, 100-101

airless, 101

dry, 101

liquid, 100-101

flame, 100 general technique, 98

hot, 100 workmanship, 140 Asbestos-cement shingle, glazed

coatings for, 19 (see also Concrete and Masonry weathering of, to produce paintable surface, 125)

Asphalt (see also Bituminous coatings)

coatings for, 8, 26, 139 B

Basic lead silico chromate pigment, 13

in corrosion-inhibiting paints, 27, 81 Benzyl cellulose, 41 Bibliography, selected, 169-172

Binder, 5 Bituminous coatings, 29-30, 82

coal tar and asphalt compared, 29, 82

cut-backs, 29

emulsions, 29

for interior water and gas pipes, 29

for roofing, 29 for underground and underwater applications, 29, 82

for waterproofing, 29

hot-applied, 29 on steel, 30, 82

paints and varnishes, 29 use over oil paints, 29

175
Blast cleaning, 112 (see also Sandblasting)

Bleeding, of asphaltic coatings, 139

Blistering, moisture as cause of, 28, 131 Blooming, 74 Blown oil, 6 Blush, 74, 94 Bodied oil (see Heat-bodied oil) Boiled oil, 5 Brass, coatings on, 83, 124

Bronze, coatings on, 83, 124 Brush application, 96-97 care of brushes, 97

choice of brush, 97

preferred for primers, 97

technique, 97

Butadiene-acrylonitrile rubber, 65 Butyl rubber, 65-66 chemical inertnes of, 65, 66 low temperature flexibility of, 65-66 G

Cadmium, coatings for, 119, 124 (see also Galvanized

surfaces)

Carboxymethyl cellulose, 41 Casein in resin emulsion paints, 16 Casein paints, 20

Catalysts (see also Curing agents)

in phenolic resin manufacture, 41, 42, 43

in polystyrene polymerization, 58

stereospecific, 37 Cathodic protection of metals, 79

Cellulose conversion to soluble derivatives, 38

derivatives, general properties of, 38

solubility of, 38

sources of, for coatings, 38

Cellulose acetate, 39-40

limited compatibility with plasticizers and resins, 39, 40 manufacture of, 39-40 ; . .ir, properties and uses, 39, 40

solvents for, 40 ; Cellulose acetate-butyrate

in airplane dopes, 40

properties and uses, 40

Cellulose acetate-propionate, 40

Cellulose butyrate, 40

Cellulose derivatives (see also specific derivative)

cellulose esters, 38-40

cellulose ethers, 40-41

cellulose mixed esters, 40 general properties of, 38

use in lacquers, 38

Cellulose nitrate, 38-39 combined with other resins, 39 flammability of, 39

in airplane dopes, 39

in lacquers, 7, 39 , manufacture of, 39 properties of, 39

uses, 39

Cellulose propionate, 40 Cement-water paints, 25, 84, 126

application of, 25, 28, 126, 131

composition, 25 curing of, 25 hiding power, 84

poor adhesion over organic coatings, 131

suitability on concrete below grade, 28, 84

surface preparation for, 25, 125, 126, 131

uses, 25 Chalking of paints, control of, 10, 107 Chalky surfaces, preparation for painting, 28, 107, 128, 131 Chlorinated rubber, 66-67

as primer for neoprene coatings, 80

in swimming pool paints, 67

in traffic paints, 67 Chlorosulfonated polyethylene, 61-62, 65

coatings, application, 62 compared with neoprene, 65 completely resistant to ozone, 62

composition, 61-62 preparation and properties, 61-62, 65

resistance to strong oxidizing agents, 62

uses, 62 Chromate treatments for metals, 124 (see also Surface

pretreatments

)

Chromium, coatings on, 83, 124, 125 Cleaning methods (see also Surface preparation, and

under specific method)

Clear coatings (see also specific type)

drying oils, 5-6 durability compared to pigmented, 77

lacquer, 7

sealer, 7-8

spirit varnish, 7 synthetic resin type, 9 varnish, 6-7 wax, 8 Coal tar pitch (see Bituminous coatings)

Coating problems, solution of, 143

acid-resistant coating, 145

coating for cinder-block basement wall, 144

coating for wooden gym floor, 145

coating system for aluminum in moderately corrosive atmosphere, 145 coating system for exterior concrete, 144

coating system for exterior wood siding in ordinary

urban environment, 143

coating system for galvanized steel in ordinary envi- ronment, 144

coating system for gypsum wallboard, 144-145

coating system for potable-water tank, 144

coating system for structural steel in moderate industrial environment, 143-144 examples of use of monograph for, 143-145

fire-retardant coating system, 145

Coatings (see also Coating systems. Paints, and specific

coating type and specific substrate)

abrasion-resistant, 50, 58, 61, 63, 64, 66, 69, 88

acid-resistant, 87, 145

adhesion of, 5

alkali-resistant, 87 appearance of, 73-75 (see also separate heading)

application methods, 96-103 (see also separate heading)

binder, 5 bituminous, 29-30

chemical-resistant, 41, 43, 48, 51, 52, 53, 56, 58, 63-66,

68-69, 83, 86-88

classification of, 5

clear, 5-9 decontaminable, 46

film-former, 5 fire-retardant, 21-24, 89, 145

for electrical applications, 19, 41, 42, 45, 48, 50, 53, 58, 61, 63, 88

176
Coatings—Continued

functional, 86-89

heat-resistant, 22, 39, 45, 55, 62, 63, 64, 65, 66, 68, 69, 88

heat-sealable, 48, 51, 53, 57, 58, 61, 69 impermeable, 49, 51, 65, 66, 67, 69, 88

luminescent, 53 mar-resistant, 88

multicolor, 21, 75, 85, 86 nonflammable or self-extinguishing, 48, 63, 64, 67, 69

organic, definition, 1 organic solvent-resistant, 87

oxidation-resistant, 65, 66, 87

ozone-resistant, 62, 65, 66, 67 pigmented, 9, 15-25 primers, 26-29 problems (see coating problems, solution of) radiation resistance of, 88-89

strippable, 8-9, 40, 41, 49, 52

synthetic resin types, 9, 85, 126, 131

textured, 75 transparent, 88

types, 5 vehicle, 5 water-resistant, 87-88

weather-resistant, 41, 42, 43, 58, 62, 63, 64, 65, 66, 67 Coatings for

aircraft propellers, 50

art objects, 53 automobiles, 3, 8, 19, 27, 42, 55 awnings, 62 basement walls, 75, 144

belts and conveyors, 64, 66 bench tops, laboratory, 61, 68, 79 blower and fan blades, 64

boats, 46, 60

boilers and stoves, 45, 47, 55 bookbinding, 50 bowling alleys, 46, 60, 78, 145

cans, 42, 50, 56, 58

cellophane, 39 chemical tank and drum linings, 19, 43, 48, 49, 52, 56,

58, 61, 63, 66, 68

"clean" rooms, 46

concrete and masonry below grade, 25, 28, 84

concrete and masonry, general (see Concrete and

Masonry)

curbs, 25 dairy barns, 25 dishwasher racks, 48 documents of importance, 53

fabrics, 39, 40, 41, 48, 49, 50, 61, 63, 65, 66, 69

fences, 25

floors (see Floor coatings)

food packaging 48, 49, 51 furniture (see Furniture coatings)

gasoline tanks, 50, 52

glass, 41, 50, 58, 63, 66

gloves, heavy duty, 62, 64 gym floors, wooden, 46, 60, 78, 145 high visibility, 14 hose (heater and brake), 64

hospital equipment, 47, 56, 83

hospital rooms, 46, 75

houses, wood siding of, 3, 17, 21, 60, 77 impact resistance, 62

industrial equipment, 43, 45, 46, 48, 56, 68

industrial plants, 11, 17, 21, 43, 45, 52, 54, 56, 62, 64,

66, 75

leather 39, 41, 48, 49, 52, 57, 58, 64, 65, 66

light fixtures, 45

linoleum, 8 low temperature flexibility 63, 65-66 machinery, 45, 63, 64 marine use, 42, 44, 46, 49, 56, 58, 60, 77 mattress springs, 19 military equipment, 43, 58 motor shells, 19

oil and grease resistance, 48, 64-65 packaging, 48, 51-52, 61 paper, 40, 41, 48, 49, 50, 51, 58, 65, 69

pipes, interior, 29

plastics, 40, 41, 50, 52, 58

plating baths, 48, 62 porches and decks, 42, 43, 44, 60 pumps, chemical, 63, 64

radiation areas, 25

refrigerators, 19, 45, 47, 56, 83

roofing, 29-30 rubber goods, 58, 62, 64, 66 sanitary rooms, 46 scrubber towers, 64 shop coats, 43 sunlight resistance, 39, 41, 65-66 swimming pools, 28, 46, 54-55, 60, 67

table tops, 3, 47 tank car interiors carrying hot caustic, 19

tarpaulins, 62

tools, 44, 64

toys, 44, 45 trim and trellis, 42, 44, 57 underground applications, 29, 43, 82 underwater applications, 29, 30, 43, 82

upholstery, 62 waterproofing, 29 water-repellancy, 55

wire, electrical, 17, 40, 41, 48, 50, 56, 57, 61, 63, 64,

65, 66, 69 Coating systems (see also Coatings and under specific

substrate)

basis for selection of, 71-76, 79, 85-86, 89 appearance, 73-75 (see also Appearance)

application limitations, 76

basic function, 73, 79, 86-89

cost, 76 nature of substrate, 72-73 type of service environment, 70

for concrete and masonry, 84-85, 144

for metals, 79-83, 144

for plaster and wallboard, 85-86, 145

for wood, 77-79, 143, 145

vinyl, 80, 81-82, 142 Cold water paints (see Cement-water paints. Whitewash

Color shade (see Shade)

Compatibility of coating systems, 79, 81, 138, 139-140 Concrete and masonry (see also Surface preparation)

chalky surfaces on, 128, 131 coatings for, 84-85, 144 cement-water paints, 25, 28, 84

latex paints, 84-85, 126

multicolor, 21, 75, 85, 86

oleoresinous paints, 28, 84, 126

synthetic resin types, 85, 126 pretreatment of, 126, 127-128

previously painted, 129-131 primers for, 28, 84

properties affecting paintability, 84, 125

surface preparation for painting, 28, 84, 125-128, 130- 131

177
Concrete and Masonry—Continued

traffic paints, 85 Copper, coatings on, 83, 124 Corrosion inhibiting pigments, 13 Corrosion inhibtion, 13, 79 by cathodic protection, 79 by red-lead pigment, 27 by zinc-rich paints, 13 mechanism of, 13, 27 primers for, 26-27, 79, 80-81 Corrosion of iron and steel mechanism of, 79, 140-141

protection against, 79-80, 140-141 Cost

in selection of coating systems, 76

of painting, 76

of sandblasting, 113 Crackle finishes, 75

Crystallinity in polymers, 33 Curing agents (see also Two-package systems)

for butyl rubber, 65

for chlorosulfonated polyethylene, 61-62

for epoxy resins, 56

for neoprene, 64

for nitrile rubber, 65

for polyamide resins, 58

for polyesters, 45

for polysulfide rubber, 66

for polyurethanes, 59 for silicone rubber, 66

for styrene-butadiene rubber, 65

Cyclized rubber, 67 D

Defects in paint films (see Paint film defects) Definitions of paint terms, 3 Dehydrated castor oil, 6 Dip-coating, 102

design of tank for, 102

in industrial finishing, 102

rate of withdrawal in, 102

viscosity control in, 102 Dopes for airplanes, 39, 40

Driers, 5 Drying oils, 5-6 (see also under Oils, and under specific

oil) blown, 6

boiled, 5 composition of, 5 dehydrated castor, 6

fish oils, 6 heat-bodied, 5

linseed, 3, 6, 20-21, 27, 44, 77

oiticica, 6

perilla, 6 raw, 5

refined,

5

semi-drying, 6 styrenated, 54

tall oil, 6 tung, 3, 6, 75 Drying time, 138-139

before placement into service, 138

between coats, 138-139

factors aff'ecting, 138

stages, 139

Dual-gun spraying, 101-102

of two-component coating systems, 101-102

Durability of coatings,

clear versus pigmented, 77

Efflorescence, 125, 127

Electrostatic spray, 100-101

airless, 101

dry, 101 general principles of, 100

liquid, 100-101

Emulsifier

polyvinyl alcohol as, 49 Emulsion cleaning, 109-110 removal of oily and water-soluble contaminants by,

109

residue, need for removal of, 109 Emulsion paints {see Latex paints)

Enamels, 16 Federal specifications for, 149 pigment volume concentration in, 74-75 Epoxy-polyamide coatings, 58

Military specifications for, 58

properties and uses, 58 Epoxy-polyamide resins, 58 Epoxy resins, 3, 55-57 amine-cured, 56

application by fluidized bed, 56, 103

classification of, 56

compatibility with other resins, 56

esterified, 56-57

properties, 55-56

reactivity, 55-56

structure, 55-56

styrenated, 57

unmodified, air-drying, 56

unmodified, banking types, 56

uses, 56, 57 Ethyl cellulose, 40-41 Extender pigments, 10 F

Federal specifications for organic coating materials

quick guide list, 149-150

sources of, 141 summary charts, 152-167

uses of, 150-151 Federal specifications, sources of, 141 Federal test methods, sources of, 141

Fill coats, for open-textured masonry, 127

Filler, for open-grain wood, 107

application of, 107 Film formation, 5 mechanism of, in latex paints, 18 Finish coats (see Topcoats)

Fire hazards in painting, precautions against, 92-93 Fire-retardant coatings, 21-24 compared -mth heat-resistant type, 22

evaluation of, 23 flame-retardant type, 22 brominated compounds in, 22 formulation of, 22-23 function and effectiveness, 21-23, 24

insulative type, 22-23 intumescent type, 22-23 mechanism of kindling, 22

selection of, 23-24

specifications for, 24

substrate effect, 23 with chopped glass fibers, 22

1.78
Fish oils, 6 Flame cleaning

of metal surfaces, 114-115

follow-up with power cleaning and priming, 114

prior removal of oil or grease, 114 removal of loose rust and mill scale from, 114 removal of old paint from, 114-115

of wood surfaces previously painted, 107 Flame spraying, 100

thermoplastic resins applied by, 100

Floor coatings, 8, 42, 43, 46, 54, 56, 58, 60, 64, 73, 78 Flow coating, 102

control similar to dip-coating 102

Fluidized bed coating, 103

applicability of, 103 hazards in, 103

principles of, 103

technique of, 103 Fluorescent pigments

in high visibihty paints, 14-15

properties of, 14 Fluorocarbon resins, 62-63 (see also specific resin)

polychlorotrifluoroethylene, 63

polytetrafluoroethylene, 62-63 Form oil, removal from concrete, 126 Functional coatings and resins, 86-89 see also Coatings)

Functionality

and polymerization, 34

in addition polymerization, 34

in condensation polymerization, 35 Fundamental information, table of sources of in monograph, 142-143 Furan resins, 68 Furniture Coatings

metal, 8, 19, 45, 46, 47, 48, 49, 56, 57

outdoor, 44, 49 wood, 3, 8, 21, 39, 42, 45, 56, 57, 60, 75 G

Galvanized surfaces coating systems for, 82, 144

durability of coatings on, 118

factory chromating of, to protect against white stain,

118, 119

inimical to paint adhesion, 118, 119

precludes further pretreatment, 118

"home cure" treatments, avoidance of, 119 painting of, 27, 82, 118-119 preparation for painting, 118-119 pretreatment of, 118-119 primers for, 27, 118

reactivity with paint vehicles as cause of early adhesion failure, 118 weathering of, as surface preparation, 118, 119

zinc dust paints for, 13, 27, 118, 119

Gel lacquers, 40

Gloss, 74-75

effect of pigment volume concentration on, 74 enhancement of with polyamides, 58 with polyurethanes, 59

factors determining, 74-75

of enamels, 16, 74 Gold flake as pigment, 14 Grout coat for open-textured masonry, 127

H

Hammertone finish, 75 Hand cleaning methods. 111

in maintenance painting, 111-112

oily contaminants spread by, 111 removal of loose rust and soil by. 111

tools employed. 111 uses of, 11-112 Hansa yellow, 12 Hardeners {see Curing agents)

Heat-bodied oil, 5

in wood primers, 26 Hiding power, 9, 10 and pigment volume concentration, 74 and refractive index, 9 of pigments, 9-10 High visibility paints, 14-15 Holdout, 8, 72 Holidays in coatings, 29, 136, 140, 142 Hot melt coatings, 40, 41, 50, 58, 67 Hot spray, 100 formulation precautions, 100

heavier film build with, 100 Hydroxyethyl cellulose, 41

I Indanthrene pigments, 12-13 Index structure and use of, 2 Intermediate coats, on steel, 81, 82

compatibility of, 139-140

Iodine number, 5 Iron blue, 11

Iron, coatings on {see Steel) Iron oxide pigments, 11 Isobutylene-isoprene copolymer, 65-66 K

Kel-F {see Polychlorotrifluoroethylene)

Knife coating, 102-103

of flat, rigid sheet, 102 Knot sealer, 50, 106

L

Lacquers, 7 automobile, 3 Chinese, 7 definition, 7 Federal specifications for, 149

furniture, 3, 38, 40

nitrocellulose, 3, 7, 39 pigmented, 15-16 Latex paints, 3, 16-20, 28, 29, 84, 85, 126, 128, 129, 130, 142 advantages and disadvantages, 16

application of, 17, 28-29, 84, 85, 126, 128, 129, 130-131,

135, 139-140, 142 chemical types, 18

acrylic, 19

blends and copolymers, 19-20 polyvinyl acetate, 19 styrene-butadiene, 18-19 comparison with solution types, 16

film formation, 18

forerunners, 16-17 formulation, 17-18

179
Latex paints—Continued

problems of applying on chalked surfaces, 128 use of surface conditioner, 128, 130

uses, 17, 19, 20

in primer-sealers, 28-19 on concrete and masonry, 28, 84-85, 126, 130-131 on plaster and wallboard, 85-86, 128, 129 on wood, 77, 139-140 Lead chromate, 13, 27 Lead, coatings on, 83 Lead silico-chromate, 13

in primers for iron and steel, 27, 81 Lexan (see Polycarbonate resin)

Linseed oil, 6 (see also Drying oils) in alkyd resins, 3, 44

in house paints, 77

in metal primers, 27

water-soluble type, 6, 20

Lithol reds, 12 Lithopone, 10 Luminescent paints

acrylic vehicles for, 53 M

Magnesium

cleaning methods for, 120-121

acid pickling, 121

alkaline cleaning, 120

electrochemical, 120 mechanical, 121

coating systems for, 83 improvement of corrosion resistance and paintability

by pretreatment, 120 pretreatments for, chemical, 121-122

caustic anodizing, 122 chrome pickle, 121 dichromate, 122

galvanic anodizing, 122

sealed chrome pickle, 121-122 pretreatments for, electrochemical (anodic), 122-124

acid chromate process, 123-124 HAE process, 122-123 Federal specification classes and grades, 123

primers for, 27-28 avoidance of acidic vehicles and lead pigments in, 27

surface preparation for painting, 120-124 Maintenance painting, 129-130 (see also Surface preparation, previously painted surfaces)

avoidance of excessive thickness build-up in, 130

mildew or mold, removal of, 130

of concrete and masonry, 130-131

of metal surfaces, 130

of plaster and wallboard, 131

of wood, 107-108, 130

periodic inspection as basis for, 129 removal of old paint, 107, 109, 110, 112-113, 130

right time for repainting, 29-130

selection of recoating system, 129

surface preparation in, 129, 130

Masonry coatings (see Concrete and masonry)

Mass tone, 11

of iron blue pigments, 11 Melamine-formaldehyde resins (see Amino resins)

Metals (see also under specific metal)

cathodic protection of, 79 coating systems for, 79-83

corrosion of, 79 maintenance painting of, 129-130

pretreatment of, 27, 105-106, 115-125 primers for, 26-28

protection against corrosion, 79 surface preparation for painting, 105-106, 108-115,

119, 120-121, 124-125, 130 Methyl cellulose, 41 Mildew on organic coatings, removal of, 127, 130

Mold (see Mildew)

Molybdated pigments, 12

Multicolor finishes, 21, 75, 85

application of, 21 formulation of, 21, 75 properties of, 21, 75

uses, 21, 75 85 N

Natural resins, 7, 68 Neoprene, 64-65

coatings, application of, 64 compared with chlorosulfonated polyethylene, 65

for abrasion resistant applications, 80

preparation, 64 primer under, 80

properties, 64

uses, 64-65

Nickel, coatings on, 83, 124, 125

Nitrile rubber, 65

Nitrocellulose (see Cellulose nitrate)

Nondrying oils, 6 Nylon resins, 57 (see also Polyamide resins)

o

Objectives and arrangement of information, 1-2, 135,

142, 145

Oil length, 6-7

Oil paints, 15, 80, 81, 126

linseed oil in, 15

Oils (see also under specific type)

drying type, 5-6 non-drying, 6 semi-drying type, 6 styrenated, 54

Oiticica oil, 6 Oleoresinous paints, 15, 80, 81, 84, 126 alkyd type, 15, 81 phenolic type, 15, 81 Opacity (see Hiding power)

Organic coatings (see also Coatings)

definition, 1

eff"ect of high energy radiation on, 88-89 nature of, 5 Organosols, 48

application of, 48

defined, 48

uses, 48

vinyl resins in, 48 P

Paint application (see also Application methods)

latex, on damp surfaces, 142 methods and equipment, 96-103

multiple-coat systems, 138-139, 141-142 precautions on metal, 142 proper conditions for, 135-136 Paint failures, premature, 142 moisture as cause of, 28, 131 Paint film defects, 29, 74, 94-95, 135, 136, 137, 139, 140, 142

180
Painting

cost of, 76 multiple-coat systems, 136 purpose of, 1 temperature and humidity conditions for, 135-136 Paint removal

by flame or torch, 107, 114-115 by liquid paint removers, 107, 110 by sandblasting, 112-113 by scraping and wirebrushing, 107, 109, 112 from concrete and masonry surfaces, 130-131 from metal surfaces, 109, 110, 112-113, 130 from plaster and wallboard, 131 from wood surfaces, 107 Paint removers, liquid

alkali-organic type, 164

alkali type, 163 Federal specifications for, 107, 150

organic solvent type, 107, 164 wax in, 107

Paints (see also Coatings and specific type)

application methods, 96-103

definition, 1 early history, 3 emulsion, 16-17 Federal specifications for, 149

fire hazard in use of, 92-93

heat-resistant, 55

house, exterior, 3, 77

latex, 16-20 mixing of, 93-94

oil-base, 15, 80, 81, 126

oleoresinous, 15, 80, 81, 84, 126

safe storage and handling of, 91-93

thixotropic, 58

toxicity of, 91-92 twentieth century developments, 3 Paint terms, defined, 3 Para reds, 12 Pebble finishes, 75

Perilla oil, 6 Phenolic paints, 15, 81 Phenolic resins, 3, 41-43

classification of, 41 general properties, 41

in coatings for steel, 79, 80-81, 82

in spar varnishes, 7, 42, 77 manufacture of, 41

modified, oil soluble, 42 pure (100% phenolic), 42-43

alcohol-soluble, oil-insoluble, thermosetting, 43

dispersion type, 43

oil-soluble, heat-reactive, 42

oil-soluble, thermoplastic, 42

uses of, 42, 43 Phosphating treatments (see Surface pretreatments)

Phthalocyanine pigments, 12 Pickhng, 110, 111, 121 (see also Acid cleaning)

Pigmented coatings (see also specific types)

classes of, 9 cold water paints, 25-26 enamels, 16

fire-retardant, 21-24

lacquers, 16

latex paints, 16-20

multicolor, 21

oil paints, 15

oleoresinous, 15 pigment function in, 9

primers, 26-29 water-solution types, 20-21 Pigments, 9-14 (see also specific pigment)

black, 14 carbon black, 14 types of, 14 vegetable blacks, 14

colored, 9, 10-12

classification, 10

natural, 10 properties of, 10

synthetic, inorganic, 11

synthetic, organic, 12

corrosion-inhibitive, 13 extender, 10 function of, 10 Federal specifications for, 9 fluorescent, 14-15

in high visibility paints, 14-15 function of, in paints, 5, 9, 10, 13 hiding power of, 9-10

list of, 9

metallic, 13-14

refractive index, role of, 9 white, hiding, 9-10 Pigment volume concentration (PVC)

critical PVC, 74

effect on gloss, 74

eff'ect on hiding power, 74

in enamels, 16, 74

of gloss and semi-gloss latex paints, 19 Plaster (see also Surface preparation)

aging of, before painting, 28-29, 85, 128

coating systems for, 85-86

multicolor finishes for, 86

primer-sealers for, 28-29, 85 surface preparation for painting, 85, 128-129, 131

Plasticizers, 5, 6 non-drying oils as, 6 Plastics, surface preparation for painting, 105

Plastisols, 48

application of, 48

defined, 48

uses for severe service, 48

vinyl resins in, 48

Polyacrylic ester rubber, 67 Polyamide resins, 57-58 adhesion of, 58

application of, 58 blended with epoxies, 58

classification of, 57

composition, 57

in corrosion-inhibiting primers, 58

linear, 57

nylon, 57

properties, 57, 58

structure, 57, 58

thermoplastic, 57-58 thermosetting, 58

uses, 57, 58 Polycarbonate resin, 69 Polychloroprene (see Neoprene)

Polychlorotrifluoroethylene, 63

coatings, application of, 63 composition and structure, 63

properties, 63

uses, 63

181
Polyester resins, 45-46 adhesion of, 45

application and curing of, 45-46

composition, 45

properties, 46 reinforcement with glass, 46

uses, 46

Polyethylene, 60-62

application of, 60, 100

chlorosulfonated, 61-62

cross-linked, 61 flame-spraying of, 100

irradiated, 61 low-and high-density types compared, 61 manufacture of, 60-61

properties, general, 60

stabilization of, 61

uses, 61 Polymeric nature of synthetic resins, 33-34 Polymerization

auto-oxidative, 34 by addition, 34-35 by condensation, 35 methods, 35-36

bulk, 35 emulsion, 36

solution, 35

suspension, 36

role of functionality in, 34 Polymer structure, 33-35

crystallinity, 33 dependence on functionality, 34

effect on properties, 33-34 Polymers, types of, 36-37

block polymer, 37 branched, 34 copolymer, 36-37

cross-linked, 33, 34

crystalline, 33 graft polymer, 37 heterpolymer, 36 homopolymer, 36

linear, 33

stereoregular, 37

thermoplastic, 33, 34

thermosetting, 33, 34

Polypropylene, 68-69 Polystyrene

copolymers, 53, 54 homopolymer, 53 polymerization from monomer, 53 styrenated-alkyd copolymer, 45, 54

styrene-butadiene copolymer, 53

in latex paints, 18-19, 53

rubber, 65

solution type, 53-54

Polysulfide rubber, 66 blended with vinyl resions, 66

Polytetrafluorethylene, 62-63 chemical inertness, 62

coatings, application of, 62, 63 composition and structure, 62 decomposition products, hazard of, 62

properties, 62-63

uses, 63 Polyurethane rubber, 66 Polyurethanes, 3, 58-60

application of, 59

history, 58

preparation, 58-59

properties, 59

types of coatings, 59

air-cured, 60

heat-cured, 59-60 moisture-cured, 60

polyisocyanate type, 59 prepolymer type, 59 single-package systems, 59-60 two-component systems, 59

uses, 60

Polyvinyl acetal, 50 Polyvinyl acetals, 50 preparation of, 50

Polyvinyl acetate, 47-48

in latex paints, 19 manufacture, 47

properties, 47-48

uses, 48

Polyvinyl alcohol, 49-50

insolubilization of, 50 preparation and properties, 49

uses, 49-50 Polyvinyl butyral, 50-51 composition, 50

cross-linking of, 50

in knot sealer, 50, 106

in safety glass, 50

in wash primers, 50-51, 117

properties, 50

uses, 50-51, 117

Polyvinyl chloride, 48

copolymers of, 48, 49, 51-52

organosols, 48

plasticization of, 48

plastisols, 48

properties, 48

stabilization of, 48

uses, 48

Polyvinyl chloride-acetate, 48-49 formulation into coatings, 49

preparation, 48-49

properties, 49

uses, 49

in chemical-resistant coatings, 49

Polyvinyl ethers, 52

composition, 52

properties, 52

uses, 52 Polyvinyl formal, 50

as toughest vinyl resin, 50 Polyvinylidene chloride, 51

acrylonitrile copolymer, 51-52 copolymers, general, 51

vinyl chloride copolymer, 51 Potable-water tanks, coatings for, 81, 82, 144 Power tool cleaning, 112

in maintenance painting, 112

safety precautions, 112 sequence of operations, 112

solvent precleaning, need for, 112

tools required, 445

Pretreatment, chemical (see Surface pretreatments)

Primers, 26-29 automotive, 27

bituminous, 29

corrosion-inhibitive, 13, 27

application of, 140 epoxy-polyamides in, 58 pigments in, 13

182
Primers—Continued

Federal specifications for, 149

for concrete and masonry, 28, 84

application of, 28 below grade, 28

for concrete and masonry—Continued

chalked surfaces, 28 Federal specifications for, 28, 84 swimming pools, 28

for metals, 26-27, 79-80, 81, 82 aluminum, 27

basic lead silicochromate-pigmented, 81

function, 26-27

galvanized, 27, 82, 118

iron and steelfi 26-27, 80-81, 82 magnesium, 27-28

oil-base oleoresinous, 80-81 phenolic resin/tung oil type, 80, 81 red lead-pigmented, 80

value of pretreatment, 27

vinyl type, 81-82

zinc yellow-iron oxide type, 80-81

for plaster and wallboard, 28-29, 85-86

latex type. 28

oleoresinous type, 29

sealing function, 28

for vinyl coating systems, 81-82, 142

for wood, 26

function, 26, 79

properties needed in, 79 wetting properties of, 141

Primer-sealers, 19, 28-29, 42, 43, 84, 85-86

for plaster and wallboard, 28-29, 85, 128

Primer surfacer, 20 Problems {see Coating problems, solution of)

PVC (see pigment volume concentration)

R

Radiation, high energy, 88-89

effects produced in organic polymers, 88-89

role of oxygen in, 88

resistance of organic coatings to, 89

sources of, 89 Raw oils, 5, 26

in wood primers, 26

effect on paint properties, 5, 26 Red lead, 13 avoidance in primers for aluminum and magnesium,

27

in primers for iron and steel, 27, 80

in vinyl primers, 82 mechanism of protection by, 27 Redwood, latex paint on, 139-140

Refined oils, 5 in wood primers, 26 Refractive index

and hiding power, 9 of common paint vehicles, 9 of extender pigments, 10

of titanium dioxide, 9 Removal of paint {see Paint, removal of)

Removers, paint, liquid {see Paint removers)

Resin emulsion paints, 16-17, 20, 28 Resins (see also Polymers and specific resin)

acrylic, 52-53

alkyd, 3, 43-45 amino, 46-47 coumarone-indene, 7 epoxy, 55-57

fluorocarbon, 62-63

functional properties of, 86-89 furan, 68

miscellaneous, 68 natural resins, 7, 68 petroleum hydrocarbon, 7 phenolic, 3, 41-43 polyamide, 57-58 polycarbonate, 69

polyester, 45-46

polyethylene, 60-62 polypropylene, 68-69

polystyrene, 53-54 polyurethanes, 58-60

silicone, 54-55

synthetic, 33-69

classification of, 37-38 polymeric nature of, 33-34

terpene, 7

vinyl, 3, 47-52

vinyl toluene, 54

water-soluble, 67-68

Roller applicator, 97-98 care of, 98

construction of, 97-98

for pigmented coatings, 97 technique of use, 98

Roller coating, industrial, 103

of flexible, flat sheet stock, 103

Rubbers, synthetic, 64-67 (see also specific type)

butadiene-acrylonitrile, 65 butadiene-styrene, 65

butyl rubber, 65

chlorinated natural rubber, 66-67 chlorosulfonated polyethylene, 65

cyclized natural rubber, 67

isobutylene-isoprene copolymer, 65 neoprene, 64-65

nitrile rubber, 65

polyacrylic ester, 67

polychloroprene, 64-65

polysulfide, 66 polyurethane, 66 SBR (or GR-S), 65

silicone, 66 s

Safety precautions when using paints, 91

falls, 93

fire, 92-93

spillage, 93

toxicity, 91-92 two-package systems, 96

Safflower oil, 6 Sandblasting, 112-114 anchor pattern of surface after, 112

as most effective method of surface preparation, 112

"brush-off" blast cleaning, 113

commercial blast cleaning, 113

cost and production rate, 113-114

dry, equipment and techniques for, 113 "near-white" blast cleaning, 113

safety precautions in, 114 treatment after, 114 vacuum blast cleaning, 113 wet blasting, 113 white metal blast cleaning, 112-113

for completed removal of all surface corrosion

and contamination, 112

183
Saran resins (see Polyvinylidene chloride copolymers)

SBR. See Styrene-butadiene rubber

Scope, 1 Sealers, 7-8 (see also Primer-sealers)

Federal specifications for, 150

for knots in wood, 50, 106

for paper and wood, 43

Selection of coating systems (see Coating systems)

Semi-drying oils, 6 (see also specific oil) safflower, 6 soybean, 6

tall oil, 6 Shellac varnish, 7 as floor sealer, 78 on asphalt, 139

Silicone resins, 3, 54,55 as water repellants, 55

in heat-resistant coatings, 55

modified, 55

silicone-alkyd resins, 45, 55

properties, 54-55

structure, 54

uses, 55

Silicone rubber, 66

Silver, coatings on, 83, 124

Silver flake as pigment, 14

Sizes, 8 Solvent cleaning, 108-109 contamination of solvent, 109

dipping, spraying, and wiping, 109

eflTective on oily contaminants, 108

prerequisite to mechanical cleaning, 108-109

usefulness for aluminum, 119

vapor degreasing, 109

effectiveness of, 109

purification of solvent In, 109

water-soluble salts not removed by, 109 Soybean oil, 6 Spar varnish, 7, 42, 56, 77

Specifications, Federal, for organic coating materials

quick guide list, 149-150 tabular summaries of, 150-167

Specifications, types and sources, 141 ASA standards, 141 ASTM test methods, 141 Federal specifications, 141 Federal test methods, 141 Military specifications, 141 • Spirit varnish, 7 Spray-coating (see under Application methods)

Spraying, cold, conventional, 98-99 adjustment of spray-gun, 99 overspray in, 98 technique of, 98 Spreading rate, 136-137

defined, 136 on concrete and masonry, 136 on metal surfaces, 137 on plaster and wallboard, 137 on wood, 136

should be uniform, 137

versus thickness, 136

versus wet film thickness, table of, 136

Stabilizers, 5 for emulsion polymerizations, 49-50

for latex paints, 49-50

for polyethylene, 61 for polyvinyl chloride, 48

Stainless steel, coatings on, 83, 125

Stains

application to wood, 107 Federal specifications for, 150

Steam cleaning, 109

strongly alkaline, for paint removal, 109

well-suited for field use, 109

Steel (see also Steel, structural; Galvanized steel) coating systems for, 79-83 corrosion of, 79 pretreatment of, 115-118 primers for, 26-27, 80-82

protection against corrosion, 79-80 surface preparation for painting, 105-106, 108-115

Steel, industrial coatings for, 82-83

Steel, stainless, coating on, 83, 124, 125

Steel, stainless, flake, as pigment, 14

Steel, structural (see also Steel, Galvanized stee)) bituminous coatings for, 82

cathodic protection of, 79 coating systems for, 80-83, 143-144

galvanized, painting of, 27, 82, 118-119 intermediate and finish coats for, 81

oil-base and oleoresinous coating systems for, 80-81, 143-144 primers for, 26-27, 80-82

vinyl coating systems for, 81-82 need for pretreatment, 81 Storage of organic coating materials, 91 Strippable coatings, 8-9 formulation, 8

uses, 8-9 vinyl resins in, 49, 52 Strontium chromate, 13 Styrenated oils, 54 Styrene-butadiene resins, 18-19, 53-54 Styrene-butadiene rubber, 65

Styrene, methyl, 54 (see also Vinyl toluene)

Styrene resins (see Polystyrene)

Surface preparation for painting (see also Surface pre- treatment, specific cleaning methods, and specific

substrate)

asbestos-cement shingle, glazed, 125

chalky surfaces, 107, 128, 130, 131

choice of method, 105, 108

cleaning methods, 108-115

acid cleaning, 110-111

alkali cleaning, 110, 120 emulsion cleaning, 109-110 flame cleaning, 107, 114-115

general, 105, 108, 124, 126

hand cleaning, 111-112

pickling, 110, 111, 121 power tool cleaning, 112

sandblasting, 112-114, 121 (see also separate heading)

solvent cleaning, 108-109 steam cleaning, 109

concrete and masonry, 28, 84, 125-128, 130-131 aging and weathering of, 125-126

for alkali-resistant synthetic resin coatings,

126

for cement-water paints, 126

for latex paints, 28, 85, 126

for oil-base and oleoresinous paints, 28, 84, 126 purpose of, 125 chalked surfaces, conditioning of, 128, 129, 130, 131

184
Surface preparation for painting—Continued

characteristics affecting paintability, 84, 125

cleaning of, 126-127

dust, dirt, and loose material, 126

efflorescence, 127 form oil, 126-127 mildew or mold, 127

concrete and masonry—Continued

glazed, roughening of, 127

open-textured, fill or grout coats for, 25, 127 patching of, 127 pretreatment of, 126, 127-128

previously painted, 128, 130-131 summary of requirements for, 128

general requirements, 105, 129 importance of, 105-106

for good adhesion, 106 masonry {see "concrete and masonry" above)

metals, 108-115, 118-121, 124-125, 130 aluminum, 119-120, 124

brass, 124

bronze, 124 cadmium, 119, 124 chromium, 124, 125

copper, 83, 124

difficult to paint, 124 galvanized iron and steel, 118-119

iron, 108-115

lead, 124-125 magnesium, 120-124

nickel, 124-125 pretreatment of, 115-120, 121-125

previously painted, 130

silver, 124

steel, 108-115 metals—Continued

steel, stainless, 124, 125

terneplate, 83, 124, 125

tin, 124-125

titanium, 124, 125

zinc, 118-119, 124

plaster, 128-129 new, 128

old, 128-129

previously painted, 131 removal of oily contaminants, 129

plastics, 105

pretreatment, chemical {see also Surface pretreatments)

of concrete surfaces, 126, 127-128

of steel surfaces, 115-119 purpose of, 105, 106, 115

types of, 105, 115

previously painted surfaces, 129-131 {see also Maintenance painting)

concrete and masonry, 130-131 cement-water paints on, 131

chalked, 128, 130, 131

latex paints on, 130-131

solvent-thinned paints on, 131 whitewash on, 131

metal, 130

localized bad areas, 130 removal of old paint, 130

plaster, 131 wallboard, 131 wood, 107-108 complete removal of old paint, 107 dependence on paint condition, 107

exterior, 107

interior, 107-108

liquid paint removers, 107

paint burn-off by torch, 107 puttying after priming, 107 summary, 132

wallboard, 128, 129 gypsum wallboard, 129 hardboard, 129

previously painted, 131 wood, 106-108

cleaning of, 106

exterior, 106

filler, 107

interior, 106-107

knots, treatment of, 106

previously painted, 107-108 priming, 106, 107

puttying, 106, 107

sanding, 106 seasoning of, 106 smoothing, 106

stain, 107 washcoat over stain, 107 Surface pretreatments, chemical {see also under specific surface)

anodic coating type, 119, 120, 122-124

acid chromate process, 123-124 chromic acid process, 119, 120 HAE process, 122-124

sulfuric acid process, 119, 120

choice of method, 108 chromate treatments, 119, 120, 121-122, 124

cold phosphoric acid type, 115

cold zinc phosphate type, 116

dilute chromic or chromic-phosphoric acid rinse to en- hance corrosion resistance, 116, 117

for aluminum and its alloys, 119-120

for concrete and masonry, 126, 127-128

for galvanized metal, 118-119

for iron and steel, 115-118

for magnesium and its alloys, 120, 121-124

for other metals, 124-125

for plastics, 105 Government specifications for, 115, 116, 117, 118, 119,

120, 121, 123, 124

hot iron phosphate type, 117

hot zinc phosphate type, 116

organic (wash primer) type, 117-118 purpose of, 105, 106, 115

types of, 105, 115

wash primer type, 117-118

zinc and aluminum phosphate type, 120 Surfacer {see Primer surfacer)

Synthetic resins {see also Polymers, and under specific

resin)

classification of, 37-38 polymeric nature of, 33-34T

Tall oil, 6 Terneplate, painting of, 83, 124, 125 Test equipment, 142 Test methods, 142 Textured finishes, 75

crackle, 75 hammertone, 75

multicolor, 21, 75

185
Textured finishes—Continued

pebble, 75

wrinkle, 75 Thickening agents

for latex paints, 49-50 Thickness of coatings, 136-138 (see also spreading rate)

and number of coats, 136

excessive, failure due to, 130, 137

factors determining, 136 measurement of, 137-138 on metallic substrates, 138

by eddy current-type gage, 138

by magnetic-type gage, 138 on non-metallic substrates, 137 wet film, 137 on wood, 136-137

related to spreading rate, 136-137 uniformity of, 137

wet, versus spreading rate, table of, 136

when repainting, 137

Thinners, use of, 5, 94-96 blush-retardant, 94 Thinning, principles of, 5, 95 importance of proper solvent balance, 95 Thinning, purpose of, 5, 94-95

Tin, coatings on, 83, 124-125 Tint tone, 11

of iron blue pigments, 11 Titanium, coatings on, 83, 124, 125 ; Titanium dioxide pigment, 3, 9, 10

control of paint chalking by, 10 Toluidine reds, 12 Toluidine yellow (see Hansa yellow)

Topcoats (see also Compatibility)

compatibility with undercoats, 81, 139-140

for metals, 80, 81, 82-83 primer protection function, 79

Toxicity of organic coating materials, 91-92, 96

Traffic paints

chlorinated rubber in, 67 Federal specifications for, 85, 149 formulation of, 85

modified phenolic resins in, 42

phenolic dispersion resins in, 43

styrene-butadiene resins in, 53 Tupg oil, 6

in phenolic paints, 15

in phenolic varnishes, 3, 7

in spar varnishes, 7, 42, 77

in water-resistant primers, 27, 28

in wrinkle finishes, 75 Tungstated pigments, 12 Two-package systems, activation of, 96

induction period for epoxy resins, 96 wash prihier, 117-118 u"

'

Ultramarine blue, 11 Urea-formaldehyde resins (see Amino resins)

V

Varnish, 6-7 alkyd, 7 bituminous, 29

composition, 6-7 Federal specifications for, 150

history, 3

oil length, 6-7 spar, 7, 42, 77

spirit, 7

stirring of, 94 Varnish resins, 7 Vegetable blacks, 14

Vehicle, 5 Vinyl coating systems

dependence on special primers, 81-82, 142

for steel in corrosive environments, 80, 81-82, 142 wash primer required in, 81, 142 Vinyl copolymers, 37, 49, 51-52 (see also specific resins)

Vinyl resins, 3, 37, 47-52 (see also Polyvinyl, and specific resin)

classification of, 47 general properties, 47

in coatings for steel, 81-82

in corrosion-resistant primers, 27

in fire-retardant paints, 22

in latex paints, 19

in plastisols and organosols, 48

in strippable coatings, 8 polyvinyl acetals, 50 polyvinyl acetate, 47-48

polyvinyl alcohol, 49-50

polyvinyl butyral, 50-51 polyvinyl chloride, 48

polyvinyl chloride-acetate, 48-49 polyvinyl ethers, 52

polyvinyl formal, 50 polyvinylidene chloride and its copolymers, 51-52 Vinyl toluene polymers and copolymers, 54 w

Wallboard

coating systems for, 43, 49, 75, 85-86, 144-145

multicolor finishes for, 86

primer-sealers for, 28-29, 85-86 surface preparation for painting, 128, 129, 131

types available, 129 Wash primer

as metal pretreatment, 27, 117-118

composition, 117, 118 drying time for optimum adhesion, 118

for mixed metal assemblies, 117 mixing and application of, 117-118 need for special primer under some vinyl paints, 118

not suitable over other pretreatments, 117 optimum coating thickness, 118

single-package type, 118 two-component type, 117-118

useful for field application, 117 wide applicability of, 124 Water (see Potable water)

Water emulsion paints {see Latex paints)

Water paints (see also under specific type)

casein paints, 20 cement-water paints, 25

early types, 20 whitewash, 25-26 Water-soluble resins, 67-68 thermoplastic types, 67

thermosetting types, 67-68 Water solution coatings, 20-21, 67-68 advantages and disadvantages, 20

application of, 20 conversion of, 20, 67 formulation of, 20

insolubilization of, 20, 67

186
Water solution coatings—Continued

melamine-acrylic base, 67

properties of, 20

trimellitic resin base, 68 Wax polish, 8

types, 8

use, 8 Weathering as means for surface preparation

of asbestos-cement shingle, 125

of concrete and masonry, 125-126

of galvanized surfaces, 118, 119 Wetting properties of coatings, 141 White lead pigments, 3, 9, 10 Whitewash, 20, 25-26

application of, 26 preparation of, 26 removal before repainting, 131

uses, 25-26 Wood

bench top coating, chemical-resistant, 79

clear finishes for, 77, 78

coatings for marine exposure, 77

coating systems for, 77-79

exterior, 77-78, 143

interior, 78-79, 145

floors, polymeric finishes for, 78, 145

furniture coatings, 77-78 house paints, 77, 143 knot sealer for, 50, 106

latex paints on, 77 moisture content for painting, 106

open-grain, filler for, 107 porch and deck coatings, 78 primers for, 26-27, 77

properties aff'ecting painting, 77 puttying of, 106, 107

preceded by priming, 106 seasoning for painting, 106

shellac as sealer or finish for, 78

stains, 107, 150

trim and trellis paints, 77 undercoaters for, 26 Workmanship in painting, 140 Wrinkle finishes, 75 z

Zinc chromate, 13

in primers for aluminum, 27

preferred pigment for magnesium primers, 27 with iron oxide in primers for steel, 27, 80-81 Zinc chromate primers (see also Primers, corrosioninhibitive) for aluminum, 27

for magnesium, 27-28

phenolic dispersion resins in, 43

Zinc, coatings on, 118-119, 124 (see also Galvanized sur- faces)

Zinc dust, 13, 14, 27

in coatings for potable water tanks, 81

in primers for galvanized metal, 27

in zinc-rich paints, 13, 14, 27, 80

Zinc dust paints, 13, 14, 27, 80, 81

as primers on galvanized surfaces, 27

for potable-water tanks, 81 good adhesion on galvanized surfaces, 14, 27, 118, 119

zinc-rich type, for corrosion inhibition, 13, 14, 27 Zinc oxide, 9, 10 avoidance in wood primers under latex paints, 77 improves mildew resistance of paints, 10

Zinc-rich paints, 13, 14, 27

as automotive primers, 27 mechanism of corrosion inhibition by, 13, 14 on steel for marine exposure, 80

vehicles for, 27, 80 Zinc sulfide, 10 Zinc surfaces (see Galvanized surfaces)

Zinc tetroxychromate, 13

in corrosion-inhibiting primers for aluminum, 27

Zinc yellow (see Zinc chromate)

i!!r GOVERNMENT PRINTING OFFICE: 1968 O 236-318

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