NBS Special pajnt Solubility

Publication -r ^•

480-40 Testing

NAT'L INST. OF STAND & TECH

NBS

PUBLICATIONS

Law Enforcement

Technology

U.S. DEPARTMENT OF

COMMERCE

National Bureau of

Standards

III. 1 IGO

.U57

1932

c. 2
ABOUT THE TECHNOLOGY ASSESSMENT PROGRAM

The Technology Assessment Program is sponsored by the Office of Development, Testing, and Dissemination of

the National Institute of Justice (NIJ), U.S. Department of Justice. The program responds to the mandate of the

Justice System Improvement Act of 1979, which created NIJ and directed it to encourage research and development

to improve the criminal justice system and to disseminate the results to Federal, State, and local agencies.

The Technology Assessment Program is an applied research effort that determines the technological needs of

justice system agencies, sets minimum performance standards for specific devices, tests commercially available equipment against those standards, and disseminates the standards and the test results to criminal justice agencies

nationwide and internationally.

The program operates through:

The Technology Assessment Program Advisory Council (TAPAC) consisting of nationally recognized criminal

justice practitioners from Federal, State, and local agencies, which assesses technological needs and sets priorities for research programs and items to be evaluated and tested. The Law Enforcement Standards Laboratory (LESL) at the National Bureau of Standards, which develops

voluntary national performance standards for compliance testing to ensure that individual items of equipment are

suitable for use by criminal justice agencies. The standards are based upon laboratory testing and evaluation of representative samples of each item of equipment to determine the key attributes, develop test methods, and establish minimum performance requirements for each essential attribute. In addition to the highly technical standards, LESL

also produces user guides that explain in non-technical terms the capabilities of available equipment.

The Technology Assessment Program Information Center (TAPIC) operated by the International Association of Chiefs of Police (lACP), which supervises a national compliance testing program conducted by independent agencies.

The standards developed by LESL serve as performance benchmarks against which commercial equipment is measured. The facilities, personnel, and testing capabilities of the independent laboratories are evaluated by LESL

prior to testing each item of equipment, and LESL helps the Information Center staff review and analyze data. Test

results are published in Consumer Product Reports designed to help justice system procurement officials make

informed purchasing decisions.

All publications issued by the National Institute of Justice, including those of the Technology Assessment

Program, are available from the National Criminal Justice Reference Service (NCJRS), which serves as a central information and reference source for the Nation's criminal justice community. For further information, or to register

with NCJRS, write to the National Institute of Justice, National Criminal Justice Reference Service, Washington, DC

20531.

Paul Cascarano, Assistant Director National Institute of Justice

ACKNOWLEDGMENTS

This report was prepared by the Law Enforcement Standards Laboratory of the National Bureau of Standards under

the direction of Paul H. Krupenie and Lawrence K. Eliason, Chief of LESL. The preparation of this report was sponsored

by the National Institute of justice, Lester D. Shubin, Standards Program Manager.
NBS Special

Publication

480-40

Paint Solubility

Testing

by

John I. Thornton

Shmuel Kraus

Bruce Lerner

Beth Hendrickson

Forensic Science Group

Department of Biomedical and Environmental

Health Sciences

School of Public Health

University of California

Berkeley, CA 94720

Nations Buretu of ^dvin

Librery, E-01 Admitv. BIdg.

MAR 1 1982

po-t G-cc ^ Cjj^jj

no. 4^60 -4o

and the

Law Enforcement Standards Laboratory

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

prepared for the

National Institute of Justice

U.S. Department of Justice

Washington, DC 20531

Issued U.S. DEPARTMENT OF COMMERCE

January 1982 Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS

Ernest Ambler, Director
see? r m

Library of Congress Catalog Card Number: 81-600176

National Bureau of Standards

Special Publication 480-40

Nat. Bur. Stand. (U.S.), Spec. Publ. 480-40, 20 pages

CODEN; XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON; 1982

Available from the National Technical Information

Service, Springfield, VA 22161.
FOREWORD

The Law Enforcement Standards Laboratory (LESL) of the National Bureau of Standards

(NBS) furnishes technical support to the National Institute of Justice (NIJ) program to strengthen law enforcement and criminal justice in the United States. LESL's function is to conduct research

that will assist law enforcement and criminal justice agencies in the selection and procurement of

quality equipment.

LESL is: (1) Subjecting existing equipment to laboratory testing and evaluation and (2) conducting research leading to the development of several series of documents, including national voluntary equipment standards, user guides, and technical reports.

• This document is a law enforcement technology report developed by LESL under the sponsorship of NIJ as part of the Technology Assessment Program, which is described on the

inside front cover of this report. Additional reports as well as other documents are being issued under the LESL program in the areas of protective equipment, communications equipment,

security systems, weapons, emergency equipment, investigative aids, vehicles, and clothing.

Technical comments and suggestions concerning this report are invited from all interested

parties. They may be addressed to the Law Enforcement Standards Laboratory, National Bureau of Standards, Washington, DC 20234.

Lawrence K. Eliason, Chief

Law Enforcement Standards Laboratory

iii
CONTENTS

Page

Foreword iii

1. Introduction 1

2. Samples and Reagents 2

A. Sample Sources and Preparation 2

B. Reagents 2

C. (1) Experimental, Household Paints 3

(2) Experimental, Automotive Paints 4

3. Results and Discussion 4

A. General 4

B. Household Paints—General Solubility 5

C. Automotive Paints—General Solubility 10

D. Automotive Paints—Sequential Solubility Scheme 14

4. Conclusions —

\

iv
PAINT SOLUBILITY TESTING

John I. Thornton, Shmuel Kraus, Bruce Lerner, and Beth Hendrickson*

University of California, Berkeley, CA 94720

This report presents the results of a study to develop criteria for the characterization of paint through

solubility testing. Experiments were conducted with common solvents to classify chemical reactivity with automobile and household paints. Seven categories of reactivity are defined, which form the basis for specifying

the extent of solubility of paints in various reagents under controlled conditions of time and temperature. The

report also describes a novel solubility test procedure by which acrylic lacquers, organic-dispersed enamels, and

water-based enamels can be distinguished through sequential testing, a characterization not previously accomplished with solubility testing.

Key words: automotive paints; household paints; sequential tests; solubility tests.

1. INTRODUCTION

The Nationwide Crime Laboratory Proficiency Testing Project conducted by the Law

Enforcement Assistance Administration and the Forensic Sciences Foundation demonstrated a need for standardization in paint solubility testing for forensic purposes.' Based on results obtained for tests of several paint samples distributed to crime laboratories throughout the

country, it was apparent that there was little or no standardization in the solubility test methods

used, reproducibility of test results, and criteria for the evaluation and reporting of results, or, for that matter, in the rationale behind the use of a particular array of solvents. For example, one

laboratory would report that a given paint sample was soluble in sulfuric acid, while another

laboratory would report that the paint was insoluble in this reagent. Reagent concentrations and

times of reaction were frequently not reported. It was apparent that there was large variation in the reagents used, in the time allowed for the reagent to act on the paint, in the temperature at which the tests were conducted, and in the criteria applied to the term "soluble." The purpose of the present study was to develop criteria to apply to solubility testing and

chemical reactivity of automotive paints, and to standardize reagent concentration and other parameters. The principal focus of the study was testing acrylic enamels used as automotive

finishes, since prior to this study these paints were reported to be virtually unreactive to all solvents when subjected to solubility tests. An ancillary objective was to apply these criteria of

solubility and reactivity to testing household paints.

* Forensic Science Group, Department of Biomedical and Environmental Health Sciences, School of Public Health.

' Crime Laboratory Proficiency Testing Research Program. Joseph L. Peterson, Ellen L. Fabricant, Kenneth S. Field, and John I. Thornton. Law Enforcement Assistance Administration, U.S. Government Printing Office, Washington, DC, Oct. 1978, 287 pp.

-1
A. Sample Sources and Preparation

Samples of vinyl, alkyd, acrylic, and epoxy paints were obtained from retail distributors in Virginia, because State law requires all components of a paint to be indicated on the label,

facilitating a confirmation of the generic type of paint. The samples tested were specifically

designated as exterior grade paint. The vinyl and acrylic paints were water-based and the alkyd

and epoxy paints were oil-based. The epoxy was cross-linked. The paints were selected to be

deeply pigmented, i.e., highly saturated with pigment, to permit a more critical assessment of

solubility and /or pigment leach. Linseed oil paint, as an example of an air-oxidizing drying oil paint, is rarely marketed now; the linseed oil paint used in this study was made by adding pigment

(25% by weight of rutile titanium dioxide) to commercially available dark boiled linseed oil by

stirring. No thinners were used; but dark boiled linseed oil contains a drier. The household paints were brushed as thin [approximately 3 mil (.075 mm)] films onto a sheet acetate substrate and

permitted to fully cure at room temperature for a minimum of 5 weeks before testing. No primers were used with any of the household paints.

The automobile paints tested were the separate chemical samples from the 1977 National Bureau of Standards' "Reference Collection of Automotive Paints."^ These automotive paints consisted of acrylic lacquers and acrylic enamels as defined by Williams.^ Acrylic enamels were of

three types: (1) organic dispersed enamels, also referred to as "solvent-thinned" enamels, (2) non- aqueous dispersion enamels, and (3) water-based enamels. The term water-based enamel is used

interchangeably with water dispersion enamel or aqueous dispersion enamel.

B. Reagents

A wide range of solvents and chemical reagents were tested for their efficacy in dissolving both household and automotive paints. These included acids, bases, and organic solvents covering

a wide range of polarity. Particular attention was given to reagents reported in the Crime

Laboratory Proficiency Project report as having applicability to paint solubility testing. All chemicals were of analytical reagent grade. Dilutions, whether of a liquid stock solution

or of a stock solid, are expressed as a weight to weight (w/w) percentage.

Stock Acids:

Acetic Acid, CH3COOH, glacial (17.4 M)

Hydrochloric Acid, HCl, cone. (11.6 M)

Nitric Acid, HNO3, cone. (15.4 M)

Phosphoric Acid, H3PO4, cone. (14.6 M)

Sulfuric Acid, H2SO4, cone. (18.0 M)

Stock Bases: Ammonium Hydroxide, NH^OH, cone. (14.8 M)

Potassium Hydroxide, KOH, 95 percent sticks Sodium Hydroxide, NaOH, 95 percent sticks

" 1977 Reference Collection of Automotive Paints, National Bureau of Standards and Manufacturers Council on Color and

Appearance, Inc.

' Automolife Finishes. Ralph A. Williams, Federation Series on Coatings Technology, Federation of Societies for Paint Technology, 121 South Broad St., Philadelphia, PA 19107, 1977, 36 pp.

2
Organic Solvents:

Acetone, b.p. 56 °C Amyl Acetate, b.p. 149°

Benzene, b.p. 80°

<-Butanol, b.p. 82°

Carbon Disulfide, b.p. 46°

Carbon Tetrachloride, b.p. 76°

Chloroform, b.p. 61°

Dichloroethane, b.p. 57°

Dichloromethane, b.p. 40°

Dimethylformaniide (DMF), b.p. 149-156°

Dimethylsulfoxide (DMSO), b.p. 189°

p-Dioxane, b.p. 101°

Ethanol, absolute, b.p. 78°

Ethyl Acetate, b.p. 77°

Methanol, absolute, b.p. 65°

Petroleum Ether, b.p. 60-80°

Piperidine, b.p. 106°

Tetrahydrofuran (THE), b.p. 64°

Toluene, b.p. 110°

Xylene (o-, m-, and yo-isomers), b.p. 135-140°

C(1) Experimental, Household Paints

The effect of each solvent on each sample of paint was evaluated at room temperature (23 °C) and, for most solvents, at elevated temperatures. The high volatility of certain solvents (e.g., carbon disulfide and acetone) presented serious practical difficulties in solubility testing at elevated temperatures. Refluxing would have addressed this problem, but it was concluded that

this would not be practical for routine use in forensic laboratories and that standardization of

refluxing procedures would create further problems.

Tests at all temperatures were observed under a low-power stereoscopic binocular microscope.

Tests at room temperature were conducted in a white porcelain spot tile. The tests at elevated temperature were conducted in tubes immersed in a water bath heated to a temperature

determined by the boiling point of the reagent. The temperature selected was close to the boiling

point, but not so close that the vapor pressure at the selected temperature would result in an

excessively rapid evaporation of the solvent. The tubes and reagents were preconditioned to the temperature of the water bath prior to the addition of the paint samples. At all temperatures, the ratio of reagent to sample was greater than 50:1 by weight. Samples

of paint approximately 1 mm X 1 mm were removed from their acetate substrate and immersed in the reagent for a period of 5 min. The reagents containing the samples were stirred at intervals of

1 min. Various reaction times were tried, but the 5-min reaction time was the most suitable; exposure times up to several hours did not alter the chemical reactivity assessed at 5 min.

Solubility or other chemical reactivity was assessed by the following criteria:

Soluble A sample was determined to be soluble if the paint was completely

disintegrated within 5 min, either as a result of solvation or chemical decomposition.

Partially soluble A sample was determined to be partially soluble if after 5 min a significant amount of pigment (more than half) was leached from the paint with some,

although incomplete, disintegration of the vehicle. Pigment leach A sample was determined to have undergone pigment leach if the paint

remained substantially intact after 5 min, but with some pigment (less than half) extracted by

the reagent. Pigment leach is distinguished from "partially soluble" in that the paint remains

substantially intact and exhibits little or no visual evidence of disintegration of the vehicle.

3
Swell A sample was said to swell if there was a significant and uniform expansion of

the paint, but with no noticeable solubility or pigment leach.

Curl A sample was said to curl if there was a tendency for the paint to curl in the

reagent, but with no noticeable solubility or pigment leach. Curling is probably an

asymmetrical form of swelling.

Discolor A sample was said to discolor if the paint turned brown or black. This

phenomenon was noted with the mineral acids, and represents a charring of the organic

matter. Care must be exercised when dealing with black or brown paints that true solvation not be confused with discoloration; although arising from different chemical considerations, the subjective appearances would be quite similar.

Soften A sample was said to soften if, after exposure to the solvent, the paint fragment

could be penetrated with a needle but was otherwise not visibly affected.

C(2) Experimental, Automotive Paints

In the case of the sequential solvent testing of automotive paints, the tests were conducted in a white porcelain spot tile. Heating, where needed, was with a 500-W infrared lamp 15 cm from

the tile. Tests were observed under a low-power stereoscopic binocular microscope. Paint samples

approximately 1 mm X 1 mm were removed from their metal substrate and placed in the spot

plate. Glacial acetic acid at room temperature was added. If the paint was an acrylic lacquer, it would dissolve within 2 min. If the paint did not dissolve, the spot plate was heated with the

infrared lamp. When the temperature was within 10 °C of the boiling point of glacial acetic acid (117 °C), the paint would dissolve within 2 min if the paint were an organic-dispersed enamel. At a distance of 15 cm from the 500-W lamp a steady state situation was achieved with

respect to temperature; the heat input was equal to the heat loss, and the temperature remained

essentially constant at approximately 107-110 °C. If no reaction occurred under heating, the acetic acid was removed by means of a pipette, a few drops of concentrated HNO-, was added, and the sample was reheated with the infrared lamp for an additional 3 min. The paint sample was then

washed in distilled water and dried. This was accomplished in situ by drawing off the acid with a

pipette, adding water, drawing off the water with a pipette, repeating the washing step, and drying

the sample by heat from the infrared lamp. A saturated solution of potassium hydroxide in tertiary butanol was then added, and the solution was heated by means of the infrared lamp. The water- based enamels then dissolved.

3. RESULTS AND DISCUSSION

A. General

Both household and automotive paints vary within a particular generic type. This variation in composition (e.g., the substitution of a methoxy group into a resin monomer) results in some

variation in solubility behavior. For example, within the acrylic enamel group, hot nitric acid will only discolor one paint sample, but will completely dissolve other samples. As a consequence, the use of solubility testing to individualize a paint or to definitively characterize an evidence paint as belonging to a particular generic type should be attempted only with caution. The principal

application of paint solubility testing is in screening or comparison.

Certain generalizations, however, may be made. For household paints, the most effective solvents are concentrated H2SO4, 30 percent (w/w) KOH, and hot HCl. These reagents will result

in either a dissolution of the paint or in a color reaction which may have some diagnostic value.

4
For automotive paints, acetone, phosphoric acid, and dimethylformamide are well suited to dis- solve the acrylic lacquers. The acrylic enamels are much more resistant to the effects of solvents

in general, but the strong mineral acids may achieve some reactivity. The failure of the acrylic

enamels to dissolve in most common reagents led to the sequential testing which was the princi

pal thrust of this study. Because of their significance the findings will be discussed in a separate

section.

B. Household Paints—General Solubility

The results of the solubility testing of the household paints at room temperature and at elevated temperatures are in tables lA through IE, 2A through 2E, and 7. Non-polar solvents|have very little effect in dissolving household paints. If a solvent had little or no effect on any paint at any temperature up to and including the boiling temperature, it will not appear in any of the

tabular data except to indicate the comprehensiveness of the testing conducted, i.e., to indicate the

logical candidate solvents within a particular group.

TaUI.K 1A. Iloiisf/idlil j)iiinls (It Ktdiii li-iiiju-i(ilun\ KiiH VUlKilrd dciil.s

Paint

H,SO,

(cone.)

Acid

HCl

(cone.) HNO,(cone.) HAc(glacial)

Vinyls

Alkyds

Acrylics

Linseed oil base

Soluble

Curls; may discolor

Soluble

Soluble; may discolor

No reaction

No reaction

No reaction

No reaction

Softens; may discolor

Curls; may discolor

Softens

Discolors

Softens

Curls

Softens

No reaction

Tahi.K IB. Household paints al room temperature, strong bases

Base

NaOH KOH NH4OH KOH NaOH

Paint (30%, w/w) (30%, w/w) (cone.) (1%, w/w) (50%. w/w)

Vinyls No reaction No reaction No reaction No reaction No reaction

Alkyds No reaction Soluble Curls; may discolor No reaction No reaction

Acrylics No reaction No reaction No reaction No reaction No reaction

Linseed oil base Soluble Soluble No reaction Softens No reaction

5
TaHI.K IC. Household paints at room temperature, non-polar solvents

Solvent

Paint C„H„ CS, CCI, Dioxane

Pet. Ether

Vinyls No reaction No reaction No reaction No reaction No reaction

Alkyds No reaction No reaction No reaction No reaction, or may soften No reaction

Acrylics Softens No reaction Softens Softens No reaction

Linseed oil base No reaction No reaction No reaction Softens Swells

TMil.K ID. Household paints at room temperature. loiv to medium polarity (dielectric constant <C6) solvents

Solvent

Paint Toluene Xylene

Amyl

acetate CHCl,

Vinyls No reaction Softens Softens Softens

Alkyds No reaction No reaction Softens No reaction

Acrylics Softens Softens Softens Softens

Linseed oil base No reaction No reaction Softens No reaction

6
7
TamI.K 2A. Household paints at 80±5°C, loiKfiiliaIrd nails

Acid

H.,SO, HCI HNO., HAc

Paint (cone.) (cone.) (cone.) (glacial)

Vinyls SnlliKlp Discolors Sf>Ttpn ^* OUI LC 119 pigment leach

Alkyds Soluble No reaction Discolors No reaction

Acrylics Soluble Soluble Soluble Softens

Linseed oil base Soluble; discolors Discolors Soluble; discolors No reaction

Tadi.K 2B. Household paints at elevated temperatu res. strong bases

Base

30% NaOH; 30% KOH; 1% KOH; 50% NaOH;

Paint 80 °C 80 "C 95 °C 95 "C

Vinyls No reaction No reaction No reaction No reaction

Alkyds Discolors Soluble Soluble Discolors

Acrylics No reaction No reaction No reaction No reaction

Linseed oil base Soluble Discolors; partially soluble Discolors No reaction

TaHI.K 2C. Household paints at elevated temperatures, non-polar solvents

Solvent

Paint C„H„, 60 °C CCI^, 60 "C Dioxane, 95 °C

Vinyls Softens No reaction Swells

Alkyds No reaction No reaction Softens

Acrylics Swells Swells Softens

Linseed oil base No reaction No reaction No reaction

TmiI.K 2D. Household paints at elevated temperatures, low to medium polarity (dielectric constant <C6) solvents

Solvent

Toluene. Xylene, Amyl acetate, CHCl,,

Paint 80 °C 95 "C 95 °C 60 °C

Vinyls No reaction Pigment leach Softens Softens

Alkyds Softens; discolors Softens No reaction No reaction

Acrylics Curls Curls Softens Softens

Linseed oil base No reaction No reaction Curls No reaction

8
9
C. Automotive Paints—General Solubility

All acrylic lacquers dissolved in room-temperature glacial acetic acid. No acrylic enamel

dissolved in this acid, even at high temperature. The results of solubility testing of acrylic lacquers

at room temperature and at elevated temperatures are in tables 3A through 4B. Historically, these

paints have not presented any problem with solubility testing, and a number of solvents are quite

efficient in dissolving these paints and distinguishing them from the acrylic enamels. The results of solubility testing of acrylic enamels at room temperature and at elevated temperatures are in tables 5A through table 6. The aqueous dispersion enamels are poorly represented in the NBS

1977 Reference Collection of Automotive Paints, owing to their recent introduction; the reactions of a single paint of this type are presented in table 6.

TaHI K 3A. Automotive paints: acrylic lacquers at room temperature, strong acids

Solvent

H.SO, HNO;, H.PO, HCl

Paint (cone.) (cone.) (cone.) (cone.)

Acrylic solution lacquer

DA77G0333 Soluble Pigment leach No reaction No reaction

NA77B0334 Partially soluble

Partially soluble, discolors No reaction No reaction

PA77L0335 Soluble No reaction Discolors Discolors

Acrylic dispersion lacquer

DB77B0334 Partially soluble

Partially soluble, discolors Discolors Discolors

Taki k 38. Automotive paints: acrylic lacquers at 80 °C. strong acids

Solvent

H_,SO,

Paint (cone.) HNO, H,PO,(cone.) (cone.)

HCl

(cone.)

Acrylic solution lacquer DA77G0333 Soluble Pigment leach Discolors Pigment leach

NA77B0334 Soluble Partially soluble, Partially sol discolors

uble No reaction

PA77L0335 Soluble Softens Partially sol uble No reaction

Acrylic dispersion lacquer DB77B0334 Soluble Partially soluble. Partially sol discolors discolors

uble. Discolors

10
TaHI.I'. 4A. Auliiiiu)li\'c /minis: iin\li< liii</iicis iil nioiii Ivmjicrdlun-. iiiiscrlldiii-iiiis snUrnls

Paint

Solvent

Acetone 30% NaOH (w/w)

AfTviic Solution lact^uer UA ( I \y\jc>oC> Soluble Soluble No reaction

Soluble Soluble No reaction

PA77I OHSS SnI II Ml p III n IP 111' ICul-llV^li

Acrylic dispersion lacquer

DB77B0334 Soluble Soluble No reaction

Tvnii. 4B. Aiiloriiolii'i' /minis: acrylic liicqners ill elevdied lem/>er/ilun\ miscellaneous solvents

Solvent

Paint DMF, 80 °C 30% NaOH (w/w), 80 °C

Acrylic solution lacquer DA77G0333 Soluble No reaction NA77B0334 Soluble No reaction

PA77L0335 Soluble No reaction

Acrylic dispersion lacquer

DB77B0334 Soluble No reaction

11
TahI.K 5A. Aiitomolive paints: acrylic enamels at room temperature, strong acids

Solvent

Paint

H.SO;

(cone.) HNO,(cone.)

H,PO,

(cone.)

HCl

(cone.)

Solvent-thinned (organic disperse

acrylic enamel

:d)

DC77F0339 Discolors Pigment leach Curls Pigment leach, discolors

FC77C0090 Curls Curls No reaction No reaction

LC77E0235 Pigment leach, discolors

Pigment leach, discolors No reaction No reaction

NC77B0308 No reaction No reaction No reaction No reaction

PC77J0255 Discolors Pigment leach No reaction Discolors

Non-aqueous dispersion enamel AN77H0313 Discolors No reaction No reaction No reaction BN77E0222 Discolors Pigment leach No reaction No reaction

CN77A0123 Discolors No reaction No reaction No reaction

FN77E0222 Discolors Pigment leach, curls No reaction No reaction

KN77C0090 No reaction No reaction No reaction No reaction

I N77Rni AO Discolors No reaction No reaction No reaction NN77B0092 Discolors Discolors Discolors Discolors

PN77F0314 Discolors Pigment leach No reaction Discolors

12
Tahi I'. 5B. Automoth'e painls: ficrYlir enamels at 80 +5 "C, strong arids

Solvent

HjSO, HNO., H;,PO, HCI

Paint (cone.) (cone.) (cone.) (eonc.)

Solvent-thinned (organic dispersed)

acrylic enamel

Non-aqueous di

DC77F0339 Discolors Soluble Curls Discolors, pigment lea

FC77C0090 Soluble Soluble No reaction No reaction

LC77E0235 Discolors Soluble No reaction No reaction

NC77B0308 Discolors Discolors Softens Discolors

PC77J0255 Discolors Soluble Soluble Discolors

ispersion enamel AN77H0313 Discolors Partially so uble Soluble Discolors

BN77E0222 Discolors Partially so uble No reaction No reaction

CN77A0123 Soluble Partially so uble No reaction No reaction

FN77E0222 Discolors Partially so uble No reaction No reaction KN77C0090 Discolors Soluble No reaction Softens

LN77B0149 Discolors Soluble No reaction No reaction

NN77B0092 Discolors Soluble Partially soluble Softens

PN77F0314 Discolors Soluble Soluble Discolors

13
TaHIK 6. Aqueous dispersion enamel (NBS #Nlf77E0054>—temperature as noted

Temperature

Solvent Room temperature 80 °C

H.SO, (cone.) Curls, pigment leach Solution yellows

HNO., (cone.) Curls, pigment leach Soluble H ,P0^ (cone.) No reaction Pigment leach

HCl (cone.) No reaction Pigment leach NaOH (30%) No reaction No reaction DMF No reaction Solution turns green

Acetone* No reaction NA

*Room temperature only; B. P. of acetone is 56.5 °C.

TaisI.K 7. Epoxy paint —temperature as noted

1 emperature

Solvent Room temperature 80 °C

HjSOj (cone.) Softens, browns Soluble

HNO;i (cone.) No reaction No reaction H ,P0, (cone.) No reaction No reaction

HCl (cone.) No reaction No reaction NaOH (30%) No reaction No reaction DMF No reaction No reaction

Acetone* No reaction NA

*Room temperature only; B. P. of acetone is 56.5 °C.

D. Automotive Paints—Sequential Solubility Scheme

The study of solubility characteristics of household paints follows the usual use of an array of

solvents to assess the responses, e.g., the degree of solubility encountered. Topcoats for automobiles, by contrast, in many instances, were found to require a sequential testing scheme. To

better express the rationale for this scheme, background information is presented on the chemical

bond types that are to be broken in solubilizing such paints. Automobiles manufactured in the United States in recent years have been finished with acrylic topcoats.* The acrylic resins may

be in either a straight-chain thermoplastic as in the acrylic lacquers, or in a cross-linked thermoset form as in the acrylic enamels.

* Williams, op cit.

14
The sequential solvent testing of automotive paints was derived from a review of the chemistry of automotive enamels by one of the authors (SK). This suggested that first if would be

necessary to break the structural cross-linkages, followed by use of a reagent which would cause

further dissolution of the partially-degraded polymer.

Acrylic lacquers are utilized for approximately 50 percent of all the automobile topcoat

production in the United States. The basic resin is comprised of methyl methacrylate, which is modified by other methacrylic or acrylic monomers. The acrylic lacquers are applied in one of two

forms: (1) solvent-thinned lacquers'* and (2) non-aqueous dispersion (NAD) lacquers.^' The latter types have slightly higher molecular weight polymers which are modified to achieve partial

solubility in aliphatic solvents. Both types are readily soluble in many organic solvents, and there

is no difference between the two types in the solubility characteristics of the paints after drying.

The acrylic enamels utilized as topcoats for automobiles are of three types: (1) solvent- thinned enamels, also referred to as organic dispersed enamels, (2) non-aqueous dispersions, and

(3) water-based enamels.*^ All three types are modified acrylic resins containing acrylic and

methacrylic acids as well as hydroxy-containing monomers. The carboxyl and hydroxyl groups

provide reactive sites for the crosslinking between polymer chains. The cross-linking agent most commonly used is butylated melamine formaldehyde.

The water-based acrylic enamels are based on polymers similar to those used in the organic

dispersed enamels, except that the percentage of hydroxy-containing monomers is reduced and the percentage of the free acids is increased. The cross-linking agent for these enamels is hexamethoxy-methyl melamine. The water-based enamels, at the present time, represent a small

portion of the total amount of acrylic enamels used in automotive applications. Their use has been

stimulated by antipollution regulations, however, particularly on the West Coast, and their use is likely to increase in the future. The types of chemical bonding involved in the cross-linked acrylic enamels are numerous. The principal types of bonds include:

1) Ester linkages between the methylol group of the melamine and the free acid in the

acrylic chain, and the ester linkage of the hydroxy-alcohol in the acrylate.

2) Ether linkages between the methylol group of the melamine and the free hydroxy groups

incorporated into the acrylic polymer.

3) Alkylamine-type linkages between the melamine and the methylol groups.

Each of these represents candidate linkages to be chemically attacked and broken in order to solubilize the otherwise insoluble polymer. The most sensitive bond to chemical attack is the ester bond. Bonds of this type are susceptible to hydrolytic attack by strong base, e.g., ethanolic KOH.

The alkylamine linkages may be broken by Hoffmann degradation. This degradation is carried out by exhaustive methylation of the amine with methyl iodide, oxidation of the product with

hydrogen peroxide, followed by pyrolysis of the product. Because of the complexities of this latter approach, the former approach was first attempted. With a slight modification (substitution of

tertiary-butanol), this method was applied successfully. Saturated ethanolic KOH in the temperature range of 80-100 °C was found to be suitable for hydrolyzing the ester bonds. However, the solution must be freshly prepared or the results are not

entirely reproducible. A saturated solution of KOH in tertiary-butanol was found to be a more

suitable reagent for the enamels; this reagent need not be freshly prepared.

The water-based enamels are not dissolved by hot alcoholic KOH unless they are first treated with a strong acid. A pretreatment with hot cone, nitric acid chemically modifies the polymer to the extent that it becomes soluble in hot alcoholic KOH.

^ Solvent-thinned lacquers are also referred to as acrylic solution or organic dispersed lacquers. NAD lacquers are also referred to as acrylic dispersion lacquers.

' Williams, op cit.

15
4. CONCLUSIONS

Prior to this study, attempts to use solvents to characterize paint consisted of one-step tests,

i.e., if a solvent failed to dissolve or react with a paint sample, it was discarded and another

reagent was tried. This approach is sufficient to characterize the solubility reaction of reagents

with household paints. The use of one reagent as an activator, with another solvent used in tanden on the same sample, is a novel approach. The present work describes a method wherein acrylic

lacquers, organic-dispersed enamels, and water-based enamels can be distinguished by sequential

tests. The method is outlined in figure 1. Primers do not interfere with this procedure. This

represents a substantial breakthrough in the use of solubility testing for the characterization of automotive paints.

AUTOMOTIVE PAINTS

I

I glaci al HAc, 25°C

soluble

yes

[no

glacial HAc,107-117°C

soluble ^ no

yes

remove acid

cone. HNO3

,

heat 2-3 min (100°C)

I H2O wash; dry

sat. KOH in t-Butanol

heat 2-3 min (100°C1

soluble

yes

Acrylic Lacquer

(No further solubility

testing necessary)

Solvent-thinned

(organic-dispersed)

Acrylic Enamel

Water-based

Acrylic Enamel

FicriU, 1. Solubililv scheme for aulnmolive paints.

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