LAMs-2798

C99

3

*- I , “

CIC-14 REPORT COLLECTION

REPFIODUCTION

COPY

LOS ALAMOS SCIENTIFIC LABORATORY

OF THEUNIVERSITYOF CALIFORNIAo LOSALAMOS NEW MEXICO

FORMULATION OF A LOW STATIC

ACCUMULATING, RESILIENT VINYL COATING

FOR EXPLOSIVE HANDLING EQUIPMENT

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LEGAL NOTICE

,.

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This report was prepared as an account of Government sponsored work. Neither the United States, nor the

Commission, nor any person acting on behalf of the Commission:

A. Makes any warranty or representation, expressed

or implied, with respect to the accuracy,completeness,or

usefulness of the information contained in this report, or

that the use of any information, apparatus, method, or process disclosed in this report may not infringe privately

owned rights; or

B. Assumes any liabilities with respect to the use

of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this report.

As used in the above, “person acting on behalf of the

Commission” includes any employee or contractor of the

Commission, or employee of such contractor, to the extent

that such employee or contractor of the Commission, or

employee of such contractor prepares, disseminates, or

provides access to, any information pursuant to his employment or contract with the Commission, or his employment with” such contractor.
LAMS-2798

UC-38, ENGINEERING AND

EQUIPMENT

TID-4500 (18th Ed.)

LOS ALAMOS SCI E.NTIFIC LABORATORY

OF THE UNIVERSITYOF CALIFORNIA LOS ALAMOS NEW MEXICO

REPORT WRITTEN: August 17, 1962

REPORT DISTRIBUTED: February 25, 1963

FORMULATION OF A LOW STATIC

ACCUMULATING, RESILIENT “VINYL COATING

FOR EXPLOSIVE HANDLING EQUIPMENT

Work done by: Report written by:

John H. Bell, Jr. John K. Donahoe

Orval L. Burnworth James S. Church

James J. Church

John K. Donahoe

Wilbur L. Johnston

Marvin A. Murphy

Jerome T. Rowen

Robert L. White

_

-al >—= .—o~l—— co

~1~

:3;; Contract W-7405 -ENG. 36 with the U. S. Atomic Energy Commission

~~(’q Q~I-E—0

All LAMS reports are informal documents, usually prepared for a special purEi

pose and primarily prepared for use wfthln the Laboratory rather than for

geneml distribution. This report has not been edIted, reviewed, or verffied

.~ 0) for accuracy. All LAMS reports express the views of the authors as of the

gECO time they were wrftten and do not necessarily reflect tbe opinions of tbe Los

— Alamos Scientific Laboratory or the fioal ophdon of tbe authors on the subject.

—

-1-
ABSTRACT

A material system has been developed for use in coating machine

tool equipment components t.caid in preventing accidental detonation

of high explosive materials during machining operations. A graphitefilled, plasticized polyvinyl chloride composition produces a resilient ccating with capability to leak-off or dissipate charges of

static electricity. The ccsting is tough, abrasion resistant, and

capable of being repaired if torn or otherwise damaged in use.

-2-
TABLE OF CONTENTS

Abstract

Chapter 1 Objectives

1.1 The Problem

1.2 Potential Solutions

1.3 Basic Specifications for a Coating

Chapter 2 Experimental Coatings

2.1 Canposition of Experimental PVC Plastisol

Coatings

2.1.1 Material.Sources and Identification

2.1.2 Formula Variations

2.2 Test lkta

2.2.1 Peel Strength of Coatings Applied to

Primed Metal Substrates

Chapter 3 Standard Process for Coating Wet Pans

3.1 Coating FonmiLations

3.2 Repair of Coatings

3.3 Other Applications of the Coating System

3.4 Storage and Age Effects

Chapter 4 Conclusion

l%ge

2

5

5

6

6

7

9

10

IL

12

13

18

19

22

23

24

25

-3-
FIGURES

Page

,lHg.1 Welded Polyethylene Wet Pan 8

Fig. 2 Block Diagrsn, Resistance Test Apparatus 14

Fig. 3 Resistance Characteristics of Graphite FiXLed PVC 15

Plastisol

Fig. 4 Variation of Resistance with Voltage for 30 w/o 16

Graphite in PVC Plastisol

Fig. 5 PVC Coated Ia.thePan 20

Fig. 6 PVC Coated Mill Fan 21

!rABLEs

Table I Basic PVC Plastisol

T%ble II Resistance of Materials

Table III Peel Strength of Coatings

Table IV Formulation for Conducting PVC Plastisol

‘I%bleV Formulation for Resilientj Non-Conducting

PVC Plastisol

-4-

Page

9

12

17

19

22
CHAJ?TER1

OBJY3CTIVES

In machining high explosive (HE) shapes, accidental detonation

of the material either by impact or the arcing of accumulated static

electricity is a distinct possibility. Therefore, the development

of a materials system to aid in the prevention of such accidents,

specifically during the machining opemtion, was required.

1.1 The Problem

The tools used to machine HE materials at

Scientific Laboratory (LASL) are equipped with

The Los Alamos

“wet pans” to catch

and dmin off the coolant fluid and the “chipd’formedduring the

machining operation. These pans have historically been made of a

non-sparking metal such as brass or aluminum. Although metallic

pans prevent any accumulation of static charges, they do not provide

protection against mechanical damage to the part if it happens to

fall from the machine chuck, nor do they provide protection against

the impact of HE with a hard object. Any material used in construction of a wet pan must (1) be resistant to corrosion or chemical

-5-
reaction with the HE or coolant; (2) survive steam cleaning operations;

and (3) be amenable to fabrication into one-of-a-kind, tailored units.

1.2 Potential Solutions

The initial.approach to the problems of impact (resulting in

either detonation or mechanical damage) and the accumulation of

static electrical charges was to fabricate the pans by hot gas welding

of rigid polyvinyl chloride (PVC) or polyethylene containing or

coated with anti-static additives. At the time, anti-static treatments for PVC and polyethylene were not reliable. A second option

was to fabricate the wet pans frcm welded aluminum sheet and subsequently apply a locally fomm.dated, anti-static PVC plastisol or

polyethylene coating.

1.3 %sic Specifications for a Coating

A satisfactory coating, as applied, should have good tipact

resistance and impact energy absorption properties, and also have

relatively good electrical conductivity. It should bond we33 to

the substrate material.with no pockets or voids to allow the accumulation of HE particles. Chemical reaction with HE cannot be tolerated, and yet the material must be cmnparatively easy to repair

in the event of accidental damage to the coating.

-6-
CHAPTER 2

EXPERIMENT COATINGS

A number of coating materials evaluated early

included neoprene sheeting, polyethylene sheeting,

plastisol formulations.

in the program

and several PVC

Neoprene sheeting (45-55 Shore ~ duraneter) was glued in place

on a primed dural substrate. Gates Engineering Co., Western Div.,

Ia3 Angeles 22, California, N-n primer, and N-29/W39 “liquid neoprene” adhesive were used for this operation. Anti-static properties were poor, and only a few pans were fabricated by this procedure

for use where the fmpact problem was considered severe.

Polyethylene sheeting was cut, formed, and hot gas (N2) welded

as a self-supporting structure. A pan

by this process is shown in F@u.re 1.

made from polyethylene for specialized

made for a milling machine

No more than six pans were

requirements while attention

was devoted to development of a “conductive” plastisol coating.

The PVC plastisol samples were all dip coated over a primed

dural or brass substrate. The primer used throughout the program

-7-
-.,.. .

--

. .. .

. .... .

.- —-—.,...

r ~-r —.. -..—/

-“..~ ,

.-

Welded Polyet~lene Wet Fan

-8-
was “Unichrome 219 PX”, a product of The United Chromium Div., Metal

and Thermit Corp., New York 17, New York. All test specimens were

prepared by a loosely standardized process: (a) oven preheat specimen

plate ~0 minutes at 173-180”c; (b) dip plate into test material with

hO seconds dwell allowed; (c) remove and drain until material flow

ceased; (d) oven fuse 25 minutes at 175-1800C or until i%ll strength

of coating is obtained; (e) trim off any ‘“dripbeads” prior to

testing. If test specimens were to be primed, the primer was applied

either by brush or spray and allowed to air dry prior to the preheat.

(Since the test program has been ccxnpleted,it has been found that

a maximum preheat time of 20 minutes at 1’i’’j-1800C will produce

maximum adhesion between substrate and coating. Newer procedures

take this fact into account.) If only one side of the test specimen

was to remain coated, excess material was trimmed off with a scalpel.

2.1 Composition of Experimental PVC Plastisol Coatings

All of the experimental coatings were derived frcm the following

basic PVC plastisol.

TABLE I

Easic PVC Plastisol

Ingredient l%rts by weight

10Q

80

20

10

4

1.5

0.5

-9-
2.1.1 l.literialSources and Identification

1. Geon 121 is a high molecular weight vinyl chloride polymer

of minus 200 mesh size especially developed for use as a plastisol

base. The manufacturer is B. F.

Ohio.

2. Flexol ~P is a primary

Goodrich Chemical Co., Cleveland 15,

plasticizer manufactured by Carbide

and

New

Carbon Chemical Co., Ditision of Union Carbide and Carbon Corp.,

York 17, New York. Chemically it is di(2-ethylhexyl)phthalate.

3. Flexol @l is another Carbide and Carbon product included

here to increase low temperature flexibility, reduce ccmpound viscosity, and control increase in viscosity with age. The chemical

identification is polyethylene glycol di(2-et~l hexoate).

4. Faraplex G60 is a polyester type plasticizer which serves

to increase hardness and abrasion resistance of the fused ccmpound.

It also acts as a light and heat stabilizer to some extent. The

Resinous Products Division of Rohm and Hass Co., Philadelphia 5,

Pennsylvania. is the source for this material.

5. Barinac (barium ricinoleate) is the primary system stabilizer. It is supplied by the National Lead Co., New York 6, New York.

6. Bentone 34 is used as a flow control agent and is an

orgonophillic bentonite manufactured by the National Lead Co.

‘i’.DCMRF is a silicone mold release fluid obtained from Dow

Corning Corp., Midland, Michigan, and is used as an internal lubricant. lY.2-200,350 centistokes viscosity is equally acceptable.

-1o-
2.1.2 Formula Variations

To this basic plastisol were added varying amounts of graphite

(llixonFlake Graphite #635, Joseph Dixon Crucible Co., Jersey City,

New Jersey), and plasticizers such as Ilexol DOP or DIDP (diisodecyl

phthalate obtained frcxnMonsanto Chemical Co., Organic Chemicals Div.,

St. Louis, Missouri, and used as a partial viscosity control agent

and primary plasticizer). Graphite loadings of 10, 20, and 30 w/o

based on the original plastisol were made, producing mixtures which

retained dip-coating capabilities. In order to obtain higher graphite

concentrateens, it was necessary to extend the “basic” plastisol with

additional plasticizer. The increase in plasticizer was varied from

10-50 w/o of the original plastisol with the Ilexol DOP, and

10-60 w/o with the DIDP. The 50% extension of the plastisol

Ilexol DOP allowed a maximum workable graphite concentration

frcnn

with

of 37.5

w/o; and the 60~ extension with DIDP allowed a workable maximum concentration of 38.3 w/o graphite. The 60j4DIDP extended, 387$graphitefilled canposition is rather viscous

bubbles. (A single pass through a 3

tendency considembly.) All.samples

and has a tendency to retain

roll paint mill will reduce this

used in the collection of data

were mixed by hand using only a spatula and beaker to mix in the

additives. The “basic” plastisol was prepared in a Hobart dough

mixer by first blending the liquid ingredients and then sifting the

dry powders slowly into the mixing bowl to prevent their agglomeration.

-1..1-
2.2 Test I.lata

Electrical resistance tests were made on a

materials including neoprene, PVC plastisol and

containing graphite or ‘Thermax” carbon

Corporation, Sterlington, Louisiana.)

TABLE II

black.

Resistance of Materials

?43terial

Neoprene (glued to dural)

“Basic” PVC Plastisol, no additives

+0.5$ Carbon Black

+lC@ Graphite

+20~ Graphite

+3@ Graphite

+60$ DIDP +38.3% Graphite

+Meas~ed on a ~ midge in air.

series of different

four PVC plastisols

(Thematomic Carbon

Resistance (Megohms)

5000*

8.4

11-12

7.6

5.4

2.1

0.1

Because the prospective use of the coated pans was to be with water

base coolants, the resistance measurements were made with tap water

as one electrical contact. A 60 cycle alternating current was used

to avoid polarization. The samples tested were 3 in. x 3 in. x

0.25 in. brass or dural plates completely coated to within 0.5 in. of

one 3 in. x 0.25 in. face. Coating thickness applied to the test

plates varied somewhat because of variations in the physical properties (primarily viscosity, and thermal conductivity) of the different

cuupositions. AU test plates were prepared by the standard process

which has been described, without modification to control the deposit

-1.2-
thickness. The higher filler content mixtures produced the greater

thicknesses. Individual sample thickness varied a maximum of

f 0.030 in. frcxsan average of 0.080 in. for the series.

The samples were immersed 1.5 in. into water contained in a

4.0 in. diabmss can. Electrical connections were made to the can

and to the sample plate. The applied voltage was measured on a

1/4$ Qmamometer. The current was measured by observing the voltage

drop across a 5000 ohm resistor wfth an oscilloscope and a

ential.amplifier. Accumcy of the current measurement was

Overall accuracy was about 5%. All measurements were made

LASL Electrical Standards Laboratory. TYgure 2 is a block

of the test apparatus.

The resistance of the graphite filled samples appears

linear with respect to filler concentration. The data are

differabout 3$.

by the

diagram

to be

plotted

as Figure 3. Resistance characteristics of 3@ graphite filled

coating is voltage dependent as shown in Figure h.

2.2.1 Peel Strength of Coatings Applied to Primed Metal Substrates

A series of tests was made on primed brass and dural substrates

in both wet and dry conditions to determine adhesion of the coating

to the substrate. The

cate the peel strength

averages for 3 samples

tests were made by LASL Group GMX-3and in&iof these coatings. Values reported are

of each type tested at 22°C.
Imx!K

RESISTANCE

Line

DIAGRAM

TEST APPARATUS

eVariac

A

-

Brass

cup

Q

l/4* Dynamometer

Tektronix 53/54GWide Band

Differential Preamplifier

Tektronix Mdel 545

oscilloscope

FIGURE 2

-14-
in

..4

20

10

9

8

7

6

5

4

3

2

1

l’,Carb

\\

1Black

RESISTANCE CHARACTERISTICS

OF GRAPHITE FILLED PVC PLASTISOL

5 10 15 20 25 3

Percent Graphite Added to “Basic” PVC

FIGURE 3

-15-
2.5

2.0

1.5

1.0

)

VARIATION OF RESISTANCE

WITH VOLTAGE FOR 30 W/O

GRAPHITE IN PVC PLASTISOL

—

Volts (60 cycle AC)

FIGURE 4

) 80 1 )

-16-
TABLE III

Peel Strength of Coatings

Peel Strength

Substrate Condition (lbs/in. of width)

Brass Dry 16.1

Brass Wet* 17.6

DJral m 5.8

D.lral Wet* 2.5

*~c~ens con~tioned in ~ter for 24 hours prior to test.

Peel test spechnens were bars coated on one side only with the “basic”

PVC plastisol +0.5$ carbon black. ASTM-903-49 procedure was followed.

-17-
CHAPTER 3

STANIMRD PROCESS FOR COATING WET!PA.NS

Although two different types of coatings

to wet pans at LASL, the basic process is the

are presently applied

same. The part is

primed, air dried, and then susPended in a =nner so as to hold

inside surfaces upward, but tilted slightly. Suspension is normally done by attaching an tiprovised combination of cables and

bars to the pan mounting lugs and arranging the resultant cradle

linkages to achieve correct pan position. Adjustments must be

made individually for each type or style of pan to produce smooth

covexage on critical surfaces. Standard preheat Is 20 minutes at

175-180”c in a mechanical convection oven. The part is immersed

in the plastisol for 30 to 50 seconds, removed, drained, and

returned to the oven for 20 to 35 minutes at 175-1800C to fuse the

coating. Ccxnpletefusion is considered to have occurred when the

coating surface acquires a polished sheen. ~sion time required

is dependent on both mass and geometry (as it affects dmin time):

-18-
Coating thickness can be increased by repeating the dip and f%sion

stages of the process. After the final fusion cycle, the coated

article is cooled and trimmed as required. (Cooling.may be by

hnmersion in

pan exterior

ation, it is

water or by air convection.) If the coating on the

does not interfere with mounting or machining operleft in place; otherwise, it is knife cut, and removed.

Surfaces which

to permit easy

Figures 5

are to be left free of coating are not primed in order

removal of the surplus coating.

and 6 are photographs of typical coated wet pans.

3.1 Coating Formulations

Two types of filled PVC plastisol formulations are currently

utilized at IASL in production coating of wet pans. The formulation

in Table IV is employed where conductivity is reqtired. The formulation for resilient, non-conducting PVC plastisol is listed in

Table V. (All of the ingredients have previously been identified

as to primary function and source.)

TABLE IV

Formulation for Conductimz PVC Plastisol

hgredient

Geon 121

l?lexolDOP

Flexol 4G0

DIDP

Paraplex G-60

E?arinac

Eentone #34

Dixon Flake Graphite #635

Farts by Weight

100

80

20

130

10

4

1.5

0.5

215

-19-
.—--- .—— — ..

-—-

~ . ..

-—- . L .W*

. .. ___

Figure 5

PVC Ccated Iathe W

-20-
~.. _ — —----.—-.. . ..

-.: —.. .—— — —— .—-n. —

. ......-..,==

,, ..+,.++-

. .— . . ..—. 4

IiiEEsE-1-’”- =~

—

-.. ---- ------ ..

.-

j

9

.

.

-. ~i==-— - . —-

. ..— —..

.—- .*

Figure 6

PVC Coated Mill Pan

-21-
TABLE V

Fozmmlation for Resilient, Non-Conducting PVC Plastisol

Ingredient

Geon 121

Flexol DOP

DIDP

Flexol @3

Raraplex G-60

Barinac

Bentone #34

DCMRF

Carbon Black

Parts by Weight

100

60

20

20

10

4

1.5

0.5

0.5

Since the Ordinance Safety Mumal specification requiring

conducting coatings with resistivity less than 1 megohm/in. has

been deleted from the LASL ccating standards, the formulation listed

in Table V has been the more popular coating material.

3.2 Repair of Coatings

Coatings which are damaged in use can be repaired in the field,

although defective parts are normally stripped and recoated. Stripping a coating from a primed substrate is difficult and usually

achieved only by mechanical peeling using a putty knife. More often,

a defective coating is carbonized by thermal abuse in an oxidizing

atmosphere, and the debris scraped away.

Preparation for field repairs can be made by knife cutting around

the flawed area, peeling the coating away within the cut, sanding the

metal, and appl@ng prtier. Much care has to be exercised if there

-22-
Is any probability of encountering HE inclusions trapped under a

flap of coating, and steam cleaning is normally required. After the

primer has been air dried, liquid plastisol is placed in the cut-out

area, and fised in place with a heated iron or exposed to a jet of

hot N2 gELS. A patch can best be blended and boded to the surrounding

sound coating by use of a heated iron or tool shaped to the coated

surface contour.

Some pans have been in regular use for 4 years with no sign of

wear or coating failure.

3.3 Other Applications of the Coating System

Work done with the coating formulations described has been by

no means limited to wet pans, although they constitute a major

fraction of applications. Other machine tool components which can

come in contact with HE have been ccated either selectively or over

all of their exposed surfaces. Lifting fixtures have been padded;

table tops, cooling grills, riser carts, mold carts, and other

assorted hardware have been coated. Minor adjustments in the plastisol formulations have occasionally been made to achieve specialized

properties. Most frequently, the Bentone 34 content has been

increased to cause pick-up of a thicker coating on a single dip. One

special modification, the addition of monomeric diallyl phthalate

and a peroxide catalyst, has been utilized to make a large gain in

-23-
toughness and impact resistance. This specially modified system

has a short life in stomge, as well as other disadvantages such

as decreased resiliency.

3.4 Storage and Age Effects

Storage life at ambient room temperatures of the formulations

listed in Tables IV and V is the order of

with a “Lightnin” type propeller mixer is

has not been disturbed for 10 to 15 days.

several months. Stirring

advisable if the plastisol

If the tank of material

has received severe thermal abuse, the contents

through a 6-8 ply bag of che se cloth to remove

,%

stirred, and allowed to stand 12 to 20 hours to

should be strained

gelled agglcxnerates,

rid itself of air

bubbles which affect coating quality. Aged material w-illdevelop a

tendency to produce slightly heavier coatings than freshly prepared

plastisol. This factor has proven to be no real problem, and a slight

reduction in part preheat time can be made to compensate for age if

required.

It is normal LASL practice to hold a 400-500 gallon tank of

the composition given in ‘IhbleV on hand. Fresh mterial is added

as required after “used” material has been strained.

-24-
CHAPTER4

CONCLUSION

A process has been developed to

impact resistant and shock absorbing

deposit and bond conductive,

PVC base coatings to metallic

substrates. These c=tings are especially valuable when exposure

to HE in massive or chip form is anticipated. There are modifications which might be made to improve conductivity, ease of

application, or both; however, development work on the program was

stopped as soon as useful formulations had been devised to meet

minimum specifications. Potential usefulness of this material

extends to coating machine components other than wet pans, which

may be involved in explosives fabrication operations.

-25-