E AP Project Summary

United States

Environmental Protection

Agency

Evaluation of Propylene

Carbonate in Air Logistics Center

(ALC) Depainting Operations

Seymour Rosenthal, Ann M. Hooper

Risk Reduction

Engineering Laboratory

Cincinnati, OH 45268

Research and Development

This report summarizes a two-phase,

laboratory-scale screening study that

evaluated solvent blends containing

propylene carbonate (PC) as a potential replacement for methyl ethyl ketone (MEK) in aircraft radome

depainting operations. The study was

conducted at Oklahoma City Air Logistics Center (OC-ALC) at Tinker Air Force

Base (TAFB). TAFB currently uses MEK

to depaint B-52 and KC-135 aircraft radomes in a ventilated booth. Because

MEK is highly volatile, many gallons

vaporize into the atmosphere during

each depainting session. Therefore, the

U.S. Environmental Protection Agency

(EPA) is supporting studies to identify

effective, nonvolatile, less toxic substitutes for MEK.

The first phase of this study screened

the performance of three solvent blends

provided by Texaco Chemical Company.* These blends contained varying

concentrations of PC, n-methyl

pyrrolidone (NMP), dibasic ester (DBE),

and other organic solvents. The performance of each blend was compared

with that of MEK—both by the paint

removal time and by a visual estimate

of the amount of paint removed without any visible substrate damage (removal efficiency). The best performer

was PC Blend 2, which contained 25%

PC, 50% NMP, and 25% DBE. This solvent blend was then compared with

MEK during the second phase of this

study. The Phase 2 tests measured

paint removal time and efficiency, paint

adhesion, flexural properties, weight

change of the substrate after paint removal, and hardness of unpainted substrate test panels.

Phase 2 test results revealed that PC

Blend 2 performed favorably in comparison with MEK in removing paint

from the fiberglass/epoxy (F/E) test panels and in subsequent paint adhesion

tests, despite an indication of possible

substrate damage. PC Blend 2 should

continue to be evaluated as a substitute in the TAFB radome depainting

operation. Additional qualification testing, required by the Air Force, and a

full-scale demonstration project are recommended before implementation.

This Project Summary was developed

by EPA’s Risk Reduction Engineering

Laboratory, Cincinnati, OH, to announce

key findings of the research project

that is fully documented in a separate

report of the same title. (See Project

Report ordering information at back.)

Introduction

This EPA Risk Reduction Engineering

Laboratory (RREL) project was jointly supported by the Waste Reduction Evaluations at Federal Sites Program (WREAFS)

and the Strategic Environmental Research

Development Program (SERDP) to provide assistance to Tinker Air Force Base,

Oklahoma. The WREAFS Program provides technical assistance and support to

federal facilities in conducting waste minimization opportunity assessments and polEPA/600/SR-94/176 September 1994

* Mention of trade names or commercial products does

not constitute endorsement or recommendation for

use.
2

lution prevention research. This project

also focuses on EPA’s 33/50 Voluntary

Reduction Program, which plans to reduce generation of 17 hazardous substances—33% by the end of 1992 and

50% by the end of 1995. One of the 17

chemicals, the solvent MEK, is used for

depainting aircraft radomes at the OC-ALC

at TAFB.

TAFB removes paint from radomes on

KC-135, EC-135, B-52, B-1, and E-3A aircraft. In a large ventilated booth, an MEK

shower loosens the paint. The MEK attacks the primer through scribed breaks in

the topcoat. The paint starts to bubble

after 30 min of continuous showering. As

the primer dissolves, the topcoat is flushed

away from the radome by the MEK shower.

Topcoat residue is filtered from the MEK.

The solvent then flows to a sump for recycling to the shower head. The operation

typically takes 1-1/2 to 3 hr. According to

TAFB, a large percentage of the MEK is

lost to the atmosphere through the booth

exhaust system. After the MEK shower,

any remaining paint residues are removed

by hand sanding. Topcoat chips are captured in a sump and sent for disposal as a

hazardous waste. In 1991, 719 lb of topcoat chips were sent for disposal and an

estimated 8,250 gal of MEK evaporated

to the atmosphere.

From previous research and background

documents, RREL has identified PC as a

possible alternative to MEK. To date, PC

had not been performance-tested as a

substitute for MEK in radome depainting

operations. EPA conducted this proof-ofconcept study to evaluate PC’s performance as a substitute for MEK.

Texaco Chemical Company provided

three PC blends.

• PC Blend 1

33-1/3 propylene carbonate

33-1/3 n-methylpyrrolidone

33-1/3 dibasic ester

• PC Blend 2

25 propylene carbonate

50 n-methyl pyrrolidone

25 dibasic ester

• PC Blend 3

15 propylene carbonate

15 n-methylpyrrolidone

15 methyl isoamy ketone

40 dibasic ester

15 dipropylene glycol monomethyl

ether

The study was conducted in two phases.

At the TAFB chemical laboratory, Phase 1

screened the three PC blends for paint

removal performance with MEK as a standard. The screening had two goals: to

determine if the PC blends removed paint

as effectively as MEK without visible damage to the F/E substrate, and to select the

best of the three blends for further testing.

At the Foster Wheeler Development Corporation (FWDC) John Blizard Research

Center Laboratory, Phase 2 tests measured paint removal time and removal efficiency under simulated depainting booth

conditions, damage to the F/E substrate,

and paint adhesion.

Phase 1: Screening Procedure

Test panels cut from a condemned radome were immersed into four beakers,

each containing one of the three solvent

blends or MEK. The backside and edges

of the test unscribed panels were masked

with aluminum foil tape to prevent damage to the honeycomb structure.

The panels were removed after 30 min

and examined for signs of bubbling of the

paint coating. Observations were recorded.

The specimens were placed back into the

bath and examined at intervals of 1, 2, 4,

8, and 24 hr after initial immersion. During

each examination, the amount of paint

removal was estimated and recorded. The

panels remained in the bath for 24 hr,

even if the paint was already removed, to

observe any possible damage to the substrate.

Phase 2: Evaluation Tests

With the use of the selected PC Blend

2, Phase 2 simulated the depainting procedure to determine the time and removal

efficiency for a spray operation, which is

more representative of the depainting operation. TAFB selected other tests to analyze the solvent’s effects on the substrate.

Tests evaluated hardness, flexural properties, paint adhesion, and weight change.

Simulated Depainting

Procedure

The simulated depainting procedure was

used for both PC Blend 2 and MEK, each

in a separate, identical unit. The apparatus consisted of a Kleer-Flo Cleanmaster

parts washer fitted with a 1/4-in.- diameter

spray nozzle. The parts washer flow rate

was approximately 7 L/min. Both units

were modified with a 0.07 hp orbital magnetic drive centrifugal pump, because the

original pump’s seal material was incompatible with both MEK and PC. The nozzle

sprayed 2-in.-square test panels made of

F/E honeycomb cut from a condemned B52 radome.

The times required for the solvents to

bubble and totally remove the paint were

recorded, and a qualitative judgment of

paint removal was recorded at various

time intervals. After bubbling, the paint

was removed by hand or with a bluntedged wooden spatula.

Hardness

Hardness tests were performed in accordance with ASTM Test Method D 2583-

87. Eight 2-in.-square test panels were

cut from an unpainted F/E “prepreg” sheet

used for another project at TAFB.

“Prepreg” refers to fiberglass fibers that

have been soaked or impregnated with a

polyester or epoxy resin. The prepreg surface is identical to that of an unpainted

radome. These tests determined indentation hardness with a Barcol Impressor,

Model No. GYZJ 934-1. Indentations were

made on the specimens and the hardness

measured. In accordance with ASTM

D 2583, 20 measurements were made on

each test panel, 10 before and 10 after

contact with the solvent. The panels were

sprayed with solvent in the depainting

simulation unit for 2 hr, and gently wiped

dry with a paper towel.

Flexural Properties Test

The flexural properties test was performed in two parts. The first part measured flexural properties of the test panels

in accordance with ASTM Test Method

D 790-92, Test Method I, Procedure A.

Sixteen test panels were prepared from

an F/E prepreg sheet provided by TAFB

and cut to test specifications. Six test panels were tested in the as-received condition, five panels were conditioned by

exposure in the MEK depainting simulator

for 2 hr, and five panels were conditioned

by exposure in the PC Blend 2 depainting

simulator for 2 hr. After drying 1 hr, the

solvent-exposed panels were subjected to

load in a tensile machine until breakage

of outer fibers occurred.

During the second part of the test, the

failed test panels were examined with a

scanning electron microscope (SEM) to

determine if the solvents damaged the

interface of the F/E and laminate structure. For SEM examination, the panels

were cut into 1-in. squares with the failure

break in the center of the square. The

squares were then mounted on an aluminum stud using carbon paint, which provides a conductive bond between the stud

and the panel. The squares were then

sputtered with gold in a vacuum chamber

to make them conductive. After about 10

min of sputtering at various angles to ensure that the gold was applied underneath

the fibers, the squares were placed in

another vacuum chamber and examined

with the SEM. The surface was examined

at 30X, 300X, and 1200X magnification;

the cross section was examined at 1200X
3

magnification to observe any fiber/matrix

interface separation. Microphotographs

were taken at each magnification to document the conditions observed. Squares of

test panels not exposed to any solvent

were also examined by the SEM to obtain

a baseline comparison.

Paint Adhesion

Paint adhesion testing was performed

in accordance with ASTM Test Method D

3359-92a, Method A using F/E with honeycomb test panels. After paint removal in

the depainting simulation unit, test panels

were repainted by TAFB personnel with

the rain erosion coatings currently applied

to B-52 radomes. The same 10- to 12-mil

thickness of coating and painting procedure used for actual radomes was applied

to these test panels. For the adhesion

tests, two incisions were made in the panels down to the substrate layer. A pressure sensitive tape was then applied to

the intersection of the cuts for a period of

90 ± 30 sec. The tape was 1-in. wide

PermaCel 99TM (manufactured by

PermaCel, New Brunswick, NJ). After tape

removal, the X-cut area was visually inspected. The adhesion is rated according

to a scale of 0A (removal beyond area of

X-cut) to 5A (no peeling or removal).

Weight Change Test

The weight change test, developed by

TAFB, determines if damage occurs to

the substrate material. A clean, unpainted

F/E prepreg test panel is weighed before

and after immersion in the solvent. A

weight loss indicates that the solvent has

attacked the substrate. A weight gain reveals that the solvent is absorbed through

microcracks in the substrate. The test

panel was immersed in the solvent for 4

hr, four times longer than the time required to strip the panel in the screening

test. After immersion, the panels were gently wiped by hand and dried in a 150 ̊F

oven for 1 hr.

A parallel experiment was also conducted on standard 2-1/2-in.-square, 316-

stainless-steel (SS) plates to validate the

drying procedure. The 316 SS is inert; it is

not affected by the solvent. A weight gain

indicates the presence of solvent residue

on the surface.

Results and Discussion

Screening Results

Table 1 presents the results of the

screening tests. Although each of the three

PC blends removed the paint, PC Blend 2

removed it most rapidly. For each solvent,

any paint remaining after the paint had

completely bubbled from the surface was

removed by hand. The paint bubbled in

one piece, which included primer layer,

rain erosion coating, and topcoat.

The inconsistency of the paint removal

times for the test panels may be explained

by the condition of the panels before

depainting. Some panels had a mottled

appearance, suggesting certain areas of

the topcoat layer were worn, exposing the

white, rain-erosion coating layer. For these

panels, paint removal would be easier,

since the paint layer was thinner than

when originally applied. Runs 1, 2, 3 used

panels with varying degrees of mottled

appearance, which explains the inconsistency of time needed to reach complete

bubbling. In Run 4, panels were selected

for similar surface conditions. These panels did not have a mottled appearance

and, therefore, provided a better comparison. As the results in Table 1 show, complete bubbling took longer in Run 4 than

in the previous runs.

The MEK did not perform as well as

expected. During the first run, significant

evaporation occurred in the MEK beaker.

To minimize the evaporation, a plastic bag

was placed over both beakers after about

8 hr into the second run. On the third and

fourth runs, a watchglass was placed over

the beakers during the entire 24-hr period. As Table 1 indicates, MEK performance declined in Runs 3 and 4 when the

beaker was covered. This behavior suggests that the paint removal properties of

MEK may be enhanced by moisture in the

air. A separate experiment conducted by

FWEI for a parallel project indicated a

mixture of 12.5% by weight water in MEK

removes paint faster and more completely

than pure MEK. When covered, the MEK/

water mixture also decreased its removal

efficiency. In TAFB’s actual depainting operation, the combination of the spraying

and the presence of air probably contribute to MEK’s performance.

After the 24-hr period, each of the test

panels were evaluated for visible substrate

damage. No substrate damage was visible, although white spots indicated some

fibers may have been exposed. The TAFB

Materials Engineer examined the panels.

Under magnification he concluded that

there may have been exposed fibers, although all panels, including the MEK, exhibited this characteristic.

Based on these results, PC Blend 2

was selected for Phase 2 testing to evaluate the effect of the solvent on the F/E

substrate.

Evaluation Test Results

Simulated Depainting Procedure

It was discovered during this exercise

that the paint system on the panels used

in the Phase 2 tests differed from the

paint used on the panels tested during

Phase 1 screening. TAFB stated that some

panels were probably painted with a neoprene-based coating, which consisted of

a yellow primer, white primer, and black

topcoat. This older paint system bonded

very well to the substrate and was extremely difficult to remove. TAFB discontinued using it over 6 years ago.

The current paint system consists of a

dark red primer, followed by a white, rainerosion coating and a black topcoat. This

paint system had been applied to the radome test panels used in the screening

tests. The paints appear identical on the

test panels. Only when the panels are

stripped can the paint system be identified.

The specimens tested in Runs 3 and 4

for the depainting simulation with PC Blend

2 had been painted using the current paint

system. The solvent removed the paint in

one piece in less than 1 hr, as in the

screening phase. In Runs 1 and 2, the PC

Blend 2 removed the neoprene paint coatTime to reach complete bubbling

hr

Solvent Run 1 Run 2 Run 3 Run 4

PC Blend 1 1 4 2 4

PC Blend 2 1 1 1 2

PC Blend 3 4 2 8 4

MEK 8 8 Not complete Not complete

bubbling bubbling

Table 1. Screening Results
4

ing in about 3-1/2 hr with the aid of vigorous scraping. PC Blend 2 appeared to

remove the neoprene paint coating more

completely than MEK. The four panels

tested with MEK had been painted with

the neoprene paint coating; they took an

average of 4-1/2 hr to remove the paint,

also with vigorous scraping.

Because of time considerations, additional runs could not be scheduled to test

panels coated with TAFB’s current paint

system. The panels used for the paint

adhesion test were, however, painted with

the current system. An additional step of

recording the paint removal time was

added to the test, and the results for the

simulated depainting tests were disregarded. Removal times for paint adhesion

test panels are provided in Table 2.

Hardness

Average hardness results are summarized in Table 3. The test objective was a

Barcol hardness of 55 or greater. As indicated in Table 3, hardness measurements

met this objective; they did not change

significantly after exposure to solvent. Measurements ranged from 75 to 85 Barcol

hardness units, with an overall average of

80.4. These results show that MEK and

PC Blend 2 do not chemically embrittle

the substrate.

Flexural Properties Test

The test results for flexural strength,

maximum strain, and modulus of elasticity, as calculated in accordance with ASTM

D790-92, are listed in Table 4.

The data presented in Table 4 demonstrate that exposure to either PC Blend 2

or MEK did not affect the flexural strength

of the panels. Although most panels failed

at approximately 72,000-psi loading, four

test panels had lower flexural strength,

failing at a loading of approximately 52,000

psi. These panels failed with a straight

break across the test panel rather than

with the zigzag pattern exhibited with the

stronger panels. Similar results in tests on

unexposed, PC Blend 2-exposed, and

MEK-exposed panels suggest that these

four test panels were cut from a weaker

PC blend 2 MEK

Run Before After Before After

1 80.0 82.6 81.4 81.4

2 79.7 79.7 79.6 81.8

3 80.4 80.7 80.0 80.5

4 79.8 79.4 78.9 79.6

† Barcol hardness.

Table 3. Average Hardness† Readings

Flexural Maximum Modulus of

strength, strain, elasticity

Solvent Run psi in/in psi

Unexposed 1 71,750 0.02520 3.64 x 106

2 70,980 0.02470 3.60 x 106

3 69,930 0.02380 3.70 x 106

4 52,390 0.02460 3.30 x 106

5 71,280 0.02410 3.60 x 106

6 72,562 0.02490 3.67 x 106

PC Blend 2 1 52,240 0.02710 3.46 x 106

2 74,670 0.02430 3.84 x 106

3 71,127 0.02505 3.66 x 106

4 51,682 0.02350 3.59 x 106

5 71,160 0.02341 3.66 x 106

MEK 1 72,544 0.02350 3.84 x 106

2 53,286 0.02380 3.63 x 106

3 71,015 0.02485 3.66 x 106

4 72,694 0.02738 3.54 x 106

5 71,433 0.02587 3.64 x 106

Table 4. Flexural Properties Summary

section of the prepreg sheet. A statistical

analysis revealed a bimodal distribution,

proving that the four panels were taken

from a different sample population than

the stronger specimens. Comparison of

individual readings within the respective

populations showed that flexural strength

was unaffected by exposure to either solvent.

The second part of the test required

inspection of the failed test panels under

a scanning electron microscope (SEM).

The SEM microphotographs indicated no

damage from either solvent to the fiber

Run PC Blend 2 MEK

1 45 30

2 21 29

3 25 25

4 25 25

Table 2. Paint Removal Times (min)
5

matrix interface or to the fibers. If damage

had occurred, the microphotographs would

have shown noticeable gaps where the

fibers interface with the matrix (most visible at the 300X magnification). Also, individual fibers appeared to be intact,

indicating that the solvent did not attack

the resin binding the fibers.

Paint Adhesion

The paint adhesion ratings were 5A for

each test panel. This rating indicates no

peeling or removal of paint by the pressure sensitive tape. This suggests complete paint adhesion after the depainting/

repainting cycle.

Weight Change Test

Table 5 presents the results of the

weight change test. The test panels exposed to both PC Blend 2 and MEK

showed weight loss, indicating slight substrate damage; this was considered to be

negligible by TAFB. A parallel experiment

with 316-SS standards confirmed that the

solvent residue had evaporated from the

surface of the test panels.

Conclusions and

Recommendations

The evaluation test results indicate that

PC Blend 2 is a potentially viable replacement for MEK. Further study must qualify

its use in TAFB’s radome depainting operations.

Other conclusions from the test results

are listed below:

• Screening tests indicated that PC

Blend 2 performed the best of the

three PC blends provided by Texaco.

• PC Blend 2 and MEK did not impact

paint adhesion.

Recommendations for future courses of

action are:

• Further evaluation of the potential

adverse effects of the solvent on the

substrate, and either

• Reformulation of the solvent blend to

eliminate or reduce any identified

damage, or full-scale demonstration

of PC Blend 2 in TAFB’s radome

depainting operation, if no damage is

identified.

In a full-scale demonstration project,

these areas should be addressed:

• Equipment compatibility with PC

Blend 2

• Waste disposal practices for PC Blend

2 and paint chips

• Procedures for removing nonvolatile

PC Blend 2 residue

The full report on this project was submitted in fulfillment of Contract No. 68-C9-

0033 by Foster Wheeler Enviresponse,

Inc., under the sponsorship of the U.S.

Environmental Protection Agency.

• PC Blend 2 removed 100% of the

paint in about the same time as MEK

and required a little more scraping for

total removal.

• PC Blend 2 showed possible damage

to the top resin layer of the F/E

substrate. The potential impact of such

damage requires further study.

• PC Blend 2 and MEK do not embrittle

the F/E substrate.

• PC Blend 2 and MEK do not affect

flexural properties of the F/E substrate.

• Examination by SEM found no

significant damage to the fibers or to

the fiber-matrix interface.

• PC Blend 2 and MEK panels exhibited

a small weight loss after immersion

for 4 hr.

Weight change

grams

Run PC Blend 2 MEK

1 -0.011 -0.008

2 -0.009 -0.006

3 -0.020 -0.013

4 -0.025 -0.009

Table 5. Weight Change Test Results
6

S. Rosenthal and A. M. Hooper served as the Foster Wheeler Enviresponse

project team.

Johnny Springer, Jr., and Kenneth R. Stone are the EPA Project Officers

(see below).

The complete report, entitled "Evaluation of Propylene Carbonate in Air

Logistics Center (ALC) Depainting Operations," (Order No. PB94-214618/

AS; Cost: $17.50, subject to change) will be available only from:

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Telephone: 703-487-4650

The EPA Project Officer(s) can be contacted at:

Risk Reduction Engineering Laboratory

U.S. Environmental Protection Agency

Cincinnati, OH 45268

United States

Environmental Protection Agency

Center for Environmental Research Information

Cincinnati, OH 45268

Official Business

Penalty for Private Use

$300

EPA/600/SR-94/176

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT No. G-35