United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-94/176 September 1994
EPA Project Summary
Evaluation of Propylene
Carbonate in Air Logistics Center
(ALC) Depainting Operations
Seymour Rosenthal, Ann M. Hooper
This report summarizes a two-phase,
laboratory-scale screening study that
evaluated solvent blends containing
propylene carbonate (PC) as a poten-
tial replacement for methyl ethyl ke-
tone (MEK) in aircraft radome
depainting operations. The study was
conducted at Oklahoma City Air Logis-
tics Center (OC-ALC) at Tinker Air Force
Base (TAFB). TAFB currently uses MEK
to depaint B-52 and KC-135 aircraft ra-
domes 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 substi-
tutes for MEK.
The first phase of this study screened
the performance of three solvent blends
provided by Texaco Chemical Com-
pany.* These blends contained varying
concentrations of PC, n-methyl
pyrrolidone (NMP), dibasic ester (DBE),
and other organic solvents. The perfor-
mance 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 with-
out any visible substrate damage (re-
moval efficiency). The best performer
was PC Blend 2, which contained 25%
PC, 50% NMP, and 25% DBE. This sol-
vent blend was then compared with
MEK during the second phase of this
* Mention of trade names or commercial products does
not constitute endorsement or recommendation for
study. The Phase 2 tests measured
paint removal time and efficiency, paint
adhesion, flexural properties, weight
change of the substrate after paint re-
moval, and hardness of unpainted sub-
strate test panels.
Phase 2 test results revealed that PC
Blend 2 performed favorably in com-
parison with MEK in removing paint
from the fiberglass/epoxy (FIE) test pan-
els and in subsequent paint adhesion
tests, despite an indication of possible
substrate damage. PC Blend 2 should
continue to be evaluated as a substi-
tute in the TAFB radome depainting
operation. Additional qualification test-
ing, required by the Air Force, and a
full-scale demonstration project are rec-
ommended 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 sup-
ported by the Waste Reduction Evalua-
tions at Federal Sites Program (WREAFS)
and the Strategic Environmental Research
Development Program (SERDP) to pro-
vide assistance to Tinker Air Force Base,
Oklahoma. The WREAFS Program pro-
vides technical assistance and support to
federal facilities in conducting waste mini-
mization opportunity assessments and pol-
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lution prevention research. This project
also focuses on EPA's 33/50 Voluntary
Reduction Program, which plans to re-
duce generation of 17 hazardous sub-
stances—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 air-
craft. In a large ventilated booth, an MEK
shower loosens the paint. The MEK at-
tacks 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 recy-
cling 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 cap-
tured in a sump and sent for disposal as a
hazardous waste. In 1991, 719 Ib of top-
coat 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-of-
concept study to evaluate PC's perfor-
mance 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 stan-
dard. The screening had two goals: to
determine if the PC blends removed paint
as effectively as MEK without visible dam-
age to the F/E substrate, and to select the
best of the three blends for further testing.
At the Foster Wheeler Development Cor-
poration (FWDC) John Blizard Research
Center Laboratory, Phase 2 tests mea-
sured paint removal time and removal ef-
ficiency under simulated depainting booth
conditions, damage to the F/E substrate,
and paint adhesion.
Phase 1: Screening Procedure
Test panels cut from a condemned ra-
dome 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 dam-
age 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 sub-
strate.
Phase 2: Evaluation Tests
With the use of the selected PC Blend
2, Phase 2 simulated the depainting pro-
cedure to determine the time and removal
efficiency for a spray operation, which is
more representative of the depainting op-
eration. TAFB selected other tests to ana-
lyze the solvent's effects on the substrate.
Tests evaluated hardness, flexural prop-
erties, 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 appara-
tus 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 mag-
netic drive centrifugal pump, because the
original pump's seal material was incom-
patible with both MEK and PC. The nozzle
sprayed 2-in.-square test panels made of
F/E honeycomb cut from a condemned B-
52 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 blunt-
edged wooden spatula.
Hardness
Hardness tests were performed in ac-
cordance 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 sur-
face is identical to that of an unpainted
radome. These tests determined indenta-
tion 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 per-
formed in two parts. The first part mea-
sured 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 pan-
els were tested in the as-received condi-
tion, 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 struc-
ture. 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 alumi-
num stud using carbon paint, which pro-
vides 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 en-
sure 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 SOX, 300X, and 1200X magnification;
the cross section was examined at 1200X
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magnification to observe any fiber/matrix
interface separation. Microphotographs
were taken at each magnification to docu-
ment 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 hon-
eycomb 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 proce-
dure used for actual radomes was applied
to these test panels. For the adhesion
tests, two incisions were made in the pan-
els down to the substrate layer. A pres-
sure 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 99™ (manufactured by
PermaCel, New Brunswick, NJ). After tape
removal, the X-cut area was visually in-
spected. The adhesion is rated according
to a scale of OA (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 re-
veals 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 re-
quired to strip the panel in the screening
test. After immersion, the panels were gen-
tly wiped by hand and dried in a 150°F
oven for 1 hr.
A parallel experiment was also con-
ducted 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 inconsis-
tency of time needed to reach complete
bubbling. In Run 4, panels were selected
for similar surface conditions. These pan-
els did not have a mottled appearance
and, therefore, provided a better compari-
son. As the results in Table 1 show, com-
plete 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 pe-
riod. As Table 1 indicates, MEK perfor-
mance declined in Runs 3 and 4 when the
beaker was covered. This behavior sug-
gests 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 op-
eration, the combination of the spraying
and the presence of air probably contrib-
ute to MEK's performance.
Table 1. Screening Results
After the 24-hr period, each of the test
panels were evaluated for visible substrate
damage. No substrate damage was vis-
ible, 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, al-
though all panels, including the MEK, ex-
hibited this characteristic.
Based on these results, PC Blend 2
was selected for Phase 2 testing to evalu-
ate 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 neo-
prene-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 ex-
tremely difficult to remove. TAFB discon-
tinued using it over 6 years ago.
The current paint system consists of a
dark red primer, followed by a white, rain-
erosion coating and a black topcoat. This
paint system had been applied to the ra-
dome 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 identi-
fied.
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 coat-
Time to reach complete bubbling
hr
Solvent
Run 1
Run 2
Run 3
Run 4
PC Blend 1
PC Blend 2
PC Blend 3
MEK
1
1
4
8
4
1
2
8
2
1
8
Not complete
bubbling
4
2
4
Not complete
bubbling
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ing in about 3-1/2 hr with the aid of vigor-
ous 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, addi-
tional 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 disre-
garded. Removal times for paint adhesion
test panels are provided in Table 2.
Hardness
Average hardness results are summa-
rized in Table 3. The test objective was a
Barcol hardness of 55 or greater. As indi-
cated in Table 3, hardness measurements
met this objective; they did not change
significantly after exposure to solvent. Mea-
surements 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 elastic-
ity, as calculated in accordance with ASTM
D790-92, are listed in Table 4.
The data presented in Table 4 demon-
strate 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
Table 2. Paint Removal Times (min)
Run PC Blend 2 MEK
1
2
3
4
45
21
25
25
30
29
25
25
Table 3. Average Hardness? Readings
PC blend 2
Table 4. Flexural Properties Summary
MEK
Run
1
2
3
4
Before
80.0
79.7
80.4
79.8
After
82.6
79.7
80.7
79.4
Before
81.4
79.6
80.0
78.9
After
81.4
81.8
80.5
79.6
f Barcol hardness.
Solvent
Unexposed
PC Blend 2
MEK
Run
1
2
3
4
5
6
1
2
3
4
5
1
2
3
4
5
Flexural
strength,
psi
71,750
70,980
69,930
52,390
71,280
72,562
52,240
74,670
71,127
51,682
71,160
72,544
53,286
71,015
72,694
71,433
Maximum
strain,
in/in
0.02520
0.02470
0.02380
0.02460
0.02410
0.02490
0.02710
0.02430
0.02505
0.02350
0.02341
0.02350
0.02380
0.02485
0.02738
0.02587
Modulus of
elasticity
psi
3.64x 106
3.60 X106
3.70x 106
3.30 x 1 06
3.60 x 106
3.67 x 106
3.46 x 106
3.84 X106
3.66x 106
3.59 x 1 06
3.66x 106
3.84 X106
3.63 x 106
3.66 x 106
3.54 x 106
3.64 X106
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 sol-
vent.
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
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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 vis-
ible at the 300X magnification). Also, indi-
vidual 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 pres-
sure sensitive tape. This suggests com-
plete paint adhesion after the depainting/
repainting cycle.
Weight Change Test
Table 5 presents the results of the
weight change test. The test panels ex-
posed to both PC Blend 2 and MEK
showed weight loss, indicating slight sub-
strate 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 replace-
ment for MEK. Further study must qualify
its use in TAFB's radome depainting op-
erations.
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.
Table 5. Weight Change Test Results
Weight change
grams
Run
1
2
3
4
PC Blend 2
-0.011
-0.009
-0.020
-0.025
MEK
-0.008
-0.006
-0.013
-0.009
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.
• 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 sub-
mitted in fulfillment of Contract No. 68-C9-
0033 by Foster Wheeler Enviresponse,
Inc., under the sponsorship of the U.S.
Environmental Protection Agency.
-------
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 ofPropylene Carbonate in Air
Logistics Center(ALC) Depainting Operations,"(OrderNo. 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
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