Project Summary

United States

Environmental Protection

Agency

Radome Depainting

Evaluation at Tinker Air

Force Base

National Risk Management

Research Laboratory

Cincinnati, OH 45268

Research and Development

Methyl ethyl ketone (MEK), a hazardous chemical, is used in chemical

depainting of aircraft radomes at Tinker

Air Force Base (TAFB). This report summarizes a laboratory scale screening

study which evaluated ten solvent solutions as a potential replacement for

MEK in TAFB depainting operations.

A test program was developed by

Pacific Environmental Services, Inc.

(PES) to determine the feasibility of using alternative solvents, less hazardous to the environment and public

health than MEK. PES prepared a test

protocol for conducting the evaluation,

which specified the stripping solutions

to be evaluated, all necessary equipment, test procedures, and method of

evaluating each test.

A qualitative approach was used to

determine the removal efficiency of

each solvent. The testing involved immersing a 2" x 2" square of an aircraft

radome (a fiberglass and epoxy or polyester composite material, in a honeycomb structure) in a beaker of the

selected solvent. The parameters evaluated included a visual assessment of

the degree of attack (% removal) on

the coating in 0.5-, 1-, 2-, 4-, 8-, and 24-

hr increments. MEK was tested to establish a baseline. Ten potential

solvents were then evaluated to determine which chemical, if any, would be

a suitable substitute for MEK.

Test results indicated that several of

the solvents stripped the paint quicker

and more efficiently than MEK (EZE

540, EZE 542, Turco 6776 Lo, Turco

6776 Thin, and Turco 6813). Although

visual examination showed no damage

to the substrate, there was some concern over potential substrate damage

due to the complete and aggressive

removal of all three coats (primer, polyurethane rain-erosion coating, and polyurethane anti-static topcoat).

Three of the Huntsman solvents

(7210-60-1, 7210-60-2, and 7210-60-5)

completely removed the top two coats

while leaving the primer untouched.

This concept of leaving the primer layer

intact is being considered by the USAF

as a suitable alternative to complete

removal. Leaving the primer layer intact would ensure that the substrate is

not damaged during the depainting operation. Additionally, there may be

some economic advantages in material

and manpower savings. Further testing of the Huntsman solvents at TAFB

is anticipated later this year.

This Project Summary was developed

by EPA’s National Risk Management

Research 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

The U.S. EPA is encouraging the development of programs to reduce the generation or emissions of hazardous wastes.

The Waste Reduction at Federal Sites

EPA/600/SR-95/079 June 1995
2

(WREAFS) program is part of this effort.

This Project Summary covers radome

depainting, a single phase of efforts to

prevent pollution at Tinker Air Force Base

where heavy aircraft such as the B-52,

KC-135, etc. are overhauled. When an

aircraft is overhauled, paint and soils are

first removed so that the underlying structure can be carefully inspected and repaired if necessary. The radome, which is

a large ellipsoid structure made of an epoxy impregnated glass over a honeycomb,

is coated with three layers of paint: primer,

rain erosion layer, and the topcoat. The

two outercoats are usually urethanes, and

the primer is an epoxy. In the present

depainting process, the radome is placed

on end in a large ventilated booth, and

the outer coats are scored so that the

solvent will penetrate to the primer. Methyl ethyl ketone (MEK) is sprayed over

the surface for a period of up to 3 hours

during which the primer dissolves, and

the topcoat chips slough off and are collected in a sump. The MEK in the sump is

recycled; evaporative losses in 1993 were

about 50,000 lb, while about 700 lb of

paint chips were sent to disposal.

Air Force personnel wanted to replace

MEK for several reasons: It is flammable

(flash point 26o

F), and has a high vapor

pressure, so that it evaporated easily,

causing high air emissions as well as solvent loss. MEK vapors cannot be captured on activated carbon, as there are

instances of it causing fires in the carbon

bed. A previous study at TAFB had demonstrated that low volatility solvent blends

could replace MEK, but the best one contained NMP, whose toxicity had become

suspect. EPA decided that other propylene carbonate blends might be effective

paint strippers, while retaining the desired

low toxicity, high flash point, and low volatility. Huntsman again made and supplied

five blends, two containing lowered concentrations of NMP, and three having

none. These were then evaluated and

compared to five commercial strippers and

MEK in laboratory tests on 2"x 2" pieces

cut from a discarded radome. The compositions of the ten strippers are given below.

• Huntsman Formulation C

#7210-60-1

40 - 60% Propylene carbonate

10 - 30% Methyl-2-pyrrolidinone

15 - 35% Dibasic ester (DuPont)*

 4 - 10% Methyl-2-hexanone

 (methylisoamyl ketone)

• Huntsman Formulation D

#7210-60-2

20 - 40% Propylene carbonate

10 - 30% Methyl-2-pyrrolidinone

25 - 45% Dibasic ester

10 - 20% Ethylene glycol diacetate

• Huntsman Formulation E

#7210-60-3

15 - 35% Propylene carbonate

40 - 60% Dibasic ester

20 - 40% Butyrolactone (gamma)

• Huntsman Formulation F

#7210-60-4

35 - 55% Propylene carbonate

30 - 50% Dibasic ester

 5 - 15% Triethylene glycol

• Huntsman Formulation G

#7210-60-5

30 - 50% Propylene carbonate

30 - 50% Dibasic ester

10 - 30% Diethylene glycol

monobutyl ether

• EZE 540

28 - 35% Benzyl Alcohol

10 - 15% Formic Acid

 1 - 5% Petroleum Hydrocarbon

• EZE 542

30 - 40% Benzyl Alcohol

5 - 10% Ethanolamine

1- 5% 2-Ethylhexyl Mercaptoacetate

1 - 5% Petroleum Hydrocarbon

• Turco 6776 Lo

15% Proprietary Ingredient No. 1

 5% Formic Acid

<5% High Boiling Aromatic Solvent

10% Proprietary Ingredient No. 2

Also contains:

Unk.% Water

• Turco 6776 Thin

10% Proprietary Ingredient No. 1

10% Formic Acid

10% Solvent Refined Hydrotreated

 Middle Distillate

10% Proprietary Ingredient No. 2

Also contains:

Unk.% Water

Unk.% Sodium Xylene Sulfonate

• Turco 6813

40% Proprietary Ingredient

<5% High Boiling Aromatic Solvent

Also contains:

Unk.% Water

Unk.% Hexynol

Unk.% Ammonium Hydroxide

(Ammonia)

Test Procedure

A test program was developed by Pacific Environmental Services, Inc. (PES)

to determine the feasibility of using these

alternative solvents, comparing them to

MEK. PES prepared a test protocol for

conducting the evaluation, which specified the stripping solutions to be evaluated, all necessary equipment, test

procedures, and method of evaluating each

test.

A qualitative approach was used to determine the removal efficiency of each solvent. The testing involved immersing a 2"

x 2" square of an aircraft radome (a fiberglass and epoxy or polyester composite

material, in a honeycomb structure) in a

beaker of the selected solvent. The parameters evaluated included a visual assessment of the degree of attack (%

removal) on the coating at 0.5-, 1-, 2-, 4-,

8-, and 24-hr intervals. MEK was included

in the test to establish a baseline. The

potential depainting compositions were

then evaluated to determine which were

suitable substitutes for MEK.

Test results indicated that several of

the commercially available compositions

stripped the paint quicker and more efficiently than MEK. These were EZE 540,

EZE 542, Turco 6776 Lo, Turco 6776

Thin, and Turco 6813. While these were

effective strippers, there is some concern

over ingredients such as formic acid (toxic,

as well as corrosive) and ammonia. Although visual examination showed that

these caused no damage to the substrate,

conversations with TAFB engineers indicated some concern over potential substrate damage due to the complete and

aggressive removal of all three coats

(primer, polyurethane rain-erosion coating,

and polyurethane anti-static topcoat). The

visual identification of the three separate

coats was easily accomplished due to the

distinctive color of each coat. (The top

coat was a dark grey, the second or middle

coat was white, while the primer coat was

red). In addition, the substrate was a distinct yellow-brown.

Three of the Huntsman solvents (7210-

60-1, 7210-60-2, and 7210-60-5) completely removed the top two coats while

leaving the primer untouched. This concept of leaving the primer layer intact is

being considered by the USAF as a suitable alternative to complete removal. Leaving the primer layer intact would ensure

that the substrate is not damaged during

the depainting operation. Additionally there

may be some economic advantages in

relation to material and manpower savings. Further testing of the Huntsman solvents at TAFB is anticipated later this

year where it is planned to strip a complete radome, and, if time allows, test the

adhesion of new topcoats to the primer.

Test Results

The removal effectiveness of the solvents varied greatly. Some of the solvents

aggressively removed all three coats of

paint, while others selectively removed only

the top two, leaving the primer coat intact.

* A mixture of dibasic esters comprising 24% dimethyl

succinate, 60% dimethyl glutarate, 15% dimethyl

adipate, plus 1% water and methanol.
3

Even though visual inspection of the radome test sections indicated no physical

signs of damage or deterioration to the

substrate upon complete removal of all

coats of paint, it may be desirable, when

the primer coat is in good condition, to

leave it intact, thus ensuring the integrity

of the substrate.

For complete removal of the first two

coats while leaving the primer coat intact,

the following solvents exhibited the best

results:

• Huntsman Corporation, Formulation E,

 7210-60-3

• Huntsman Corporation, Formulation F,

 7210-60-4

• Huntsman Corporation, Formulation

 G, 7210-60-5

For complete removal of all three coats

the following solvents exhibited the best

results:

• EZE Products, Inc., EZE 540**

• EZE Products, Inc., EZE 542

• Turco Products, Inc., Turco

 6776 Thin**

• Turco Products, Inc., Turco 6776 Lo**

**Required a final waterwash to completely remove the

primer coat.

Costs

Purchase costs for each of the solvents

used in this test program are presented in

Table 1. The costs for the commercially

available strippers are as quoted, while

those of the Huntsman formulations are

calculated from the cost of the individual

components multiplied by their percentage in the final blend; no cost of blending

the mixtures or a profit margin for the

supplier of the blends are included.

Cost effectiveness of each solvent must

also consider solvent loss, waste disposal,

depainting time, degree of paint removal,

subsequent surface treatment and related

factors. Such an evaluation was beyond

the scope of this study.

Conclusions

These screening tests indicate that there

are viable replacements for MEK which

effectively remove the two topcoats while

leaving the primer relatively intact. Further

testing must be done to evaluate new top

coat adhesion to primer which has been

exposed to the stripper in a full scale

depainting test; two radomes and solvent

blends have been procured for trials

planned for late 1994. The results of these

full scale trials will then be evaluated by

TAFB engineers to determine if installation on a test aircraft is justified.

The full report was submitted in fulfillment of Contract No. 68-D2-0062 by PES,

Research Triangle Park, NC, under the

sponsorship of the U.S. Environmental Protection Agency.

 Solvent/Blend Approximate Cost $/gallon

MEK 5.00

EZE 540 14.00

EZE 542 18.00

Turco 6776 Lo 17.00

Turco 6776 Thin 17.00

Turco 6813 17.00

Huntsman No. 1 12.00

Huntsman No. 2 12.00

Huntsman No. 3 9.00

Huntsman No. 4 9.00

Huntsman No. 5 9.00

Table 1. Purchase Cost Comparison of Selected Solvents
4

United States

Environmental Protection Agency

Center for Environmental Research Information

Cincinnati, OH 45268

Official Business

Penalty for Private Use

$300

EPA/600/SR-95/079

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT No. G-35

This Summary was authored by the staff of Southern Research Institute,

Birmingham, AL 35205; Pacific Environmental Services, Inc., Research

Triangle Park, NC 27709; and EPA author, S. Garry Howell, National Risk

Management Research Laboratory, Cincinnati, OH 45268.

James S. Bridges is the EPA Project Officer (see below).

The complete report, entitled "Radome Depainting Evaluation at Tinker Air

Force Base," (Order No. PB95-230835; 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 can be contacted at:

National Risk Management Research Laboratory

U.S. Environmental Protection Agency

Cincinnati, OH 45268