PROCESSES

P2 Option: N-Methylpyrrolidinone (NMP)

P2 Option: Abrasive Blasting Media

Background: Vapor Degreasing

1,1,1-trichloroethane is used to clean metal surfaces and to remove carbonaceous material. Clean, grease free parts are a prerequisite to many of the maintenance operations. Typical degreasing operations are performed in heated vats called vapor degreasers that contain a liquid phase and a vapor phase of degreasing solvent. The metallic part, which has deposits of oily residues, is suspended in the vapor blanket for a specified time. The vapor condenses on the surface of the dirty part and trickles down to the liquid taking with it any oily deposits, thereby resulting in a clean part. During this process hundreds of pounds of the solvent are lost as vapor each year, contributing to air pollution. 1,1,1-trichloroethane has been identified as an ODS and thus is being phased out of production. MEK is normally applied to surfaces that require painting after they have been cleaned with 1,1,1-trichloroethane.

P2 Option: Isopropyl Alcohol (IPA)/Cyclohexane Vapor Degreaser

P2 Option: Aqueous Cleaners

P2 Option: Aqueous Cleaners

Potential Problems of the Aqueous Cleaners

1. Proper approval must precede use of aqueous cleaning methods.
2. Depending on the materials being cleaned, the material being removed, and the cleaning methods used there may be potential for corrosion of parts. Proper use of additives is imperative to eliminate corrosion.
3. NADEP Jacksonville found that the wastewater generated from their aqueous cleaners did not meet requirements to discharge to the wastewater treatment plant. Therefore they are required to drum their water at a cost of $1.21/pound. They typically generate 56,000 pounds/year of this wastewater.

Research: Alternative Solvent Progress

Ongoing projects exist to explore the use of chemicals that can be substituted as alternatives to degreasing agents and maskant removers currently being used. The search for the replacement of 1,1,1-trichloroethane as a degreaser has been the topic of much research. The objective of several of these research studies is not only to develop a degreaser that will work as efficiently as 1,1,1-trichloroethane, but also to reduce air emissions. A number of chemicals have been suggested to replace 1,1,1-trichloroethane. Similar studies have been undertaken for replacing solvents such as MEK which are used to clean surfaces for painting purposes.

Although a suitable substitute may be found, it is important to keep in mind what limitations the new chemical may have. Some chemicals may work better on a selected application while performing poorly on others. Performance testing will be required for those chemicals that are chosen as a suitable substitute for 1,1,1-trichloroethane and MEK. Some of the products under consideration are shown in Table 1.2.1. A more comprehensive list of substitutes is available in the DLA Product Guide, which is accessible through the DLA web site and the P2 Technical Library (see http://128.174.5.51/denix/DOD/Library/Air/Airmgt/aqch4.html#Appendix F.

Table 1.2.1: Possible Substitutes for 1,1,1-trichoroethane
(Heavy Degreaser/Maskant Remover)

Background: Liquid Oxygen (LOX), Gaseous Oxygen (GOX), And Liquid Nitrogen (LIN)

Liquid oxygen (LOX), gaseous oxygen (GOX), and/or liquid nitrogen (LIN) systems and components are found on virtually every aircraft weapons system and on numerous types of support equipment and production plants. In the Air Force at least 25 Technical Orders (TOs) call for the use of Ozone Depleting Substance (ODS) solvents, mainly CFC-113 (Freon 113), to clean various components of these systems and equipment. Typical components include tubing, plumbing, fittings, gauges, converters, environmental controls, etc. New non-ozone depleting compounds are needed that will clean LOX/GOX/LIN components and remove virtually all hydrocarbon residue from any material that will come in contact with the LOX/GOX/LIN product. Additionally, due to the extremely low temperature of LOX and LIN, the cleaning process must leave no residual moisture that could freeze and damage the component or interfere with the flow of the LOX/LIN within the system(s).

Currently CFC-113, trichlorotrifluoroethane (C₂F₃Cl₃), is used for cleaning LOX/GOX/LIN system components in hand-wipe applications by the Air Force. CFC-113 works well for cleaning components that are removed from systems and for cleaning components left in place on LOX/GOX/LIN systems. CFC-113 however, is a Class I, Group I ODS.

P2 Option: Recovery of CFC-113

Solvent Substitution for CFC-113 in LOX/GOX/LIN

The 1990 Clean Air Act Amendments specifically direct that the production of all Class I chemicals to cease by 1 January 2000. Effective 1 January 1996, specific annual production limitations for each Class I substance, which includes CFC-113, were imposed. For these reasons it is important to find an acceptable substitute cleaner for CFC-113 and similar ODS cleaners.

Research is underway to identify environmentally acceptable chemical substitutes for use as cleaners for LOX/GOX/LIN system components in hand-wipe applications at Air Force bases. The substitutes are to be used for quick servicing maintenance activities. Much coordinated work between the Air Force, Navy, and NASA is ongoing with regard to substitute cleaners for LOX/GOX/LIN system components. This work will be summarized and recommendations will be made regarding how to proceed.

Potential Substitute Cleaners for LOX/GOX/LIN System Components

There are a variety of commercially available substitute cleaners for CFC-113 which are all less detrimental with respect to ozone depletion. These substitute cleaners are listed below in order of products that have undergone the most testing and show promise for cleaning LOX/GOX/LIN system components. The list is not necessarily complete, but it does provide a starting point for CFC-113 replacement. This list was compiled from commercial and government literature and word-of-mouth sources.

Substitute Cleaner - Trade Name (Other Data)

(1) HFE (hydrofluoroether) compounds, 3M (potentially expensive, approximately $180/gallon)
(2) Navy Oxygen Cleaner - Aqueous solution containing surfactants, (many cleaning steps)
(3) HCFC 141b (production and consumption banned effective 1 January 2003)
(4) Crown, Heavy Duty Cleaner & Degreaser #8260
(5) LPS Laboratories, Zero-Tri
(6) Dow Corning, OS-10
(7) CRC, QD Contact Cleaner 02180
(8) Ethyl lactate (flash point 47 degrees C)
(9) Fine Organics, FO 655
(10) Sprayon Products, 749 N.O.D. Cleaner/Degreaser
(11) Borothene (works extremely well for vapor degreasing)
(12) N-methyl-pyrrolidone (biodegradable, capable vapor degreaser)
(13) Miller-Stephenson, MS-990 Solvent Flux Remover

Ongoing Air Force Efforts to Find Viable Substitute Cleaners for CFC-113 for LOX/GOX/LIN System Components

The Air Force is coordinating their replacement cleaner efforts with NASA and the Navy. The Air Force is currently considering the following CFC-113 replacement cleaners:

1. Navy Oxygen Cleaner is an aqueous mixture containing surfactants that are recirculated during the cleaning process. The Navy Oxygen Cleaner is good for use on removable parts that can be cleaned in a shop location. There are many rinsing and drying steps to the cleaning process using the Navy Oxygen Cleaner, with many checks. Because of this, use of the Navy Oxygen Cleaner is time consuming.
2. HCFC 141b, dichlorofluoroethane (C₂H₃FCl₂), has an ozone depletion potential of 0.12, and is banned for production and consumption effective 1 January 2003 and thus could be used as a temporary measure. There are two versions of HCFC 141b under consideration by the Air Force. The first is pure HCFC 141b, and the second is an aerosol recipe containing 5% isopropyl alcohol (IPA). HCFC 141b with 5% IPA has been used when slow drying times are required.
3. HFE compounds (hydrofluoroethers) are manufactured by 3M. These compounds are not ozone depleting substances and have been found initially to work well for cleaning LOX/GOX/LIN system components. However the current cost of HFE compounds for cleaning purposes is rather high (~$180/gallon).

Demonstration of cleaning efficiency for these potential replacement cleaners is being carried out by NASA at the White Sands Testing Facility (WSTF) in Las Cruces, NM, by Dr. Harold Beeson. In addition, the Naval Sea Systems Command is also carrying out demonstration testing of these and other replacement cleaners in Arlington, VA. The lead at Naval Sea Systems Command is Neal Antin (NAVSEA/03Y2A), (703) 602-5552, x205. According to John King, Air Force SA-ALC, if the replacement non-ODS cleaners for LOX/GOX/LIN system components are demonstrated as acceptable for NASA and Naval Sea Systems Command submarine applications, they will be acceptable for Air Force base requirements.

Other Efforts to Find Viable Substitute Cleaners for CFC-113 for LOX/GOX/LIN System Components

There are two projects ongoing at Wright Laboratory that may provide additional data and solutions for CFC-113 replacement cleaners for LOX/GOX/LIN system components:

1. Development of a CFC-free oxygen plasma cleaning process for aircraft oxygen systems. POC: P. Mykytiuk, WL/MLSE, (937) 255-3953.
2. Development and demonstration of use of laser technology for cleaning oils and other contaminants from oxygen lines of aircraft and ground support equipment without the use of ozone depleting chemicals.
   POC: R. Hull, WL/MLPJ, (513) 255-3898, x 3165.

Background: Alternatives to P-D-680

The military specification for P-D-680 is held by the Army Tank-Automotive and Armaments Command. Information on commercially available environmentally compliant solvent alternatives to P-D-680 can be found in U.S. Army Technical Advisory Message 92, "Substitutes P-D-680," 5 November 1996. The message lists several alternatives that meet P-D-680 performance parameters. These alternatives consist of highly refined aliphatic hydrocarbon compounds. The specification for P-D-680 is being revised to allow use of more environmentally compliant compounds. However, there is no aqueous based cleaner that meets the P-D-680 specification, essentially by definition. Therefore, to get approval for a water based cleaning process approval would be needed from the owner of the document that called out P-D-680.

P2 Option: Cyclonic Filter System and Cross Flow Filter System

P2 Option: Microbial Water-Based Cleaning

P2 Option: Aqueous Parts Washer

Potential Problems of Aqueous Cleaning

1. Depending on the materials being cleaned and the cleaning methods used there may be potential for corrosion of parts.
2. Aqueous cleaners have not been approved for all applications nor for all Services. See Appendix B on approval authority.

P2 Option: Replace P-D-680 Type II with P-D-680 Type III

P2 Option: Solvent Substitution

Many bases throughout the Air Force have begun to use alternatives to P-D-680, Type II. Wright Laboratory has put together a table of these alternatives with pros and cons. A more comprehensive list of substitutes is available in the DLA Product Guide, which is accessible through the DLA web site (see http://128.174.5.51/denix/DOD/Library/Air/Airmgt/aqch4.html#Appendix C) and the Joint Service P2 Technical Library (see http://128.174.5.51/denix/DOD/Library/Air/Airmgt/aqch4.html#Appendix F).

Table 1.4.1: Possible Substitutes for P-D-680 Type II and Their Properties

P2 Option: Solvent Substitution

A wide variety of substitute solvents are commercially available that have a low vapor pressure, which means less of the solvent evaporates and pollutes the atmosphere. Table 1.5.1 lists several common substitutes. A more comprehensive list of substitutes is available in the DLA Product Guide, which is accessible through the DLA web site and the P2 Technical Library (see Appendix F). They have varying capabilities to clean metal components. The choice of solvent depends on the specific application. The effectiveness of a particular solvent at removing targeted contaminates must be evaluated on test parts, then qualified for use.

One disadvantage of solvents with a low vapor pressure is that more time must be allowed for residual solvent to evaporate after cleaning is completed. This delays further processing of the component. Another disadvantage is that several of the replacement solvents have a noxious odor, even in low concentrations, so adequate ventilation is essential.

Table 1.5.1: Possible Substitute Solvents for Cleaning

Background: Mechanical Cleaning

Abrasive Blasting

Abrasive blasting is an alternative to solvents for cleaning. In the blasting process, particulate media is propelled by compressed gases or a liquid to impinge on the contaminated surface. No toxic or hazardous chemicals are used; however, the blasting media can become contaminated with the material being blasted from the surface. There are several different types of blasting media, some multi-purpose and others single purpose. Several types of blasting media are described below:

Mineral Grit/Sand Blasting

Mineral grit and sand are effective blasting media because of their hard, abrasive qualities. However, the media and cleaning residue become mixed and difficult to separate. If the cleaning residue mixture is hazardous, it can be costly to properly dispose of the media/residue mixture.

Steel Shot/Grit

Steel shot is similar to bird shot. It is rough on a substrate but not nearly as rough as sand. Shot and grit can be mixed to achieve the desired performance. Advantages are that it can be magnetically separated from the residue and reused, and it does not create a hazardous dust. Wastes are kept to a minimum. Steel shot stripping is best done in an enclosure/glovebox or with sturdy, full body personal protective equipment.

Plastic Media

Plastic media is relatively easy on substrates and can be reused. The media can be tailored to a range of applications by using plastic beads of varying size and hardness. Systems are relatively inexpensive.

Plastic Foam

Oils, greases, dirt, and even paint can be removed from parts by blasting with small bits of urethane foam. If the soil is moist (i.e. oil and grease), the foam bits will absorb it. The foam can then be washed, dried and reused. Waste streams are similar to a parts washer with the advantage that the part itself does not get wet. For paint or scale removal, the foam can be wet to reduce dust generation.

P2 Option: Ministeam Cleaners

Background: Aqueous Parts Washers

Aqueous, hot water parts washers use a combination of hot water and detergent to remove contaminants from parts. Most systems separate oil and solids from the cleaning solution that allows a batch of water and detergent to be used repeatedly before becoming too soiled to be effective. Wastes from this cleaning process include the spent detergent solution, oil, and solids/sludge. The quantity of waste is typically much less than the waste generated from solvent cleaning operations. One disadvantage to water-based parts washing is an increased potential for rust formation on the parts being cleaned. However, there are many detergent formulations available that include rust inhibitors to minimize this problem.

P2 Option: Aqueous Parts Washers

Background: Widespread use of ODSs for electronics cleaning is now being replaced by a variety of materials, such as IPA, or processes, such as is described below.

P2 Option: Integrated Closed Loop Cleaning

P2 Option: Benzyl Alcohol

P2 Option: Plastic Media Blasting

P2 Option: PMB or Benzyl Alcohol

P2 Option: Ultra High Pressure Water Jet Paint Removal

P2 Option: Pressure Water Stripping and Media Blasting Techniques

Wright Laboratory has several paint removal programs including: LARPS; alternate chemical paint strippers; biodegradable plastic media; gel-encapsulated enzyme activated coatings removal; "smart" stripping processes; and next generation energetic stripping. "Smart" stripping is the ability to strip with a vision system that can adjust the stripping media to ensure a good job. The speed of the water gun can be reduced to get a deeper strip or increased to reduce wear on the substrate if the paint is peeling fast. Also, "smart" stripping can select between coatings. It has the ability to remove the topcoat, but leave the primer coat on the substrate.

P2 Option: Sodium Bicarbonate Blasting

Background: Alternative Solvents

Methylene chloride, a suspected carcinogen, is commonly used for stripping of organic coatings. The CAA90 identified methylene chloride as a HAP and as such, it is subject to regulation under the Aerospace NESHAP. The Aerospace NESHAP requires elimination of organic HAP emissions from depainting operations by September 1998.

Over the past several years, various companies have developed paint strippers to replace methylene chloride. The majority of these strippers contain benzyl alcohol. Both alkaline/amine and acid activated strippers have been formulated. The acid activated strippers have been successfully used in the commercial sector. However, these strippers are not considered to be acceptable for military applications because of their potential to induce hydrogen embrittlement in high strength steel.

Several benzyl alcohol based strippers have been used for military applications, with varying degrees of success. Examples of these strippers are Turco 6813 (polyurethane), McGean Rohco E1058 (polysufide), Eldorado SR-145 (polyurethane/polysufide), and Eldorado PR-3133 (polyurethane). Many other products have been tested and used for military applications. In formulating environmentally preferred strippers, a key point that must be considered is the stripper’s ability to attack the various types of coating systems that are present on military aircraft. A fair degree of success has been achieved with epoxy primers, polysulfide coatings, and polyurethane topcoats.

A more difficult problem has surfaced when stripping aged Koroflex primer (TT-P-2760). The Koroflex primer is a polyurethane coating that is applied to larger aircraft (e.g. transport aircraft) to improve the flexibility of the coating system, thereby increasing its resistance to cracking and adhesion loss due to flexing of the structure during flight. The Koroflex primer also provides a barrier, thus lending an increased level of protection to the substrate. In many cases, the Koroflex primer is applied directly to the metal and overcoated with MIL-C-85285 or MIL-C-83286 topcoat. This coating system has been found to be extremely difficult to remove. Methylene chloride is marginally effective on Koroflex primer, while the environmentally preferred strippers are even less effective on Koroflex. Because many of the weapon systems in the DoD inventory use Koroflex primer, there is a need to develop environmentally preferred strippers that can strip the primer in a reasonable period of time.

In 1993, Grumman St. Augustine Corporation (GSAC) initiated a pollution prevention project to replace the methylene chloride based paint stripper that is currently used at the site. A total of eighteen different paint strippers were evaluated. Testing was conducted in the laboratory and field to ensure performance in an operational environment. Testing included: coating removal rates, sandwich corrosion, intergranular attack/end grain pitting and hydrogen embrittlement. Stripping efficiency was evaluated on several different coating schemes: Epoxy Primer (MIL-P-23377TY1CL3) + Polyurethane Topcoat (MIL-C-83286), Epoxy Primer + Polysulfide (MIL-S-81733) + Polyurethane Topcoat, Epoxy Primer + Koroflex (TT-P-2760TY1CL2) + Polyurethane Topcoat.

A summary of the paint stripper evaluation results is presented in Table (1). Of the eighteen strippers tested, several of the strippers met the GSAC requirements. These include McGean Rohco E1058, Turco 6813, Eldorado SR-145 and Eldorado PR-3133.

Table 2.1.1: Environmentally Preferred Paint Stripper Evaluation Results

Stripping Efficiency

Stripping efficiency was a somewhat qualitative test with respect to stripping times. Of the strippers that passed the corrosion tests, the SR-145 was the top performer. This stripper was found to be effective on both polyurethane and polysulfide coatings in relatively cold weather. Eldorado PR-3133 was very effective on polyurethane and less effective on polysulfide coatings. The PR-3133 can be used to strip Koroflex primer.

The McGean Rohco E1058 was formulated specifically for polysulfide coatings and, as such, is effective on polysulfide/polyurethane coating systems. However, in cases where coating thickness’ are excessive (> 12 mils), the polyurethane must be stripped before the E1058 can effectively remove the polysulfide. Turco 6813 tends to be more temperature sensitive than the Eldorado SR-145 and PR-3133. However, this stripper will effectively strip epoxy/polyurethane coatings at 70 ° F. Additionally, it was found to effectively strip Koroflex primer with somewhat decreased strip rates. Strip times vary according to temperature, type of coatings, overall coating thickness, and age of coatings. In general, the benzyl alcohol based strippers require significantly longer dwell times compared to methylene chloride based strippers. A summary of the recommended stripping guidelines is depicted in Table 2.1.2. The stripping efficiency data clearly indicates that Koroflex with a polyurethane topcoat is extremely difficult to remove. Using the Turco 6813, this coating took four applications of the stripper and 22 hours to strip. Note that the Koroflex strips much more readily when an epoxy primer is used between the base metal and Koroflex.

Hydrogen Embrittlement

Hydrogen embrittlement test results were extremely inconsistent. As previously mentioned, a number of different test methods were used. This test was rerun several times in an effort to obtain consistent data. Of the test methods used, the ASTM F519 1(a) loaded at 75% notch ultimate tensile strength (UTS) and ASTM F519 1(d) loaded at 65% of the predetermined breaking strength are considered to be the most stringent. In general, if a stripper passed either of these tests at least once, it was considered to be an overall passing result. Two of the strippers tested met this criteria: Turco 6813 and McGean Rohco E1058.

SR-145, PR-3133, A-29SCW and Turco 6017 were evaluated using ASTM F519 1(a) at a sustained load of 45% notched tensile strength and step loaded to 90% after 200 hours. All of the specimens exposed to the Turco 6017 failed under 150 hours. The specimens exposed to the Eldorado SR-145, Eldorado PR-3133 and CEE BEE A-29SCW reached the 200 hour mark and failed at various times under the 90% loading (4.8 to 44.9 hours). The failed specimens were then metallurgically analyzed to determine the degree of embrittlement within the specimen. The ranking from best to worst was SR-145 (minimal), PR-3133 (minimal), A-29SCW (slightly higher), and Turco 6017 (significant). The test criteria used here is consistent with the Air Force hydrogen embrittlement test often used for qualification of maintenance chemicals. Therefore, the SR-145 and PR-3133 would be considered acceptable for some programs. Northrop Grumman is presently rerunning the hydrogen embrittlement test using ASTM F519 1(a) loaded at 75% UTS.

Table 2.1.2: Stripping Guidelines for Turco 6813 and McGean Rohco E1058

*Topcoat and Primer previously stripped with Turco 6813.

Background: Mechanical Stripping/Abrasive Blasting

Abrasive blasting is an alternative to solvents for depainting. In the blasting process, particulate media is propelled by compressed gases or a liquid to impinge on the contaminated surface. No toxic or hazardous chemicals are used; however, the blasting media can become contaminated with the material being blasted from the surface. There are several different types of blasting media, some multi-purpose and others single purpose. Some of the various types of blasting media are described below:

Mineral Grit/Sand Blasting

Mineral grit and sand are effective blasting media because of their hard, abrasive qualities. However, the media and cleaning residue become mixed and difficult to separate. If the cleaning residue mixture is hazardous, it can be costly to properly dispose of the media/residue mixture.

Steel Shot/Grit

Steel shot is similar to bird shot. It is rough on a substrate but not nearly as rough as sand. Shot and grit can be mixed to achieve the desired performance. Advantages are that it can be magnetically separated from the residue and reused, and it does not create a hazardous dust. Wastes are kept to a minimum. Steel shot stripping is best done in an enclosure/glovebox or with sturdy, full body personal protective equipment.

Plastic Media

Plastic media is popular for paint stripping because it is relatively easy on substrates and can be reused. The media can be tailored to a range of paint stripping applications by using plastic beads of varying size and hardness. Systems are relatively inexpensive.

Plastic Foam

Oils, greases, dirt, even paint can be removed from parts by blasting with small bits of urethane foam. If the soil is moist (i.e. oil and grease), the foam bits will absorb it. The foam can then be washed, dried and reused. Waste streams are similar to a parts washer with the advantage that the part itself does not get wet. For paint or scale removal, the foam can be wet to reduce dust generation.

Dry Ice (CO₂)

Developed for aircraft paint stripping, this system has some interesting characteristics. First, there is no excess waste generated. The CO₂ pellets sublime after contact with the part, dissipating as CO₂ gas; paint and soil falls to the ground. Second, the sublimation of pellets on impact helps to lift paint from the surrounding substrate. First generation systems were slow but recent modifications have improved the speed. Once optimized, paint stripping proceeds at 1 sq ft/min.

A process has been developed by McDonnell-Douglas Aerospace, Cold Jet Inc., and Maxwell Laboratories Inc., that combines CO₂ pellets with a flashlamp. The flashlamp provides heat to destroy a coating's cohesive bonds; the pellets cool and clean the surface. The process has been successfully applied to aluminum and composite materials used in aircraft.

The limitation of large-scale CO₂ blasting is high cost. A capital investment of $170,000 to $250,000 is necessary for the pelletizer. More sophisticated models may cost upwards of $400,000. These costs do not include expenses for auxiliary equipment or installation.

Wheat Starch

This relatively inexpensive media was also developed for aircraft paint stripping. The crystalline wheat starch used in this system can be reused and actually becomes more aggressive as it breaks down. Wheat starch is very forgiving of error and is easy on substrates. Crystalline wheat starch molecules are different from the naturally occurring polymer so explosion potential is not a problem.

Walnut Shells and Other Food By-Products

These unusual media are used, among other things, to clean carbon deposits from engine parts. Ground walnut shells are gentle on the substrate and inexpensive. Peanut shells and ground corn cobs are also used as blast media.

Bicarbonate of Sodium Stripping (BOSS)

BOSS eliminates the toxic stripping solvents such as toluene, xylene, MEK, and acetone from the stripping process. One disadvantage is grit entrapment behind panels and in cracks that contributes to corrosion, particularly of aluminum structures. Even when no corrosion is yet present, the entrapped powder gives a false impression of corrosion. This can be disturbing to system users. The media is not recyclable and disposal (usually into the sewer) requires large amounts of water. Care must be taken to keep contaminants (e.g. heavy metals) out of the waste stream.

Vacuum Blasting

This is actually a blast system that can use a variety of media such as some of those described above. A special vacuum head is held against the substrate. The media is accelerated to the head using a vacuum instead of compressed air. After impact the vacuum pulls the media back to the holding bin where it is continuously reused. This reduces environmental impact and worker exposure to hazardous substances and allows other nearby operations to continue uninterrupted.

Needle Guns (Chipping)

This system uses multiple, reciprocating needles, to pummel paint, dirt and scale from surfaces. The needles are contained in a vacuum head that is held up against the part during cleaning. The vacuum system captures the removed soil, thus eliminating worker and environmental exposure. A major advantage is that no excess wastes are generated. The technique is popular in applications such as removing lead-based paint from bridges.

Brush Removal

3M company developed a machine with rotating brushes that mechanically cleans copper metal sheets with pumice. The machine replaced an existing chemical surface preparation process that used sequential spraying with ammonium persulphate, phosphoric acid and sulfuric acid. The chemical process generated 40,000 lbs/yr of hazardous liquid waste. With the mechanical cleaning machine, the fine abrasive pumice generates a nonhazardous sludge that can be sent to a conventional landfill. POC: Mike Koeningsberger, (612) 778-4523.

The following tables compare depainting technologies and are excerpts from a study conducted by Acurex Environmental Corporation and US Air Force Armstrong Laboratory. The study was funded by the US Air Force Armstrong Laboratory under contract number F08637-95-D6003, Task Order D5303. This information is an average of data collected from several airframes at each of the following Air Logistic Centers: Tinker AFB, Hill AFB, McClellan AFB, Kelly AFB, Robins AFB, and Wright-Patterson AFB.

Table 2.1.3: Relative Life-Cycle Cost:
Normalized Operating Costs ($/ft²), Production Rates (ft²/hr), and Capital Conversion Costs ($)

Note that these were results from certain airframes at particular locations. Costs vary widely between weapon systems. For example, the cost for wheat starch and plastic media blasting on some weapon systems has been reported at ten times the cost presented in table 2.1.3. Therefore it is important to obtain information which corresponds to a particular weapon system. Costs may also vary to some extent between sites. Therefore, when looking at costs from another site, even though it is your type of weapon system, determine if location specific costs will be different. Also, when comparing calculated numbers, such as cost per square foot, from different sources, determine what factors went into the calculations.

Table 2.1.4: Stripping Technology Comparison

One alternative to stripping and reapplication of chromated primer is to consider implementation of a coating system that does not require stripping of the coating to the metal surface. For instance, the weapon system manufacturer would be required to spray the corrosion inhibiting primer (chromium, or non-chromium) followed by a coating system that is designed to be stripped. The coating system would be an intermediate coat that is readily attacked by a specific type of solvent (such as alcohols) and a polyurethane topcoat. In most cases, the rework activity would then be limited to stripping and reapplication of the nonchromated coatings. This would eliminate the chromium in the paint stripper residue and waste associated with painting operations in addition to airborne emissions of hexavalent chromium.

The rationale behind removal of organic coatings has been to expose the bare metal in order to facilitate evaluation of the health of the metal substrate. As an alternate, Thiokol has developed a process that will allow for non-destructive inspection (NDI) of critical structures without removal of the coating. The technique that has been developed will comprehensively evaluate the residual strength of critical structures and determine the location of cracks and/or corrosion. This will further allow for the continued monitoring of the health of the vehicle by detecting changes such as crack growth, corrosion depth and microstructural changes.

The NDI technique involves the use of two technologies which are sensitive to the condition of metallic structures: eddy current measurements and ultrasonic sensing. These evaluation techniques can be computer automated for data acquisition, analysis and archiving. Ultrasound, which is sensitive to discontinuities in elastic structures, and electromagnetic eddy-currents, which are sensitive to the conductivity or magnetic permeability in the local of the probe, represent complementary means of detecting anomalous conditions in metals. The sensing probes used in both techniques can be tailored for specific structure and defect geometries.

P2 Option: FLASHJET^®

The Army is preparing to test the FLASHJET^® over the next two years at the Mesa, AZ facility. In the first year the system will be tried on helicopters and components, and in the second year on tanks and other ground equipment. How the FLASHJET^® will perform on the irregular surfaces of many of the Army vehicles will be investigated.

Appliqués are thin polymer films backed by a pressure-sensitive adhesive that can be peeled off in sheets, eliminating the need for abrasive blasting, chemical coating removal, or other techniques. The Boeing Company and 3M have been demonstrating the feasibility of replacing most, if not all painting of an aircraft with this technology. The work is being performed as part of a Technology Maturation contract from the Joint Strike Fighter (JSF) Paintless Airplane Program (JPAP). This technology involves films backed by adhesives.

Tinker AFB initiated a project to develop a faster acting stripper for Koroflex primer. The project is a collaborative effort between the US EPA, the US Air Force, and industry. The project was completed in 1996 and a production version of the alternate stripper(s) will be available in 1998.

The first phase of the project involved identifying potential candidate strippers for Koroflex. During this phase of the program, the chemical mechanism for degradation of the polymeric bonds was used as a basis for identifying alternative solvents. Six different types of solvents systems were identified as potential replacements for methylene chloride in paint stripping: benzyl alcohol, n-methyl-2-pyrrolidinone, dibasic ester mixtures, propylene carbonate, n-amyl acetate, and methyl isoamyl ketone. Several commercially available strippers were identified. Thirteen strippers were selected as candidate for further evaluation.

The evaluation phase of the project is currently underway. Each candidate stripper will be evaluated based on several performance parameters. These include: appearance, biodegradability, toxicity, pH, evaporation, viscosity, flash point, cold stability, consistency, flow, removal power/rinsability, corrosion, hydrogen embrittlement, storage stability, and density. Once this testing is complete, the Air Force will select three stripper candidates that will undergo full scale testing in field operations. After completion of this phase, the strippers will be evaluated for effectiveness in removing other coatings such as epoxy primers (BMS 10-11, MIL-P-23377) or self priming topcoats (TT-P-2760).