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LOW TOXICITY PAINT STRIPPING OF ALUMINUM AND COMPOSITE SUBSTRATES Nona Larson Boeing Aerospace Seattle, Washington INTRODUCTION Part 1 The effective chemical paint strippers for aerospace coatings are toxic and present a hazard to both personnel and the environment. They contain phenol, methylene chloride and hexavalent chromium materials which have been targeted by governmental regulations for future elimination. mese strippers are also detrimental to composites. This report focuses on impact damage inflicted on composite substrates by some of the mechanical methods meant to replace chemicals. OBJECTIVE The objective of this program is to find benign altemates to hazardous chemical paint strippers. APPROACH The approach with the least impact on production was taken for this project; i.e., identify and evaluate commercial chemical strippers which did not contain the targeted materials; if none were available, develop a chemical stripper; and finally evaluate non- chemical methods. To accomplish this work, the following three tasks were identified: Task 1: Evaluate Chemical Strippers Identify coatings and substrates to be evaluated; contact commercial sources of strippers; evaluate strippers based upon their ability to strip the coating, length of time it takes, and damage sustained by the substrate. If necessary, blend Boeing proprietary formulas. Task 2: Evaluate Non-Chemical Stripping Methods Investigate the mechanical, radiation and other stripping methods being tested throughout industry and evaluate only those which appear to offer solutions to aerospace problems. Task 3: Specification Coverage Change Boeing specifications for abrasive blasting to include effective and non-damaging paint removal methods. This will include Process Specification Departures for interim solutions and specific applications. CHEMICAL STRIPPER ALTERNATIVES Conclusions from Boeing Document D180- 30690-4, Comprehensive Chemical Reduction Research Projects Final Report 1989, summarize the work from Tasks 1 and 2. No low toxicity chemical paint strippers were I found in this program which can be recommended to replace those presently used; however, the four strippers: Brulin EXP 2187 mod., ManGill LP 4566, and Turco 6088 may be used effectively on selected coatings. For example, EXP 2187 mod. is extremely effective on removing melamine enamel and military epoxy primer; Turco 6088 is effective in removing phenolic resin varnish. The mechanical methods appear to be the best overall approach to removing the high performance aerospace and commercial coatings. Results are shown in Tables 1 and 2. Recommendationsmade in D180-30690-4 state that we should monitor (rather than duplicate) data on alternate stripped methods, such as plastic media blasting and laser paint removal. 53
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LOW TOXICITY PAINT STRIPPING OF ALUMINUM AND COMPOSITE SUBSTRATES
Nona Larson Boeing Aerospace
INTRODUCTION Part 1
The effective chemical paint strippers for aerospace coatings are toxic and present a hazard to both personnel and the environment. They contain phenol, methylene chloride and hexavalent chromium materials which have been targeted by governmental regulations for future elimination. mese strippers are also detrimental to composites. This report focuses on impact damage inflicted on composite substrates by some of the mechanical methods meant to replace chemicals.
The objective of this program is to find benign altemates to hazardous chemical paint strippers.
The approach with the least impact on production was taken for this project; i.e., identify and evaluate commercial chemical strippers which did not contain the targeted materials; if none were available, develop a chemical stripper; and finally evaluate non- chemical methods. To accomplish this work, the following three tasks were identified:
Task 1: Evaluate Chemical Strippers
Identify coatings and substrates to be evaluated; contact commercial sources of strippers; evaluate strippers based upon their ability to strip the coating, length of time it takes, and damage sustained by the substrate. If necessary, blend Boeing proprietary formulas.
Task 2: Evaluate Non-Chemical Stripping Methods
Investigate the mechanical, radiation and other stripping methods being tested throughout industry and evaluate only those which appear to offer solutions to aerospace problems.
Task 3: Specification Coverage
Change Boeing specifications for abrasive blasting to include effective and non-damaging paint removal methods. This will include Process Specification Departures for interim solutions and specific applications.
CHEMICAL STRIPPER ALTERNATIVES
Conclusions from Boeing Document D180- 30690-4, Comprehensive Chemical Reduction Research Projects Final Report 1989, summarize the work from Tasks 1 and 2. No low toxicity chemical paint strippers were I
found in this program which can be recommended to replace those presently used; however, the four strippers: Brulin EXP 2187 mod., ManGill LP 4566, and Turco 6088 may be used effectively on selected coatings. For example, EXP 2187 mod. is extremely effective on removing melamine enamel and military epoxy primer; Turco 6088 is effective in removing phenolic resin varnish. The mechanical methods appear to be the best overall approach to removing the high performance aerospace and commercial coatings. Results are shown in Tables 1 and 2.
Recommendations made in D180-30690-4 state that we should monitor (rather than duplicate) data on alternate stripped methods, such as plastic media blasting and laser paint removal.
For 1990 this program was to continue on a monitoring basis. Processes included were: plastic media, sodium bicarbonate, and carbon dioxide blasting, high pressure water jet, xenon lamp and laser paint removal. Envirostrip wheat starch media was not available in 1989.
Wheat Starch Media
In April 1990, a new paint stripped media was introduced'. This media, made of 100% crystallized wheat starch, is non-toxic, biodegradable in the true sense of the word, and made out of a renewable resource (as opposed to petroleum based plastics). Since this product was new, independent research for us to monitor, we started our own evaluation program. The first step in the study of this new process was to send samples to the vendor (Ogilvie Mills). When they came back, these samples looked good enough to pursue this process more actively than program planning had anticipated. Media characteristics and results are summarized in Table 3.
The coatings we tested were chosen because they were the most difficult to remove of the military and commercial airplane specification coatings. Some of these were brought to us on scrapped or test parts as a challenge because the owners did not believe that the wheat starch media would remove the coating. Table 4 lists the coatings removed, and Table 5 lists the effects.
Wheat starch blasting caused the least damage of any other blasting method we tested directly. Comparisons were made by blasting identical panels (composites laid up at the same time by the same person), or by blasting different parts of a single large panel. Figures 1 to 18 show these comparisons. These cross-
sections were photographed at 200x.
No damage was detected on 0.020-inch thick aluminum, with the exception of some deformation of the clad surface. Figures 19 and 20 show results of stripping the thin aluminum panels. Sandwich corrosion tests were performed per D6-17487, Certification Testing of Aircraft Maintenance Materials. As expected, the media passed both in the crystalline state and dissolved in deionized water.
Discussion of Envirostrip Wheat Starch Blasting
It is not difficult to explain the differences in results obtained with this and other blasting media. Figures 21 & 22 show the fracture surfaces of new and used media. A particle hitting the paint surface at a pressure above approximately 30 psi will break. The glassy fractures shown in the figures explain why the media remains effective cycle after cycle. It becomes more effective as the particles get smaller simply because there are more sharp edges per pound of media in the blast stream. The limiting factor for size of this media is drag. When the particles break down enough that dust hangs in the air, the operator cannot see through it. A dust separation system easily removes this.
The fact that the particle breaks at around 30 psi makes the media very forgiving. Turning up the pressure will increase the flow rate, but will not make the media itself more aggressive.
One common concern when using "wheat" in a blasting system is the so called "silo effect," where a high concentration of wheat dust will spontaneously combust. This is not expected to be a problem because crystallized starch molecules are different in structure than the naturally occurring polymer. Nevertheless,
we asked the vendor to supply data on the explosive limits of the dust. The following table summarizes the finds.
Envirostrip Properties: Explosive Limits
Media Size Ignition Min. Explosive Mesh (US Std) Temperature Concentration
There are many i9teresting and useful properties to this media which we discovered during our study.
"C oz per @
Envirostrip does not remove alodine. The aloding remains after depainting passes 7day salt spray exposure, but it is dehydrated so paint adhesion suffers. This can be remedied by a quick dip in an alodine tank. Ten seconds should be sufficient.
New media is less aggressive than used media. When stripping Kevlar, new media gives the operator more control than the used media.
c. Large media works better on the elastomeric coatings, while small media works better on the more brittle coatings, Le., epoxy primers and topcoats.
The media works best by breaking through the paint surface and peeling the paint up from the edges of the stripped area.
Some of the harder coatings @e., BMS 10-11 type 1 epoxy primer) will break the media down faster than others. This is not true of Mil-P- 23377.
DiscussionlResults of Other Methods
There are a number of paint stripping methods currently commercially available. Each of these has good and bad points. The information listed herein has been gathered from a variety of sources in addition to the data generated by this project.
Xenon Flashlamp &painting
This method does not lend itself well to production use, Coated surfaces are exposed to high intensity pulses of light. A special head to focus the light must be used for each different part configuration. The three primary drawbacks are that it is not very effective on light colored coatings, it treats composite substrates the same as paints, and acutely toxic gases are released when polyurethane paints are broken down without adequate oxygen flow. An oil smut is left behind the flashlamp, so a cleaning step such as carbon dioxide pellet blasting is required. Dark, low-gloss topcoats (Le., MILC-83286) can be removed at rates up to one square foot per minute. Light colors are removed much more slowly, and high gloss white is not efficiently removed. No damage to metal substrates has been found with this process; the metallic surface completely reflects the xenon flash. Composite surfaces do not reflect the flash, and are therefore removed in layers analogous to paint.
This method is similar to the Xenon flashlamp method, but is more easily controlled. Laser depainting is geared toward robotic applications where capital cost is high, but some applications justify the cost. Optimization is difficult due to the uncontrollable variations in paint thickness. Light colored and high-gloss topcoats are removed less efficiently than darker coatings, but they CM be removed in a reasonable amount of time. Two companies are in the process of making this method commercially available.
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Sodium Bicarbonate Blasting
This method, more than any other mentioned in this report, is a case of trading off pros and cons. The media is very effective without causing much impact damage to the substrate. Since rinsing is very difficult, there is a high potential for corrosion problems. The sodium bicarbonate breaks down in water, forming sodium sesquicarbonate, which has a pH of approximately 10. Leaving this on aluminum pacts will be very detrimental to the part. The usual solution to this problem is to use a dilute acid rinse. Small parts are well suited to this, but large pacts or stmclures are not. The dilute acid rinse also creates more hazardous waste, driving up the cost of the total paint removal process.
The media itself is inexpensive at about $0.50 per pound. Unfortunately, it is not recyclable. Enormous amounts of water are required to dissolve the spent media before sewering. To keep the dust down, it is blasted with water which contributes to the corrosion potential. The positive aspect of non-recyclable media is that a dedicated facility is not required.
Carbon Dioxide Pellet Blasting
COz pellet blasting has been the subject of an enormous amount of testing. This process is ideal in the sense that no toxic substances are generated or released, a dedicated facility is not required, and precleaning or surface preparation of the painted surface is not needed. However, there are some very serious areas of concern with this process. Some of which limit the use of C q blasting to steel or very thick aluminum pacts. Very high pressures ace used to accelerate the particles. The particles impact the surface at extremely high velocities, high enough to leave dents in 0.020 inch thick aluminum.
There are two systems commercially available to produce COz pellets for blasting. The more widely used system extrudes the pellets, producing small cyclinders. They are not optimally shaped for removing paint, and
unfortunately the optimization of the system that has been performed to date has consisted of changing pressure, impingement angle and stand-off distance. There has been some planning to optimize the process from the front end, beginning with producing pellets with different shapes, sizes, andlor hardness. This new approach to optimization should increase the strip rates. Previous rates have been slower than acceptable for production operations.
Plastic Media Blasting
Plastic media blasting has been studied in great detail. There are a number of complete programs, both military and commercial, which report widely varying results (Ref. 2 to 12). The process is partly accepted (one cycle only) by the FAA, and it is widely used by the military (Ref. 13 &. 14). There are seven types of media, varying from soft to hard and mild to aggressive. The soft media requires a longer dwell time, so it does not necessarily cause less damage than the harder types. Plastic media is quite aggressive on composites, causing an unacceptable amount of erosion andlor fiber damage, with the exceptions of graphite or boron epoxy.
Ice Crystal Blasting
The Canadian Navy funded a program to devise a way to remove coatings from the interiors of submarines. This method must be safe in a confined environment with minimal air flow. This method is very effective on the interior coatings it was designed to remove. The company which invented the process is now optimizing to remove aerospace and other high performance exterior coatings. Progress is being made in this area. The coatings can be removed with very little damage to any substrate tested, including composites and thin aluminum. This process will be watched closely during this program, In addition to being effective, it has the attractive properties of requiring no precleaning- of the painted surface, minimizes the waste generated, does not require a dedicated facility, and is
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completely non-hazardous to personnel.
Two primacy concerns have been repeatedly expressed about any blasting method for paint removal of structures: crack closure and media entrapment.
Crack closure has been a concern ever since the early days of plastic media blasting. Research has shown (15) that different plastic media types do not close cracks, but the media can become lodged in the crack and prevent it from showing up during dye penetrant inspection. This is more likely to happen with plastic media than wiq other types that are water soluble.
Media entrapment is of serious concern to some, while others do not consider it a major problem. Any accelerated particles will f i d ways to enter openings. This can add weight to an aircraft and interfere with moving parts. Ideally, these areas would all be masked before stripping, but in reality, this is not always done properly. The other side of this issue was expressed very well by a shop foreman (16) who said, "I can live with that," comparing media entrapment to residual chemical paint stripper that was oozing out of faying surfaces on the repainted airplane parked behind him.
During 1990 this program investigated a number of mechanical paint stripping methods. Most of these methods were evaluated by monitoring research done by others. Due to the newness of the Ogilvie Mills Envirostrip media, there was no outside work to monitor. Therefore, we did our own evaluation. Overall, this media proved to be the best currently available technology. Clearly there is no panacea, but this method is more widely applicable than any other. Recommendations for using this method on aircraft are awaiting fatigue testing, which the Boeing Commercial
Airplane Group plans to do in 1991.
On the other hand, chemical strippers will remain a problem, especially when pH is a consideration: There is simply no low toxicity, low vapor pressure analogy for methylene chloride and phenol. When tank stripping is possible, the N-methyl-2- pyrrolidone based formulations can be used at elevated temperatures. However, these cases can usually be mechanically stripped, making preferential the wheat starch media. When pH is not a factor, concentrated, low molecular weight organic acids will work, but they are relatively slow and unpleasant to use. Therefore, it can be concluded that a highly innovative approach will be required for developing a good neutral pH, room temperature and chemical aerospace coating stripper.
FLUIDIZED BED ABSORBENT CLEANING
After demonstrating the feasibility of non- solvent substitutes during 1989, the objective in 1990 was to develop methods for their use that were suitable for industrial scale-up. The most promising materials at that time were absorbents such as starch and uncalcined diatomaceous earth. This could lead to the conclusion that very fine particulate matter is best, but developments in 1989 dealt primarily with substitution materials usable in the same approximate manner as a wipe solvent. While it can be generalized that a finer particle provides more surface area per volume, density and other properties become major considerations with other delivery systems. When fluidized bed technology was investigated as a possible means to scale-up and automate absorbent cleaning, the absorbent media had to be completely re- evaluated.
While it was theorized that the particles would aggregate with oil absorption and gain
sufficient bulk and density to drop off the test panels, in reality the adhesion of the Zyglo test oil to the panel was the dominant force in the system. Aggregated particle mass never reached the point where it could exert the tensile and sheer stresses necessary to overcome the oil-panel bond. This precipitated a search for denser, larger absorbent materials which were also sewerable. Flours, cornmeal, and finally grain cereals were tried. Whole grains cereals proved to possess the right balance of particle size, absorbency and density. Specifically, Bear Mush, branch h o l e wheat cereal was used as the active absorbent in our most successful trials.
"%e actual fluidized bed material used was "Envirostrip," a modified wheat starch resembling large sugar crystals. The Envirostrip crystals provided the abrasiveness and bulk necessary to remove the oil laden wheat particles without manual assistance. This is a critical consideration for scale-up and prospective process automation.
The media selection thus dictated the procedure which evolved to: 1) Immersion of racked parts or sheet stock in the wheat cereal; 2) Removal of the wheat/oil conglomerate in the fluidized bed turbulent phase. Zyglo penetrant inspection oil was used for artificial contamination because it could be observed during bed operation; traditional cleaning tests indicated comparable properties with the MILL7870 protective oil commonly used by our sheet stock suppliers.
The lab scale bed diameter was 14 cm with a total height of 43 cm. 301 CRES screening was used as the air spreader (funnel). Oillwheat removal occurred almost instantaneously when the bed was turbulent.
Operating parameters were determined experimentally, and it was found that the most efficient cleaning was in a region referred to as "rapid bubbling." Some spouting occurs in this region. Combined with the rapid rolling of the bed, this spouting results in the
maximum contact of the media and soiled
of pressure as a function of flow rate for the
function of velocity.
surface. Figure 23 shows the characterization f
system. Figure 24 shows pressure as a r
I Scale-up of the fluidized bed can be calculated from the best fit curves of the plots shown in Figures 25 through 29. These plots are based on experimental values, with the exception of AP. AP was calculated using Ergun's equation. (4) The more commonly used equation established by Baeyens and Geldart (5) was determined to be inappropriate for this situation. Their equation does not hold true when the bed volume is small, or when the fluid and particle densities are orders of magnitude different, both of which were true of our system.
Figure 25 shows the operating air velocities at varying bed heights. Using the best fit curve for this plot, a bed height of one meter will require an air velocity of approximately 0.28 meters per second for efficient operation.
Scale-up to a bed height of one meter should be achievable using available plant air. Multiple ports may be necessary to achieve the operating flow rate. This size bed will approximate the size of many vapor degreasers.
Another non-solvent substitute investigated in 1990 was an oil absorbing cloth made by 3M called "Oil Sorbent, Type T-151." This cloth is made of 1/4 inch thick coarsely felted polypropylene. 3M markets this material for cleaning oil spills, and it is available to our shops for that purpose.
Our interest in this material was precipitated by an inquiry to Environmental Affairs from the Everett Production Drawing area. This area uses large quantitities of Freon to remove finger prints from the mylar drawing film. T- 151 samples were tested in a variety of oil removing situations including the coated mylar film. It performed very well demonstrating a high degree of competitiveness for any oil
accretion. On the basis of the initial work, it is concluded that these cloths could replace the naphthdpetroleum distillate preclean now 8. Kelley, Stephen, "Methods for employed in a two-step solvent cleaning Mechanically Removing Paint from process involving a non-polar precleaning Aircraft Structures," Robotic Solutions solvent followed by a polar final cleaning in Aerospace Manufacturing Roboics solvent. International/S.M.E., Orlando, FL,
"ALCIMABEB Ogden, UT.
Lem, Ruben, "Envirostrip, A Non- Petroleum Based Natural Dry Blast Media Engineered for the Aerospace Industry," WDhdust ry Advanced Coatings Removal Conference, Atlanta, GA, May, 1990
D204-144361, "Impact Study of Plastic Bead Paint Removal on Corrosion Prevention and Control," The W i g Company, Boeing Aerospace, Seattle, WA, March, 1987.
Panciera, H., "Surface Finish Removal from Advanced Composites Prior to Repair and Refinishing, Naval Air Rework Facility, Alameda, CA.
"Coating Removal via Plastic Media Blasting, "WAVAIR Engineering Support Office, Materials Engineering Division, Naval Air Rework Facility, Pensawla, FL.
NESO Code 34132, "Preliminary Report on Plastic Media Paint Stripping from Graphite Epoxy Surfaces, "Materials Engineering Lab, March, 1984.
86-E3B2-19, "Impact Study of Plastic Bead Paint Removal on Corrosion Prevention and Control, "The Boeing Company, W i n g Aerospace, Seattle, WA.
Project No. 00-143, "Paint Stripping of F-4 Aircraft and Component Parts Using Mechanical Methods,
9. NOOO19-83-G-0049 IMP Contract No., Project WBS-235, "Automated Painting and Stripping Project, Sixth Quarterly Report," Department of the Navy, prepared by Grumman Aerospace Corp. (Confidential).
10. 00-143 Stage 2 PRAM Project, Roberts, R.A., "Mechanical Paint Removal Process," Interim report on stripping paint from the first F-4E prototype at Hill AFB, UT, July, 1984, and May, 1985.
11. AFWAL-TR-85-4138, Childers, Sidney, et al., "Evaluation of the Effects of a Plastic Bead Paint Removal Process on Properties of Aircraft Structural Materials," December, 1985.
12. Bullington, J.B., Williams, D.R., "Organic Coating Removal via Multiple Plastic Media Blast Cycles on Clad Aluminum Airframe Skins," Corpus Christi Army Depot Chemical Branch, Engineering Branch, Corpus Christi. TX.
13. A.21 Process Standard, "Plastic Media Blast Cleaning and Paint Removal," Training Gu ide for Corpus C hrisd Armv Deuot
14. Manufacturing Operating Instruction 8- 1951985937, "Plastic Media Blast Cleaning for CH-47D Modification Program," July, 1985.
15. 8516 8-427, "Flourescent Penetrant Detection of Fatique Cracks After Plastic Bead Paint Removal," Boeing Vertol, Philadelphia, PA, December 1985.
MDSR 330036-1, "PMB Stripping of A i r c r a f t , " M a n u f a c t u r i n g Development, Boeing Vertol Company, Philadelphia, PA, May 1986