BARING IT ALL. Coatings Removal Options Uncovered. by Wayne N. Schmitz

Transcription

1 BARING IT ALL Coatings Removal Options Uncovered Four key paint-strippeing technologies stand out. Which is right for your application? T by Wayne N. Schmitz ime is rapidly running out on the use of toxic methylene chloride paint stripping processes. Fortunately, several viable alternatives are now available and, most importantly, they can be used effectively on both metal and composite substrates. The challenge, therefore, is for operators of depainting facilities to shift from a search for a technology that works to a decision as to which process to buy. The Front Runners There are approximately 12 paint stripping technologies currently offered as environmentally acceptable alternatives to the use of phenol-based methylene chloride chemicals. Of these, four appear to be emerging as leaders, though they vary widely in their methods of coatings removal: Flashlamp/Carbon Dioxide (CO 2) Pellet Blasting Laser Paint Stripping Soft Chemicals/High-Pressure Water Blasting Wheat Starch Dry Media Blasting The actual performance characteristics of each process are based on the manufacturers claims, previously demonstrated under controlled conditions. Flashlamp/CO 2 Pellet Blasting This process is actually a hybrid of two established technologies: dry ice (CO 2) pellet blasting, and the heat removal of surface coatings by the pulsed light energy from a xenon flashlamp. In principle, pulsed-light energy bursts bum away the surface coating while a stream of dry ice pellets sweeps away the carbonized residue and keeps the substrate temperature below 200 F The entire process is contained within a shroud, through which a high volume/high velocity vacuum flow is maintained. This flow transports all removed paint into a series of filters and an activated charcoal scrubber, effectively rendering the system discharge nonhazardous. Laser Paint Stripping Laser paint stripping, like the flashlamp/co 2 pellet blasting process, uses high-intensity, pulsed-light energy bursts to remove the paint film. This system also employs an effluent capture system to vacuum away the paint residue. However, the similarity of the two processes ends here. The pulsed-light energy is generated by a pulsed CO 2 laser and the energy 1 Parts July/August 1997

2 footprint is approximately one square centimeter, compared to the flashlamp which measures one by 12 inches. While the flashlamp is pulsed four to six times per second, the laser is pulsed up to 1000 times per second. Pulsing the laser very rapidly and moving it quickly over the surface yields a strip rate similar to that of the pellet blasting process. Because no cooling media are used in the laser process, substrate temperature is controlled by limiting the thickness of paint film removal to one millimeter per pulse pass. Numerous passes are required to remove normal coating thickness. Soft Chemical/High-Pressure Water Blasting Soft chemical technology uses non-methylene chloride-based chemicals (sometimes called environmentally acceptable ) to soften surface coatings, which are then blasted away with high-pressure water jets at 6000 pounds per square inch. The active ingredient in the chemical softener is usually formic acid or benzal alcohol. Wheat Starch Dry Media Blasting Wheat starch blasting is, in many respects, an outgrowth from the plastic media blasting (PMB) process. The primary difference is the stripping media, while the blasting, recovery and recycling systems are essentially the same. The primary advantages of switching to wheat starch media are better process control, reduced damage potential (especially on composite substrates), and reduced media consumption. As the wheat starch media is blasted onto the coating surface, it fractures into smaller pieces that are recycled and reused until they are too small to erode the paint effectively. Unlike the plastic media, which becomes rounded and less efficient each time it is recycled, wheat starch particles shatter into smaller pieces, each having sharper cutting edges. Therefore the strip rate with wheat starch remains fairly constant as compared to that of PMB. Best-Buy Considerations The problem that confronts the potential customer, for any of the offered paint stripping technologies, is how to compare the performances and economic characteristics of each accurately and choose a process that best fits their needs. All too often, emphasis is placed only on strip rate (square feet 2 Parts July/August 1997

3 per minute) or system cost. While these issues are important, they can easily be offset by some other critical considerations. cals/hiigh-pressure water blasting and wheat starch dry media blasting require many hours of aircraft preparation to prevent media intrusion and to provide a clean, dry surface to preclude media contamination. Pre-Strip Preparation These tasks are virtually nonexis- Because flashlamp/co 2 pellet tent for both the flashlamp/co 2 pel- blasting and laser paint stripping let blasting and laser paint stripping processes. Conversely, soft chemiprocesses use no media or chemicals to break down the surface coating, 3 Parts July/August 1997

4 the aircraft need not be washed or dried to remove surface soils. Masking surface openings, aerodynamic fillers, and door seals is also not required since there is no media to penetrate or contaminate the structure. Eliminating these tasks saves about one-third of the touch labor costs and materials, and reduces the throughput time Post-Strip Cleanup These tasks and costs vary widely. The absence of any media, masking materials, or media penetration into the airframe when using either the flashlamp blasting or laser paint stripping virtually eliminates post-strip clean-up. Both pulsed-light stripping processes, however, do require a small amount of manual paint removal since the stripping head may not be able to access all areas of the plane. Both chemicals/water blasting and wheat starch blasting require removal of mask materials and manual removal of paint around door openings, etc. Both also leave the aircraft surface coated with chemical residue or media dust that must be thoroughly rinsed away. Each may tend to destroy form-in-place seals, aerodynamic fillers, and butt gap sealant that must now be repaired. Also, wheat starch recycling and its resultant breakdown can leave dust in the airframe. Dedicated Facility Question By definition, this means that, whether for personnel safety, media containment or recycling, or stripping environment control, a special facility must be used only for a particular paint stripping process. Because pellet blasting uses no media, the lamp-focused energy range is only a few inches, and all the removed coating is contained and collected at the stripping source, a dedicated facility is not required. Safety requirements do dictate that personnel working within 10 feet of the stripping head wear ultravioletrated, tinted glasses and ear protection due to the noise level of the CO 2 pellet blast nozzle. This process can be implemented out or inside large maintenance hangars in which other aircraft are being serviced, and temperature and humidity have no impact on the process. The laser process employs a very high-energy light beam, dictating the need for a dedicated, closed facility. The process must be robotically applied and controlled by an operator safely outside the stripping cell, and facility doors must be equipped with interlocks to shut the laser down in the event of unauthorized entry. Soft chemicals/high-pressure water blasting also requires a dedicated facility. Soft chemicals work best when the temperature is at 40 C or higher and they are allowed to penetrate for 12 to 20 hours. Add to this a complex recovery or recycling system for the removed paint, chemical and water slurry, as well as a robotically applied, high-pressure water blast system, and a very specialized hangar is required. Wheat starch blasting creates a great volume of dust which must be 4 Parts July/August 1997

5 contained and from which the operator must be protected, usually by wearing a sealed suit and helmet supplied with breathing air. In addition to containing the dust, a dedicated facility is required to capture, separate, recycle and prepare the spent media and removed paint residue for disposal as hazardous waste. Worker Safety Personnel safety is affected not only by the direct interface between workers and the process, but also by the hazards posed as the process is applied to the aircraft surface. A consideration often voiced by potential users of the pellet blasting and laser paint stripping processes is the possibility of fire or explosion. This is legitimate since both remove the surface coating by instantaneously raising its temperature to the point at which it explodes off the surface and, in the case of laser stripping, is actually incinerated. The possibility of fire or explosion with the pellet blasting process is prevented by the absence of oxygen within the stripping head shroud. The CO 2 pellets turn to gas on contact with the structure surface, creating a continuous fire extinguisher. CO 2 stripping tests on surfaces covered with hydraulic fluid, lubricating oil, flammable solvents, and jet engine fuel have failed to cause ignition. Laser stripping employs an air knife across the surface to negate the plasma plume formed when airflow is not present. The additional mass of air created by the collection system vacuum may prevent ignition. Because laser systems, to date, have been developed only.to strip off-aircraft components such as radomes and helicopter rotor blades, no significant data is available to assess the fire and explosion hazard. Worker safety for off-aircraft component stripping is assured by keeping all personnel out of the facility during operations. Though not as hazardous as some solvents, soft chemicals, especially formic acid, still pose a health risk to workers, who must wear protective clothing when applying them. Since high-pressure water blasting follows manual application of the paint and chemical removal process, robotic application assures worker safety. Application of the wheat starch blasting demands that- workers wear a sealed suit and helmet supplied. Roboric application incorporating a capture shroud could eliminate these requirements. Worker Acceptance Worker acceptance of any technology is tied directly to their comfort level when using it, how difficult it is to apply, and how well it works. In this context, the technologies are quite different. Within its present application lim- 5 Parts July/August 1997

6 itations, laser stripping of off-aircraft components should receive wide acceptance. Since all operations are robotic, pre-cleaning and post-strip cleanup is virtually eliminated and the operator controls the process from a booth. Depending on the application, pellet blasting should receive the same level of worker acceptance, especially if the process is controlled from an enclosed booth. Even using a semimanual application, worker acceptance will he assured, since safety equipment is minimal, lightweight, and comfortable. Worker acceptance of soft chemicals/high-pressure blasting and wheat starch dry media blasting processes is likely to be less enthusiastic. Pre-strip preparation and post-strip cleanup tasks are extensive and cumbersome, and protective clothing and breath- ing apparatus are required. Media Recycling Soft chemicals/water blasting and wheat starch both require recycling. Though the chemicals cannot always be reused, the water used to blast the chemicals and paint off the surface may be purified and reused. Wheat starch is recycled until it becomes fine dust no longer capable of paint erosion. Recycled water has no impact on process performance; however, recycled wheat starch tends to strip at a different rate each time it is reused. Waste Volume and Disposal Costs With no media to dispose of and hazardous waste limited to the ashlike paint residue trapped in the effluent capture system filters, waste With some technologies, such as lazer stripping and pellet blasting, the paint removal process is controlled from an environmentally regulated booth. 6 Parts July/August 1997

7 volume and disposal costs are low for the pellet and laser processes. Such is not the case for soft chemicals/high-pressure water or wheat starch. Though not in the same physical state, the volume of stripped paint is still large for both. Added to the paint is the spent and contaminated chemical softener. Even though the wheat starch is reused several times, it too must be disposed of. The problem is that as the media shatters into dust it can no longer be separated from the paint particles and must, therefore, be disposed of as hazardous waste. With current disposal costs ranging up to $1000 per 55-gallon drum, it becomes a significant element in the total cost. Repaint Adhesion Characteristics This is another element of the total paint stripping cost equation in which water and media blasting do not fare well. While pellet blasting and laser stripping, employing no particulate media, leave the surface extremely clean, the other two technologies leave a chemical residue or dust coating. Neither is acceptable, necessitating extensive post-strip rinsing and thorough surface preparation before painting. Repaint adhesion characteristics are outstanding after coatings removal with the pulsed-light energy systems. Repaint adhesion following water and dry media blasting is average, at best. If any trace of softener or starch remains in lap joints or surface intersections, the new paint will pop off the surface in a short period of time. Versatility, Compatibility One key consideration that will influence the purchasing decision is versatility and compatibility with existing maintenance facilities. Almost without exception, major airline operators lack the capital for highly specialized, single-application robots or to build new facilities. Regarding versatility, each technology has one function: paint stripping. The issues of safety, process application and control, and facility usage may prevent the performance of other maintenance tasks while these processes are at work. System Reliability, Supportability Costs System reliability is one of the most critical considerations. A strong suit of pellet blasting is that the technology is mature. Flashlamp systems have been around for over 25 years; CO 2 pellet blasting for about 10. All other system elements are off-the-shelf hardware that has been around even longer. Laser stripping systems large enough to strip entire aircraft have yet to be 7 Parts July/August 1997

8 developed. Subsequently, there is no existing data to determine reliability, maintainability, or supportability. The soft chemicals/water blasting and wheat starch blasting processes both employ off-the-shelf hardware like pumps, compressors and filters, so reliability should not be a major concern. Supportability costs are made of all the consumables and scheduled maintenance activities required. Unexpected failures notwithstanding, the pellet blasting process requires a continuous supply of materials, like spare flashlamps, liquid CO 2, and deionized water. The costs for water and dry media rise according to the quantities used. Conclusions Each technology has some recognizable advantages and drawbacks. There are, however, gray areas - what one operator may consider an advantage can be seen in a negative light by another. The reality is that no one process is capable of performing every task satisfactorily, all the time, in all applications and to satisfy every need. The cost of any technology is negotiable. Making a selection based on an advertised price could be a very costly mistake. The complexity of the customer s needs, the wide range of configurations, and the fact that so many technologies are being offered in a highly competitive market all accommodate pricing flexibility. Life cycle cost or cost of ownership really determines the best buy, Up-front system price is obviously a primary consideration, but it rapidly declines as a relevant cost issue. What really stands out on the ledger sheet year after year is: What does it cost to use this process? Does the technology make me more competitive? Can I expand or upgrade capabilities as the market changes? Will the technology continue to meet tightening environmental regulation with minimal changes or costs? Is the system reliable and easy to maintain or does it hurt the production schedule? Is the process operator-friendly? Can the process be used for other purposes and increase its worth? The final decision, however, should be based on a complete cost analysis of every viable candidate. n Author s Bio Wayne N. Schmitz has been A member of the McDonnell Douglas Product Support division for more than 30 years. He currently works in the Tactical Aircraft and Missile Systems division minimizing the use of hazardous materials, reducing toxic waste, and serving as program manager of the Flashjet Coatings Removal Process. 8 Parts July/August 1997