industry survey and research of depaint methods …infohouse.p2ric.org/ref/51/50104.pdf · industry...

2003 Aerospace Coatings Removal and Coatings Conference 1

INDUSTRY SURVEY AND RESEARCH OF DEPAINT METHODS Daniel W. DeKruif, Southwest Research Institute (SwRI®)

ABSTRACT

There are a wide variety of processes and technologies available for aircraft coating removal and preparation. Selecting the proper process for a specific application requires knowledge of the various known technologies. Process selection criteria typically include initial requirements, operational requirements, economics, and performance with respect to the end user’s needs. As part of a research study funded from private industry, SwRI surveyed the coatings technology field and applicable industries to formulate a summary of available technologies suitable for aircraft coating removal and preparation. Each method was then researched via the Internet, process demonstrations, trade magazines, journals, conferences, and interviews with suppliers and end users. The results were used to produce a report evaluating each process with respect to the customer-specified selection criteria. DRY BLAST MEDIA (DMB)1,2

Types Dry Media Blasting (DMB) designates a broad range of processes that use high-velocity particles (media), propelled by compressed air to remove surface coatings or impart surface finishes. The air and media are mixed and metered through a pressurized blast pot, and delivered to the surface through pipes and/or thick rubber hoses before exiting the blast nozzles. Generally, the spent media is recovered, separated from the waste, and reused. Numerous types of media are available to address specific applications, and in this document only medias intended for use on airframes are considered. In general, harder media types are more effective for coating removal and impart more damage to the substrate. However, just as important as media selection is the blast process itself. Parameters such as blast pressure, nozzle standoff, angle of attack, and dwell time will have a large effect on the overall process. Proper operator training and certification are critical to the operation’s success. Most organizations create documents, such as the Air Force’s TO 1-1-8, to define blast parameters for internal use.

1 Joint Paint Removal Study, Joint Policy Coordinating Group on Depot Maintenance, Tasking Directive 1-90, Final Report on Plastic Media Blasting, Joint Depot Maintenance Analysis Group, Technology Assessment Group, June 1994. 2 Plastic Media Blasting (PMB) Paint Stripping, Joint Service Pollution Prevention Opportunity Handbook, August, 2002. Retrieved February 13, 2003, from http://p2library.nfesc.navy.mil/P2_Opportunity_Handbook/5_5.html.



PLASTIC MEDIA3

DMB is widely used for coating removal on both military and commercial aircraft. The majority of these processes use plastic media, which is sometimes referred to as PMB. These media are DOD certified under MIL-P-85891 “Plastic Media for the Removal of Organic Coatings” and are classified as Type I though Type VIII4,5. The media, being an engineered product, is continually improved for such characteristics as better efficiency, lower breakdown rate, and less surface residue. There are significant variations in the media quality between vendors, so testing is recommended for each application. Type V media, an acrylic thermoplastic, is the most commonly used variety for aluminum aircraft surfaces due to its relatively high efficiency, low breakdown rate and minimal substrate impact. With proper blast parameters and technique, soft-clad aluminum surfaces have been shown to significantly retain their cladding through repeated de-paint cycles. Type V media is commonly used on surfaces down to 0.032 inches thick and has been approved for thin-skins down to 0.016 inches. Several weapons systems have discontinued manual application of Type V and dry media blasting in general on thin aluminum skins due to surface deformation. However, automated systems can be an advantage on thin-skins due to better process control and reduced dwell time. Significant testing of Type V media has been performed with composite materials showing mixed results6. As usual, blast parameters and techniques are critical parameters determining blast efficacy and substrate damage. Glass-epoxy composites have generally responded poorly to Type V media with significant resin removal and surface fiber breakage. Graphite-epoxy composites faired better, but still require precise control of the blast process. Consequently, manual blast facilities have implemented less aggressive medias (such as starch-based media) and processes for composite surfaces. Type VIII media, which is being marketed by US Technology as Magic, is a new plastic hybrid of amino thermoset resin and reinforcing fiber7. It has been tested to be more effective (higher strip rate) than Type V with similar breakdown rates. The major advantage touted for Magic is its minimal substrate impact. It has shown lower Almen strip heights8 and better surface roughness than Type V for aluminum substrates. Additionally, the Navy has tested Type VIII on bismaleimide composites and found no significant delamination or strength differences after five blast cycles.

3 Polymedia® Custom Engineered for the PMB Industry, US Technology Corporation, (n.d.). Retrieved January 21, 2003, from http://www.ustechnology.com/poly.htm. 4 MIL-P-85891A, “Plastic Media, For Removal of Organic Coatings”, April 1, 1992. 5 MIL-P-85891A, Amendment 2, “Plastic Media, For Removal of Organic Coatings”, June 26, 1998. 6 Polymedia-Lite™ Evaluation for Composite Structures, (n.d.) Retrieved January 21, 2003, from http://www.ml.afrl.af.mil/ctio/pdf/projects-depaint14.pdf 7 Magic® for Unmatched Performance, US Technology Corporation, (n.d.). Retrieved January 21, 2003, from http://www.ustechnology.com/magic.htm. 8 Anonymous, “Procedures for Using Standard Shot Penning Test Strip: SAE J443”, Engineering Society for Advancing Mobility Land, Sea, Air and Space, PA, USA. (1984)



Organic Media

In addition to plastic, other media types are available, most notably organic based products including starches, apricot pits, walnut shells, and pecan shells. These offer such benefits as being biodegradable, less expensive, and imparting lower residual stress to the substrate. They are generally less aggressive though resulting in slower stripping rates. Wheat starch9,10,11,12 is the most common organic media used for aerospace applications. Wheat starch is especially attractive for composite surfaces due to its excellent controllability characteristics and minimal substrate damage. Since wheat starch is a less aggressive media then Type V, it permits more flexibility when manually stripping specific layers13. Selective stripping has several advantages:

• It can be used in touch-up applications where coating imperfections are repaired without reapplication of the entire coating system.

• It is possible to remove topcoats leaving high-chromate primers intact thereby reducing hazardous waste.

• Selective stripping may reduce substrate damage and increase the number of possible de-paint cycles on a surface.

The two major disadvantages of wheat starch are its moisture susceptibility and low strip rates as compared with plastic media. Wheat starch in high humidity situations or where condensation is possible such as in a depressurizing blast pot has a tendency to bond and swell. This is especially important with long-term storage hoppers. However, careful blast system design can accommodate for water absorption. Another recently developed organic media is corn-based starch14,15. ADM is marketing a corn hybrid media as EnviroStrip XL. Corn has significantly better water absorption characteristics when compared to wheat. Wet corn media, will return to a usable material after drying unlike wheat. Preliminary testing with corn based starches have shown strip rates at least as high as wheat although still considerably lower than Type V. Corn media has low substrate impact

9 ENVIROStrip® 30/50 Wheat Starch Media Aviation Grade (Code 1603), (n.d.). Retrieved January 21, 2003, from http://www.envirostrip.com/envipds.htm 10 Monette, Denis, and Oestreich, John, ENVIROStrip® Plus Wheat Starch Dry Stripping Media, ADM/Ogilvie, 1999, retrieved from http://www.envirostrip.com/envitechrep.htm. 11 Le Blanc, Paul, Koutlakis, George, and Monette, Denis, New Developments in Starch-Based Abrasive Media: An Overview of Research and Development Activities at Archer Daniels Midland, ADM/Ogilvie, 1998, retrieved from http://www.envirostrip.com/envitechrep.htm. 12 Evaluation of Type V, Polymedia-Lite™ DMB Processes for Depainting 0.025 Inch 2024-T3 Aluminum Alloy, (n.d.). Retrieved January 21, 2003, from http://www.ml.afrl.af.mil/ctio/pdf/projects-depaint07.pdf. 13 EnviroStrip News Update, Fall/Winter 1999/2000, ADM/Ogilvie, Montreal, Quebec. Retrieved from http://www.envirostrip.com/envinewsletter.htm. 14 Monette, Denis, ENVIROStrip Strip® XL Corn Hybrid Polymer Dry Stripping Media, ADM/Ogilvie, 995 Mill Street, Montreal, Quebec, Canada. 15 ENVIROStrip® XL Corn Hybrid Polymer, Aviation Grade (Code 1660), (n.d.). Retrieved January 21, 2003, from http://www.envirostrip.com/envipds.htm



similar to wheat starch as well. ADM is marketing EnviroStrip XL as a direct replacement for plastic media, since it can operate on the same equipment with little modification.

Other Media

Another recent advance in media technology is sponge-encapsulated media. This technology, developed by Sponge-Jet, uses a urethane sponge embedded with abrasive particles16. Sponge media is available in many varieties with a broad range of characteristics. White Sponge Media is the most common aerospace product for composite surfaces. It uses a urea abrasive similar to Type II media. White sponge media is not as aggressive as plastic media, but it does have several advantages. First, it is similar to organic media in that it is gentle on composite surfaces, but still has a reasonable strip rate. Second, it is capable of producing selective stripping. Finally and probably most significant is that it produces about 10% of the dust of conventional media. This might make it an excellent choice for touch up and repair operations outside of a blast facility. Sponge media has some special equipment requirements and is usually not a direct replacement for plastic or organic media. Sodium bicarbonate17 in dry media form has been tested on many weapons systems over the past decade. Sodium bicarbonate is softer and denser than plastic media. While, it is relatively safe on most substrates, it is not widely used by the Air Force due to its low strip rate and potential for corrosion at higher temperatures. Sodium bicarbonate is often used as an abrasive in wet blasting systems.

Table 1 – Important Characteristics of Media Types1

Media Type Material Hardness Strip Rate @ 30 PSI (ft/min)

Consumption @ 30 PSI

Type II Urea 54-62 Barcol 2.5 8% Type V Acrylic 46-54 Barcol 2 8% Type VIII Blended Amino 60 Barcol 3 5% Corn Starch3 Corn 80 Shore D 0.4-1.2 5% Wheat Starch3 Wheat 80 Shore D 0.3-0.8 7% Sodium Bicarbonate

Sodium Bicarbonate

53-64 Brinell 0.3 N/A2

SpongeJet White

Urethane Sponge, Urea

N/A2 1-2 12%

1 – Type I, III, IV, and VI are omitted since they are not intended for use on airframes. 2 – Not available at time of publication. 3 – Corn or Wheat starch mixed with 10% acrylic is considered Type VII media.

16 Sponge Blast™ Unleashing a New Dimension in Blasting Media, US Technology Corporation, (n.d.). Retrieved January 21, 2003, from http://www.ustechnology.com/sponge.htm. 17 Joint Paint Removal Study, Joint Policy Coordinating Group on Depot Maintenance, Tasking Directive 1-90, Final Report for Sodium Bicarbonate Paint Stripping, Joint Depot Maintenance Analysis Group, Technology Assessment Division, February 1995.



Requirements

Set-up

The initial equipment requirements for dry media blast systems are generally high. Beyond the required media delivery system (blast pots, hoses, nozzles etc.), there must be media recovery, recycling, and storage systems. In order to maintain compliant media, the recycling system usually separates the media based on size, density, and ferrous content. Additionally most dry media blast operations produce significant dust that must be evacuated from the facility via reverse-pulse dust collectors.

Operation

Operators in a blast environment require forced air respirators, protective clothing and hearing protection. Additional safety measures may be required depending on the facility including carbon monoxide monitors, safety harnesses, and personnel monitoring. Other factors to consider are staffing and blast operation training. Aircraft preparation and de-preparation are major considerations in any dry media blast process. Considerable effort is required to prevent media ingress and blasting of sensitive areas such as antennas. Often this preparation is more time consuming than the actual blast process. De-preparation may include another process such as high-pressure water or sanding for cleanup of masked areas. The aircraft de-preparation usually involves a wash-down to remove media residue. This process may be performed in a separate facility.

Safety

All dry media blast systems have to be designed with safety in mind. There are airborne particulate concerns from the media, paint and substrate. Special attention must be paid to chromium-based primers and heavy metals such as cadmium that may be generated from fasteners. National Institute for Occupational Safety and Health (NIOSHA) approved supplied air respirators are required for most dry blasting; the one exception may be sponge based media. Personal protective equipment (PPE) is also required. Facility ventilation is generally necessary to maintain visibility, prevent an explosive atmosphere and to reduce particulate emission. Another important consideration is fall protection, especially when operating on top of an aircraft. Media under foot can create a slippery surface. Consequently, many facilities require safety harnesses and/or proper guarding when de-painting the upper surfaces of an aircraft.

Economics

Initial capital equipment costs for blast facilities vary widely based heavily on the size of the facility. Ventilation, in particular, increases non-linearly with size since a specific cross-section airflow rate must be maintained. System costs can range from $100,000 for a small booth for de-



painting components up to $10,000,000 for a facility to handle cargo aircraft. Fighter aircraft facilities generally run in the $1,000,000 - $2,000,000 range. Note that automation expenditures are not included in the equipment cost estimates. Operational cost will vary considerably based on location, application, and process. Two data points cited in references for plastic media blasting are for F-4 Aircraft de-painted at OO-ALC and C-5 aircraft de-painted at SA-ALC. The F-4 analysis showed per plane operational costs of $13,316 in 1987. The C-5 data gives a per plane cost of $151,000 in 1993. Some important considerations for operational costs are: burdened labor rate, media cost and consumption, facility maintenance, utility usage, consumable usage, waste removal. Performance

Controllability

Controllability varies with media type. Plastic media generally has poor controllability when used in manual applications. Selective stripping is not easily accomplished and stripping of composites requires well-trained operators and careful monitoring of blast parameters. Other media such as wheat, corn, and sponge jet have better controllability and are capable of selective stripping.

Effectiveness

Effectiveness also varies with media type. Plastic media blasting is the fastest de-paint process currently available. It can be even more effective with an automated system. The softer, more forgiving media types are less efficient. Nearly a factor of ten exists between the fastest and slowest stripping rates (Table 1). The challenge is to optimize the many system variables to get the best combination of speed, cost, and substrate integrity.

Versatility

Since sponge-jet is a near dustless process, use of white sponge media may allow application in non-ventilated facilities. Such versatility may offer the opportunity for rework of the coating system during the painting process.

Adaptability and Scale Up

DMB is one of the most widely used processes for full aircraft de-paint. It scales well, and systems have been installed to manually depaint aircraft as large as a C-5. Additional blast nozzles can be added via supplemental blast pots or multi-port pots. It is important to note that equipment and operational costs scale with facility size.



DMB is also one of the easiest technologies to automate because of its forgiving process parameters. Robotic systems have been built for removing paint from small automotive components to full frame fighter aircraft. These systems remove the worker from the blast environment and increase production by allowing one operator to control multiple nozzles. There will likely be some areas that can’t be reached by robotics due to geometry and fixturing, but the blast equipment can be used to manually touch up prior to de-preparation. WET PROCESSES

For many years, methylene chloride was used as the primary method for stripping paint from aircraft. Regulations passed within the past ten years have reduced the exposure limits and hazardous waste tonnage permissible in the aircraft de-paint industry. These new regulations have inspired the development of environmentally friendly technologies that perform similarly to methylene chloride, while reducing the waste by-products normally produced during paint removal via wet processing. This study considers the predominant wet processes used today, namely, high and medium pressure water paint stripping, and an alternative to methylene chloride called benzyl alcohol. The wet processes can be both closed and open loop, but each case requires proper treatment of the wastewater to remove solid waste and minimize contaminants. Though the technology involved with wastewater treatment is quite extensive, this document will only deal with treatment technologies that are in use or are currently under study in the de-paint industry. Benzyl Alcohol Formulations18,19 Benzyl alcohol paint stripping techniques were developed to meet changes in environmental law that reduced the acceptable levels of hazardous air pollutants (HAP). Benzyl alcohol solutions are divided into acidic or basic formulations. Acidic formulations are comprised of 25-35% benzyl alcohol and 10-15% formic acid, which is used as an accelerator. The overall pH of the acidic formulation is 2.5. Basic formulations are comprised of 30-50% benzyl alcohol and 5-10% amine or ammonia compounds, resulting in an overall pH of 11.0. Acidic formulations work faster than basic formulations, though the ambient temperature affects the performance. Cooler temperatures increase the reaction time, thereby requiring the use of a temperature-controlled booth to optimize the paint removal operation. Acidic formulations have some drawbacks, notably that they are not recommended for use on high strength steel or magnesium substrates. Additionally, composite substrates, such as fiber reinforced composites, must be masked or removed. The reaction caused by a benzyl alcohol formulation will last for several hours; hence it will take longer to delaminate the paint when compared to methylene chloride.

18 Benzyl Alcohol Paint Stripping, Joint Service Pollution Prevention Opportunity Handbook, August, 2002. Retrieved February 13, 2003, from http://p2library.nfesc.navy.mil/P2_Opportunity_Handbook/5_5.html. 19 John Adams, Naval Air Systems Command, personal communication, February 28, 2003.



After ample time is given for the reaction, the benzyl alcohol formulation rinses easily with water. The wastewater is collected for treatment, separating the solid waste so that it can be drummed and sent to the landfill. Wastewater handling complexity depends on the final pH and type of paint removed. One wastewater handling technique that simplifies the wastewater treatment process and reduces operating costs is ultraviolet oxidation (UVO) – a photocatalysis technique used to decompose organic contaminants. This wastewater treatment method was successfully tested at McClellan Air Force Base20 and is currently being investigated by the Naval Air Systems Command (NAVAIR). The system under investigation at NAVAIR is a patented process developed by Purifics® Environmental Technologies, Inc.21. Purifics® is a supplier of a closed-loop titanium dioxide (TiO2) photocatalyst slurry process. The treatment method utilizes ultraviolet light and titanium dioxide as a catalyst (similar to an automobile catalyst except the TiO2 uses light energy for activation). The catalyst absorbs photons (light energy) and through a chemical reaction forms two hydroxyl radicals. These hydroxyl radicals oxidize organic contaminants into carbon dioxide and water. The TiO2 is reclaimed through a patented process, and the clean water is returned to the system. Though TiO2 is inexpensive, the system is provided with enough catalyst to last the lifetime of the unit. The photocatalysis method avoids some of the drawbacks of conventional UV systems, such as fouling, poor efficiency due to turbidity, frequent quartz sleeve replacement, and high-intensity ultraviolet lighting. The photocatalysis process destroys most of the benzyl alcohol formulation, and the only by-products of the reaction are carbon dioxide and water. Note that it is recommended to coalesce the paint chips stripped from the aircraft prior to the wastewater treatment system.

Requirements

Set-up The size of the de-paint facility dictates the amount of infrastructure modification and capital investments when implementing a benzyl alcohol chemical stripping process. As with any chemical process, the aircraft must be stripped in an enclosure in order to control the release of volatile organic chemical (VOC) emissions and wastewater. In addition to the booth, the minimum requirements for such a system are:

• Concrete floor complete with water-flushed trenches to evacuate the sludge (benzyl alcohol formulation, water, and paint chips) that is removed from the aircraft.

• Solid particle separator, sized for the planned effluent volume • Holding Tanks, used to adjust the pH of the wastewater

20 Kingsley, Gordon B., and Clay, Robert, “Depaint and Alodining Wastewater Treatment and Recycle”, (n.d.), Retrieved February 27, 2003, from https://www.denix.osd.mil/denix/Public/Library/PRO97/casestd2.html. 21 Photo-Cat® Water Treatment, (n.d.), Retrieved February 19, 2003, from http://www.purifics.com/photocat.html.



• A collection of pumps to transport the wastewater between individual processes • Electric service to power the pumps and ancillary equipment • Control system with redundant safety methods • Air handling equipment to filter the VOC’s • Wastewater treatment system

Operation Aircraft preparation prior to paint removal with benzyl alcohol formulations is a necessity. All areas that can be damaged by the chemical mixture must be masked or removed. In order to facilitate paint removal, the top layers of the aircraft should be scuffed to allow easier access of the benzyl alcohol formulation to the primer coat. Safety Due to the acidic or alkaline nature of benzyl alcohol formulations, application requires the use of skin and eye protection. In addition, proper handling and waste disposal procedures are needed to ensure operator safety. Benzyl alcohol formulations have flash points ranging from 150ºF to over 200ºF, so they are classified as combustible materials. Therefore, their use should be coordinated with the local fire department. In addition to the use of a ventilation system, respirators must be used to minimize ingestion of ammonia or formic acid. Economics Initial costs for a benzyl alcohol stripping process depend on the size of the facility. Given the need for air handling equipment, pumping stations, infrastructure modifications, and environmentally controlled stripping booth (to name a few), the costs for the facility may run as much as $8 million dollars. Three plants that use the benzyl alcohol process cost nearly $24 million, with one of those facilities under development at NAVAIR. In addition to these initial requirements, the estimated cost for a Purifics® wastewater treatment system is approximately $400,000. The annual operational costs for the benzyl alcohol hydrogen peroxide process at NAVAIR are approximately $3 million. This cost covers labor, utilities, permits and safety regulations, reagent chemicals required by the process, etc. A wastewater treatment plant may be designed to handle wastewater for the entire facility, so additional costs not attributed to the paint removal process will be incurred. Though use of the Purifics® wastewater treatment facility is currently under development at NAVAIR, once the process is proven with the Purifics® purification system, the operating costs are expected to drop to about $2.1-2.2 million per year.



Performance

Controllability Layer removal depends on several factors, such as the benzyl alcohol formulation, the reaction time, ambient temperature, and whether any chemical resistant layers of paint exist. Given the number of variables involved with this method, it is difficult to remove individual layers, making this technique less attractive for precision stripping operations. Effectiveness Provided that the temperature is above 65 degrees Fahrenheit, the acidic formulation is capable of delaminating paint from the airframe. The addition of hydrogen peroxide permits a more aggressive reaction, resulting is less time to delaminate the paint. However, hydrogen peroxide may corrode the substrate, comprising the integrity of the aircraft. Versatility The very nature of the application requires administering the benzyl alcohol solution across the entire surface of the aircraft. Since benzyl alcohol formulations have excellent adherence with vertical surfaces, any part of the aircraft is accessible. However, the use of this stripping technique is not recommended for composite structures. Adaptability and Scale Up Multiple benzyl alcohol formulations are available that are effective for topcoat removal, but differ in performance when removing primer coats. Hence, care must be taken to select the optimal formulation based on coating type. Benzyl alcohol stripping can be scaled to handle small off-airframe components to full size aircraft. However, the required facilities for paint removal when using benzyl alcohol formulation must be sized appropriately, generating an initial facilities cost that may be unacceptable. Automation could be used to apply chemicals to the aircraft, but it may not be as cost-effective in chemical stripping as it would be for other processes. Vapormatting22,23 The Vapormatt process, called Vapormatting, utilizes a mixture of water and abrasive media accelerated from a nozzle via compressed air to clean and/or remove surface coatings. This commercial process is in use for a variety of purposes, such as cleaning fast-food equipment and stripping aluminum, polyester and nickel graphite abradable coatings. It was developed as an alternative to chemical stripping methods, and is considered an environmentally friendly closed loop surface cleaning and conditioning process.

22 Alan Parker, Hydratec Systems Ltd - U.K., personal communication, February 21, 2003 23 Vapormatt WaterWave. (n.d.) Retrieved February 21, 2003, from http://www.vapormatt.com/products_waterwave_page.htm.



Vapormatting allows for the use of a variety of abrasives, from talcum powder to Type V plastic media to ruby hard alumina. Moreover, since the particles are suspended in water, no dust or static electricity is created. As the water buffers and lubricates the particles on impact, very fine finishes are possible without substrate damage. Furthermore, nozzle pressure can be regulated permitting selective layer removal without damaging the substrate. To aid with the inevitable problem of hydraulic fluids on the surface of the aircraft, the slurry in Vapormatting can be mixed with biodegradable degreasing compounds such that the aircraft can be degreased and stripped simultaneously. Vapormatting is a closed loop process; water loss is attributed only to evaporation. A cyclone is used to separate the media and debris. Reusable media drops to the bottom of the cyclone, whereas the water, damaged media, and paint debris come out of the top. The wastewater is then pumped into a filter that continuously removes the oil and blast debris. The filter is designed as a “moving bed”, such that the wastewater is deposited onto the paper as it moves linearly from the drive to the idle drum. The media is separated from the blast debris during the filtering process and is reintroduced to the system following a secondary separation. Larger systems are available to handle blast booth type applications. Vapormatt Incorporated has designed a “water wave” process that extends the functionality of the filtering process via oscillating subsurface mechanism. Called the “WaterWave”, this system is modular by design, allowing for large booth construction while segmenting the wastewater removal. It is built in an enclosed booth, and the parts are cleaned/stripped on top of an open grating. The wastewater is collected beneath the floor in a series of troughs, and an oscillating wiping mechanism transports the wastewater to the filtering equipment. The modular design of the waterwave has the added benefit of lessening the load on the oscillating wiping mechanism.

Requirements

Set-up Infrastructure modification and capital investments depend on the need of the de-paint facility. For example, the stand-alone enclosed manual unit, called the Vapormaster 1717, the following utilities are required:

• 20 amp service of 380 or 415 volt, 50 or 60 Hz, 3 phase and neutral • compressed air supply of 40 scfm and 100 psi • clean, fresh water supply of 30-90 psi • floor level drain with grit trap that can intermittently handle 40 liters per minute • 8” diameter air vent to external atmosphere



The WaterWave system is modular by design, so the services needed to efficiently operate the system depend on the number of units employed. The required utilities are:

• 20 to 40 amp service (depending on size of installation) of 380 or 415 volt, 50 or 60 Hz, 3 phase and neutral

• Compressed air supply at 100 psi and 120 scfm per Mirus gun (an extra 80 scfm is needed to use the optional pressure generator)

• Clean, fresh water supply of 30-90 psi • Floor level drain with grit trap that can intermittently handle 40 liters per minute • Ventilation to suit the size and type of installation • A level surface that is capable of supporting a point pressure of 500 pounds per sq. ft.

Operation The stand-alone systems are similar to glove boxes, which require minimal operator training to utilize the machine. With proper training, operators become efficient in 3-4 hours. Since waste is not automatically drummed in the stand-alone or with the WaterWave systems, a waste removal process must be established. In addition to the operator requirements, a level surface must be available in order to install the equipment. A clean fresh water supply must also be present. Safety Unlike dry media blast systems, Vapormatting does not generate any dust or static electricity. Therefore, in manual operations, personal protection requires waterproof clothing and footwear, and full head protection. Since chemicals can be added to the Vapormatting process to mitigate corrosion of the substrate, a breathing apparatus may be required. As with any high-pressure application, the greatest risk to the operator is flesh exposure to the high-pressure slurry. For that reason, operators need to be trained on how to use the equipment safely. Economics The initial cost for the integration of a WaterWave modular system will depend on the size of the facility. For large-scale processes, such as a 5 x 8 unit WaterWave system, the slurry recycling equipment will need to be modified and developed to handle the increased volume of wastewater. The overall system cost will be based on the number of individual WaterWave units, the design and development costs associated with a new slurry recycling device, and any additional automation such as a robotic installation. The costs for a large-scale 5 x 8 unit WaterWave facility may run as much as $5 million dollars. Labor costs will be associated with machine use, waste handling, and part preparation. Labor and utility rates will vary depending on location and state laws.



Performance

Controllability Vapormatting is available as either a manual or automatic process. In the manual process, control depends on operator training, system pressure, and paint hardness. Accurate control over the slurry pressure and nozzle raster speeds is possible with the automated systems. Since this process utilizes a water-based slurry, visual feedback in large automated systems may be compromised from splash back. Raster speeds and the type of media are the major factors in determining the coating removal rate. Since this process has not been utilized on off-airframe components, controllability of selective layer removal will need to be determined. Effectiveness Vapormatting is used by airlines to clean aircraft wheel rims without removing the underlying anodized coating. The process is also used to clean heat shields. Actual paint removal performance for a variety of airframe substrates is unknown. It is anticipated that this paint removal technique will vary in performance due to difficulties with coating variations and operator error. Automation may provide a more repeatable system, yet more testing is needed to verify paint thickness removal versus slurry and pressure. Versatility Vapormatting is capable of utilizing a wide variety of abrasives due to the unique filtering design. The flexibility with the choice of abrasive allows the facility to remove many different types of surface materials. Vapormattting is commonly used for cleaning surfaces, such as fast food grills; however, it has found use in paint removal applications, such as removing the protective layer from aircraft wheel rims. Actual performance removing paint from composite structures has not been determined. Adaptability and Scale Up Current off-the-shelf manual and automatic blasting booth units are available in sizes up to 1700mm x 2280mm (67” x 90”). In addition to these stand-alone models, Vapormatt, Inc. offers a scalable water collection and filtration system called the “WaterWave”, which is primarily used for manual blasting of large components. The WaterWave is a modular system that can be sized to meet the proposed paint removal facility requirements, and, with some additional design and development, it can be used as an automated de-paint facility. However, significant changes in the infrastructure will be necessary. Depending on the size, several pump stations may be needed to maintain water pressure. Additionally, the WaterWave technology would need to be altered to handle a larger volume of wastewater. Though Vapormatting has found many applications in the cleaning and abrading industry, it has not been utilized to remove paint from full aircraft or off-air-frame components.



High and Medium Pressure Water Stripping (Aqua Miser)24,25,26 Another wet process similar to the Vapormatting technique is the Aqua Miser Bicarbonate of Soda Stripping (B.O.S.S.) system available from Carolina Equipment and Supply Company (CESCO). This is an open loop system that uses high-pressure water sprayed onto the surface via a handheld spray gun. The system is capable of mixing an abrasive with the high-pressure water at the nozzle, to facilitate removal of more stubborn coatings and/or corrosion. Water is pressurized to approximate 15,000 psi for the small unit (model E-25M and D-44) and 40,000 psi for the large system (model E-75M and D-115). There are multiple types of abrasives that can be utilized with the Aqua Miser system, notably bicarbonate of soda formulations, aluminum oxide, and glass bead. CESCO supplies the following three proprietary bicarbonate of soda formulations:

• Boss Blast – Sodium bicarbonate (NaHCO3) formulation that is good for removing paint from metallic and composite substrates,

• Boss Blast II – Sodium Sesquicarbonate (Na2CO3-NaHCO3-2H2O) formulation that effectively removes corrosion and paint from metallic and composite surfaces, and

• Boss Blast III – A water-soluble non-toxic proprietary media with a neutral pH. This is the most aggressive formulation and it is primarily used to remove heavy rust and paint from steel surfaces.

The Aqua Miser B.O.S.S. system can be purchased as a portable unit, making it attractive for quick deployment and touch-up applications. To provide a complete process when using portable models, Aqua Miser offers wastewater-handling equipment that filters out sludge and discharges filtered water into the sewer. A variety of spray guns are available for multiple applications.

Requirements

Set-up The Aqua Miser blast system is designed for convenience, lessoning the requirements for building modifications or ancillary capital investments. The Aqua Miser B.O.S.S will require some basic utilities. 24 High and Medium Pressure Water Paint Stripping Processes, Joint Service Pollution Prevention Opportunity Handbook, August, 2002. Retrieved February 13, 2003, from http://p2library.nfesc.navy.mil/P2_Opportunity_Handbook/5_5.html. 25 Waterjet Stripping, Joint Service Pollution Prevention Opportunity Handbook, August, 2002. Retrieved February 13, 2003, from http://p2library.nfesc.navy.mil/P2_Opportunity_Handbook/5_5.html. 26 Larry Fulmer, President, Carolina Equipment & Supply Company, Inc., personal communication, January 27, 2003.



Electrically Powered Units: • Compressed Air Supply (30 SCFM at 100 psi) • Clean Fresh Water (6 GPM at 45 psi for E-25M) or (6 GPM at 60 psi for E-75M) • 70 amp service of 460 volt, 60 cycle 3 phase electricity

Diesel Powered Units: • Clean Fresh Water (6 GPM at 45 psi for D-44) or (6 GPM at 60 psi for D-115) • (Note – Diesel model comes with its own air compressor)

The Aqua Miser B.O.S.S. is an open loop system that requires the use of wastewater handling equipment to meet state and federal regulations. CESCO offers several models that treat the wastewater before it is discharged into the sewer. These same models can be incorporated into a B.O.S.S. Blast Containment Room, which is a closed loop system that utilizes the high-pressure water stripping technology. Actual wastewater treatment equipment will vary depending on the proposed or existing facility needs. Operation Electrical energy is required to power the E-25M and the E-75M Aqua Miser B.O.S.S. systems. The availability of diesel fuel is required when utilizing the D-44 and the D-115 models. The diesel units have a built in air compressor, but the electrically powered units require the availability of compressed air. CESCO recommends that two operators be present when using the Aqua Miser B.O.S.S. system. Actual labor requirements depend on the size of the objects to be de-painted and the desired stripping rate. In addition to the labor, the aircraft must be prepared prior to paint removal to protect sensitive areas. Once the bulk stripping process is completed, the aircraft must be washed down to remove any remaining abrasive residue. The presence of the residue does not damage the substrate, but it can cause runs on the surface after the aircraft is painted if not rinsed properly. Safety Operators in a blast environment require shin and foot guards, waterproof clothing, and hearing protection. Additional safety measures may be required depending on the facility. Appropriate safety measures should be established prior to transporting diesel fuel. Economics The capital investment for an electrically powered Aqua Miser B.O.S.S. system is approximately $65,000 for the E-25M, or $70,000 for the E-75M. Diesel models range anywhere from $53,000 for the D-44 to $78,000 for the D-115. Each of these models is available with the base accessories, such as two spray guns, and a high-pressure hose assembly. Optional equipment is available to meet specific needs.



In addition to the base models, wastewater treatment equipment is an additional expense. Manual equipment, such as the AquaKlean C50M & C80M, will range from $8,000 to $10,000, depending on the capacity. The automatic AquaKlean models are significantly more expensive, from $40,000 for an 18-gpm unit (C100A), to $53,000 for a 30-gpm unit (C150A). Initial costs for the B.O.S.S. Blast Containment room, including a waste reclaim floor and a wastewater treatment system is approximately $100,000 for the 10 foot wide by 15 foot long by 10-foot high model. For the 20 foot wide by 30 foot long by 15-foot high model, the cost is approximately $175,000. Operating costs would include the following:

• Electric power for a 70 amp, 460 volt, 60 hertz, 3 phase line (to provide energy to the system and a 25 horsepower motor)

• Electric power for the AquaKlean systems, capable of providing energy to a 2 to 5 horsepower motor

• Compressed Air Supply System, capable of providing continuous 30 scfm at 100 psi service

• Control center (if using the containment room) • Labor costs for one supervisor and two operators

Actual costs will vary depending on local utility costs and labor rates.

Performance

Controllability When the Aqua Miser B.O.S.S. is used as a manual system, control will depend on operator skill. Given appropriate training, the high water pressure is capable of removing the top layers of paint down to the primer. The addition of a light abrasive, like the Boss Blast, permits the removal of the primer coat. Due to variability inherent in a manual process, it may not be possible to remove individual paint layers. Effectiveness The Aqua Miser B.O.S.S. system is highly effective when using water mixed with abrasive. When using only water, the primer coat remains, which may be beneficial in some applications. Wastewater treatment is centrifuged and treated, but the level of cleanliness may be less than that achieved with an ultraviolet system. Some facilities actually soften the paint with chemical strippers prior to blasting to improve effectiveness. Versatility Unlike an automated process, the spray gun is capable of accessing areas of the aircraft that would be difficult or impossible in robotic applications. The operator can control the dwell time to remove stubborn coatings and adhesives, such as glue and filler.



Adaptability and Scale Up Nozzle based systems have been successfully automated in the past. The automated wastewater handling and treatment system (B.O.S.S. Blast Containment room) can be augmented with a robotic system to increase the productivity of the stripping process. As with DMB processes, an automated system would likely require manual touch up depending on the item to be de-painted. TOUCH METHODS

The primary touch processes commonly in use at de-paint facilities are sanding and grinding. Sanding is used for paint finishing applications, whereas grinding is used for paint removal tasks. Though the objectives of sanding and grinding are different, the tools involved are similar; the difference being in the type of abrasive used. For this report, the tools involved with sanding or grinding will be lumped together as one type of touch process. Abrasives will be identified as applicable to sanding, grinding, or both. Planing is an alternative paint removal process that may find use in aerospace applications. This report provides a brief discussion about planing tools and how they can be ported to the aerospace de-paint industry. Types

Sanding and Grinding

Sanding and grinding are two of the original paint removal and paint preparation methods. Sanding is a smoothing or dressing process that occurs by rubbing an abrasive material over the painted surface. Sanding is used commercially and in the military for preparing substrates for paint application and to prepare a painted surface for an additional layer of paint. The dust generated by the sanding process for military applications is typically captured by a vacuum system and disposed of as hazardous waste. The sandpaper or abrasive is disposed of as solid hazardous waste. The grinding process typically occurs in multiple steps. Highly abrasive material is used during the beginning of the grinding process. Less abrasive materials are selected as the coating is removed so as not to damage the substrate. De-paint quality is typically dependent on the experience/skill level of the operator. Substrates, particularly composites, can be damaged due to the operator’s lack of experience. Hand grinding is a slower process than power tool assisted grinding but is less likely to cause damage to the substrate. Though the potential exists to de-paint an entire aircraft by grinding, it is currently viewed as a secondary touchup process to complement other methods of de-painting due to economic considerations.

Planing

By definition, “planing a surface” is to make it smooth and even by sliding a cutter across the surface in a linear direction. Planing is a method that is typically used to reduce material thickness or to remove paint and scale from a variety of substrates. Under appropriate



circumstances, planing may be a rapid paint removal method. Planing is commonly utilized in the woodworking industry for substrate preparation and paint removal. Planing requires a flat surface to operate efficiently and to control the depth of paint removal. Though depth control may be accurate, the effectiveness of planing may be compromised when encountering contoured surfaces. Requirements

Set-up

Initial requirements are abrasive stock and power tools. In order to utilize power-assisted sanding, grinding, and planing methods several off-the-shelf tools will be required. For example, power-assisted sanding and grinding is accomplished with a Right Angle Grinder, Right Die Grinder, or Random Orbital/Dual Action Grinder. Each tool has particular attributes that make them ideal for specific applications. For instance, the right angle grinder provides an additional handle to allow for higher load applications. The right die grinder is similar in concept to the right angle grinder; however, the lack of the additional handle relegates this tool to less aggressive applications. A random orbital tool operates as a dual action motion device. First, the pad oscillates in a small circular orbit, while attached to a shaft that is parallel to the main drive assembly. The main drive assembly is also offset, generating an elliptical orbit. Though the offsets are small, the combination of motions produces a random motion to the sanding/grinding pad, which evenly distributes the load across the substrate allowing for uniform paint removal. Unlike the aforementioned tools (right die grinder), the random orbital sander will cease operation under excessive pressure, thereby minimizing the potential for substrate damage. Random orbital tools are available with several styles of grips, allowing for more or less control of the tool depending on the accuracy of the application. Palm-grip models are the easiest to hold and control, since the tool fits into the hand allowing for maneuverability against contoured surfaces. Right-angle models can be purchased for applications requiring more power at reduced sanding speeds. The larger motor permits the operator to push down on the surface without losing motor power, making the tool useful for more resistant coatings. In-line sanders require the use of two hands to operate, utilize the most powerful motors, and are commonly used to remove paint from large flat surfaces. The sanding and grinding tools use shop vacuum to extract the paint dust or collect dust into a canister that attaches to the tool. The abrasive pads provided by 3M come equipped with a number of holes, which must line up with the holes in the backup pad in order to efficiently remove dust. The abrasive pads are attached to the backup pad with the traditional “hook and loop” method, where the fibers on the back of the abrasive pad become intertwined with loops on the interface pad (much like a Velcro attachment). Hook and loop attachment was created to combat situations where dust compromised adhesive backed abrasive pads. 3M has recently introduced a new method of non-adhesive attachment called Hookit™ II. This method is simply the inverse of the hook and loop method, as the hooks are present on the back of the abrasive pad, and the loops are located on the backup pad. Hookit™ II was designed to



enhance the life of the abrasive pad (by up to four times), as the former hook and loop system was determined to wear out prematurely. Another product that shows promise for more efficient dust extraction is the DustMaster™ Supreme Abrasive by Clayton Associates, Inc. This pad is constructed as an open mesh covered with abrasive. The open mesh allows for approximately 135 holes per square centimeter, or about 28,000 holes per pad. The dust travels a maximum of ½ millimeter to reach the nearest hole and is then vacuumed through the interface pad. In order to enhance the durability of the system, the open-mesh abrasive is attached to an interface pad that has 21 holes, and this pad is attached to a backup pad called a “yellow crown”, that has a more common 6 holes for dust extraction. Moreover, the open-mesh abrasive attaches to the interface pad with a similar technique as the Hookit™ II, permitting longer attachment interface life. The use of an open-mesh design mitigates the caking and pilling of dust, which is commonly found on standard five to eight hole abrasive pads. Since the DustMaster Supreme is an open-mesh pad, it may lack strength when compared to a fully solid pad. Therefore, SwRI recommends conducting a study on the integrity of the pad under common loading conditions. Planing is similar to sanding and grinding in that it attacks the coating by chipping and scraping. Tools used for planing are less forgiving than sanding and grinding tools since there is no compliance in the blades. Planing is an aggressive process that if not properly controlled can rapidly remove substrate material. An example of a planing tool is the Metabo Planer Model No. Ho 0882. It has an adjustable cut depth of 0 to 3 millimeters and an 82-millimeter cutting width. Such tools may be useful for off-airframe components with flat surfaces. Like sanding and/or grinding tools, the quality of paint removal is operator dependant. Unlike the aforementioned tools, excessive labor may be involved to continuously adjust the blades for variable cut depths, which is commonly found in paint preparation and removal operations. Use of such a device is dependant on surface geometry and paint removal needs.

Operation

For manual sanding, personnel will require training in order to determine the appropriate abrasive to use with a particular coating layer and substrate. The quality of the de-painting is typically dependent on the experience/skill level of the operator. Substrates, particularly composite substrates, may be damaged due to the operator’s lack of experience. Automated sanding will require technical operators skilled in both computer controls and proper sanding techniques. Though abrasive pads find frequent use in aggressive applications, such as weld line grinding, there are varieties available that may be used on composite materials, which is more susceptible to sanding and grinding loads. These pads are configured to allow for extra compliance when



encountering contoured surfaces while simultaneously maintaining a long life with sharp wear-resistant materials such as aluminum oxide and silicon carbide. 3M is one of the leading manufacturers of abrasive technology, and have developed a series of abrasive pads that offer promise when removing paint from a composite substrate. The following is a short list of some of the most promising pads and discs for removal of paint from composite substrates.

3M Roloc Bristle Disc This is a disc comprised of many abrasive impregnated strands. As the disc wears, fresh abrasive is exposed, allowing for optimal performance throughout the life of the disc. This product is available in a variety of sizes and abrasive grades, permitting its use for fine finishing operations, paint and rust removal, and gasket removal. The small size of the disc (1” to 3” in diameter) allows the operator to access tight areas such as grooves and blind corners. Moreover, the abrasive impregnated strands have some compliance, which resists excessive loading; subsequently producing a consistent finish with only light pressure. This disc is fabricated with two types of abrasive, in grades 50, 80, and 120, and it is used on steel and aluminum substrates. 3M Bristle Discs Like the Roloc disc, the Bristle disc is comprised of abrasive impregnated bristles. However, the bristles are radially aligned with the axis of rotation. Multiple discs can be stacked on one spindle allowing for greater coverage. The bristles are flexible, and conform to the contour of the surface, allowing for fine finish work while reducing the possible for operator damage due to too much applied force. These discs are good for scuffing the metal before painting, removing gasket material, and selective paint removal. The 6-inch bristle discs come in 3 grades, 36, 50 and 80 with the Cubitron™ abrasive grain. The 3-inch brushes use the same abrasive and come in grades 50, 80, and 120. 3M Green Corps (Type 27) This product line comes in several varieties. The Green Corps Depressed Center Wheel is ½” thick, has an attachment point beneath the plane of the disc, and is comprised of Cubitron™ abrasive grain. This allows for a highly robust physical attachment while simultaneously removing heavy stock, slag, and burrs on mild steel, cast iron, and stainless steel. It is available in grades 24 and 36. This pad was designed to be competitive with fiber discs and high-performance grinding wheels. The Green Corps Flexible Grinding Wheel is similar in construction to the Depressed Center Wheel, but it is only ? ” thick. This version is used in metal fabrication and general maintenance. It is available in grades 36 through 80.



3M Imperial Disc The Imperial Disc is a flat disc used with right angle die and orbital sanders. It is available in grades 36E, 40E, and P80E-P120E. The discs come in three sizes: 5, 6, and 8 inches. The purple discs contain Cubitron™ abrasive and are supported by a durable E weight paper backing. The Imperial discs are attached via 3M™ Stikit™ or Hookit™ II attachment system. 3M Production Disc The production disc is useful for finish sanding of plastic filler, rough featheredging paint, fine featheredging paint, sanding putties, and final primer/surface sanding. It is available in grades 80-P500, in both 5 and 6-inch diameters. It is gold in color and meant to be used for dry sanding only. 3M Type D Disc (Open Coat) This version is recommended for paint removal, metal work, and fiberglass grinding (marine applications). It is available in grades 16 through 36, and comes in 5, 7 and 9-inch diameters abrasive discs. The discs are brown in color, and contain a durable fiber backing.

In addition to the sanding discs, 3M makes a variety of scuff sanding pads and discs. The scuff products are generally used for cleaning, polishing, removing gasket adhesive, and preparing a surface for painting. Some product examples are: ScotchBrite Scuff Rolls, ScotchBrite Discs, and ScotchBrite wheels. One recent innovation by 3M is the 3M SandBlaster sanding paper and sanding sponge. The SandBlaster material is used to prepare bare wood, metal or drywall. The material is an electrostatically coated ceramic aluminum oxide mineral blend. 3M promotes this product as having the strength to outlast and outperform conventional sandpaper on such materials as paint, wood, metal, plastic and fiberglass. Mirlon abrasive pads (scuff pads) are one of the fastest growing technologies in the abrasives industry. Also known as "Synthetic Steel Wool" or a "Steel Wool Substitute" these pads are being commonly used anywhere you would normally use steel wool. The primary advantages are that they last longer than traditional steel wool and do not rust when exposed to water or water based solutions in rubbing out finishes. Mirlon Abrasive Pads are available in two types, very fine or ultra fine. They are supplied in a 6" x 9" pad and are packaged in convenient job packs containing 2 pads as well as boxes of 20 pads. Automated sanding, polishing, and deburring technologies are available. However, the traditional approach is to use a robot to manipulate small parts that are moved up against various sanding discs and grinding wheels.



More advanced automated approaches utilize a robot with specialized tooling and sensors to handle abrasive pads and to perform complex sanding motions. This approach to paint removal requires costly hardware and software development that is specific to the application and mimics human sanding techniques.

Economics

Electric or pneumatic power hand tools range from $75 to $500 and may use air or electricity. Quality handheld planers will cost approximately $200. A roll of 3M Imperial Discs (125 pieces) with the Stikit attachment system will cost approximately $50 to $100 depending on the disc diameter. 3M Roloc Bristle Discs usually come in boxes of 25 at an estimated cost of $30-$40. For each case, the backing pads are an additional charge of $20-30. Fully automated robotic sanding systems may cost as much as one million dollars depending on the size and complexity of the system.

Safety

Heavy gloves, eye protection, and respiration equipment is required for power sanding, grinding, and planing operations. In addition, air reclamation may be required to maintain a minimum air quality level. Note: All of these grinding wheels REQUIRE the use of a guarded tool per ANSI Standard B7.1. These wheels are NOT to be used as cut-off wheels. Performance

Controllability

Hand sanding, though a tedious process, allows for accurate control on the amount of paint removed from the substrate. Power tool assisted sanding is a faster process than the hand sanding approach but the paint removal rate may be too aggressive, diminishing the ability of the operator to control the paint removal rate. The two primary variables for controlling paint removal are the type of abrasive and the specific attributes of the power tool.

Effectiveness

Sanding is very effective for removing paint on flat or nearly flat surfaces. Contoured surfaces present more of a challenge for power assisted sanding techniques due to point or line contact with the substrate. Sanding is not an effective means for removing paint in blind corners. Additionally, sanding is typically not a good method for rapidly removing paint uniformly.



Planing is very effective for removing paint on flat or nearly flat surfaces. Like power-assisted sanding, point or line contacts are possible when engaging contoured surfaces, potentially causing damage to the substrate. Due to the nature of the method, blind corners or depressed features cannot be accessed with planing tools.

Versatility

For surfaces that can be accessed, sanding, grinding, and planing are all quite versatile, since the techniques are deployed with portable equipment. The tools are generally low in mass, allowing for integration with a robotic end-effecter. Additionally, the utility requirements are either electric or pneumatic power, which are commonly found in de-paint facilities.

Adaptability and Scale Up

The touch methods for coating removal investigated in this section are appropriate for treating localized areas and for touchup paint removal. However, none of the aforementioned methods are economically feasible for de-painting large areas. Automating the touch processes for large areas would be challenging since a variety of material grades are required to properly remove paint without damaging the substrate. For a fully automated sanding system, additional variables, such as sanding forces, must be considered. ADVANCED TECHNIQUES

As preventative maintenance gains more importance with an aging military fleet, so do the costs associated with maintaining the flight readiness of aircraft. New technologies emerged from the need for less costly means of paint removal and waste material processing. This section considers two technologies that have been implemented or are under development at military facilities across the United States. The technology has been refined over the past several years, making them more feasible considerations for de-paint production use. Laser Ablation27,28,29 Laser paint stripping was developed in the late 1980’s, and incorporated into several military facilities across the country. These facilities use laser paint stripping for a variety of tasks, such as helicopter rotor blade cleaning, and radome stripping. Laser paint stripping operates by applying a square or rectangular focused beam of energy onto the surface. Due to the extreme temperatures, the laser beam is pulsed at a specific frequency. Each pulse removes a small

27 Joint Paint Removal Study, Joint Policy Coordinating Group on Depot Maintenance, Tasking Directive 1-90, Final Report on Laser Paint Removal, Joint Depot Maintenance Analysis Group, Technology Assessment Division, (n.d.). Retrieved January 31, 2003, from http://www.jdmag.wpafb.af.mil/lasbod2.pdf. 28 Pollution Prevention Success Stories, Hill AFB. January 1999, Retrieved January 31,2003 from http://www.afcee.brooks.af.mil/pro-act/success/hill/hill.pdf. 29 Ralph Miller, General Lasertronics Corporation, personal communication at General Lasertronics Inc. facility, February 13, 2003.



amount of paint in a process called “ablation”. By definition, ablating a surface is to remove a coating by cutting, eroding, melting, evaporating, or vaporizing. The laser energy, pulse duration, and power are constant during the paint removal process. Once the paint is removed from the surface, the paint ash residue is vacuumed into a filtration system where it is accumulated for non-hazardous waste disposal. This technique of paint removal is a non-intrusive, low-energy process that requires a minimal amount of surface preparation and post processing activities. The waste from this process is less than the initial volume of paint applied, making laser paint removal an attractive alternative to existing chemical stripping techniques. Laser paint removal eliminates the use of hazardous materials and associated large volumes of hazardous waste common with some other methods. Substrate damage due to corrosion is eliminated, since the laser does not initiate a chemical reaction with the atmosphere. Laser stripping is a reproducible process, in that the surface can be inspected automatically for sufficient paint removal. In a robotic system, the laser is located on the end-effector, which houses a spectral camera. The camera is used to compare the surface color after a small amount of paint is removed with the control color stored in the computer. Since the laser generates high temperatures, the laser cannot be applied to a specific location for an extended period of time without damaging the substrate. Therefore, one unit researched rasterizes the area to be stripped into a grid of thirty 1-centimeter squares. The laser stripping system follows a pre-described pattern in the rasterization. If more paint needs to be removed at a particular point, the laser stripping system returns to the incomplete square once it has a chance to cool. The rastering technique is especially important for the removal of coatings from composite substrates. Unlike a PMB system, which speeds up or slows down based on the amount of paint removal, laser stripping will revisit specific locations of the surface until the color of the substrate matches the control. This can extend the processing times, reducing the overall volumetric paint removal rate.

Requirements

Set-up An automated paint removal system that ablates paint via a pulsed laser requires the integration of a robot, multispectral camera, end effector development, waste management system, system controller, and an interconnected safety system. The robot and laser can be purchased as off-the-shelf components, but additional development costs will be required in order to integrate the robot into the paint removal system. In addition to the hardware, there will be minimal system training costs. The waste removal system will need to extract the ash as it is being ablated from the substrate via a vacuum system. Hence, compressors and blowers of sufficient capacity will be needed to ensure that the waste stream is being collected. Waste is processed by first separating the waste into particulate matter and vapors. Once the particles are filtered out, they are dried, placed in storage containers, and sent to the landfill as non-hazardous waste. The vapors are oxidized and



converted into carbon dioxide, nitrogen, and water vapor. Since this process relies on the use of a high-power laser, the paint removal booth must be classified as a Class IV Laser enclosure. Operation Operation of a laser ablation system requires an adequate power supply in order to use a laser of 400 watts. Since the laser ablation system is approximately 10-15% efficient at the wall outlet, a 220 VAC 30 amp power supply is recommended. The lamps in the laser ablation system must be changed every 500 hours of operation. The diode pump has a life of approximately 10,000 hours. As part of the laser ablation process, an air cleaning system (HEPA) is needed to filter the paint ash from the air. Additionally, ash removed via ablation, must be collected in a waste management system. In the laser ablation process, the waste is normally less than the original amount of paint applied, at approximately 30% waste and 70% vaporized. Since a Class IV laser is in use during laser ablation, eye protection is required to prevent the potential for injury from direct or reflected rays. Safety Class IV lasers are high power devices, and the direct beam or even diffuse reactions are hazardous to the eyes and skin. Class IV lasers are also considered a fire hazard, so care must be taken as to the type of reaction when struck by the laser. The operator is required to wear eye protection if they are within 20 feet of the system. All reflective material must be removed from the enclosure, since eye damage can occur from reflected beams of class IV laser light. In order to ensure the maximum possible safety, enclosure interlocks are recommended to shut the laser down (or conceal the laser), when an operator enters the enclosure. Since fuel vapors can be present behind panels on aircraft, laser ablation system used on aircraft must be equipped with sufficient vacuum recovery heads. Providing sufficient airflow through the head, the laser units have been certified as Class I Div 1. Economics When the Laser Automated Decoating System (LADS) was installed at Hill Air Force Base in Ogden, Utah, it required an investment of approximately $6 million dollars for the laser removal system. The Government spent an additional $2 million dollars over a period of two years to develop the system to usable unit. Today a laser ablation systems, at a power capacity of 400 watts and a footprint of 25 sq. ft., will cost approximately $400 thousand dollars. The actual laser unit costs about $200 thousand dollars, and the rest of the cost accounts for development, installation, and integration of the system along with the support and control equipment.



Operating costs would include the following:

• Electrical power for a 400 Watt Class IV Laser, 200 hp air supply system, chiller, power supply rated at 30 amps @ 220VAC, robot, and control center (Cost depends on usage and local electrical costs)

• Labor costs for one supervisor and two operators • Cost of several bulbs and filters over a year’s time

Performance

Controllability Paint stripping via laser ablation is highly effective at layer removal, since the stripping process is controlled by color recognition software. With the advent of modern color cameras, it is possible to accurately match colors with minimal error. In addition, a small amount of material is removed from each pass, mitigating the potential for “hot spots” or areas where the paint surface is uneven. Plus, though programming changes, the system can remove paint and other materials of variable thickness. Effectiveness The laser ablation system successfully removed 90-95% of sealant from an A-10 wing, and 99% of the filler material in an F-22. The paint removal rate is approximately 3.5 sq. ft. per minute when ablating a 0.001” thick layer. Though the laser ablation system is effective with paint removal, incompatible materials may not be removed. In this situation, manual clean-up costs will increase to finish stripping the aircraft. Versatility In addition to paint, the laser ablation system can remove glues and sealants. Care must be taken to ensure that the material to be removed is not flammable (such as bondo). Moreover, stripping of room temperature vulcanized (RTV) material is not recommended, as it “bubbles up” when exposed to the laser. Adaptability and Scale Up The system is comprised of components that are installed outside the operations booth. Though the area of paint removal is small, it is scaleable to handle stripping larger systems. More study is needed to determine the change in power requirements when supplying booths that can house an unassembled aircraft. Laser ablation systems are open to automation, which is the recommended technique when using this process.



Flashjet™ 30,31 Flashjet™ uses a combination of two processes previously researched for paint removal. The first process utilizes a quartz tube filled with xenon gas, which pulses when energized to remove paint from the object’s surface. The second process is CO2 blasting, which is used by Flashjet™ to perform several functions. Since the CO2 has to be kept extremely cold to maintain solid form, the process cools the object’s surface as well as removing soot from the de-painted area. The CO2 process also cleans the lamp and provides a non-combustible condition at the work head. Flashjet™ was patented in 1991 by McDonnell Douglas Aerospace (MDA), which is now part of Boeing St. Louis. Flashjet™ has since been purchased from Boeing St. Louis by a group of private investors under the name Flash Tech, Inc. PAR Systems provides the automation for Flash Tech, which is generally a robot mounted on either a gantry or mobile lift unit. There are Flashjet™ systems currently in production use at WR-AFB, GA (radomes), Kingsville, TX (T-45’s), Corpus Christi, TX (helicopters), and Mesa, AZ (helicopters). There are also prototype models in demonstration use in Singapore and at NADEPJAX, FL

Requirements

Set-up Flashjet™ requires several pieces of equipment for operation. In addition to the actual flashlamp work head, a facility also requires the basic components listed below.

• CO2 pelletizer unit with CO2 storage system • 200 hp air supply system • Chiller for cooling water • Power supply providing 100 amps @ 480V-3 phase for the flashlamp • Effluent Capture System (ECS) including pre-filter and HEPA filter followed by charcoal

canister for handling process waste • Robotic system with gantry or mobile unit • Process control center

In addition to these components, a building may have to be constructed if existing space is not available. Flash Tech would design and construct a system considering a customer’s specific needs and the object(s) to be de-painted. The system could be either purchased or leased from Flash Tech by the customer. No specific certification is required for system operation, although there should be one individual present for each shift familiar enough with the system to be capable of basic troubleshooting.

30 Wayne Schmitz, Director, Technical Services, Flash Jet Inc, personal communication, February 25, 2003 31 Hank Dingfelder, Boeing St. Louis, personal communication, January 27, 2003.



This individual may be an employee of the customer, however a Boeing employee oversees the system at WR-AFB. Operation Electrical energy is required to power many of the components listed in the above section. Besides electrical requirements, the WR-AFB system also uses approximately 11,500 lb. of CO2 per day and several bulbs over the course of a year. For de-paint of radomes at WR-AFB, there is generally one Boeing employee supervising two NC grade operators. The operators are separated from the de-paint process during the majority of the operation, so safety equipment is only required when the operators must enter the work area. Safety The WR-AFB system was thought to be safe enough to stand near during use, but test results indicated booties, gloves, and a dust mask are required inside the work area. Fortunately, most of the time is spent inside the control room where no safety equipment is required. The Flashjet™ does produce some noise and bright lights, which are potential safety concerns. Flash Tech personnel recommended the use of shaded eye protection for those onlookers who chose to view the process up close (within several feet). The prototype unit at NADEPJAX was only demonstrated off-hours due to noise complaints by production staff. Flashjet™ could cause damage to fiberglass, composites, and possibly even thin metals if misused. However, if used properly and stripping down to bare substrate isn’t necessary, there is little chance of damage. Testing has been performed on metals and composites and results can be found in studies such as the Naval Air Warfare Center Technical Report on Aircraft De-painting Technology. Paint waste is recovered from the work head and contained by the ECS providing clean air discharge to the atmosphere. The paint waste requires disposal as hazardous waste. Economics The system at WR-AFB initially cost approximately $3.5-4M, and an additional $500K for the building. Although the system is mainly used on radomes, the gantry robot is capable of stripping much larger objects. The WR system’s operating costs include the following:

• Electrical power for a CO2 pelletizer unit, 200 hp air supply system, chiller, power supply rated at 100 amps @ 480V-3 phase, robot, and control center (Cost depends on usage and local electrical costs)

• Approximately 11,500 lb. CO2 per day (1999 CO2 cost was $0.05/lb.) • Labor costs for one supervisor and two NC operators • Cost of several bulbs and filters over a year’s time



Performance

Controllability The flashlamp moves across the object’s surface pulsing at intervals to remove the paint from one patch at a time. Due to the nature of the process, it is difficult to select layer removal. Operators can adjust speed and intensity, but often repeat the depaint process to achieve desired stripping. The difficulty in doing this is to repeat the process enough times to remove the desired amount of coatings in some areas without damaging the substrate in other areas. Effectiveness In one study Flashjet™ was shown to remove coatings at 2.0 to 4.0 sq. ft. per minute to the primer and 1.6 to 2.8 sq. ft. per minute to the substrate. Paint thickness of course greatly affects de-paint time. Radome coating removal can range from a few hours to a couple days depending on the paint. The systems at NADEPJAX and WR-AFB left behind areas of raw substrate and primer after one pass. Some areas still showed darker where graphics had been as well. The helicopters at Corpus Christi appeared to have more uniform coating removal results. Since the paint removal results from a flash, small shadows of paint were left behind most of the numerous rivets found on the surface of the helicopters. Versatility The process is effective on most sprayed coatings, but not as effective on fillers. Since the head is a large surface requiring fairly accurate positioning, the system works best with large relatively flat areas. Due to its geometry and robotic manipulator, Flashjet™ is somewhat limited in the areas it can reach on aircraft. The process can remove coating from all areas of the simple geometry of a radome except the top several inches of the tip, which has to be sanded later prior to painting. Flashjet™ is estimated to be capable of de-painting approximately 60% of the surface areas on helicopters, and over 80% of the area on a P3. Adaptability and Scale Up Flashjet™ systems are installed automated. Due to the size of the work head, necessary support equipment lines, and precision requirements, it has not been feasible to construct a manual unit. Even if an operator controlled system was requested by a customer, it is likely Flash Tech would have to use some kind of robotics to manipulate the work head through operator input.



EXPERIMENTAL METHODS32

Microspheres33 Microspheres are tiny spherical particles that can be made from a variety of materials such as glass, ceramic, carbon, or plastics. Microspheres are used widely in the medical industry where the process is typically referred to as micro-encapsulation. However, they have been successfully used in other industries such as plastics, printing, and painting. Microspheres can be used to change the physical properties of materials, such as weight, toughness, durability, and surface smoothness. Microspheres are typically composed of a small amount of hydrocarbon liquid, encapsulated in a plastic shell. When heat is applied to the coating, they enlarge due to the volatility of the hydrocarbon by 30 to 50 times. The expansion properties of the spheres enable new processes for making foams, and three-dimensional (raised) prints. In the paint industry, microspheres are commonly used as filler material, to reduce the bulk density of paints, while providing better application properties. In the area of coatings removal, microspheres have been successfully used as a release agent. In this case, unexpanded microspheres are mixed into the paint, and when dried they remain unexpanded. When the paint is to be removed, it is heated to at least one hundred degrees C. Then, the coating cracks under the pressure due to the expansion of the microspheres. One heating method employed under experimentation was to dip the painted part into a high temperature water bath to both activate the microspheres and wash away the paint residue. This technique could be adapted to aircraft by using a spray application of the water. However, safety issues arise when spraying high temperature water. Another experimental technique was to mix ceramic microspheres with the hydrocarbon (in this case, pentane gas) microspheres. The paint was removed after applying microwave radiation for a few seconds. The microwave radiation essentially heated up the ceramic, which in turn heated the pentane microspheres. Of course in an aircraft application, microwaves do pose other technical and safety challenges, such as equipment radiation hardening, and personnel shielding. Microspheres have finite life spans due to the semi-permeability of the plastic shell. New microsphere formulations should be explored to maximize the lifespan of the microspheres. While certain brands of microsphere have been tested and found to be compatible with many chemicals and solvents, the reaction to the chemicals in RAM coatings is unknown. Testing should be performed to determine compatibility or to formulate customized microspheres for use with the RAM coating. It is also unknown how the high metallic content of the RAM coatings will interact with the expansion of the microspheres. Additional testing would also be necessary to determine the longevity of the composite substrate with repeated coating removals. Given the

32 Ted Bates, Boeing Company, personal communication, February 13, 2003 33 Expancel® Microspheres Introduction. (n.d.) Retrieved February 12, 2003, from www.expancel.com/english/about/Default.htm



satisfactory completion of the above testing, microspheres may offer the potential for a low-cost, commercially available method for removing aircraft coatings. Zero Added Waste Cutting, Abrading, and Drilling (ZAWCAD)34 35 Also known as cryogenic blasting, ZAWCAD is a technology developed by the Idaho National Engineering and Environmental Laboratory and was later spun off to form ZawTech International. The technology uses liquid nitrogen stored at low temperature and very high pressure (60,000 psi), which is ejected through one or more small orifices either as a jet for cutting applications or as a spray for cleaning/de-paint applications. At this pressure, the nitrogen becomes something between a solid and a super-critical fluid. When ejected from the nozzle, the nitrogen acts much like a dry media, which impacts the surface and abrades the paint. Additionally, the nitrogen expands at a very fast rate as it turns into a gaseous state. The expansion has the effect of blasting the paint from the surface. Due to the Joule-Thompson effect, it also heats up, rather than cools down like most expanding gases. A ZAWCAD system was reportedly sold to the Boeing Company through Progressive Technologies, for use in coatings removal at the Boeing refurbishment plant in Wichita, Kansas. This is the only known commercial installation of the technology for coatings removal. The advantages to using liquid nitrogen are numerous. There are no secondary waste streams such as water, or media. Nitrogen is plentiful, however there are refining, liquefying, and storage costs. Nitrogen is safe, posing no hazard of explosion, fire or oxidation reaction. This technology not only removes paint coatings, but also rubberized coatings such as wing tank sealant. Cleanup is much faster than other technologies. One disadvantage is that the liquid nitrogen cutting jet is so powerful that is poses a serious safety hazard to human operators. The main disadvantage is that according to sources at Boeing, the nitrogen expansion effect would disintegrate composite materials. For this reason, it is unsuitable for use on composite skinned aircraft. Ultrasonic Microcavitation36 This technology is currently under development at Kansas State University under Dr. Shamir Madanshetty. The project is being funded with money from a private company, which owns the rights to the technology. The technology uses a shaped ultrasonic pulse to create micro-bubbles on the surface of the part. When the trailing edge of the pulse passes, the micro-bubbles implode, blasting the coating from the surface. The technology has been tested on aircraft parts by submerging them into a tank of water, which acts as the sound pulse wave medium. The inventors believe that the technology

34 DOE News Media Advisory. (1998). Retrieved February 12, 2003, from newsdesk.inel.gov/press_releases/1998/przawtechspinsout2.html 35 Beckman, Mary, Featured Research: A Super Cool Cutting Tool. (1999). Retrieved February 12, 2003, from www.inel.gov/featurestories/8-99zawcad.shtml 36 Dr. Shamir Madanshetty, Kansas State University, personal communication, February 20, 2003



could be reconfigured to work without the full submersion of the part. Rather, a wand, much like the head of steam cleaning machine, would create enough wetted surface with the part to carry the sound wave. It is believed that the wand would not have to seal against the surface or even conform closely to the surface for the technology to work. The portable hand tool has not yet been developed, but should be under development by the end of 2003. Inventors claim that the technology would work on all surface profiles, most coatings (stiff or flexible), and any substrate. Moreover, the inventors make the assertion that the substrate will suffer no damage. Currently, the gentleness of the technology is being demonstrated by removing ink from Xerox copies, multiple times before the paper begins to degrade. Inventors also claim that the scaled up hand tool strip rate is dependent on the size of the tool, yielding possibly up to 1 sq. ft. per minute. There are several advantages when utilizing this technology, such as low energy requirements, safe operation, and minimal waste. Most of the water used in the process is to be recovered by the wand through a vacuum. The water could then be cleaned and reused. The small amount of wastewater from the process is that which may not be recovered at the tool. There are no known disadvantages. This technology seems very promising and worth further development. CONCLUSION

There are many different coatings removal technologies and various adjustable parameters within each process. Prior to selection of a depaint method, organizations must first determine details of their own application and the relative importance of each criteria researched within this document. The requirements and performance criteria weights could be applied to the various processes in the form of a comparison matrix to aid in deciding which coating removal process would be most feasible. Each facility may have multiple applications, each with respective weights of the criteria and resulting optimal processes.

industry survey and research of depaint methods …infohouse.p2ric.org/ref/51/50104.pdf · industry...

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