An environmentally safe and effective paint removal process for aircraft

JOM, Mar 2001 by Kozol, Joseph

To reduce hazardous waste from fleet and depot aircraft paint stripping and to conform to regulations banning toxic chemical paint strippers, the U.S. Naval Air Systems Team (materials division, depots, and headquarters) teamed with the U.S. Air Force at Warner Robins Air Logistics Center for concept development, characterization, and demonstration of a mature, advanced paint-removal system, the Boeing xenon/flashlamp CO^sub 2^ (Flashjet(R)) process. Extensive metallic and composite-materials testing was conducted. This paper describes the development and characterization program leading to authorization of the process for use on fixed-wing navy aircraft.

INTRODUCTION

During the life cycle of military aircraft, paint stripping and recoating are required periodically for inspection, maintenance, and repair, as well as for changes in paint schemes and special-- purpose coatings. The paint systems used on military aircraft have included epoxy primers and polyurethane topcoats, which are more difficult to remove than the enamels or acrylics used earlier. Because chemical paint strippers used for aircraft have historically contained toxic, hazardous components, aircraft depainting operations are a major source of hazardous-waste generation for the U.S. Department of Defense. Federal and state agencies have begun to restrict the use of these hazardous materials, and government directives require significant reductions in hazardous-waste generation.

When choosing a paint-stripping method for aircraft, a primary concern is whether the method will damage or degrade the properties of the underlying metallic or composite substrate. The U.S. Naval Air Systems Team (laboratories, depots, and headquarters) partnered with the U.S. Air Force at Warner Robins Air Logistics Center to investigate mature, advanced paint-removal technologies. The result of that investigation was a joint navy-air force-industry program for development, evaluation, and qualification of the Boeing synergistic xenon flashlamp/carbon-dioxide pellet (Flashjet(R)) coatings-removal process for aircraft depainting, shown schematically in Figure 1. In addition to the Boeing Flashjet stripping-head assembly illustrated in Figure 2, the system is comprised of a control console, power module, dry-ice particle delivery system, liquid CO^sub 2^ storage facility, compressor, and effluentcapture system. Paint stripping with the Flashjet process was shown to reduce the hazardous-waste stream significantly. The qualification process involved establishing acceptable, reproducible process parameters and testing to demonstrate that the Flashjet process canbe used to remove navy aircraft paint systems selectively to primer or substrate. Also demonstrated was that the process would not damage the mechanical properties of the thin, structural-aluminum alloys or graphite/epoxy substrates used in navy aircraft.

PROCESS OPTIMIZATION

Under a program initiated in 1991 by the U.S. Air Force at Warner Robins Air Logistics Center in Georgia, McDonnell Douglas Aerospace (now the Boeing Company) was contracted to develop and demonstrate the Flashjet process to provide efficient strip rates without damage to sensitive substrates.1 With the sponsorship of the Strategic Environmental Research and Development Program, the Navy teamed with Warner Robins in 1992 to leverage funds and optimize operating parameters for maximum coatings-removal rates and good process control.2 The operating parameters under consideration were input voltage, flash-repetition rate, stripping-- head rate of travel, standoff distance and CO^sub 2^ delivery pressure, feed rate, and impingement angle.

ENVIRONMENTAL SAFETY AND EFFECTIVENESS

Environmental issues were addressed by Boeing during operation of the Flashjet process. The system was demonstrated to capture airborne contaminants and to meet National Emissions Standards for Hazardous Air Pollutants and U.S. Occupational Safety and Health Administration requirements for hazardous air-pollution emissions. In addition, operational-safety requirements were established for ultraviolet-radiation protection and hearing protection.

MATERIALS CHARACTERIZATION

With air force and navy supervision, extensive testing was performed to evaluate the effects of flashlamp/CO^sub 2^ paint-stripping assemblies on the mechanical properties of aircraft aluminum alloys and polymer-composite substrates.2 For these test programs, aluminum alloy and graphite/epoxy laminate panels were painted with epoxy primer and polyurethane topcoat, typical of military paint schemes. Panels were air dried for seven days followed by artificial aging/curing for another seven days at 66 deg C. Paint was removed by the Flashjet process to the primer, to the substrate, and to an abusive saturation condition defined by an increased number of passes (increased dwell time) after stripping to the substrate. Substrate temperatures were recorded using adhesivebacked indicators to detect peak temperatures reached in stripping. Panels equipped with thermocouples embedded from the (unpainted) underside were used to confirm that temperature peaks lasted only on the order of milliseconds, indicating that exposure times and temperatures would not be expected to cause degradation of properties. Subsequent mechanical-properties tests confirmed that degradation would not occur.