The invention described herein generally relates to processes for stripping coatings from various substrates such as metals, i.e., aluminum and steel; composites such as plastics, carbon-based substrates, ceramic tiles, wood, glass, fiberglass building materials and the like. Paint strippers typically used for removal of polymeric coatings contain solvents, water, surfactants, corrosion inhibitors, and viscosity modifiers. Solvents are typically volatile liquids or low-vapor pressure solvents that are not biodegradable. The surfactants are used to promote wetting and penetration of the coating. Viscosity modifiers are predominantly cellulose derivatives, though finely divided silica, clay and synthetic polymers can be used. Corrosion inhibitors are specific to the stripper composition and for protecting the metallic substrates involved. The more environmentally acceptable paint removers contain low vapor pressure solvents containing ammonia, amines, or organic acids; see U.S. Pat. No. 5,215,675 to Wilkins et al., which relates to a stripping composition containing peroxide. U.S. Pat. No. 3,355,385 relates to a process that uses hydrogen peroxide as the bond release agent for stripping organic coatings.
Automobiles and aircraft are painted to protect the substrates from corrosion and to enhance the cosmetic appearance to help market the product. The prior art has utilized many different compositions and methods for the removal of paint from different substrates. For example, some paint stripping compositions utilized highly caustic, alkaline solutions as demonstrated by Murphy in U.S. Pat. No. 3,766,076 and by Sullivan in U.S. Pat. No. 3,980,587. However, caustic solutions aggressively attack most metal substrates such as aluminum, steel, copper, zinc and chromium. The prior art utilizes various other volatile organic solvents, such as pyrrolidones as reported in U.S. Pat. No. 4,120,810 and phenyl ethers or ethoxylated alcohols as disclosed in U.S. Pat. No. 4,619,706.
Other known stripping methods use blasting processes with materials like plastics, starches, water and sometimes solvents. These methods not only produce large amounts of waste, which must be treated, but also generate a significant amount of airborne pollutants. With the proliferation of resinous coatings for the protection of metal and composite surfaces has come the need for a method of removing these types of coatings from their substrates without damaging the underlying material. It has been observed, for example, that strong acids are effective in removing these coatings, but their indiscriminate oxidation of the metal substrates makes acids undesirable and unacceptable in the majority of cases.
Several other techniques have been developed in attempts to satisfactorily remove paint coatings. One technique is to debond or dissolve the organic coating in a chemical solvent. While these solvents are often effective for debonding the paint coating from the substrate, they generate chemical waste such as stripping sludges, which result in disposal and pollution problems. Some organic stripping materials such as methylene chloride are effective as stripping agents and do little harm to underlying substrates. However, these chemicals pose additional problems such as toxicity, high volatility and flammability. While efforts have been made to minimize the problems associated with solvents, organic chemicals are still relatively inefficient, requiring large volumes of materials that must be disposed of in an approved manner upon completion of the stripping procedure. For example, conventional aircraft paint removal methods are increasingly under fire from many regulatory agencies, posing a problem for commercial and military operators. Moreover, some solvents such as methylene chloride are suspected human carcinogen agents, and the U.S. Environmental Protection Agency has classified these solvents as hazardous air pollutants. The costs associated with chemical stripping are astronomical, and the process is labor-intensive, consuming from a few hundred hours to as much as a few thousand hours depending on the aircraft part and the size of the aircraft. In addition, operational costs include special safety apparel, the cost of waste containment/disposal and facility/equipment maintenance costs. In most current practices, operational difficulties include the need for multiple chemical applications to remove the coatings, and a large degree of hand sanding in areas where the coatings remain after the chemical stripping.
More recently, other methods with lower environmental impact have been developed and generally fall into three categories: (a) the use of less toxic chemicals; (b) impact blasting of the paint coating including: plastic media blasting (PMB), laser stripping, aqua jet, CO2-pellet blasting, starch or vegetable media blasting or ice-blasting; and, (c) vaporization of the paint via concentrated heat and/or light energy, e.g., laser stripping. Combinations of these methods also have been evaluated, including the FlashJet process wherein a pulsed xenon flash lamp initially vaporizes the paint coating into a fine ash residue. This residue is subsequently swept by a frozen carbon dioxide-pellet blast into a multistage collection system of filters and activated charcoal scrubber. However, these alternative methods, some of which are at a relatively mature stage of development, are not without problems. For example, the PMB procedure must be done in a closed system because of particulate generation and the potential for air pollution. The use of water as an impact medium as in the ice-blasting procedure, however, partially circumvents this difficulty. In general, the use of high-speed media jets in blasting procedures poses personnel safety and compliance issues. This is true even in the recently developed FlashJet process.
In view of these problems, there is a pressing need to develop novel approaches to paint stripping that are environmentally benign, pose minimal personnel safety problems, and are inexpensive. For delicate substrates of composites and honeycombs (e.g., radomes, helicopter rotor blades), the aforementioned methods like PMB or media blasting are not compatible because they will cause considerable damage to the substrate being stripped.
A laboratory version of a photochemical process for paint stripping has been described in Proceedings of the Electrochemical Society, Vol. 98-5, pp. 232–235, 1998). This study showed that the aqueous peroxide medium could be photochemically decomposed by UV light to generate gas bubbles within the polymeric coating. The present invention concerns a improved method for stripping paint based on either UV light and IR radiation in combination, or IR radiation alone. Modifications in the aqueous stripping medium to better facilitate coating debonding are also described.