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Selectively Strippable Silyl-Containing Aerospace Topcoats using Environmentally Friendly Fluoride Salts
Dr. Erick Iezzi | U.S. Naval Research Laboratory (NRL)
The topcoat and primer used on the exterior of Department of Defense (DOD) aircraft and components are removed (i.e., stripped) using chemicals, abrasive materials, or thermal treatments. All of these methods generate hazardous waste, are labor intensive, consuming both energy and resources. Chemical stripping is one of the most effective methods of coating removal, although it remains the most dangerous to individuals due to dermal exposures and inhalation of hazardous chemicals, in addition to causing environmental damage from air and water contamination. Similar to the other methods, chemical stripping cannot selectively remove the polyurethane topcoat from the underlying non-metallic substrate (e.g., anti-corrosive epoxy primer, carbon-fiber reinforce polymeric composite) without damaging or completely removing the substrate. To address this issue, we aim to develop novel stimuli-responsive silyl-containing aerospace topcoats that possess similar properties as current aircraft topcoats, then demonstrate, as proof-of-concept, that these topcoats can be selectively degraded and removed from a non-metallic substrate, without damaging the substrate, using an environmentally friendly chemical stripper.
The proposed topcoats will be designed as two-component (2K) polyurethanes and will possess silyl-based chemical triggers that are selectively activated on-demand with a fluoride ion stimulus. Upon activation, a cascading breakage of bonds will occur throughout the cross-linked network to enable the coatings to degrade and delaminate from a substrate. These topcoats will be resistant to disassembly with chloride ion from the environment, in addition to aqueous strong acid and base solutions. All silyl-containing trigger molecules will be synthesized and their structures characterized, then incorporated into clear and pigmented topcoat formulations. These formulations will incorporate renewable raw materials, such as bio-based isocyanates and solvents, to the maximum extent possible. The topcoat formulations will be applied on substrates, such as epoxy-primed aluminum panels and release film, and characterization of the cured topcoats will include surface and bulk chemical analysis, thermal and mechanical properties, and resistance to degradation from artificial sunlight. Fluoride ions will be supplied from an organic or inorganic fluoride salt in a non-aqueous, aprotic organic solvent, and topcoat removal experiments will be conducted by immersing samples in these solutions under static and dynamic conditions.
An advanced coating that demonstrates similar properties to traditional cross-linked polyurethane coatings, yet can be selectively degraded on-demand with a fluoride salt, would advance science in the area of stimuli-responsive materials. For the DoD, development of an advanced aerospace topcoat and environmentally friendly chemical stripper would enable depots to eliminate the use of hazardous paint strippers, such as methylene chloride, reduce the emission of hazardous air pollutants (HAPs), decrease heavy metal exposures to maintenance personnel, and potentially decrease water consumption and pollution. The silyl-containing coating technology would also reduce or eliminate damage to several non-metallic substrates caused by current chemical strippers, in addition to reducing maintenance labor costs and improving operational readiness through decreased processing time. Finally, selective stripping will potentially allow for a life-cycle extension of intact anti-corrosive primers, thus reducing hazardous material (e.g., hexavalent chromium) usage during the re-painting of aircraft.