The objective of this project was to develop novel stimuli-responsive silyl-containing polyurethane topcoats that possess similar properties (i.e. thermal, mechanical, weatherability) as current Department of Defense (DoD) aircraft topcoats, then demonstrate, as a proof-of-concept, that these topcoats can be selectively removed from a strongly adhered substrate (e.g. epoxy primer) using a mild and environmentally friendly fluoride salt solution. The fluoride salt solution should not damage or remove the underlying substrate, thereby demonstrating that the topcoat is selectively strippable. Furthermore, the fluoride salt should not alter the chemical structure or mechanical properties of the substrate upon removal of the topcoat.
The polyurethane topcoats used on the exterior of DoD aircraft and ground support equipment are removed with hazardous chemical strippers, such as methylene chloride, abrasive materials, such as blast media and sand paper, or thermal ablation. However, all of these methods also damage the underlying nonmetallic substrate (e.g. anti-corrosive epoxy primer, carbon fiber reinforced composite) upon removal of the topcoat. Furthermore, a chemical stripper that is environmentally friendly, can selectively remove the polyurethane topcoat with minimal applications, and will not damage the underlying non-metallic substrate, does not currently exist.
Herein, the project team successfully demonstrated, as a proof-of-concept, that stimuli-responsive silyl-containing polyurethane (Silyl-PU) coatings can provide similar thermal, mechanical, and performance properties (e.g. up to 60% GE Impact flexibility) as a MIL-PRF-85285 qualified polyurethane topcoat, yet also be selectively and completely removed (within 20 minutes) from a strongly adhered epoxy primer using an environmentally friendly fluoride salt solution under static conditions at room temperature (Figure A1). Removal of the topcoat did not affect the chemical structure of the epoxy primer according to infrared analysis.
The Silyl-PU technology, as a topcoat on aircraft, could prevent exposure of hazardous materials (e.g. hexavalent chromium) to workers and the environment by maintaining an intact epoxy primer upon selective removal of the topcoat, thereby reducing costs associated with hazardous waste disposal and reapplication of the primer.