Objective

The polyurethane topcoat and anti-corrosive epoxy primer on the exterior skin of Department of Defense (DoD) aircraft and ground support equipment (GSE) are currently removed with chemical, abrasive, or thermal treatments. Chemical stripping is the most cost effective, especially when stripping large aircraft, yet generates large quantities of hazardous waste, pollutes the air, and certain strippers are extremely hazardous to workers. Abrasive materials generate hazardous airborne particles, are labor intensive, and may require expensive containments, whereas thermal ablation with lasers is expensive, energy consuming, and safety concerns persist. None of these methods can selectively remove the polyurethane topcoat without also damaging or removing the underlying polymeric substrate (e.g., epoxy primer, composite part), which presents issues when conducting field repairs of coatings and during de-painting of aircraft at depots. To address these issues, the team plans to develop stimuli-responsive aerospace topcoats that are selectively degraded and completely removed from the underlying polymeric substrate, without affecting said substrate, upon activation with a mild fluoride salt stripper.  

Technical Approach

The stimuli-responsive topcoats will possess similar aliphatic chains and linkages as current aerospace topcoats, yet also contain silyl bonds that act as selective chemical triggers to enable degradation. These silyl-containing topcoats will be based on unique binders, such as synthesized aliphatic silyl diols or silyl-containing organosilane polymers, and fabrication will occur by spraying white and gray-colored formulas of these molecules either with or without the use of aliphatic isocyanates. The properties (e.g., thermal, mechanical) of the resulting cross-linked camouflage gray and gloss white silyl-containing topcoats will be determined and optimized, then the best candidates will be tested to the majority of MIL-PRF-85285, Type II (support equipment) and Type IV (aircraft with extended weatherability) performance requirements. New fluoride salts, in the form of ionic liquids, will be developed to selectively activate (i.e., break) the silyl bonds within the topcoats, thereby facilitating cascading bond cleavages throughout the network to enable complete degradation and removal within 24 hours. Unlike fluorinated alkyls, fluoride salts are not per- or polyfluoroalkyl substances (PFAS), thus environmental persistence and toxicity should not be a concern. These fluoride salts, as hazardous air pollutant (HAP) free solutions, will be designed to possess several key properties (e.g., high activity, stability, low corrosivity) that are found in current aircraft paint strippers, and the time of topcoat removal from an epoxy primer or composite substrate will be determined via static dwelling and agitated topical treatment. After topcoat removal, the chemical structure and mechanical properties of the polymeric substrates will be determined to show they were not affected by the fluoride salt solutions.  

Benefits

A topcoat that provides unique mechanical properties and durability, yet can be selectively degraded on-demand, would advance science in the field of stimuli-responsive cross-linked networks. For the DoD, a topcoat that meets the performance requirements for use on aircraft and GSE, is selectively and completely removed with an environmentally friendly chemical stripper, and does not damage the underlying polymeric substrate upon removal, would have numerous benefits to workers who perform field repairs of aerospace coatings or complete de-painting of assets at depots. These benefits include: reduced worker exposure to hazardous materials (e.g., hexavalent chromium) in epoxy primers, reduced generation of hazardous waste, extended service-life for intact substrates, reduced labor and material costs associated with de-painting and painting, and increased operational readiness for assets by reducing turn-around-times. (Anticipated Project Completion - 2028)

 

Distribution Statement A: Approved for public release: distribution is unlimited.

Publications

Iezzi, E. B., G. C. Daniels, K. B. Sutyak, and E. Camerino. 2024. Impact of Cross-Linker Structure on the Properties of Durable and Selectively Degradable Silyl-Containing Polyurethane Networks. ACS Applied Polymer Materials, 6(14):8178–8190.