For over 30 years, the U.S. Department of Defense (DoD) has utilized solvent-borne polyurethane topcoats on the outer mold line (OML) (i.e., outer surface) of aviation assets and support equipment. However, these topcoats contain isocyanate-functional molecules and other chemicals that are hazardous to the environment. Exposure to isocyanates during polyurethane topcoat spray applications can result in numerous health issues, such as watery eyes, difficulty breathing, and sensitization. Assets must be isolated to prevent individuals from exposure to isocyanates in airborne mists (from spraying) and while the topcoat cures. Traditional polyurethane aerospace topcoats also possess hazardous air pollutants (HAP), which are carcinogenic, including high levels (i.e., up to 420 g/l) of volatile organic compounds (VOC), which evaporate into the atmosphere and pollute the air.
To address these environmental issues, the Naval Research Laboratory (NRL) developed two novel and environmentally friendly topcoat technologies: 1) a single-component (1K) polysiloxane topcoat, which is an all-in-one can system that does not require the metering and mixing of components; and 2) a two-component (2K) polysiloxane topcoat, which requires metering and mixing of components. These polysiloxane topcoats are free of hazardous isocyanates, free of HAP, and lower in VOC compared to DoD-qualified polyurethane aerospace topcoats. The polysiloxane technology was licensed to Niles Chemical Paint and Coatings (NCP Coatings) for optimization and commercialization, and multi-gallon quantities of camouflage (i.e., low-gloss) gray, semi-gloss gray, and gloss white topcoats were produced for testing to MIL-PRF-85285, MIL-PRF-32239, MIL-PRF-81352, and extended laboratory performance requirements, including application on scrap aircraft components and the entire OML of active DoD assets. These assets were then utilized in the field (e.g., at depots and aboard aircraft carriers) under normal operating conditions for a period of 1-2 years, and assets painted with a MIL-PRF-85285 qualified 2K polyurethane topcoat were used for comparative purposes. During all demonstrations, the isocyanate-free and HAP-free polysiloxane topcoats demonstrated equivalent, and in a few instances better, field performance (e.g., corrosion resistance, weatherability) compared to the qualified polyurethane topcoat. Use of the polysiloxane topcoats on assets also resulted in reduced VOC emissions, reduced preparation and application times, and a reduction in the generation of hazardous paint waste. These performance characteristics validated that the polysiloxane topcoats can be an environmentally friendly drop-in replacement for MIL-PRF-85285 qualified 2K polyurethane topcoats without sacrificing performance.
The 1K camouflage gray and 2K gloss white polysiloxane topcoats, as produced by NCP Coatings, where approved by the Naval Air Systems Command and listed on the qualified products database (QPD) as MIL-PRF-85285F, Type I/IV, Class NH, Form S, Grade N and Type II, Class NH, Form M, Grade N products, respectively, on August 19, 2022. Plans to revise MIL-PRF-81352 are also underway; therefore, the 1K camouflage gray polysiloxane topcoat can be fully qualified and listed on the QPD as an aircraft touch-up topcoat. Currently, the cost of the polysiloxane topcoats are similar to qualified Type IV polyurethanes.
This project was a collaboration between individuals at NRL, Naval Air Warfare Center – Aircraft Division, Air Force Research Laboratory, NCP Coatings, Navy and Marine Corps installations, and Air Force bases.