Numerous military aircraft and shipboard surfaces, such as radomes, antennas, gun shields, wing leading edges, and helicopter blade leading edges, are coated with a specialized erosion-resistant protective coating possessing strict performance requirements. These protective coatings must provide excellent rain erosion resistance, superior mechanical properties, good adhesion to the substrate and meet a host of other metrics outlined in MIL-PRF-32239 and SAE AMS-C-83231A. Historical protective coatings that meet these metrics are often polyurethane-based and contain large quantities of volatile organic compounds (VOCs), hazardous air pollutants (HAPs), and isocyanates which are hazardous and may be prohibited for use in the near future under the Prohibited and Controlled Chemical List (PCCL).
A drastic reduction in VOCs, HAPs, and other hazardous compounds in such coatings will lead to significant environmental and occupational safety improvements, as well as increased coating application productivity associated with reduced application and cure times. Protective tapes, an alternative to rain erosion protective coatings, have recently been developed to reduce applicator occupational safety hazards; however, these tapes have shown a drastic decrease in performance compared to conventional coating systems. Currently no wear resistant coating or tape alternative can meet the desired requirements for VOCs, HAPs, and isocyanates while meeting the performance metrics under MIL-PRF-32239A and SAE AMS-C-83231A.
The objective of Luna’s limited scope program was to develop a rapid curing rain erosion coating that has low VOC content and is non-HAPs via the use of exempt solvent technologies. Luna’s goal was to develop a rain erosion coating based on a unique glycidyl carbamate (GC) hybrid resin chemistry that offers epoxy-type rapid reactivity and adhesion combined with excellent erosion, flexibility, weathering, and mechanical properties typical of polyurethane systems.
Luna focused on coating chemistry and full formulation development followed by property validation per MIL-PRF-32239A and SAE AMS-C-83231A, including rain erosion testing. Variants were initially screened for erosion performance using a custom ultrasonic cavitation test method developed by Luna. Other key metrics that were tested include impact resistance, low temperature flexibility, dry/wet adhesion, elongation, and tensile strength. Luna assessed top performing coatings for application time, coating buildup, and cure time prior to full rain erosion testing at the University of Dayton Research Institute (UDRI) per SAE AMS-C-83231A requirements.
The limited scope program was enabled by the completion of several individual tasks that culminated in the demonstration of highly flexible GC coating variants. The Luna team synthesized and formulated a variety of resins and coatings intended to bolster the toughness and impact resistance over prior GC coatings by optimizing the resin chemistry, investigating new curative packages, and carefully selecting additives for toughness and durability. To date, Luna’s efforts have resulted in several formulations that show excellent flexibility, adhesion, and impact resistance and are comparable to a commercial rain erosion coating system.
The program demonstrated that there is significant room to create GC-based environmentally-friendly rain erosion systems and reduce risk associated with hazardous materials during coating application. Additional work will be needed to optimize coatings for rain erosion performance and future funding will allow Luna to move the technology from the development phase toward SAE AMSC-83231A qualification and full Department of Defense transition.
With a successful ultra-low VOC and non-HAP rain erosion coating, the return-on-investment for DoD Components will be significant – in terms of lowered sustainment and environmental costs associated with radome protective coatings on aircraft. These benefits are associated to the large quantity of USAF and Navy aircraft radomes and antennas processed annually at maintenance depots. Other commercial markets for application are widespread, including leading edges of aircraft wings and rotary blades, hypersonic missiles, lift rockets and spacecraft, and wind turbine blades. Luna will continue to leverage collaborative efforts with USAF, UDRI, aircraft maintenance depots, Prime integrators, and coating manufacturers to facilitate DoD transition.