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The erosion resistant protective coatings used on military aircraft and shipboard surfaces have stringent performance requirements. These coatings are frequently applied on-site and under ambient conditions as multi-coat systems on metal alloys and composite substrates. Currently, the solvent-borne two-component polyurethane (PU) coatings are the undisputed systems of choice to meet the stringent on-site application/curing and performance requirements. These erosion resistant coatings have high volatile organic compound (VOC) and hazardous air pollutant (HAP) emissions and use hazardous and toxic isocyanate compounds.
The contemporary water-borne polyurethane coatings that promise significant lowering of VOCs and HAPs are not suitable for these Department of Defense (DoD) applications due to their inefficient film formation as well as longer drying times that render them unacceptable for multi-coat on-site applications and ambient cure conditions. The present commercial polyurethane coatings technology is based on isocyanate compounds as primary building blocks. The use of isocyanate compounds, both at manufacturing and application sites, and their related environmental, health, and safety-related costs are large burdens to the DoD.
The overall objective of this project is to design, develop, and evaluate innovative Non-Isocyanate Polyurethane (NIPU) coating systems for environmentally sustainable rain erosion coatings that meet or exceed performance requirements specified in SAE AMS-C83231A as well as the additional desirable performance properties as per MIL-PRF-32239. The project will also develop a strategic transition plan for implementation of this new coating technology in the field through cooperative development with the industrial partners and the end-user DoD sites.
This project will leverage cyclic carbonate/amine chemistry and build on earlier successful exploratory research. The project team will develop a platform of amine-functional NIPU building blocks that are customized for high-performance rain erosion coating systems with low or no VOCs or HAPs. Specifically, the team will develop two distinct types of coating systems: (1) two-component high-solid coatings (2K-HS NIPU system) and (2) 100% solid UV-curable coatings (UV-NIPU). The 2K-HS-NIPU coating systems will comprise amine-functional NIPU (NIPU-PA) in one can and epoxy functional crosslinkers in the second. These systems when cured at ambient temperature with epoxy-functional crosslinkers, will produce highly durable films with customizable performance properties. For UV-NIPU coating systems, a wide range of (meth)acrylate functional NIPU oligomers will be developed from NIPU-PA. 100% solid UV-curable coating compositions will be formulated using the conventional reactive diluents. These systems will be spray applied on-site and cured using the portable UV sources. UV-curable coatings, which have not previously been used for such military applications, can now be used because of the commercial availability of portable UV-cure equipment. Such companies as Boeing have a strong interest and development plan to harness this technology. UV-cure coatings promise 100% solid rapid curing with significantly lower environmental footprint.
Both the proposed 2K-HS-NIPU and UV-NIPU systems are completely free from hazardous isocyanate compounds, both at the manufacturing as well as at the application sites. Furthermore, 2K-HS-NIPU will have substantially reduced VOCs and HAPs while UV-NIPU will be completely free of any volatiles. The project will establish a baseline lifecycle framework and carry out assessment of human health and environmental impacts of the NIPU coatings during their manufacturing, application, and curing stages. By carefully designing the morphology of NIPU oligomers and their functional group contents, the performance requirements as per the prescribed specifications can be customized. (Anticipated Project Completion - 2019)