Erosion resistant protective coatings used on military aircrafts 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, solvent-borne two-component polyurethanes coatings are the systems of choice to meet on-site application, curing, and performance requirements. Important environmental issues are associated with these erosion-resistant coatings, including a significant environmental burden due to their high volatile organic compounds (VOCs) and hazardous air pollutant (HAP) emissions and the use of hazardous and toxic isocyanate compounds.
Contemporary water-borne polyurethane coatings that promise significant lowering of VOCs and HAPs are not suitable for some DoD applications due to inefficient film formation and longer drying times when compared to solvent-borne coatings 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 enormous burdens to the DoD.
Dr. Vijay Mannari and his team from the School of Engineering Technology at Eastern Michigan University are underway with a SERDP project to design, develop, and evaluate innovative non-isocyanate polyurethane (NIPU) coating systems for environmentally sustainable rain erosion coatings that meet or exceed performance requirements. Their project also includes the development of a strategic transition plan for implementation of this new coating technology in the field through cooperative development with industrial partners and the end-user DoD sites.
By leveraging cyclic carbonate/amine chemistry and building on previous research, Dr. Mannari plans to develop a platform of amine-functional NIPU building blocks that are customized for high-performance rain erosion coating systems with low or no VOC or HAPs, with two distinct system types:
- Two-component high-solid coatings (2K-HS NIPU system)
- 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, with low or no VOC or HAPs, 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 methacrylate 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 can be spray-applied on-site and cured using portable UV sources.
Both 2K-HS-NIPU and UV-NIPU systems are completely free from hazardous isocyanate compounds at manufacturing as well as at the application sites. Also, 2K-HS-NIPU will have substantially reduced VOCs or HAPs while UV-NIPU will be completely free of any volatile compounds. The technology has strong technology transfer potential with current DoD and aerospace projects. UV-cure coatings promise 100% solid rapid curing with a significantly lower environmental footprint.