Elastomeric polyurethane coatings are used to provide rain-erosion resistance and other specialty functions for the exterior of Department of Defense (DoD) aircraft. However, these coatings contain isocyanate-functional polymers, which can result in numerous health issues for workers who are exposed to these hazardous molecules during spray applications. Elastomeric polyurethane coatings are also high in volatile organic compounds, in the form of organic solvents, which can pollute the air. Furthermore, polyurethane specialty coatings have been observed to wrinkle and partially delaminate during active service, which likely results from degradation of the polymer network as a result of constant exposure to sunlight and extreme environmental conditions.
During this project, the research team synthesized several organosilane polymers with different chain lengths and alkoxysilane functionality at the terminus, then used these polymers to form isocyanate-free polysiloxane coatings via reaction with atmospheric moisture. The thermal and tensile properties of the coatings as unfilled samples were determined and compared to a qualified polyurethane elastomeric coating, then two organosilane polymers were down-selected and used to form unfilled and filled polysiloxane coatings for testing to rain-erosion resistance and other specialty coating performance requirements.
Polysiloxane coatings demonstrated elongation values up to 263.2%, whereas the greatest tensile strength was 23.1 MegaPascals. The onset degradation temperature for these coatings ranged from 196.2-288.3 °C, and in general, coatings based on polymers with terminal trimethoxysilane groups demonstrated greater stability, which is likely due to increased crosslink density compared to coatings based on polymers with terminal methyldimethoxysilane groups. An unfilled polysiloxane coating based on a methyldimethoxysilane- terminated polymer showed limited stability to artificial sunlight and hot/humid conditions, whereas a filled polysiloxane coating based on a trimethoxysilane-terminated polymer demonstrated sufficient tensile strength, elongation, and degradation resistance to both artificial sunlight and hot/humid conditions. The filled coating lacked the required tear strength for use in specialty applications, although modifications to the polymer could resolve this issue.
Polysiloxane coatings based trimethoxysilane-terminated polymers may have potential applications in specialty aircraft coatings, which would enable the DoD to eliminate hazardous isocyanate-functional polymers.