Synthesis of non-isocyanate polyurethanes and their application in radiation-curable aerospace coatings

Although the current polyurethane coatings used in the aerospace applications meet the stringent performance requirements, they don't comply with the environmental demands, e.g., low VOC contents. Moreover, containing a significant amount of solvent, they usually need long drying times, which causes significant production burdens. Furthermore, many of these coatings contain isocyanates, which are hazardous and are likely to be regulated soon. In this study, a series of sustainable UV-curable non-isocyanate urethane acrylate (NIPU-AC) oligomers, with different structures and acrylate equivalent weights, were synthesized and used as a primary building block of UV-curable coatings for aerospace applications. Synthesis of the NIPU-AC oligomers was carried out in three steps: First, multi-functional cyclic carbonates (MFCC) were prepared by carbonation of aliphatic epoxy compounds under mild temperature and pressure conditions in the presence of a catalyst. In the next step, amineterminated polyurethane oligomers (PUPAs) were synthesized by the reaction of MFCCs with a stoichiometric excess amount of aliphatic and cycloaliphatic amines. (meth)acrylate functionality was then introduced by direct reaction of amine groups at the chain ends of PUPAs with methacrylic anhydride (MAAH). The amine value titration and FTIR confirmed the successful synthesis of NIPU-AC oligomers with > 95% yield. Finally, UVcurable NIPU coatings, as a new generation of sustainable coatings for aerospace applications, were developed and evaluated for aerospace-critical performance properties, such as low-temperature flexibility and resistance to specific chemicals/fluids. The results showed that flexibility at -54 degrees C (no cracks or delamination at 1/8), and good chemical resistance (MEK double rubs > 90, and no significant change in the appearance regarding the fluids) could be reached through the proper design of NIPU-ACs, selection of appropriate reactive diluents, and UV-cure conditions. These fast-curable NIPU coatings, which contain low VOCs and HAPS and no isocyanates, could be sustainable alternatives for current aerospace polyurethane coatings.