Thermally curable polyurethanes with main-chain benzoxazine and pendant carboxylic acid groups and their use as electrically insulating coatings

This work describes the preparation of thermally curable benzoxazine-modified polyurethanes with pendant carboxylic acid moieties, which have potential as organic coatings with enhanced thermal stability and electrical insulation properties. Thermoplastic poly(urethane-co-benzoxazine)s (PUBs) were synthesized via the polyaddition reaction of a benzoxazine diol chain extender with isocyanate-terminated urethane oligomers made from poly(tetramethylene ether)glycol at various molecular weights, dimethylol propionic acid, and toluene diisocyanate. Spectroscopic methods were utilized to characterize the proposed structures of the starting materials and final polymeric coatings and confirmation of co-presence of urethane bonds and benzoxazine rings. The curing temperature of the cross-linked PUBs (XPUBs) was evaluated using differential scanning calorimetry analysis. A catalytic effect of free carboxylic acids was observed, resulting in a lowering of the curing temperature by up to 30 °C. The XPUBs exhibited excellent thermal stability and reduced flammability compared to common polyurethane due to the formation of phenolic cross-linked linkages. The tensile strength and modulus of the XPUBs were also improved, while the extensibility of the polymeric coatings was mostly preserved. The key parameters that determine the possible application of these materials as electrical insulator coatings were very promising. The XPUBs displayed high dielectric strength of up to 38 kV/mm, low dielectric constant of less than 3, and a dissipation factor of approximately 6.