The study on curing and weight-loss mechanisms of benzoxazine during thermal curing process

Based on the weight loss and small molecule volatiles released during the thermal curing process of three kinds of benzoxazines, bisphenol A/aniline type benzoxazine (BA-a), p-cresol/diamine type benzoxazine (MPH-mda) and 2,4-xylenol/4,4′-methylenebis-(2,6-diisopropyl) aniline type benzoxazine (DMPH-mdpa), their curing mechanisms and the weight-loss mechanisms were studied. The weight-loss ratios and structure changes of these benzoxazines during the curing process were studied by weighing method, Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The volatiles released from these benzoxazines during different curing stages were collected by a self-made experimental device. The structures of these volatiles were characterized by FTIR and proton nuclear magnetic resonance spectroscopy (1H NMR). The results showed that the mixtures of imine, triazine, aniline and Schiff base were released from the cracking of the intermediate during the thermal curing of BA-a. Compared with BA-a, the weight loss of MPH-mda was smaller, and its volatile was a small amount of stable diamine type Schiff base. For DMPH-mdpa, because all active reaction points on benzene ring were occupied by alkyl groups, it cannot form effective polymer structures after ring-opening reactions, which led to that DMPH-mdpa had the most weight loss during the curing process, and mainly generated phenol type volatiles.