Preparation and Properties of a Phosphorus-Free Intrinsic Flame-Retardant and High Thermal Resistant Polybenzoxazine with Good Degradability

Polybenzoxazines are a type of thermosetting resin with good thermal resistance. A flame-retardant and degradable polybenzoxazine (ERY-a) was synthesized by a special bisphenol containing a bridging diacetal structure (ERY) and aniline. A systematic comparative study was conducted between ERY-a and bisphenol A/aniline-type benzoxazine (BA-a). Their chemical structures were confirmed by Fourier transform infrared (FTIR) spectra and nuclear magnetic resonance (NMR) spectra. The curing behaviors and polymerization reactions were analyzed by differential scanning calorimetry (DSC) and FTIR spectra. Dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) were used to investigate their thermal properties, and the results showed that the cured ERY-a, referred to as P(ERY-a), exhibited a notable glass transition temperature Tg of 297 °C and a char yield of 51% under N2 at 800 °C, both of which were significantly higher than those of cured BA-a. Furthermore, the flame retardancy of P(ERY-a) and ERY-a-based composites was assessed through microcalorimetry (MCC) and vertical combustion testing. The findings highlighted that the superior flame retardancy and low heat release capacity (HRC) of P(ERY-a) were due to the presence of the diacetal structure. More importantly, the cross-linked P(ERY-a) can be degraded in a solvent via the cleavage of the diacetal structure, and optimal degradation conditions were systematically studied. Moreover, the carbon fiber from the ERY-a-based composite can be easily recycled through the degradation of ERY-a.