Thermosetting resins with high heat resistance are difficult to degrade because of their cross-linking structures. In this work, a kind of diphenol (ACE) with a diacetal strucure is used to synthesize two novel benzoxazine monomers, ACE/aniline-type benzoxazine (ACE-a) and ACE/p-methylaniline-type benzoxazine. Their chemical structures are characterized by Fourier transform infrared (FTIR) spectra and NMR spectra, and the results show that the diacetal structure is successfully introduced. Their curing behaviors and polymerization reactions are studied by differential scanning calorimetry and FTIR spectra. Different cross-linking structures containing the diacetal are proposed for two polybenzoxazines. Their thermal properties are analyzed by dynamic mechanical analysis and thermogravimetric analysis, and the results show that they have excellent heat resistance and thermal stability. For cured ACE-a [P­(ACE-a)], its T g reaches as high as 301 °C and the char yield at 800 °C under N2 is 55%. Additionally, these polybenzoxazines can be completely degraded under mild acidic conditions, and the effects on the degradation are investigated systematically. Furthermore, ACE-a is used as a matrix to prepare fiber-reinforced composites with carbon fiber or glass fiber, and then, these fibers can be easily recovered from the composites by degrading the resin.

A comparative study on the flame retardancy of two polybenzoxazines with and without a diacetal structure, p(A) and p(B), is firstly carried out by microscale combustion calorimeter (MCC). It is proved that the presence of the diacetal structure can endow p(A) with good flame retardancy with a low heat release capacity (HRC). Then, the monomer of this diacetal containing p(A), A‐boz, is used to modify a typical diamine type benzoxazine (M‐boz) to improve its flame retardancy. The effects of A‐boz with different ratios on curing behavior, heat resistance and thermal stability of the co‐polybenzoxazines of M‐boz and A‐boz, p(M/A)s, are analyzed by differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA). Furthermore, the flame retardancy of p(M/A)s is investigated by UL‐94 test, limiting oxygen index (LOI) and MCC, and the degradation and flame retardant mechanisms are investigated by scanning electronic microscopy (SEM), Raman spectra and TGA‐FTIR. When the addition amount of A is 20 wt%, the LOI value of p(M/A) is as high as 39.5% and its flame retardant rating reaches UL‐94 V‐0. In addition, the mechanical properties of p(M/A)s are tested. The results suggest that the Tg and char yield at 800°C in N2 of p(M/A)s increase from 210 to 243°C and 45% to 51%, respectively. This work suggests that the incorporation of the diacetal structure is an effective strategy to improve the flame retardancy of resins.

Benzoxazine resins have attracted much attention because of their excellent properties. As a new kind of thermosetting resin, their gelation and vitrification behaviors during the curing process are worth studying for promoting their development, but few research works have been done on this subject. In this work, an ordinary diamine-type benzoxazine resin (PH-ddm) was chosen as a research object. Its curing kinetics were studied by differential scanning calorimetry (DSC) and a phenomenological model was used to get equal curing degree curves reflecting the relationship among curing degree, curing temperature and time. Moreover, a gelation curve and a vitrification curve of PHddm based on an Arrhenius equation and a DiBenedetto equation were obtained by parallel plate rheometer and modulated DSC (MDSC), respectively. Then, a well-known time-temperature-transformation (TTT) diagram was plotted. Besides, the rheological behavior of PH-ddm was analyzed through rotatory viscometer testing and modified double Arrhenius equation to obtain the equal viscosity curves. A TTT-viscosity diagram was obtained through combining the equal viscosity curves with the TTT diagram. This is the first time for using the TTT diagram to investigate the curing process of benzoxazine resins. Our results provide an effective method to optimizing molding conditions of the benzoxazine resins.