Multiple catalytic polymerization of phthalonitrile resin bearing benzoxazine moiety: Greatly reduced curing temperature

Abstract

Phthalonitrile resin is a high-performance thermosetting resin with excellent thermal stability, good mechanical properties, low water absorption and low dielectric constant, thus it shows great potential in aerospace, marine, and electronic packaging. However, the extremely high polymerization temperature and long curing cycle limit the practical application of phthalonitrile resin. It is revealed that the active species released by ring-opening of benzoxazine can promote the polymerization of phthalonitrile and the polymerization temperature of phthalonitrile resin containing benzoxazine (BAph) could be reduced to 240–280 ℃, but there is still difficultly in practical application. In this study, dicyandiamide (DCA), which contains reactive nitrile group and active amino groups, was selected as the curing agent to catalyze the curing reaction of BAph. The reaction activity, curing process and curing behavior of BAph-dca were investigated by differential scanning calorimetry (DSC), gelatin time (tgel) measurement and dynamic rheological analysis (DRA). The polymerization mechanism of BAph-dca was studied by Fourier transform infrared spectroscopy (FTIR) and DSC. It was shown that with 10 wt% of DCA, the initial exothermic temperature decreased from 222.9 ℃ (neat BAph) to 159.4 ℃, the peak temperature of ring-opening polymerization of benzoxazine (Tp1) decreased from 255.9 ℃ to 203.2 ℃, and the peak temperature of nitrile polymerization (Tp2) decreased from 288.5 ℃ to 271.6 ℃. Importantly, polymer films can be obtained at 200 ℃, which is substantially lower than the curing temperature of typical phthalonitrile resin. Moreover, the polymer films cured at 200 ℃ showed good thermal stability. The polymer film with 3 wt% of DCA showed initial decomposition temperature (Td5) of the 412 ℃ and the char yield (Yc) of 72.09 % at 800 ℃. This study is of great significance in reducing the polymerization temperature of phthalonitrile resin and promote its practical application.