Melamine modified phthalonitrile resins: Synthesis, polymerization and properties

Phthalonitrile resin, as a thermosetting resin, possesses high chemical resistance, high thermal resistance, high thermo-oxidative stability and good mechanical properties. However, the extremely high curing temperature needed to achieve high curing degree severely limits its large-scale application. It is found that introduction of benzoxazine moities into phthalonitrile would increase the reactivity and decrease the curing temperature. In this work, phthalonitrile resin with melamine moiety (BAph-m) is synthesized by taking advantage of benzoxazine chemistry using melamine as amine source. Its chemical structure is confirmed by Fourier infrared spectroscopy (FTIR), 1H and 13C nuclear magnetic resonance (1H NMR) spectra. Its curing reactivity, polymerization mechanism and the properties of the resultant polymers are systematically studied. It is found that the introduction of melamine reduces the content of oxazine ring, leaving active groups like phenolic hydroxyls and secondary amines as build-in catalysts. Consequently, the modified resins show much higher reactivity. The exothermic peaks corresponding to ROP of benzoxazine ring and curing of phthalonitrile groups of BAph-30 m decreased to 197.4 °C and 278.2 °C, 77.7 °C and 30.1 °C lower than those of neat BAph. The gelation time shortened from 892 s for BAph-2m to 231 s of BAph-30 m. Moreover, it is found that the build-in active groups could catalyze the reaction of benzoxazine moities and phthalonitrile groups to form phenolic Mannich bridge structures, triazine rings, phthalocyanine rings and isoindoline. The modified resins show very high thermal stability with BAph-30m-280 showing initial decomposition temperature (Td5) of 492 °C and char yield (Yc) of 79.9%, which is 35 °C and 6.8% higher than those of BAph-280.