A polar polyimide as multifunctional flame retardant for epoxy resin through constructing intimate 3D interpenetrating polymer network

With the rapid development and application of 5G technology, design and fabrication of lightweight polymer materials with excellent flame retardancy, low dielectric constant, and high thermal insulation is a worthy but great challenge. In this study, a polar polyimide (PI) contain amide group was designed and incorporated into epoxy resin (EP) to fabricate multifunctional materials combing a small molecule named phenoxycycloposphazene (HPCTP). Due to the synergistic effect of PI and HPCTP in flame retardant, EP blend passed UL-94 V-0 rating only needing 3 wt% PI and 1 wt% HPCTP, meanwhile the limited oxygen index (LOI) obviously increased from 23.8% to 35.0%. More importantly, the dielectric constant obviously decreased at a wide range from 10−1 to 107 Hz and the thermal conductivity also reduced by more than 45% from 0.236 to 0.127 W m−1 K−1. Due to the good compatibility and reactivity between PI and EP, PI was uniformly dispersed and connected EP to construct intimate 3D interpenetrating polymer network, and thus the tensile and flexural properties were slightly enhanced. Moreover, combustion parameters, including peak heat release rate (PHRR), total heat release (THR), smoke produce rate (SPR) and total smoke production (TSP), all indicated that EP composites significantly reduced fire risk. Overall, this study offers a promising strategy to fabricate flame retardant EP composites with outstanding comprehensive performances.