Phthalonitrile-Schiff base epoxy resin: unique gas-condensed phase synergy endows excellent flame retardancy

Epoxy resin is widely applied for its excellent mechanical properties and chemical stability, yet its intrinsic flammability greatly limits further applications. Herein, an intrinsically flame-retardant epoxy resin (PNDEP) with phthalonitrile and Schiff base structures was synthesized via a facile, efficient three-step reaction. Then, 4,4'-diaminodiphenylmethane (DDM) was adopted as the curing agent to obtain cured PNDEP/DDM resin through thermal curing. Compared with commercial bisphenol A diglycidyl ether (DGEBA) epoxy resin, cured PNDEP/DDM resin showed improved comprehensive performance, which was ascribed to the higher crosslinking density, as well as the rigid isoindoline and triazine structures formed. Notably, its glass transition temperature reached 214 °C, Young’s modulus hit 5630 MPa, and char yield at 700 °C achieved 56%. Moreover, PNDEP/DDM featured a distinctive gas-condensed phase synergistic flame-retardant mechanism. Non-combustible gases produced during combustion induced foaming to form a dense and robust 3D porous sponge-like char layer, thus endowing the material with excellent flame retardancy. Specifically, it passed the UL-94 V-0 vertical burning test. And its peak heat release rate and total heat release reduced by 88.8% and 84.2%, respectively, compared with that of DGEBA/DDM system. Owing to its superior comprehensive properties, cured PNDEP/DDM resin has broad prospects in high-end fields like electronic packaging and aerospace engineering.