Phosphazene as an effective flame retardant for rapid curing epoxy resins

Abstract

A rapid curing, low viscosity epoxy amine resin developed for high pressure resin transfer moulding composite fabrication possessing good thermal and mechanical properties and excellent flame retardancy after cure is presented here. The resin system consists of diglycidyl ether of bisphenol F (DGEBF), an epoxy phenolic novolac resin and diethyl toluene diamine (DETDA), while rapid cure is enabled by 2-ethyl-4-methylimidazole (2E4MI) catalysis. The fire-retardant additive, a crystalline powder, hexaphenoxy cyclotriphosphazene (HPCTP) produces only a small increase in viscosity and regardless of additive concentration, the formulations are less than 200 mPas at processing temperatures of between 80 and 125 °C. Using the UL-94 vertical burn test, a V0 rating and a limiting oxygen index (LOI) of 36.2% were achieved using only 10 phr HPCTP. Indeed at 20 phr of HPCTP an LOI of 43.6% was achieved. The HPCTP additive remains miscible in the network after cure which reduces reactivity modestly and the glass transition temperature (Tg), but the flexural properties, including modulus, strength and strain to failure, either increase modestly or were unaffected.