Synthesis of bio‐based reactive N/P flame retardant and its curing and flame retarding behavior in epoxy resins

In this study, a pioneering bio‐based nitrogen–phosphorus flame retardant and curing accelerator named oxime‐phosphazene hexakis [(4‐(hydroxyimino) 2‐methoxy) phenoxy] cyclotriphosphazene (HAPV) was successfully synthesized using hexachlorocyclotriphosphazene and vanillin. When 5 wt % HAPV was added into epoxy, the limiting oxygen index increased from 22% to 27% and passed UL‐94 V‐0 (UL is defined as Underwriters Laboratories) rating. Meanwhile, with the addition 5 wt% HAPV, the apparent activation energy (Ea) of HAPV/EP decreased from 20.22 to 67.15 kJ/mol, and the pHRR value was suppressed from 581.21 to 330.18 kW/m2. It was due to that the HAPV quenched the combustion chain reaction through the gas phase and condensed phase, forming a dense char layer for blocking the heat transfer. Overall, the study provides an environmentally friendly epoxy flame retardant curing accelerator that shows great potential in epoxy flame retardant applications. The schematic of the preparation and flame retardant mechanism of HAPV‐EP composite material.