Flame-Retardant Epoxy Resins Enabled by a Reactive Cyclophosphazene Compositional Mixture

The inherent flammability of epoxy resins (EPs) limits their use in safety-critical fields. To improve their fire safety, a reactive phosphorus/nitrogen-containing flame retardant, denoted as CP-NH2, was prepared via the nucleophilic substitution of hexachlorocyclotriphosphazene. CP-NH2 is a compositional mixture of cyclophosphazene derivatives with different side-group types and substitution ratios. It contains amino and Schiff-base groups, which can participate in the curing and charring processes of EP. The incorporation of CP-NH2 markedly improved the flame retardancy of EP thermosets while maintaining good mechanical properties. At a phosphorus content of 0.31 wt %, the EP thermosets achieved a UL-94 V-0 rating and a limiting oxygen index of 30.2%. Compared with neat EP, the total heat release and total smoke production of EP-3CP decreased by 32% and 20%, respectively. Its tensile strength also increased by 13%. The flame-retardant mechanism was investigated by combining experimental thermal analysis with molecular simulations. The results indicate that CP-NH2 improves the flame retardancy of EP through a condensed-phase charring mechanism. The incorporation of CP-NH2 promotes the formation of a dense and continuous intumescent char layer during combustion. This protective layer acts as an effective barrier, retarding heat transfer, restricting oxygen diffusion, and suppressing the release of volatile degradation products. Overall, CP-NH2 provides an efficient route to improve the fire safety and mechanical performance of EP thermosets at a low phosphorus content. This work provides a facile and effective strategy for designing high-performance reactive intumescent flame retardants for fire-safe epoxy materials.