Synthesis of a cyclotriphosphazene derivative containing DOPO for simultaneous improvement in flame retardancy, toughness and strength of epoxy thermosets

Improving the flame retardancy of epoxy thermosets without sacrificing mechanical properties, thermal decomposition stability and glass transition temperature (T_g) remains a challenge. In this study, a novel halogen-free flame retardant containing phosphorus/nitrogen (N₃P₃-DI) was synthesized from cyclotriphosphazene, p-hydroxybenzaldehyde, itaconic anhydride and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). Subsequently, N₃P₃-DI was used for flame-retardant modification of epoxy resin (EP). After the introduction of 10 wt% N₃P₃-DI, the EP thermoset achieved a limiting oxygen index of 29.6% and UL-94 V-0 rating. The peak heat release rate and total smoke production of the N₃P₃-DI-modified EP thermosets decreased by 48.4% and 27.5%, respectively, compared to those of the pure EP thermoset. Furthermore, the flame retardancy effect of N₃P₃-DI was confirmed by analysis of pyrolysis volatiles and the residual char of the thermoset. Compared with the pure EP thermoset, the N₃P₃-DI-modified EP thermoset also exhibited an increase of both the impact strength and flexural strength by 46.3% and 31.2%, respectively, demonstrating the effect of simultaneous toughening and strengthening. This is mainly due to the introduction of N₃P₃-DI reducing the crosslinking density of the thermoset network and increasing the free volume, while also improving the rigidity of the network chain. In addition, the modified EP thermosets still maintained high levels of T_g and thermal decomposition temperature. Therefore, this study provides an attractive strategy to simultaneously improve the flame retardancy, toughness and strength of epoxy thermosets, showing great prospects for future advanced applications in composites. © 2025 Society of Chemical Industry.