Improved flame retardancy of epoxy resin composites modified with a low additive content of silica-microencapsulated phosphazene flame retardant

Epoxy resin (EP) composites with improved flame retardancy were fabricated. To solve the problem of the large addition of phosphazene flame retardants, we designed a system based on phosphorus–nitrogen–silicon synergistic flame-retardant Si(H) microcapsules with silica as the shell and hexaphenoxycyclotriphosphazene as the core. Si(H)/EP composites were manufactured with two coupling agents to promote interactions. In cone calorimetric tests, heat and gas releases of Si(H)/EP composites were reduced, and fire hazard was minimized. Residual analysis indicated the strong rigidity and mechanical robustness. The mechanism included phosphorus quenching effect and nitrogenous diluting effect of flame inhibition effect in gas phase, and phosphorus charring effect and silicic barrier and protective effect in solid phase. This research provides an effective flame-retardant method for EP composites with a low additive content. [Display omitted] •Flame-retardant EP composites were fabricated based on Si(H) microcapsules.•Only a low additive content realized an obvious improvement of flame retardancy.•Phosphorus–nitrogen–silicon synergistic flame-retardant effect was realized.•The flame-retardant mechanism included four effects in the gas and solid phases.