A P/N/S-Containing Fire Retardant Enables Flame-Retardant, Mechanically Robust and Transparent Epoxy Resin

Despite great advances, it remains a grand challenge to design a fire retardant for the creation of transparent fire-retardant epoxy resin (EP). To address this issue, a phosphorus/nitrogen/sulfur-containing ionic flame retardant, TD, composed of a diphenylphosphinate unit and a 2-thiophenethylammonium unit, was synthesized and incorporated into EP. The EP composite containing only 3 wt % TD (0.25 wt % phosphorus) achieved a vertical burning (UL-94) V-0 rating, and the limiting oxygen index (LOI) increased to 24.7%. Cone calorimetry results showed that, relative to neat EP, the EP/5TD composite achieved notable reductions in heat and smoke generation. Its peak heat release rate (pHRR) and total heat release (THR) decreased by 44.0% and 24.0%, while the peak yields of carbon monoxide (pCOP) and carbon dioxide (pCO2P) decreased by 23.5% and 58.6%. Notably, TD not only significantly enhanced fire safety and mechanical properties of EP but also effectively maintained thermal properties and optical transparency. The flame-retardant efficacy of TD was ascribed to the combined action of P, N, and S, which enabled multiple mechanisms, including condensed-phase char barrier formation, gas-phase dilution/cooling, and radical scavenging effects. This work develops highly effective multielement flame retardants for preparing fire-safe epoxy resins with superior comprehensive performances.