A High‐Efficient DOPO‐Based Flame Retardant as a Co‐Curing Agent for Simultaneously Enhancing the Fire Safety and Mechanical Properties of Epoxy Resin

Abstract Simultaneously enhancing the fire safety and mechanical properties of epoxy resin (EP) remains a persistent challenge. Herein, a high‐efficient phosphaphenanthrene‐based flame retardant (FNP) is synthesized using 3,5‐diamino‐1,2,4‐triazole, 4‐formylbenzoic acid, and 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide. Due to the presence of active amine groups, FNP is employed as a co‐curing agent for fabricating EP composites with outstanding fire safety and mechanical properties. EP containing 8 wt% FNP (EP/8FNP) achieves a vertical burning (UL‐94) V‐0 rating with a limiting oxygen index of 31%. Meanwhile, FNP declines the peak heat release rate, total heat release, and total smoke release of EP/8FNP by 41.1%, 31.8%, and 16.0%, respectively, compared to those of unmodified EP. The increased fire safety of EP/FNP composites is because FNP promotes the formation of an intumescent, compact, and cross‐linking char layer for EP/FNP composites, and releases P‐containing substances and noncombustible gases in the gas phase during combustion. In addition, EP/8FNP exhibits 20.3% and 5.4% increase in the flexural strength and modulus compared with those of pure EP. Furthermore, FNP enhances the glass transition temperature of EP/FNP composites from 141.6 °C for pure EP to 147.3 °C for EP/8FNP. Therefore, this work is conducive to the future development of fabricating fire‐safe EP composites with enhanced mechanical properties.