DOPO-Triazole Synergistic Epoxy Monomer: A Strategy to Overcome the Flame-Retardancy/Toughness Trade-Off

Epoxy resins (EP) are widely used in aerospace, electronics, and coatings due to their excellent mechanical and thermal properties. However, their inherent flammability and brittleness limit high-end applications. In this work, a novel reactive flame retardant epoxy monomer (EP-DVGA) containing DOPO and triazole units was designed and synthesized via a molecular engineering strategy. The chemical structure was confirmed by FTIR and NMR. A series of modified epoxy thermosets were prepared by co-curing EP-DVGA with bisphenol A epoxy resin (E51) using DDM as curing agent. The results showed that EP-DVGA significantly enhanced flame retardancy: At 16.31 wt% loading, the limiting oxygen index increased from 25.9% to 34.3% with UL-94 V-0 rating, and cone calorimetry revealed 73.2% and 69.2% reductions in peak heat release rate and total heat release, respectively. Mechanistic studies demonstrated a dual flame retardant effect involving phosphorus radical quenching in the gas phase and formation of a dense graphitized char layer in the condensed phase. Remarkably, EP-DVGA also improved mechanical properties—impact strength increased by 47% and tensile strength by 33.1% at optimal loadings—attributed to energy dissipation through reversible hydrogen bonding and π–π interactions. This molecular design successfully overcomes the traditional trade-off between flame retardancy and mechanical performance, offering a promising strategy for developing high-performance intrinsically flame retardant epoxy materials.