A high-entropy strategy for activating DOPO at ultralow loadings toward flame-retardant epoxy with reduced smoke and improved mechanics

Epoxy resins are widely used in structural composites and protective coatings, but their high flammability and release of smoke and toxic gases remain major safety concerns. Conventional transition-metal oxides have been explored for char formation and smoke suppression, but limited active-site diversity and redox flexibility restrict their performance; meanwhile, low-loading DOPO-modified epoxy systems still struggle to balance flame retardancy, smoke suppression, toxicity reduction, and mechanical performance. Here, spinel-structured Mn–Co–Cr–Fe–Ni–Cu high-entropy oxides (HEOs) were introduced into a DOPO-modified epoxy system to construct low-loading multifunctional flame-retardant composites. The defect-rich HEOs contain multivalent transition-metal sites and oxygen-vacancy-related structures, which can assist DOPO in regulating combustion reactions, promoting catalytic carbonization, and suppressing volatile degradation products. At the same total additive loading, partial replacement of DOPO by 0.5 wt% HEOs enabled EP/2.0DOPO/0.5HEOs to show the best overall fire-safety performance among the investigated formulations. Compared with neat epoxy, the pHRR decreased from 1460 to 1186 kW/m 2 , while the THR, peak SPR, TSP, and peak COP were reduced by approximately 52%, 52%, 12%, and 40%, respectively, indicating suppressed heat release, reduced peak smoke emission, and lower CO generation. In terms of mechanical performance, EP/2.0DOPO/0.1HEOs exhibited the highest tensile and impact strengths of 73.79 MPa and 38.37 kJ/m 2 , respectively. This work demonstrates that low-loading HEOs can serve as multifunctional catalytic additives for improving fire safety, smoke suppression, toxicity reduction, and mechanical robustness of epoxy composites.