A Bio‐Based Diphenylphosphine Oxide Derivative Synthesized via a Facile One‐Step Method as an Efficient Flame‐Retardant Curing Agent for Epoxy Resins

ABSTRACT In this study, a bio‐based flame retardant, DPO‐ITA, was efficiently synthesized from diphenylphosphine oxide (DPO) and itaconic acid (ITA) via a facile one‐step reaction. The incorporation of DPO‐ITA increased the initial thermal decomposition temperature and glass transition temperature compared with those of the unmodified epoxy resin (EP‐0). At a phosphorus content of 2.3 wt%, EP/DPO‐ITA 2.3 achieved a UL‐94 V‐0 rating and a limiting oxygen index of 39.5%. Cone calorimetry results showed that EP/DPO‐ITA 2.3 exhibited significantly reduced peak heat release rate, total heat release, average effective heat of combustion, and fire growth rate, with reductions of 65.1%, 25.7%, 16.1%, and 74.5%, respectively, compared with EP‐0. The enhanced flame retardancy arises from the synergistic action of DPO‐ITA in both condensed and gas phases, including the formation of a dense, highly graphitized char layer, quenching of reactive radicals by phosphorus‐containing species, and dilution of flammable volatiles by nonflammable gases released during decomposition. Meanwhile, EP/DPO‐ITA 2.3 exhibited tensile strength comparable to EP‐0, while its flexural strength was significantly enhanced by 47.03%. This work demonstrates that DPO‐ITA is an effective bio‐based flame retardant for epoxy resins and provides insights into the development of high‐efficiency flame‐retardant materials.