Development of a Highly Transparent Flame‐Retardant Epoxy Resin Based on a <scp>DPO</scp> ‐Derived Epoxide

ABSTRACT With the aim of developing a high‐performance flame‐retardant epoxy resin (EP), (oxiran‐2‐ylmethyl)diphenylphosphine oxide (DPO‐EP) was synthesized via the epoxidation of allyldiphenylphosphane oxide and subsequently employed as a reactive flame retardant. Flame‐retardant EP samples with various phosphorus contents were prepared by curing blends of diglycidyl ether of bisphenol A (DGEBA) and DPO‐EP, using 4,4′‐diaminodiphenylsulfone (DDS) as the curing agent. Among these, the EP sample containing 0.6 wt% phosphorus (EP/DPO‐EP/0.6) exhibited a limiting oxygen index (LOI) value of 32.9% and achieved a UL‐94 V‐0 rating. In the cone calorimeter test, EP/DPO‐EP/0.6 showed significant reductions in peak heat release rate (57.3%) and total heat release (14.0%) compared to pure EP. Mechanistic studies revealed that DPO‐EP primarily exerted its flame‐retardant effect through the release of phosphorus‐containing radicals and non‐flammable gases in the gas phase, while simultaneously facilitating the formation of a compact char layer in the condensed phase. Furthermore, the incorporation of DPO‐EP accelerated the curing process of EP without compromising its transparency. In addition, the dielectric constant and dielectric loss of EP were reduced by the introduction of DPO‐EP. The combination of high flame retardancy, excellent transparency, and improved dielectric properties renders DPO‐EP‐modified EP a promising candidate for advanced applications in electronic and optical fields.