Synthesis of a P/N/B-Containing Halogen-Free Modifier for High-Efficiency Flame Retardancy, Reinforcement and Toughening of Epoxy Thermosets

Although epoxy resins (EP) have been widely used in many fields, performing halogen-free flame-retardant modification on them while improving their mechanical properties still holds significant research value. Herein, a halogen-free P/N/B-containing modifier (DVB) was synthesized via mild nucleophilic substitution, esterification, and Schiff base reactions using diphenylphosphinyl chloride, vanilline, phenylboronic acid, and 3-amino-1,2-propanediol as precursors. Its efficacy on bisphenol A epoxy resin/4,4′-diaminodiphenylmethane thermosets was systematically evaluated, with comparisons to a phosphorus-only intermediate (DV). At 3 wt% DVB loading (0.18 wt% P content), the modified EP exhibited a limiting oxygen index of 28.5% and achieved UL-94 V-0 classification. Relative to pristine EP, EP/5% DVB demonstrated a reduction of 41.8% in peak heat release rate, 27.7% in total heat release, and 30.0% in total smoke production, outperforming EP/5% DV (17.8%, 15.8%, and 12.2% reductions, respectively). Furthermore, EP/DVB thermosets displayed maximal improvements of 37.0% in flexural strength and 59.4% in impact strength, coupled with a glass transition temperature retention rate of 98%. Collectively, DVB presents considerable application potential for synergistically enhancing the flame retardancy and mechanical performance of epoxy resins.