Abstract Due to the plasticizing effect, the phosphorus‐derived flame retardants often imparted flame retardancy to thermosetting resins at the expense of heat resistance. To address this issue, the phosphorous/nitrogen flame retardant (DOPO‐TGIC) and 4,4′‐diphenyl methane diisocyanate (MDI) were covalently introduced into the cross‐linked network of vinyl ester resin (VER) by the reaction of the OH groups in DOPO‐TGIC and VER and the NCO groups in MDI. Thus, an intrinsically flame‐retardant VER was obtained. The as‐prepared VER sample exhibited increased glass‐transition temperature (Tg) relative to virgin VER sample because DOPO‐TGIC served as an additional cross‐linking point during curing. Meanwhile, the P/N‐containing VER sample featured superior flame‐retardant properties, with a UL‐94V‐0 classification. Compared with VER sample, the peak heat release rate (PHRR) and total heat release (THR) of the P/N‐containing VER sample were reduced by 55.6 and 44.0, respectively. The improved flame retardancy was mainly due to the suppressed burning reaction in the gas phase and improved carbonization capacity in the condensed phase. Therefore, this work proposes a novel and effective strategy for addressing the trade‐off between Tg and flame retardancy of thermosetting resins.
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