Inherent flame-retardant vinyl ester resins with improved heat resistance enabled by additional P/N-containing cross-linking points

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 ( T g ) 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 T g and flame retardancy of thermosetting resins.