Reactive phosphorus-containing monomers combined with modified diatomite fillers: A double-barreled approach to highly fire-safe vinyl ester resins

Vinyl ester resins (VER) are widely used in various fields because of their excellent corrosion resistance, mechanical properties, and processability. However, their inherent flammability severely limits further development. To address this issue, a reactive flame-retardant co-monomer (DHP) containing phosphorus in two different chemical environments (+3 and +5 oxidation states) was synthesized and combined with modified diatomite to prepare flame-retardant VER composites. The results showed that at a DHP loading of 30%, the composite achieved an LOI of 27.1% and passed the UL-94 V0 vertical burning test. When 5% DHP was replaced with 5% modified diatomite, the composite maintained a V0 rating, with the LOI increasing to 27.3%. Furthermore, the peak heat release rate (pHRR) and total heat release (THR) decreased from 426.2 kW/m² and 39.7 MJ/m² to 357.3 kW/m² and 33.3 MJ/m², respectively, corresponding to reductions of 35.8% and 42.0%, respectively, compared with those of neat VER. Analysis of the flame-retardant mechanism revealed that during pyrolysis, part of the phosphorus in DHP entered the gas phase as phosphorus-containing free radicals, inhibiting combustion reactions by quenching active radicals. The remaining phosphorus generated phosphoric or polyphosphoric acids, catalyzing matrix charring and forming a dense, continuous char layer that impeded heat and mass transfer. The incorporation of modified diatomite further promoted char formation, enhancing the condensed-phase flame-retardant effect. This study proposes a synergistic flame-retardant strategy combining a dual-valence phosphorus-containing co-monomer with modified diatomite, offering a novel approach for developing high-performance flame-retardant VERs.