Enhanced fire safety of intrinsically flame-retardant unsaturated polyester and its fiber-reinforced composites via phosphotungstic acid-modified zirconium phosphate

Unsaturated polyester resin (UPR) suffers from high flammability and the release of toxic smoke during combustion. Although intrinsically flame retardant UPR (FRUP2), containing low-oxidation-state phosphorus structures, exhibits excellent gas-phase flame inhibition, it inevitably aggravates incomplete combustion, leading to excessive smoke and CO release. To address this limitation, a novel flame retardant (phosphotungstic acid-modified zirconium phosphate (HPW/ZrP)) was synthesized and introduced into FRUP2 and its glass fiber-reinforced composites. At an optimal loading of 3 wt%, FRUP2-HPW/ZrP achieved a limiting oxygen index (LOI) of 24.2 % and a UL-94 V-0 rating. Cone calorimetry revealed significant reductions in both fire and smoke hazards: the peak heat release rate (PHRR) and total heat release (THR) decreased by 20.28 % and 9.10 %, respectively, while total smoke production (TSP) and CO yield (COY) were reduced by 18.87 % and 74.65 %, compared with FRUP2. The enhanced performance is attributed to the strong acidic sites on HPW/ZrP, which catalyze the rapid formation of a dense and graphitized char layer, effectively inhibiting heat and mass transfer and mitigating the smoke toxicity induced by low-oxidation-state phosphorus. In addition, HPW/ZrP improved the mechanical performance of FRUP2. For glass fiber-reinforced composites, HPW/ZrP maintained excellent flame retardancy and smoke suppression without compromising tensile strength. This study provides an effective strategy to overcome the smoke toxicity issue of phosphorus-containing intrinsically flame retardant UPR, offering a promising pathway toward fire-safe, high-performance polymer composites.