Synergistic intumescent flame retardant system based on zinc hydroxystannate and piperazine pyrophosphate for high-impact polystyrene composites with improved fire safety

High-impact polystyrene (HIPS) is widely used in household appliances, electronic equipment casings, packaging materials, and automotive parts due to its excellent dimensional stability, fluidity, electrical insulation, and rigidity. However, its high flammability and significant smoke emission during combustion limit its application in fire-sensitive environments. In this study, a synergistic intumescent flame retardant system composed of hollow cubic zinc hydroxystannate (H-ZHS), piperazine pyrophosphate (PAPP), and melamine polyphosphate (MPP) was developed to improve the fire safety of HIPS composites. Thermogravimetric analysis (TGA), limiting oxygen index (LOI), UL-94 vertical combustion, and cone calorimetry (CCT) were employed to evaluate the thermal stability, flame retardancy, and smoke suppression properties. The optimised composite (30 wt% total flame retardant loading with a PAPP/MPP mass ratio of 7:3 and 2 wt% H-ZHS) achieved a UL94-V0 rating (1.6 mm thickness) and an LOI of 34.3%. Compared to pure HIPS, the peak heat release rate (pHRR), total heat release (THR), and total smoke production (TSP) decreased by 74.1 ± 3.2%, 20.6 ± 1.8%, and 65.3%, respectively. The synergistic mechanism involved condensed-phase char formation catalysed by H-ZHS and gas-phase flame inhibition by non-flammable gases (e.g. NH₃) from PAPP/MPP decomposition. This work provides a practical strategy for developing halogen-free flame-retardant HIPS composites with balanced fire safety and mechanical properties.