The Effects of Sodium Antimonate on the Flame Retardancy and Mechanical Properties of Thermoplastic Polyurethane Composites

Thermoplastic polyurethane (TPU) is highly versatile yet inherently flammable, restricting its use in fire-safe applications. This study incorporates sodium antimonate (SA) into TPU via melt blending (0–10 wt%). Through a comprehensive suite of analytical techniques—including thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), universal testing, and cone calorimetry—the effects of SA on the various properties of the material were investigated. Quantitative analysis reveals that 10 wt% SA delivers the best flame retardancy, reducing the peak heat release rate by ~50% (from 605 to 310 kW/m2) and total heat release by ~40% (from 32 to 19 MJ/m2), while increasing the char residue from 9.84% to 13.89%. However, to retain mechanical robustness, the SA content must be capped at ≤5 wt%. This ensures that the material retains 30% of its fracture elongation and a tensile strength of 1.7 MPa; higher loadings cause severe embrittlement due to particle agglomeration. Mechanistically, SA acts via a dual-phase mode—promoting a dense insulating char layer in the condensed phase and quenching reactive radicals in the gas phase. These findings establish a practical balance between fire safety and mechanical performance, offering a clear guideline for the rational design of flame-retardant TPU composites.