Thermal Degradation Kinetics and Flame-Retardant Synergy in Zinc Borate/Red Phosphorus-Filled Thermoplastic Polyurethane Composites

Abstract

Thermoplastic polyurethane (TPU) is widely used in engineering and industrial applications due to its excellent mechanical properties and versatility; however, its inherent flammability and thermal degradation behavior limit its use in fire-sensitive environments. In this study, flame-retardant TPU composites containing red phosphorus (RP) and zinc borate (ZnB) were prepared via melt compounding to investigate their combined effects on thermal stability, fire performance, and mechanical properties. Thermogravimetric analysis demonstrated that RP significantly increased char yield and altered the degradation pathway, whereas ZnB further enhanced condensed-phase stability by forming boron-rich protective layers. Non-isothermal degradation kinetics, evaluated using model-free methods, revealed conversion-dependent activation energies, indicating a complex multi-step degradation mechanism. RP-containing composites exhibited higher activation energies at advanced conversion levels, suggesting improved resistance to thermal degradation. Mechanical testing showed that RP increased tensile strength and modulus, whereas the incorporation of ZnB into hybrid systems reduced tensile strength and elongation at break. Overall, the RP/ZnB hybrid system offers a promising approach to achieving a balance among flame retardancy, thermal stability, mechanical performance, and processability in TPU composites.