Exploiting High Molecular Weight: A Polymeric Phosphoramide Conferring Efficient Flame Retardancy and Water-Resistance to Thermoplastic Polyurethane

A polymeric phosphoramide flame retardant (PPPA) with a benzene ring in the main chain was successfully synthesized in this study. Its molecular structure, chemical composition, molecular weight, and thermal stability were characterized, and its effects on the properties of thermoplastic polyurethane elastomer (TPU) were systematically investigated. It was found that the addition of PPPA significantly enhanced the flame retardancy of TPU, with the limiting oxygen index (LOI) reaching 28.0% and passing the UL 94 V-0 rating without dripping at an addition level of 9%. PPPA facilitates the formation of a highly graphitized, phosphorus-rich, and compact char layer through catalytic carbonization, effectively blocking heat transfer and suppressing the release of flammable gases. In comparison with pure TPU, for the TPU/9% PPPA composite, the PHRR (peak heat release rate) is lowered by 78.3% and the THR (total heat release) is reduced by 37.9%. Incorporating PPPA results in decreased mechanical properties of TPU, more notably at higher loading. Moreover, the TPU/PPPA composites exhibited excellent water resistance, maintaining stable flame-retardant properties after immersion in hot water at 85 °C for 144 h, benefiting from the high macromolecular weight of PPPA, which resist leaching and migration.