Robust charring construction of thermoplastic polyurethane: Cyclotriphosphazene structure bearing alkynyl and its impacts on thermal degradation, burning behavior and toughness

As versatile engineering polymer, thermoplastic polyurethane (TPU) is up against the challenge of reducing fire hazard and maintaining excellent mechanics due to the intrinsic flammability and poor charring capability. Without relying on metal-containing hybrid, this study developed a single-component compound HCAP with pronounced charring capability with 71 % at 800 °C due to the self-reinforcement derived from the crosslinking reaction of alkynyl group, which also enabled TPU to possess excellent charring capability, flame retardancy, anti-dripping, and flexibility. The incorporation of HCAP decomposed to catalyze forming charring layer, meanwhile, alkynyl group started crosslinking reaction to further strengthen the layer, which enormously inhibited the decomposition of hard and soft segments at high temperature. Only adding 5 wt% HCAP increased the char residue of TPU from 2.9 % to 18.8 % at 800 °C, as well as 27.5 % LOI value and UL-94 V-0 rating accompanied by anti-dripping were also achieved. Along with some dilution effect in gas phase, noticeable reduction was observed for heat release and smoke release behavior. Moreover, the introduction of HCAP brought out plasticization for TPU, which resulted in increased elongation at break by 76.4 % and tensile toughness by 45 %. This improvement endowed TPU with expanded application in safety protection field.