Surface micro-regulation for ammonium polyphosphate: Construction of a fire-safe thermoplastic polyurethane composite based on smoke suppression strategy of char layer reinforcement

Thermoplastic polyurethane (TPU) is highly flammable and exhibits dripping behavior when ignited. Improving its flame retardancy is critical to broadening its application. However, the application of ammonium polyphosphate (APP) to TPU tends to result in the undesirable consequence of excessive smoke emission. Herein, a one-pot method was used to surface micro-structure modification of APP with piperazine and tannic acid to construct a core-shell intumescent flame retardant (APT), which was then introduced into TPU by melt blending to obtain TPU/APT composites. Compared to TPU, TPU/7APT achieves a UL-94  V -0 rating without dripping and reduces the peak heat release rate by 80.0 % and total heat release by 37.8 %. Furthermore, at the same additive level, the total smoke production of TPU/7APT is reduced by 22.8 % compared to TPU/7APP. Through the analysis of the char layer and gas-phase pyrolysis products of TPU/APT, the flame-retardant and smoke-suppression mechanism is explored. This modification strategy addresses the limitations of APP application in TPU, providing valuable insights for the development of high-performance TPU composites.