Green Preparation of Ammonium Polyphosphate With Shell-Core Structure for Enhancing Fire Safety of Thermoplastic Polyurethane

In the recent years, the development of high-performance flame retardants (FRs) for thermoplastic polyurethane (TPU) has become increasingly important due to its wide applications in various industries and the growing demand for fire safety. In this study, ammonium polyphosphate (APP) was used as the core, whereas chitosan (CS) and nano-silica (SiO2) served as the shell to create a shell-core-structured FR through a self-assembly process in an aqueous phase. APP, APP@CS, and APP@CS@SiO2 were incorporated into TPU at a concentration of 20 wt%, and their effects on TPU properties were compared. The findings showed that APP@CS@SiO2 exhibited superior flame retardancy for TPU. Compared to pure TPU, the TPU/APP@CS@SiO2 showed reductions in peak heat release rate, total heat release, peak smoke production rate, total smoke production, peak CO production rate, and peak CO2 production rate by 58%, 80%, 80.5%, 89.1%, 20.4%, and 60.1%, respectively. Additionally, APP@CS@SiO2 effectively inhibited the thermal degradation of TPU and enhanced its thermal stability, resulting in the residual carbon content of the composite rising to 28.5%, which is about 20 times higher than that of pure TPU. In conclusion, APP@CS@SiO2 significantly improved the fire safety of TPU by efficiently catalyzing char formation in the condensed phase, diluting flammable gases, and hindering gas-phase free radical chain reactions, thereby opening new avenues for the practical application of TPU composites.