Design of novel double-layer coated ammonium polyphosphate and its application in flame retardant thermoplastic polyurethanes

A multilayer core-shell flame retardant (APP@SiO2@UiO-66-NH2(Zr)) was developed by wrapping ammonium polyphosphate (APP) with a zirconium-based metal-organic framework (UiO-66-NH2(Zr)), tetraethyl orthosilicate, and gamma-(2,3-epoxypropoxy) propytrimethoxysilane. The flame retardant was applied to thermoplastic polyurethane (TPU) to improve the smoke suppression and fire resistance of TPU. It is well established that thermal radiation and smoke are important factors leading to fire losses and casualties. Compared with the pure TPU, the total heat release, total smoke production, CO yield, and CO2 yield of TPU/APP@SiO2@UiO-66-NH2(Zr) composites were reduced by 87.8 %, 87.7 %, 52.4 %, and 76.8 %, respectively. In addition, P-O/P--O, C-C/ C--C, and Si-O-Si/SiO2 protective layers appeared in the residual carbon structure because of the multicomponent P/N/Si/UiO-66-NH2(Zr) flame retardant effect. Moreover, the adsorption of UiO-66-NH2(Zr) and the barrier of the protective char layer reduced the generation of combustible volatiles during combustion. In summary, APP@SiO2@UiO-66-NH2(Zr) played a flame-retardant role in both the gas and condensed phases, which improved the flame retardancy, smoke suppression, and toxicity reduction performance of TPU.