Ammonium polyphosphate/montmorillonite nanocomposite with a completely exfoliated structure and charring–foaming agent flame retardant thermoplastic polyurethane

A completely exfoliated structure of ammonium polyphosphate/montmorillonite (APP–MMT) nanocomposite was prepared and applied to thermoplastic polyurethane (TPU) to prepare TPU composites containing APP–MMT and a charring–foaming agent (CFA), after which the properties were compared with those of TPU/APP/CFA composites. The thermal decompositions of TPU composites were investigated by thermal gravimetric analysis. The synergistic effect between APP–MMT and CFA was superior to that of APP and CFA, and the char residue was greatly improved. The flame retardancy of TPU composites was studied by limited oxygen index (LOI) measurements, UL-94 testing, and cone calorimeter test. The APP–MMT nanocomposite promoted the compactness and strength of the char layer formed by the combustion of the TPU composites. The CFA improved the thermal stability of TPU. Compared with TPU/APP/CFA composites, the peak release rate (pHRR) of TPU/APP–MMT/CFA composites were lower and the LOI were higher. When the APP–MMT/CFA compound proportion was 3:1 in weight, the LOI was 25.4%, the pHRR and total heat release of the TPU composites decreased by 78.2% and 36.6%, respectively, and the smoke production and CO emission were restrained. The mechanical properties and morphology of TPU composites were analyzed by tensile strength, elongation at break, and scanning electron microscopy. The APP–MMT composite improved the dispersion of CFA in the TPU, and the mechanical properties of TPU/APP–MMT/CFA composites were higher than that of TPU with only CFA. The structure of residual carbon was studied by X-ray photoelectron spectroscopy, and it was confirmed that the APP–MMT composite promoted the formation of a good protective carbon layer during the combustion of the TPU composites and improved its flame retardancy.