Effects of nanoclays on the thermal stability and flame retardancy of microcellular thermoplastic polyurethane nanocomposites

Microcellular nanocomposite foams based on thermoplastic polyurethane (TPU) and organically modified layered clays were prepared via an efficient foaming method, microcellular injection molding (MIM). The morphology of the nanoclays was characterized by X-ray diffraction (XRD) and transmission electron microscopy. The results revealed that the layered nanoclays in the nanocomposite foams achieved a well dispersed morphology within the cell walls due to the MIM foaming process. Postburn SEM images showed that TPU/NC10 foam (with 10 wt% nanoclay) was able to maintain its foam porosity, original shape, and microcellular structure due to the synergistic effects of its microcellular structure and the fully dispersed nanoclays. Thermal stability and flame resistance were assessed by thermogravimetric analysis (TGA) and microscale combustion calorimetry, respectively. TGA results indicated that the nanocomposite foams had a significant decline in the thermal degradation rate compared to the unfilled TPU/NR foam. The flame resistance tests showed that the heat release capacity (HRC) of TPU/NC10 foams was substantially lower (27% lower) than that of the unfilled TPU/NR foam. The char yield of the TPU/NC10 foam was 19.14%, which was almost 18 times higher than that of the unfilled TPU/NR foam. With the presence of the microcellular structure, the HRC values decreased from 266.8 J/g k for solid TPU/NC10 to 235.5 J/g k for TPU/NC10 foams. This work combines two flame retardant mechanisms—the thermal barrier effect of the nanoclay layers and the microcellular self-intumescing system—in the nanocomposite foams. Our study also provides substantial motivation to produce novel, environmentally-benign, and flame-retardant TPU foam using microcellular injection molding. POLYM. COMPOS., 39:E1429–E1440, 2018.