Thermal degradation, flame retardance and mechanical properties of thermoplastic polyurethane composites based on aluminum hypophosphite

Aluminum hypophosphite (AP) was used to prepare flame-retarded thermoplastic polyurethane (FR-TPU) composites, and their flame retardancy, thermal degradation and mechanical properties were investigated by limiting oxygen index (LOI), vertical burning test (UL-94), thermogravimetric analysis (TGA), cone calorimeter (CC) test, Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and tensile test. TPU containing 30 wt% of AP could reach a V-0 rating in the UL-94 test, and its LOI value was 30.2. TGA tests revealed that AP enhanced the formation of residual chars at high temperatures, and slightly affected the thermal stability of TPU at high temperatures. The combustion tests indicated that AP affected the burning behavior of TPU. The peak of heat release rate (PHRR), total heat release (THR) and mass loss rate (MLR) greatly reduced due to the incorporation of AP. The tensile test results showed that both the tensile strength and the elongation at break slightly decreased with the addition of AP. The digital photos and SEM micrographs vitrified that AP facilitated the formation of more compact intumescent char layer. Based on these results mentioned above, the flame-retarding mechanism of AP was discussed. Both the self-charring during the decomposing process of AP and its facilitation to the charring of TPU led to the great improvement in the flame retardancy of TPU.