Fire behavior and mechanical properties of phosphorus-containing silica gel coated ammonium polyphosphate as fire retardants in thermoplastic polyurethane (TPU)

To enhance its properties, ammonium polyphosphate (APP) was subjected to modification using a synthesized phosphorus-containing silica gel, resulting in MAPP. Ammonium polyphosphate (APP) was modified by synthesized phosphorus-containing silica gel, resulting in MAPP, which was then used to form thermoplastic polyurethane composites. MAPP exhibited better thermal stability with a smaller dispersion size, lower initial decomposition temperature and better resistance to heat at high temperatures than APP. At the same time, MAPP showed more obvious advantages in improving the char yield, flame retardant performance and mechanical properties of TPU. TPU/5%MAPP showed a limiting oxygen index of 29.1% during burning tests, significantly better than TPU/5%APP (27.3%). The cone calorimeter test results also indicated that the incorporation of MAPP resulted in a significant reduction in key parameters such as peak heat release rate, total heat release, and total smoke release and an increase of the char yield of TPU composite. The TG result showed that the T_max of the TPU/5%MAPP composite increased and the mass loss rate decreased compared with that of TPU/5%APP. Moreover, the tensile strength of the TPU/5%MAPP composite increased by 18.5% compared with the TPU/5%APP composite due to the improved dispersibility of MAPP than APP. An analysis of the fire resistance mechanism showed that the improved flame retardance in the TPU/5%MAPP composite resulted from the structural changes in the char residue. This study provides an efficient and extensible approach for the synthesis of high-performance flame retardant TPU materials.