Phosphate-Functionalized Silica for Improved Flame Retardancy and Thermal Stability of Thermoplastic Polyurethane

In the current work, a synergistic flame retardant based on phosphate-functionalized silica (PhosFS) has been synthesized by utilizing alkyne–azide click chemistry between 2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane, 4-(azidomethyl)-1-oxide, and propargyl-functionalized silica (PFS). Phosphate-functionalized silica was characterized using various techniques such as solid-state 31P nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis, and inductively coupled plasma–mass spectrometry. PhosFS, when incorporated into thermoplastic polyurethane (TPU), showed superior thermal stability and higher char yield in comparison with TPU. The flammability of the composites was analyzed by UL-94 vertical burning and limiting oxygen index (LOI) test. With the addition of 10 wt % PhosFS in TPU, the LOI value increased to 28.5% and the V-0 rating was achieved by UL-94. The plausible mechanism for the flame retardance property of composites has been proposed on the basis of results obtained from FTIR and scanning electron microscopy–energy dispersive X-ray spectroscopy. TPU composites based on phosphate-functionalized silica exhibited better flame retardancy due to the formation of continuous and highly entangled char.