Study on sol–gel-modified ammonium polyphosphate and its application in the flame-retardant polyurethane composites

In this study, organic–inorganic-modified ammonium polyphosphate (APP) flame retardant (FR) was prepared by a sol–gel method that used tetraethoxysilane and vinyltriethoxysilane as precursors. Then modified APP (VTSAPP) was added into polyurethane (PU) to fabricate the FR PU/VTSAPP composites. Crucially, the structure and performance of VTSAPP were systematically characterized. The results showed that a dense hybrid silica gel layer structure was formed on the surface of APP. Ideally, the limiting oxygen index (LOI) value of PU/20VTSAPP composite increased from 19.5% for pure PU to 30% and the vertical burning test (UL-94) reached up to V-0 level. Moreover, the addition of VTSAPP dramatically decreased the released smoke of PU during combustion compared to the counterpart of APP. The FR mechanism of VTSAPP was deeply explored. It is confidently shown that phosphorus and silicon compounds of VTSAPP contributed to the formation of a more continuous and compact char and improved the thermal stability of the char layer. The results of this study distinctly confirmed that VTSAPP was an effective FR agent for PU. Highlights The modified APP flame retardant was prepared by a sol–gel method that used TEOS and VTES as precursors. The modified APP was added into the PU matrix to prepared flame-retardant PU composites. The modified APP not only enhanced the flame retardancy of PU but also reduced the smoke emission. The flame-retardant mechanism of modified APP in PU was deeply explored.