Remarkably improving the fire-safety of polypropylene by synergism of functionalized ZrP nanosheet and N-alkoxy hindered amine

Realizing the high efficiency of halogen-free flame retardant is one of the biggest challenges and the most urgent issues in the development of high performance polypropylene (PP). Coupling the catalytic carbonization and free-radical quenching mechanism is proposed to be an efficient method to address this issue. In this work, a functionalized zirconium phosphate (F-ZrP) with good catalytic carbonization capability was synthesized, and was combined with N-alkoxy hindered amine (NOR) to flame-retard PP. The composition showed high flame-retardant efficiency. When the content of NOR, F-ZrP and ammonium polyphosphate were 0.2 wt%, 4.95 wt% and 14.85 wt%, respectively, the flame retardant PP reached a limiting oxygen index of 36.0% and achieved a UL-94 V-0 rating. Meanwhile, the peak heat release rate, average heat release rate, total heat release, peak smoke production rate and total smoke production were declined by 95.6%, 90.6%, 73.3%, 93.5% and 80.6%, respectively. It revealed that F-ZrP formed closed micron-scale char-cages via interfacial carbonization. In the charcages, the nitroxyl radicals generated by NOR efficiently quenched the active free-radicals produced by PP and induce their cyclization reaction. Under the further catalytic effect of ZrP, graphitized crystalline char with extraordinary thermostability and barrier effect was generated quickly and protected the polymer from burning.