To improve the flame retardant efficiency of intumescent flame retardants, some nano-scale additives as synergists were always added. In this work, nano silicon and piperazine pyrophosphate/melamine polyphosphate into polypropylene was investigated, which was obviously efficient to improve thermal stability and flame retardancy. The optimal synergistic ratio corresponded to 1 wt% SiO2, 10 wt% piperazine pyrophosphate, and 5 wt% melamine polyphosphate. The system passed the UL-94 V-1 classification and had a limiting oxygen index value of 34.5%. It exhibited a decrease on peak heat release rate and peak smoke production rate by 81% and 80%, respectively. Based on characterizations of scanning electron microscope, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy, the synergistic mechanism was investigated. A reactive cross-linking network between SiO2 and piperazine pyrophosphate/melamine polyphosphate during burning was formed to enhance the structure of carbon protective layer. Nano silicon acted as a foaming nucleating agent to promote the formation of porous dense carbon layer in intumescent flame retardants during burning. The synergistic strategy of SiO2 and piperazine pyrophosphate/melamine polyphosphate provided new compounded system for the design of polymeric materials with excellent flame retardancy, great thermal stability, and low release of heat and smoke.
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