Synergistic Effect of SiO ₂ on Intumescent Flame-retardant Polypropylene

The flammability of polypropylene and its intumescent flame retardant composites consisting of ammonium polyphosphate (APP), pentaerythritol (PER) and SiO ₂ were studied and the synergistic effect of SiO ₂ , as well as the synergistic mechanism was discussed in detail. For the composites, the content of SiO ₂ varied from 0 to 12 wt.% where the total addition of APP/PER/SiO ₂ was constant at 30 wt.%. The flammability properties were characterized by Limiting Oxygen Index (LOI) and UL-94 tests, while the synergistic mechanism of SiO ₂ was investigated by a combination of thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectrometry and scanning electron microscopy (SEM). The results showed that SiO ₂ had a synergistic effect on the PP/APP/PER system, with an optimum addition at 3.5 wt.%. The introduction of SiO ₂ improved the flame retardant properties of the PP/APP/PER system and the strength of the char layer.