Carbonization mechanism of polypropylene catalyzed by Co compounds combined with phosphorus-doped graphene to improve its fire safety performance

Polypropylene (PP), as a non-charring polymer, is highly flammable without obvious char residue after combustion. Catalyzing carbonization of the polymer during combustion has become a valid method for improving its flame retardancy. In this work, two novel combined catalysts were designed and their effects on the thermal stability and flame retardancy of PP were researched. Thermogravimetric analysis results indicated that the combination of Co compounds and P atom-doped reduced graphene oxide (PRGO) markedly improved the thermal stability of PP. Besides, the char residue of PP was increased with the addition of combined catalysts. The char yield was enhanced from 1.09 wt % for pure PP to 7.10 wt % for PP/Co-MOF/PRGO. Combustion behaviors of PP and its composites were investigated by the cone calorimeter test. The incorporation of combined catalyst resulted in the reductions of peak heat release rate, total heat release, CO and CO2 releases of PP during combustion. The presence of carbon spheres in the residual char has been confirmed by X-ray diffraction and scanning electron microscope. The flame-retardant mechanism was summarized based on the results of gas chromatography-mass spectrometry and char analysis. This research extends the method of catalyzing the formation of high-quality carbonaceous protection layers to improve the flame-retardant performance of polymers.