Synergistic effect of fly ash on hydroxymethylated lignin-containing flame retardant polypropylene: Flame retardancy and thermal stability

In this study, fly ash (FA) was used as a synergist to enhance the fire resistance of intumescent flame retardant polypropylene containing hydroxymethylated lignin. Effects of fly ash on flame retardancy and thermal stability properties of polypropylene (PP) composites were investigated using a limiting oxygen index (LOI) meter, UL-94 vertical combustion tester, cone calorimeter, and thermogravimetric analyzer. The results showed that during combustion, incorporation of FA can effectively solve the dripping problem of the PP composites. Addition of 0.5% loading amount fly ash into the PP composite (0.5FA/IFR/PP) enhanced the flame retardancy significantly. Additionally, its LOI value increased from 28% to 33%, and UL-94 vertical burning passed V-0 grade from V-2 rate. Notably, owing to addition of the fly ash, the peak of heat release rate of the pure PP decreased by 58.0%. Moreover, the release of CO2 and CO was largely suppressed, and higher char residue was obtained for 0.5FA/IFR/PP. Fire safety of the PP composites was significantly improved. From the analysis of charring residues, a possible flame retardant mechanism was proposed. That is, incorporation of fly ash increased the formation of a denser and intact char layer, which is important in inhibiting heat and flammable gases generated during combustion of underlined matrix.