Flame retardancy of polypropylene filled with expandable graphite and magnesium hydroxide: The impact of particle size of expandable graphite and its mechanism

Polypropylene (PP) was modified by expandable graphite (EG) and magnesium hydroxide (MH) to enhance its flame retardancy. The impact of EG particle size on fire behavior of PP/MH/EG ternary composite was studied in detail. The results reveal that EG synergizes markedly with MH in flame-retarding PP. The PP/MH/EG composite with larger EG particle size displays better flame retardancy, smoke inhibition, acid resistivity, processability, and less toxic gas release upon combustion. The striking contrast derives from differences of charring behavior in condensed phase. The composite with larger EG particle size can produce intumescent char with larger expansion volume and better compactness and continuity, which hinders heat transfer inside polymer composite. Owing to much smaller expansion volume, the composite containing smaller EG particle size cannot generate effective fireproof barrier. As a consequence, this composite exhibits worse fire retardance and smoke inhibition in contrast with the composite with larger EG particle size.