Synergistic effects of red phosphorus masterbatch with expandable graphite on the flammability and thermal stability of polypropylene/thermoplastic polyurethane blends

The flammability characterization and synergistic effects of red phosphorus masterbatch (RPM) with expandable graphite (EG) in flame-retardant polypropylene (PP)/thermoplastic polyurethane (TPU) composites are investigated by limiting oxygen index, UL-94 testing, cone calorimeter tests, thermal gravimetric analysis, Fourier transform infrared (FTIR), and scanning electron microscopy. The results show that the flame retardancy of PP/TPU/EG/RPM composites is greatly influenced by RPM. The synergistic effects between RPM and EG take place in the flame-retardant composites. The presence of RPM with EG decreases significantly the heat release rates and total heat release, and UL-94 V-0 rating is achieved when suitable amount of RPM substitutes for EG in the PP/TPU/EG/RPM composites. The T (onset) and T (10 wt%) of the composites are improved because of the presence of RPM. The FTIR spectra show that the incorporation of RPM improves the thermo-oxidative stability of PP/TPU at higher temperatures. The morphological observations indicate the reinforcement of thermal stability, and flame-retardant performance is attributed to the compact and stable char layers promoted by RPM with EG acted as an effective heat barrier and thermal insulation.