Synergistic effect of expandable graphite and intumescent flame retardants on the flame retardancy and thermal stability of polypropylene

In this article, the investigation mainly focuses on the synergistic effect of expandable graphite (EG) and intumescent flame retardants in improving the flame retardancy of polypropylene (PP). The ratio of melamine polyphosphate (MPP) and dipentaerythritol (DPER) has been studied by limiting oxygen index (LOI) and vertical burning test (UL-94) tests. The results demonstrate that the optimal ratio of MPP and DPER in flame-retarding PP was 3/1. Except that, the incorporation of EG into the PP/MPP/DPER system can greatly improve the flame-retardant properties of PP materials. When the content of EG is 10 wt%, LOI value of PP composites reaches 33.2 % and obtains V-0 rating in UL-94 tests. Furthermore, Fourier transform infrared spectra (FTIR) indicate that the main char-formation process of this system occurs at 350–400 °C. And char residue for PP composites after combustion was systematically analyzed by FTIR and X-ray photoelectron spectroscopy spectra. Based on these facts, a potential condensed flame-retardant mechanism was primarily proposed.