Effects of zinc and nickel salts in intumescent flame-retardant polypropylene

Variable amounts of zinc and nickel salts, such as ZnSO₄· 7H₂O and NiSO₄·6H₂O, have been incorporated into blends of polypropylene (PP)/ammonium polyphosphate (APP)/dipentaerythritol (DPER) with the aim of studying their effect on intumescent flame retardance (IFR). The PP/IFR/salt composites have been prepared using a twin-screw extruder, and their IFR behaviours have been evaluated through limiting oxygen index (LOI), vertical burning tests (UL-94), and cone calorimeter tests (CONE). The results show that, at an appropriate level, zinc and nickel salts can increase the LOI and decrease the heat release rate (HRR). The composites have been studied with the aid of thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The flame-retardant mechanism of the PP/IFR/salts system is also discussed in terms of catalytic charring. ZnSO₄·7H₂O has been shown to be the most effective among the aforementioned metal salts, which has proved to be strongly associated with its low melting point and the interaction between DPER and SO₄^2-.