Synergistic effect of zinc hydroxystannate with intumescent flame-retardants on fire retardancy and thermal behavior of polypropylene

The flammability and thermal degradation properties of polypropylene (PP) composites containing zinc hydroxystannate (ZHS) and intumescent flame retardant additives (IFR), i.e. ammonium polyphosphate (APP) and pentaerythritol (PER) were characterized respectively by limiting oxygen index (LOI), UL-94 measurements, Cone calorimeter test (CCT) and Thermogravimetry analysis (TGA) in this work. A synergistic effect in flame retardancy was observed when ZHS was used in combination with APP and PER. The experimental data indicated that ZHS enhanced the LOI value, UL-94 ratings and restricted the dripping of the composites. The PP/IFR composites passed the UL-94 V-0 rating test in the presence of 1 wt% ZHS. The CCT tests indicated that the heat release rate (HRR), peak rate of heat release (PHRR) and mass loss rate (MLR) values of the PP/IFR/ZHS samples were much lower than those of the PP/IFR and pure PP samples. The TGA results showed that ZHS could accelerate the char formation of IFR, therefore, greatly increase the thermal stability of PP composites. The Fourier transformed infrared spectra (FTIR) revealed that the flame retardant mechanism of ZHS could be ascribed to its catalysis degradation of the PP resin, which promoted the formation of charred layers with the P-O-P and P-O-C complexes in the condensed phase. SEM observation further indicated that ZHS could promote forming stable and compact intumescent char layer and effectively protect the underlying polymer from burning.