Flammability characteristics and flame retardant mechanism of phosphate-intercalated hydrotalcite in halogen-free flame retardant EVA blends

The flammability characteristics and flame retardant mechanism of phosphate-intercalated hydrotalcite (MgAl-PO4) in the halogen-free flame retardant ethylene vinyl acetate (EVA) blends have been studied by X-ray diffraction (XRD), Fourier transfer infrared (FTIR) spectroscopy, cone calorimeter test (CCT), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), limiting oxygen index (LOI) and UL-94 tests. The results show that the hydrotalcite MgAl-PO4 intercalated by phosphate possesses the enhanced thermal stability and flame retardant properties compared with ordinary carbonate-intercalated hydrotalcite MgAl-CO3 in the EVA blends. The CCT tests indicate that the heat release rate (HRR) and mass loss rate (MLR) values of the EVA/MgAl-PO4 samples are much lower than those of the EVA/MgAl-CO3 samples. The TGA data show that the thermal degradation rates of MgAl-PO4 and EVA/MgAl-PO4 samples are much slower and leave more charred residues than those of MgAl-CO3 and its corresponding EVA blends. The LOI values of EVA/MgAl-PO4 samples are 2% higher than those of the corresponding EVA/MgAl-CO3 samples at the range of 40-60 wt% loadings, while the EVA sample with 55 wt% MgAl-PO4 can reach the UL-94 V-1 rating. The dynamic FTIR spectra reveal that the flame retardant mechanism of MgAl-PO4 can be ascribed to its catalysis degradation of the EVA resin, which promotes the formation of charred layers with the P-O-P and P-O-C complexes in the condensed phase. The SEM observations give further evidence of this mechanism that the compact charred layers formed from the EVA/MgAl-PO4 sample effectively protect the underlying polymer from burning.