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

Abstract

The flammability characteristics and flame retardant mechanism of phosphate-intercalated hydrotalcite (MgAl-PO 4) 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-PO 4 intercalated by phosphate possesses the enhanced thermal stability and flame retardant properties compared with ordinary carbonate-intercalated hydrotalcite MgAl-CO 3 in the EVA blends. The CCT tests indicate that the heat release rate (HRR) and mass loss rate (MLR) values of the EVA/MgAl-PO 4 samples are much lower than those of the EVA/MgAl-CO 3 samples. The TGA data show that the thermal degradation rates of MgAl-PO 4 and EVA/MgAl-PO 4 samples are much slower and leave more charred residues than those of MgAl-CO 3 and its corresponding EVA blends. The LOI values of EVA/MgAl-PO 4 samples are 2% higher than those of the corresponding EVA/MgAl-CO 3 samples at the range of 40–60 wt% loadings, while the EVA sample with 55 wt% MgAl-PO 4 can reach the UL-94 V-1 rating. The dynamic FTIR spectra reveal that the flame retardant mechanism of MgAl-PO 4 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-PO 4 sample effectively protect the underlying polymer from burning.