Synergistic effects of sepiolite on the flame retardant properties and thermal degradation behaviors of polyamide 66/aluminum diethylphosphinate composites

The effect of sepiolite as a synergistic agent on the flame retardancy and thermal degradation behavior of polyamide 6,6 (PA66)/aluminium diethylphosphinate (AlPi) composites were investigated by limiting oxygen index (LOI), vertical burning (UL-94) tests, thermogravimetric analysis/infrared spectrometry (TG-IR) and cone calorimeter tests (cone). The morphologies and the chemical compositions of char residue were investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectra. The results demonstrated that the specimens with the thickness of 1.6 mm passed UL-94 V-0 flammability rating and the LOI value was as high as 33.4% when the loading amount of AlPi was 14 wt %. However, when sepiolite (3 wt % based on flame retardant) incorporated into the AlPi/PA66 composites and the total loading amount of AlPi/sepiolite mixture was only 10 wt %, the samples with the thickness of 1.6 mm successfully passed UL-94 V-0 flammability rating and the LOI value reached 32.5%. It was deduced that the combination of AlPi with sepiolite possessed an evident synergistic effect and enhanced the flame retardant efficiency for PA66 materials. The TG-IR and FTIR tests of the char residues revealed that the incorporation of sepiolite changed the degradation process of the PA66/AlPi system and possibly formed diethylphosphinic acid, siliconphosphate and magnesiumphosphate in condense phase, which enhanced the thermal stability and char yield of PA66 materials at high temperature. The results of cone tests revealed that AlPi combined with sepiolite efficiently reduced the fire behavior parameters, such as heat release rate (HRR), total heat release (THR) and so on. The digital photographs and morphological structures of char residues demonstrated that sepiolite benefited to the formation of more sufficient, compact and homogeneous char layer on the materials surface during burning. The analysis of XPS indicated that sepiolite promoted the formation of flame retardant elements rich char layer on the materials surface, which increased the strength of the char layer and enhanced the flame retardancy of PA66 polymeric materials.