Synthesis and characterization of inherently flame retardant polyamide 6 based on a phosphine oxide derivative

To improve the flame retardancy of polyamide 6 (PA 6), 2,3-dicarboxy propyl diphenyl phosphine oxide (DPDPO) as the reactive flame retardant was introduced into the molecular chain of PA 6. The chemical structure, thermal property, mechanical property and flame retardancy of flame retardant PA 6 were characterized by Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, differential scanning calorimetry, thermogravimetric analysis, tensile test, vertical burning test and limiting oxygen index (LOI) test. The flame-retardant mechanism was analyzed by cone calorimetry and pyrolysis-gas chromatography-mass spectrometry. The introduction of DPDPO reduced the melting point and crystallization of flame retardant PA 6, but the flame retardancy of flame retardant PA 6 improved. The flame retardant PA 6 with 5 wt% DPDPO could pass the V-0 rating (UL 94) with the LOI value of 31.7%. The results indicated that DPDPO promotes the degradation of PA 6 at high temperature, and mainly act by the quenching effect of phosphorus-containing radicals in the gas phase. In addition, the reduction of caprolactam produced during degradation also contributes to the improved flame retardancy. The inherently flame retardant PA 6 shows the prospect in the fields of fiber and textile.