The study on flame retardancy synergetic mechanism of magnesium oxide for PA66/AlPi composite

The flame retardant plays extraordinary important role in the most commonly used polymer materials of polyamide 6,6 (PA66). In this work, micron-sized magnesium oxide (MgO) as a synergist was used to combine with aluminium diethylphosphinate (AlPi) to synergistically enhance the flame retardancy of PA66. The synergetic system by AlPi and MgO improved the flame retardancy of PA66 matrix when the total loading amount only was 10 wt%. The LOI value of the composite was as high as 32.3% and the sample in vertical burning testing obtained UL-94 V-0 rating with the 1.6 mm thickness. The cone results of PA66/AlPi/MgO composite, such as HRR and THR, indicated that the AlPi/MgO synergetic system effectively reduced the fire hazard. The MgO reacted with diethylphosphinic acid produced by the degradation of AlPi to create the magnesium phosphate during the burning, which promoted the formation of char yield and enhanced the thermal stability of PA66 material. The microstructure of char residues exhibited that the char layer became more compact and homogeneous on the surface of PA66 during burning, which was also benefit to efficiently protect the composites from heat and oxygen. This work provided a new method of using synergetic system to enhance the flame retardancy of PA66 materials.