Preparation of High Fire Safety Polyamide 66 Using Polyphosphazene to Promote Char Formation of Diethyl Aluminum Phosphinate

The development of Polyamide 66 (PA66) with high fire safety is of paramount importance. In this work, a new polyphosphazene flame retardant (HDP) was synthesized. HDP provides an excellent barrier effect in the condensed phase and contributes effective flame inhibition in the gas phase. ADP was subsequently incorporated to construct a synergistic HDP/ADP flame-retardant system for preparing high-fire-safety PA66 composites (PA66/HDP/ADP). HDP contributed to the char formation and decreased heat release rate (HRR) of PA66/ADP. The peak heat release rate (PHRR), total heat release (THR), and maximum average rate of heat emission (MARHE) of PA66/HDP₂/ADP₈ containing 2% HDP and 8% ADP were reduced by 68.6%, 15.6%, and 61.1%, respectively, while achieving a UL 94 V-0 rating and a limiting oxygen index (LOI) of 29.5%. A synergistic effect of 54.8% was observed between HDP and ADP in terms of the PHRR. PA66/HDP_6.5/ADP_3.5 achieved a UL 94 V-0 burning rating while extending the ignition time by 21 s. In addition, the PA66/HDP_6.5/ADP_3.5 and PA66/HDP₂/ADP₈ composites not only exhibit a substantial reduction in PHRR, but their time to PHRR (T-PHRR) is also extended by more than 150 s. In conclusion, the HDP/ADP synergistic system markedly improves the fire safety of PA66 during combustion.