Synthesis and Characterization of a DOPO-Based Macromolecular Flame Retardant and Its Application on the Flame Retardancy of Polyamide 66

To enhance the flame retardancy of polyamide 66 (PA66), this study developed a macromolecular flame retardant (PA–DDP) using nylon 66 salt and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DDP). The mechanical property, flame retardancy, and flame retardant mechanism of flame retardant PA66 (FRPA66) were characterized. In a vertical combustion test, FRPA66 with a P concentration of 4000 ppm performs well in terms of flame retardancy, receiving a V-0 grade (UL-94). The cone calorimeter results indicate significantly lower heat release of FRPA66 than from pure PA66. The peak heat release rate and total heat release of FRPA66 with 6000 ppm P are 47.8% and 21.5% lower, respectively, than those of PA66. Moreover, PA–DDP exhibits flame retardant properties in gas and condensed phases. Phosphorus-containing radicals in the pyrolysis product of PA–DDP capture active radicals, thereby interrupting the combustion chain reaction and playing a flame-retardant role in the gas phase. The break of the P–C bond in PA–DDP promotes the droplet drop to carry away heat of combustion timely in the condensed phase, rather than forming a carbon layer to cut off heat transfer and oxygen supply.