Phosphorus–Nitrogen Flame Retardant for Enhanced Fire Resistance in Unsaturated Polyester Resin

Herein, a novel flame retardant, 2,2,4,4,6,6-[1-(9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-1-di(4-aminophenyl)-1-methyl)]-cyclotriphosphazene, named DPHP, was synthesized and used to flame retard the unsaturated polyester resin (UPR). On the one hand, the phosphorus element in DPHP can interrupt the chain reaction by the P-containing free radicals generated during combustion in the gas phase, and the P-containing compounds generated can catalyze the matrix into char in the condensed phase, and make the generated char denser and more continuous than pristine UPR. On the other hand, the nitrogen element in DPHP can produce non-flammable gas during combustion, and dilute oxygen and combustible volatile products, so DPHP played a role in the gas and condensed phases, improving the fire safety of the UPR matrix. As a result, 10DPHP/UPR showed better flame retardancy with a limiting oxygen index of 29.6%, and the peak heat release rate (pHRR) and total heat release rate (THR) decreased by 63.86% and 30.65%. The flame-retardant mechanism of DPHP in UPR was analyzed and discussed.