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A "trinity" reactive flame retardant (TRFR) was successfully synthesized from pentaerythritol, phosphorus oxychloride (POC), and p-aminobenzoic acid in two steps. The flame retardant polyamide 66 (PA66) was prepared by polymerizing TRFR with PA66 salt; the structural changes during the heating process, the morphology, and composition after combustion of flame retardant PA66 were analyzed. Fourier transform infrared, scanning electron microscopy, and Raman analysis results showed that the TRFR structure on flame retardant PA66 decomposed at the temperature of 25-550 C, forming compounds containing phosphorus, carbon, and nitrogen, respectively. These compounds promoted the dehydration of the combustion surface to form char, increased the char formation rate, and produced nonflammable gases, resulting in a dense, porous, noncombustible carbon layer. The carbon layer may isolate oxygen and heat, thereby preventing the polymer from sustainability of combustion. When the TRFR salt content was 3%, TRFR flame retardant PA66 has excellent flame retardancy with limited oxygen index value of 29 and UL94 of V-0 rating.

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