Intrinsic Flame Retardant Polyamide 6 Based on High-Temperature Self-Cross-Linking Carbonization and Catalytic Synergistic Carbonization

Although polyamide 6 (PA6) is a widely used engineering plastic, its high flammability with severe melt dripping poses significant fire risks, and common additive flame retardants often deteriorate its mechanical properties. Herein, an intrinsic flame-retardant PA6 copolymer was successfully synthesized by incorporating a phosphorus-containing monomer (PD) derived from DOPO and a Schiff base intermediate, and the copolymer was named PA6-co-PDx and was prepared by melt polycondensation. The incorporation of PD significantly enhanced the flame retardancy of PA6, with PA6-co-PD40 passing a UL-94 V-0 rating and a limiting oxygen index of 26.1%. Cone calorimetry revealed remarkable reductions in the peak heat release rate (53.6%) and total heat release (27.6%), indicating suppressed combustion intensity. The flame-retardant mechanism was attributed to the synergistic effect of high-temperature self-cross-linking and catalytic char formation in the condensed phase, along with gas-phase radical quenching by phosphorus-containing species. Although the introduction of PD slightly reduced the crystallinity and mechanical properties, the copolymer retained acceptable processability, demonstrating great potential for applications requiring high flame retardancy.