Suppress thermal degradation of DDP in copolymerized PA66 with better intrinsic flame retardancy by regulating prepolymerization

Abstract

Intrinsic flame-retardant polyamide 66 (PA66) is highly desirable for solving processing challenges in high-fire-safety applications, but the poor thermal stability of phosphorus-based intrinsic flame retardant DDP ((6H-Dibenz [c,e] [1,2] oxaphosphorin-6-ylmethyl)-p-oxide-butanedioic acid) causes severe thermal degradation during PA66 copolymerization, limiting its application. Herein, a synergistic strategy of introducing caprolactam (CPL) as the third monomer and optimizing the polymerization process was proposed to address this issue. By reducing the temperature of each polymerization stage, DDP was more completely incorporated into the PA66 molecular chains and PA66-5PA6-nDDP copolymers (5 wt% PA6, n wt% DDP) were obtained confirming by ¹H NMR, ³¹P NMR and FTIR. The incorporation of DDP improved the flame retardancy of PA66. PA66-5PA6-5DDP achieved a limiting oxygen index (LOI) of 29.0% and passed UL-94 V-0 rating, with a 22.4% reduction in peak heat release rate (pHRR) compared to PA66-5PA6. Notably, the copolymers showed potential spinnability fibers with smooth surface and uniform element distribution (diameter ≈ 30 μm) were obtained via rapid stretching simulation, and the tensile strength and modulus of PA66-5PA6-5DDP fibers increased by 25.9% and 57.1% respectively compared to PA66-5PA6 fibers, which is conducive to expanding the application of flame-retardant PA66 in textile and special fiber fields. Systematic investigations on flame-retardant mechanism via TG-FTIR, Py-GC/MS and SEM-EDS revealed a gas-condensed phase synergistic effect. A unique layered char structure was observed. Specifically, P and O elements were enriched in the outer layer, forming a P—O conjugated barrier, whereas C elements were concentrated in the inner layer and contributed to maintaining char integrity. This layered structure can efficiently block heat/mass transfer in the outer layer and maintain the integrity of the char layer via the inner layer, thereby enhancing the flame-retardant efficiency. This work provides an effective strategy for preparing intact flame-retardant PA66 copolymers with balanced flame retardancy, spinnability and mechanical properties, and deepens the understanding of phosphorus-based flame-retardant mechanisms in PA66.