A reactive flame retardant based on functionalized α-amino-Ε-caprolactam with a P-C-N bond structure for copolymerized flame-retardant polyamide

As one of the most widely used polymer materials in the world, polyamide 6 (PA6) requires enhanced flame retardancy due to its expanding applications. Here, a reactive flame retardant, P-Ph-N ACL, based on functionalized α-amino-Ε-caprolactam is synthesized through a two-step Kabachnik-Fields reaction, using α-amino-Ε-caprolactam (ACL), 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), and benzaldehyde as the reactants. As a reactive flame retardant, P-Ph-N ACL exhibits excellent flame-retardant performance, thermal stability, and reactivity. A series of copolymerized flame-retardant polyamide (P-Ph-N PA) are synthesized via hydrolytic copolycondensation using Ε-caprolactam (CPL) and P-Ph-N ACL as raw materials. As the amount of the P-Ph-N ACL increases, the flame-retardant performance of P-Ph-N PA improves significantly. When 8 wt% of P-Ph-N ACL is added, the limiting oxygen index (LOI) of P-Ph-N PA-8 increases from 21% for pure PA6 to 35%. In vertical combustion tests, P-Ph-N PA-8 self-extinguishes within 7 s after ignition, achieving the UL94 V-0 rating. The cooperative effects of gas-phase and condensed-phase flame-retardant mode of actions collectively contribute to the enhanced flame retardancy of P-Ph-N PA. Furthermore, P-Ph-N PA-8 maintains a relatively high molecular weight and crystallinity, along with commendable thermal stability and mechanical properties. © 2025