Synthesis of Polyamide 66 Copolymers with Inherent Flame Retardancy and Enhanced Mechanical Properties

Flame-retardant modification of polyamide 66 (PA66) typically leads to a reduction in mechanical properties, as the steric hindrance effect of flame retardants weakens the intermolecular forces and affects the crystallization behavior, thereby limiting its broader applications. In this work, a reactive flame retardant (TAP), composed of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and triazole structures, was synthesized and employed to fabricate inherently flame-retardant PA66 copolymers via a straightforward synthetic route. Incorporation of 4 wt % TAP into PA66 (denoted as PA66-co-TAP-4) endowed the copolymer with a limiting oxygen index (LOI) of 30.2% and a V-0 rating in the vertical burning (UL94) test. Compared with pure PA66 (80.9 MPa), PA66-co-TAP-4 achieved a higher tensile strength of 85.6 MPa. Synergistic effects of gas-phase radical quenching, fuel dilution, condensed-phase barrier action, and TAP-copolymer hydrogen-bonding interactions render PA66-co-TAPs with superior flame resistance and improved mechanical performance. This work provides a viable strategy to enhance the fire safety of PA66, thereby, significantly expanding its practical application value.