Synthesis of a phosphorus-containing nickel(ii) complex towards suppressing heat release and enhancing the fire safety of polyamide 6

Abstract

Polyamide 6 (PA6) is inherently flammable and releases substantial heat during combustion, which restricts its widespread application. While conventional 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-based flame retardants exhibit notable efficiency, their ability to catalyze char formation in PA6 remains limited, hindering effective heat suppression. To address this challenge, a phosphorus-containing transition metal complex, BPMNi, was synthesized by strategically integrating nickel(II) with a tailored DOPO-derived ligand. The introduction of the metal element aimed to enhance catalytic carbonization during combustion. After melt blending, the composite containing BPMNi (PA6/BPMNi-3) achieved a limiting oxygen index of 31.5% and passed the vertical burning test with a V-0 classification. Its peak heat release rate and total heat release values were reduced to 653 kW m⁻² and 40.0 MJ m⁻², representing decreases of 40.3% and 55.2%, relative to neat PA6. The flame-retardant behavior and suppression of heat release stemmed from a dual-action mechanism. In the gas phase, high-energy free radicals were captured, terminating the chain reaction of the combustion process. Meanwhile, in the condensed phase, a robust and thermally stable char layer formed on the PA6 matrix surface, acting as a protective shield.