Polyphosphamide Containing Triazine and Melamine Cyanurate for Flame-Retardant PA6

A poly­(triazine-phosphamide) BPGA constituted of benzene melamine and phenyl phosphonic dichloride was synthesized and characterized, and the synergistic flame-retardant properties and mechanism of BPGA/melamine cyanurate (MCA) in polyamide 6 (PA6) were investigated. The combination of BPGA and MCA with a mass ratio of 7/7 exerted the best flame-retardant effects in improving flame retardancy (higher LOI value), self-extinguishing property (higher UL 94 grade), and higher quality retention (more char residue). BPGA/MCA showed a good synergistic charring effect, forming more high-temperature thermally stable residues with rich P–N cross-linked structures due to the interactions or reactions between BPGA and MCA in condensed phase; especially, the former significantly affected the thermal decomposition behavior of the matrix. In the gas phase, the combination of BPGA and MCA supplied a sustained high content of noncombustible gases in flame during the main degradation stage of the matrix, which facilitated the dilution of the concentration of flammable gases and oxygen. Meanwhile, P-containing radicals resulting from the thermal decomposition of BPGA could quench the free-radical chain reactions during combustion, thereby suppressing the flame. Thus, this lifted flame-retardant efficiency mainly contributed to the gas-phase flame inhibition mechanism without ignoring the shielding effect of char layers in the condensed phase.