Preparation of a Macromolecular Flame Retardant with a Phosphine Oxide Structure and Its Application in Polyamide 6

In this study, a novel macromolecular flame retardant (MFR) with a phosphine oxide structure is successfully synthesized to improve the flame retardancy of polyamide 6 (PA6). Following this, the flame-retardant polyamide 6 (FR–PA6) is prepared via melt blending the MFR with PA6. Results indicate that the introduction of MFR has little effect on the melting and crystallization temperature of FR–PA6. While it slightly reduces the thermal stability of PA6, MFR significantly enhances its flame retardancy. The limiting oxygen index of FR–PA6 increases from 21.8% to 28.2%, and it successfully passes the UL-94 V-0 rating when it contains 0.5 wt% of phosphorus. Compared with pure PA6, the av-EHC of FR–PA6 is reduced by 32.2% and the SEA is increased by 66.7%. The MFR showed a flame-retardant mechanism in both the gas phase and the condensed phase. In the gas phase, the decomposition of MFR releases phosphorus-containing free radicals to interrupt the combustion chain reaction and reduces the concentration of the combustible caprolactam. In the condensed phase, the MFR promotes faster formation of melt droplets during combustion, taking heat away from the burning PA6 timely.