Synergistic effect of layered yttrium phenylphosphonate and aluminum diethylphosphonate in flame retarding polyamide 66

A novel layered rare-earth flame retardant, yttrium phenylphosphonate (YPP), was synthesized via a self-assembly solution route and employed as a synergistic agent with aluminum diethylphosphinate (AlPi) to enhance the fire safety of polyamide 66 (PA66). While the incorporation of 8.5 wt.% AlPi alone improved the Limiting Oxygen Index (LOI) to 29.0%, it failed to achieve a V-0 rating. Incorporating merely 0.5 wt.% YPP into the system (S7: 8.0 wt.% AlPi / 0.5 wt.% YPP) resulted in a substantial performance leap, achieving a UL-94 V-0 rating with no melt dripping and an LOI of 32.3%. Relative to neat PA66, the optimized composite demonstrated a 70.2% reduction in Peak Heat Release Rate (PHRR) and a 15.0% decrease in Total Heat Release (THR). Concurrently, the char yield at 800°C increased significantly from 1.6% to 8.3%. Thermogravimetric analysis coupled with infrared spectroscopy (TG-IR) and X-ray photoelectron spectroscopy (XPS) revealed a dual-phase mechanism: YPP promotes the formation of a thermally stable, phosphorus-yttrium crosslinked char network in the condensed phase, while AlPi releases phosphorus radicals to quench combustion in the gas phase. These findings establish the AlPi/YPP system as a highly efficient, halogen-free flame retardant strategy for engineering plastics, offering superior fire safety at significantly reduced additive loadings.