Enhancing the flame retardancy of polyamide 6 through ZDP-mediated suppression of ADP agglomeration and enhanced phosphorus–carbon retention

To address the critical trade-off between high flame retardancy and robust mechanical properties in polyamide 6 (PA6), zinc diethylphosphinate (ZDP) was introduced as a physical and chemical synergist for aluminum diethylphosphinate (ADP). A series of halogen-free flame-retardant PA6 composites were fabricated via melt-compounding at a total loading of 12 wt%. The composite with a ZDP-to-ADP mass ratio of 8:4 (PA6–8Z4A) achieved the best overall balance among the tested formulations. It reached a UL-94 V-0 rating, increased the limiting oxygen index, and suppressed smoke generation. Thermogravimetric analysis of the corresponding 8Z4A flame-retardant mixture revealed a strong non-linear charring synergy, with a nitrogen char residue of 62.41% at 750 °C. TGA-FTIR, SEM-EDS, and XPS results were consistent with enhanced retention of phosphorus- and carbon-containing species in the condensed phase and the formation of a denser protective char. In addition, the lower melting point of ZDP improved the physical dispersion of ADP aggregates, thereby effectively maintaining acceptable mechanical integrity of the PA6 matrix.