Synthesis of phosphorus‐nitrogen hybrid flame retardant and investigation of its efficient flame‐retardant behavior in PA6/PA66

Abstract To improve the flame retardancy of PA6/PA66 copolymer, a phosphorus‐nitrogen flame retardant DDP‐M was successfully synthesized via melt condensation reaction between 2‐[(5‐oxo‐6H‐phosphanthridin‐5‐yl)methyl]butanedioic acid (DDP) and melamine (Mel). Its structure, product composition, and thermal stability were characterized by FTIR, nuclear magnetic resonance spectroscopy (NMR), matrix‐assisted laser desorption and ionization time‐of‐flight mass spectrometry (MALDI‐TOF‐MS), and thermogravimetric analysis (TGA). The results suggested that DDP‐M was a hybrid flame retardant. A series of flame‐retardant PA6/PA66 composites were prepared by melt extrusion. Their thermal stability and flame retardancy were assessed by differential scanning calorimetry (DSC), TGA, limiting oxygen index (LOI), and vertical burning test (UL‐94). The combustion behavior demonstrated that DDP‐M can effectively improve flame retardancy, achieving 32% of LOI value and UL‐94 V‐0 rating with only 9 wt.% addition. No melt dripping was accomplished by adding 12% DDP‐M. The high dose (15%) harmed the flame‐retardant properties. The results of pyrolysis gaseous productions analysis and residual carbon analysis revealed the flame‐retardant mechanism of DDP‐M was a synergistic effect of melamine inducing gaseous phase flame retardancy and highly condensed P‐N char layers inducing condensed phase flame retardancy. The composites retained good mechanical properties due to the good dispersibility and compatibility between polyamide and DDP‐M.