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

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.