Synthesis and application of novel imidazole-containing phosphorus–nitrogen flame retardants in epoxy resins

A novel 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-based phosphorus–nitrogen synergistic flame-retardant NA/DOPO-Chl was designed and synthesized from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide,paraformaldehyde, chloroacetyl chloride, and N-(3-aminopropyl)-imidazole and developed and applied to enhance the flame retardancy of epoxy resin. FTIR, NMR, and high-resolution mass spectrometry were used to characterize the chemical structures. The flame-retardant epoxy composites were systematically evaluated using thermogravimetric analysis, limiting oxygen index measurement, vertical burning test (UL-94), cone calorimetry, and scanning electron microscopy. With the incorporation of only 3.0 wt% of the flame retardant, corresponding to a phosphorus content of 0.23 wt%, the epoxy composite achieved a V-0 rating in the UL-94 test and exhibited a limiting oxygen index of 28.3%. Cone calorimetry results revealed that the peak heat release rate and total heat release were reduced by 46.15% and 48.55%, respectively, indicating substantially suppressed fire hazards. Furthermore, the mechanical properties of the modified epoxy were notably enhanced, demonstrating a balance between flame retardancy and mechanical performance. This study provides an efficient strategy for developing high-performance epoxy resins with excellent fire safety and retained mechanical integrity, showing promising potential for applications in advanced electronic packaging and composite materials.