A Liquid Phosphaphenanthrene-Derived Imidazole for Improved Flame Retardancy and Smoke Suppression of Epoxy Resin

The fabrication of high-performance thermosetting polymers with intrinsic flame retardancy and smoke suppression is necessary but challenging. Herein, we have successfully synthesized a liquid phosphaphenanthrene/imidazole-containing curing agent (DOPO-AI) for epoxy resin (EP) by the Atherton–Todd reaction of 1-(3-aminopropyl)imidazole (AI) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). The as-prepared EP/DOPO-AI samples exhibit comparable mechanical properties to the reference EP/AI-6 sample, with slightly decreased glass transition temperatures. Impressively, the EP/DOPO-AI sample can pass the UL-94 V-0 rating and achieve a limited oxygen index (LOI) value of 37.0%. Additionally, the EP/DOPO-AI sample shows 26.3%, 36.6%, and 24.4% reductions in the total heat release (THR), peak of heat release rate (pHRR), and total smoke production (TSP) relative to the EP/AI-6 sample, indicating a significantly reduced fire hazard. The significant enhancements in flame retardancy and smoke suppression are attributed to the condensed- and gaseous-phase effects of DOPO-AI. This work demonstrates a feasible methodology for creating high-performance EPs with inherent flame retardancy and smoke suppression, holding great promise in the future.