Flame-Retardant and Smoke-Suppressive Epoxy Resins with Enhanced Storage and Thermal Stability via Curing with a Phosphorus-Containing Imidazole Derivative

Abstract

Addressing the flammability of epoxy resins and simplifying the production process remain huge challenges. In this work, an imidazole-derived phosphorus/nitrogen-containing latent curing agent (ATIM) was synthesized using nitrilotrimethylene triphosphonic acid (ATMP) and 1-methylimidazole as raw materials for fabricating a single-component flame-retardant epoxy (EP). The ATIM-cured EP (EP/ATIM) showed desired flame-retardant properties, and its UL-94 classification reached V-0 when ATIM content increased to 13 wt %. The limiting oxygen index (LOI) of EP/ATIM-15 (ATIM content: 15 wt %) reached 33.0%, and its peak heat release rate (PHRR) and peak smoke production rate (PSPR) were 51.9 and 41.4% lower than those of the imidazole (IM)-cured EP (EP/IM-7, IM content: 7 wt %), respectively, which demonstrated considerable progress in both flame resistance and smoke reduction properties. EP/ATIM exhibited a higher onset decomposition temperature and greater char residue compared with EP/IM-7, demonstrating superior thermal stability. The shelf life of EP/ATIM-15 could be up to 7 days, confirming that the introduction of P-containing acid endowed EP/ATIM with increased storage stability. Thus, this study offers a feasible strategy for enhancing the storage stability, fire retardant performance, and thermal properties of a single-component epoxy resin.