A liquid phosphorus-containing imidazole derivative as flame-retardant curing agent for epoxy resin with enhanced thermal latency, mechanical, and flame-retardant performances.

• A liquid phosphorus-containing imidazole derivate (DA) was synthesized and used as flame-retardant latent curing agent for EP. • DA exhibits good intermiscibility with EP, enhanced thermal latency, moderate curing temperature, and fast curing rate. • DA significantly enhanced the flame retardancy and mechanical properties of EP. The development of phosphorus-containing flame retardants combining good compatibility with matrix, low curing temperature, and mechanically reinforcing effect has remained a major challenge. Herein, we reported the synthesis of a liquid flame-retardant curing agent (DA) via the nucleophilic substitution between diphenylphosphinic chloride and 1-(3-aminopropyl)-imidazole (AI). DA exhibited good blending and latency towards epoxy resin (EP) at room temperature. According to DSC studies, DA could rapidly cure EP at moderate temperature. Compared with EP/AI sample, EP/DA samples displayed comparable or higher glass transition temperature (T g) and enhanced mechanical properties due to the introduction of rigid diphenylphosphinyl group and improved cross-linking density. Moreover, DA improved the flame-retardant performances of EP thermoset. For instance, the LOI and UL94 rating of EP/DA-16 sample achieved 37.2 % and V-0, respectively. In addition, the peak of heat release rate (PHRR), average of heat release rate (AHRR), fire growth rate (FIGRA), and total heat release (THR) for EP/DA-16 sample reduced by 32 %, 42 %, 28 % and 27 % in comparison to EP/AI sample, respectively. DA was characterized by its good compatibility with EP, moderate curing temperature, fast curing rate, suitable thermal latency, mechanical reinforcing and flame-retardant effects, and thus it had a broad application prospect in various industrial fields. [ABSTRACT FROM AUTHOR]