Facile construction of one-component intrinsic flame-retardant epoxy resin system with fast curing ability using imidazole-blocked bismaleimide

Latent curing epoxy resin (EP) system with satisfactory flame retardancy has attracted rather considerable interest both in academic and industrial communities over the years. Herein, imidazole-blocked bismaleimide compounds were synthesized through the Michael addition reaction of N,N'-bismaleimide-4,4'-diphenylmethane (BDM) with imidazole (IM), 2-methylimidazole (2MI), and 2-ethyl-4-methylimidazole (EMI), respectively, and were simultaneously used as latent curing agent and flame retardant synergist for 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) based EP (DOPO-EP). The storage stability study disclosed that imidazoleblocked bismaleimide compounds kept chemically inert towards epoxy at room temperature, resulting in long pot life over 40 days. When heated, imidazole-blocked bismaleimide compounds underwent retro-Michael addition reaction to regenerate BDM and the corresponding imidazoles. The fast curing ability was thus achieved via chain polymerization of epoxy initiated by thermally generated free imidazoles. Moreover, poly-bismaleimide component derived from the homopolymerization of thermally generated BDM was incorporated into the cross-linked EP network, resulted in enhanced thermal and charring performance of epoxy matrix. Compared with the contrast samples EP/2MI-BDM and DOPO-EP/2MI thermosets, DOPO-EP/2MI-BDM thermoset exhibited better flame retardancy with remarkably higher limiting oxygen index (LOI) value and UL94 rating as well as lower peak of heat release rate (pk-HRR), average of heat release rate (av-HRR) and total heat release (THR) from cone calorimeter test. The flame-retardant mechanism study revealed the synergistic flame-retardant effect of phosphaphenanthrene and bismaleimide in condensed phase.