Thermally Responsive Imidazole-Based Diels–Alder Microbeads as a Latent Curing Agent for Epoxy Resins

A single-component epoxy system is necessary for industrial applications because of its convenience, productivity, and cost. Thus, there is a need for a latent curing agent that secures storage stability while simultaneously triggering an immediate curing reaction above a certain temperature. Herein, a microbead-type latent curing agent (BMI–DA, average bead size: 2.4 μm) based on benzyl 2-methylimidazole (BMI) incorporated with thermally responsive Diels–Alder (DA) linkages is presented. According to thermal and spectroscopic analyses, the BMI-based DA microbeads consist of DA linkages formed between tetrafunctional furan (TFu) and bismaleimide (BM) and 2:1 adducts of BM and BMI. Consequently, the physical protection of the dynamic DA network and chemical protection owing to the inductive effect acted simultaneously, thereby effectively suppressing the reactivity of BMI. Compared to pure BMI, which solidifies within 2 days at 20 °C, the BMI–DA microbeads remain stable for 30 days in the presence of YDF-170 epoxy resin. The curing reaction is triggered when the DA network collapses, and BMI is exposed. Moreover, owing to the flexible chains of the DA network, the tensile and lap shear strengths of the epoxy cured with BMI–DA microbeads increased by 49 and 19%, respectively, compared to those of cured epoxy/pure BMI.