Catalyst-free synthesis of isocyanate-blocked imidazole and its use as high-performance latent hardener for epoxy resin

Catalyst-free synthesis in three kinds of isocyanate-blocked imidazole compound was successfully achieved by chemically connecting 2-ethyl-4-methylimidazole (2E4MI) with hexamethylene diisocyanate (HDI), trimethylhexamethylene diisocyanate (TMHDI) or 4,4′-diphenylmethane diisocyanate (MDI), which were tested as the latent hardeners for epoxy (EP) resins. The chemical structures of the synthesized HDI-2E4MI, TMHDI-2E4MI and MDI-2E4MI were identified with Fourier transform infrared spectroscope (FTIR), 1H and 13C nuclear magnetic resonance (1H NMR and 13C NMR). Based on characterizations including thermogravimetry/differential scanning calorimetry (TG/DSC) simultaneous testing and hot-stage FTIR, it was found that both HDI-2E4MI and TMHDI-2E4MI exhibited two-step deblocking behavior, whereas the deblocking process of MDI-2E4MI was completed only via one step. Curing kinetics of HDI-2E4MI/EP, TMHDI-2E4MI/EP and MDI-2E4MI/EP systems were then studied by non-isothermal DSC using Kissinger model. The results revealed that MDI-2E4MI/EP system could be cured via a two-step reaction with the highest curing rate due to the high reactivity of MDI-2E4MI, which was much different from the four-step curing behaviors of both HDI-2E4MI/EP and TMHDI-2E4MI/EP systems. Compared to the reference 2E4MI, noticeably, the three synthesized hardeners showed the outstanding latent features with lower curing activity and longer shelf life and the EP systems cured by the hardeners exhibited desirable mechanical properties and thermal stability. This study provided a feasible approach to synthesize high-performance imidazole-based latent hardener for wide practical applications in a greenway via the blocking treatment through tailoring the isocyanate structure.