Imidazole Derivatives with an Intramolecular Hydrogen Bond as Thermal Latent Curing Agents for Thermosetting Resins.

Epoxy resins are important thermosetting resins widely used in industrial applications. Though imidazoles as curing agents have attracted particular attention because of their high reactivity in chain polymerizations with epoxides, polymerization of a liquid epoxy resin containing imidazoles proceeds gradually even at room temperature. This makes it difficult to use such mixtures as one-component materials for industrial applications. To improve the shelf life of the mixutures, we have developed a very simple and powerful thermal latent curing agent, a 2-(2-hydroxyphenyl)imidazole derivative (1), having an intramolecular hydrogen bond between the phenolic hydroxyl group and the nitrogen atom of the imidazole ring, leading to suppression of reactivity of 1 toward epoxy resins at room temperature. It was confirmed that high reactivity of 1 toward epoxy resins at 150 °C was based on breakage of the intramolecular hydrogen, whereas the epoxy resin composition showed long-term storage stability at room temperature.