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

Abstract

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.

O‐Fmoc‐protected 3‐hydroxy‐2‐butenoate derivatives as thermal latent curing agents for thermosetting epoxy resins

“…Epoxy resin compositions containing ionic liquids had long‐term storage stability and good curability around 150 °C, although some needed higher temperatures (above 200 °C) for activation. On the other hand, our group has also proposed a thermal latent curing agent where the nucleophilicity of an imidazole ring is suppressed by intramolecular hydrogen bonding . The agent is compatible with conventional epoxy resins and shows good curability as well as long‐term storage stability.…”

Imidazobenzoxazin-5-ones as thermal latent curing agents for thermosetting epoxy resins

“…By contrast, our group has proposed another liquid‐type thermal latent curing agent where retro ‐Michael addition reactions are used to generate imidazoles . Furthermore, our group has also reported that 2‐(2‐hydroxyphenyl)imidazole derivatives work as a thermal latent curing agent, which is compatible in conventional epoxy resins and shows good curability and superior storage stability …”

“…These results indicate that the polarity around 1 would largely influence its hydrolysis. Therefore, we considered that if the polarity were increased by generation of hydroxy groups or alkoxylates in an epoxy curing system, in situ generation of the imidazole derivative 2 having high curability from 1 would be accerelated …”

“…14 Furthermore, our group has also reported that 2-(2-hydroxyphenyl)imidazole derivatives work as a thermal latent curing agent, which is compatible in conventional epoxy resins and shows good curability and superior storage stability. 15 Meanwhile, benzoxazines derived from a similar chemical compound with 2-(2-hydroxyphenyl) imidazole have been developed as a novel type of phenolic resin. Kimura et al studied bisphenol-A-based benzoxazine-epoxy resin compositions and their curing system with additives, and they reported that the benzoxazine had good thermal stability with good compatibility towards epoxy resins.…”

Latent curing systems stabilized by reaction equilibrium in homogeneous mixtures of benzoxazine and amine

“…The blocking of the active groups (such as amines, thiols, and phenols) of hardeners with thermally removable groups (Fig. 1b)17181920212223 is the main approach for achieving the liquid latent system. Sometimes, these methods may induce sluggish liberation of the active groups, resulting in slow curing processes that are undesirable in commercial uses.…”