Exploring synthesis techniques for an imidazole-based one-component epoxy latent curing agent: Chemical capping and mechano-chemical capsuling

Abstract

The increasing demand for one-component epoxy is driven by its eco-friendliness, convenience, and cost-effectiveness, underscoring the importance of developing latent curing agents that trigger reactions under specific conditions. This study introduces an imidazole-based latent curing agent (ICM-SU) characterized by superior storage stability and low-temperature curing capabilities, achieved through dual protections of chemical capping and mechano-chemical capsuling. The process commences with the synthesis of isophorone diisocyanate capped 2-methylimidazole (ICM), wherein the reactivity of 2-methylimidazole (2MI) is controlled via the inductive effect of isophorone diisocyanate, achieved through chemical capping. Subsequently, ICM-SU is produced by uniformly coating it with silica nanoparticles and a urea shell through mechano-fusion system’s dry processing for mechano-chemical capsuling, effectively isolating it from contact with epoxy. The curing mechanism and chemical structure of ICM-SU are identified in four stages: de-capping, de-shelling, formation of epoxy-imidazole adducts, and chain growth. Notably, the melting point of the urea shell (122 °C) closely matches the curing onset temperature, suggesting that the exposure of epoxy to restored 2MI initiates the curing process. As a result, ICM-SU demonstrates significantly enhances storage stability at 20 °C for over 42 days, a stark improvement over 2MI, which solidifies within 8 h. Moreover, specimens cured with ICM-SU exhibits a 32 % increase in tensile strength and a 25 % improvement in Young’s modulus compared to those cured with 2MI, attributed to the reinforcing filler effect of silica nanoparticles.