Bio-based, high-latent-efficiency cross-linking accelerator with steric hindrance and salt formation effects

Abstract

One-component epoxy resins have attracted significant attention due to their improved processing, environmental friendliness, and quality stability. However, the development of green and high-latent-efficiency systems remains an urgent requirement. In this work, a bio-derived, high-latent-efficiency accelerator was facilely synthesized via a Michael addition reaction between an abundant bioresource, itaconic acid, and imidazole. To confirm the latent mechanism, another accelerator without a carboxyl group was synthesized from dimethyl itaconate and imidazole and used together with imidazole as a control. The chemical structures of the two synthesized accelerators were thoroughly characterized using FTIR, ¹H NMR, ¹³C NMR and TOF-MS spectra. The latent and curing behavior of the one-component epoxy resins formulated from bisphenol A epoxy, dicyandiamide and the accelerators were studied using DSC and rheological methods. The results showed that the itaconic acid-modified imidazole system exhibited a long storage life of over 210 days at room temperature and could be cured rapidly at elevated temperatures. In contrast, the imidazole- and dimethyl itaconate-modified imidazole systems demonstrated shorter storage lives of less than 1 day and 8 days, respectively. The excellent latent efficiency of the itaconic acid-modified imidazole (IAIM) was attributed to the synergistic inhibition of imidazole reactivity toward epoxy groups through the steric hindrance effect from the main structure of itaconic acid and the salt formation effect from the carboxyl groups. This work presents a green and simple strategy for synthesizing high-efficiency latent curing accelerators.