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

Abstract

One-component epoxy resins have captured great attention due to their improved processing, environmental friendliness, quality stability, etc., but developing green and high-latent-efficiency ones is still urgent required. In this work, a bio-derived, high-latent-efficiency accelerator was facilely synthesized via Michael addition reaction of an abundant bioresource itaconic acid and imidazole. To confirm the latent mechanism, another accelerator without carboxyl group was synthesized from dimethyl itaconate and imidazole, and used together with imidazole as the controls. The chemical structures of the two synthesized accelerators were characterized in detail by FTIR, 1H NMR, 13C NMR and TOF-MS spectra. The latent and curing behavior of the one-component epoxy resins from bisphenol A epoxy, dicyandiamide and accelerators were studied by DSC and rheology, and the results showed that itaconic acid-modified imidazole systems possess long storage life at room temperature of above 210 days and can be cured rapidly at elevated temperatures. Otherwise, the imidazole system and dimethyl itaconate-modified imidazole system have short storage life below 1 day and 8 days, respectively. The excellent latent efficiency of itaconic acid-modified imidazole (IAIM) was attributed to the synergistical inhibit of the reactivity of imidazole toward epoxy groups through the steric-hindrance effect from the main structure of itaconic acid and the salt-formation effect from the carboxyl groups of itaconic acid. This work provides a green and simple strategy to synthesize high-efficiency latent curing accelerator.