Design and synthesis of thiol-terminated imidothioether oligomer as thermally latent hardener and modifier for high-performance epoxy/thiol thermosets

Conventional base-catalyzed epoxy/aliphatic thiol systems have a poor storage stability, and are impossible to be one-pot systems. A widely accepted solution is to use the latent catalysts, which are usually insoluble anionic initiators sensitive to impurities and moisture, and have many limitations in practical use. In this work, a soluble thiol-terminated imidothioether oligomer (TPI) was synthesized and used as a latent hardener and modifier to prepare one-pot epoxy/thiol systems without the latent catalysts. The latent behavior and curing mechanism of epoxy systems (EP/TPI series), consisted of bisphenol A diglycidyl ether (DGEBA), pentaerythritol tetrakis(mercaptoacetate) (PETMP) and TPI, were proposed. The thiol exchange reaction between TPI and PETMP preceded the curing reaction of DGEBA, and produced aromatic thiols, which were unreactive at ambient conditions and reacted with DGEBA to form the thermosets at temperatures above 130 °C. At the loading of 24.4 wt% TPI, the epoxy system (EP/TPI-3)had a shelf life of 13 days and was completely cured within 20 min at 180 °C. Meanwhile, the catalyst-free curing process of EP/TPI-3 system was insensitive to impurities and moisture. Furthermore, the thermal and mechanical properties of EP/TPI-3 thermoset were simultaneously improved, thereinto, the glass transition temperature, Young’s modulus and ultimate tensile strength were increased by 16.7 °C, 12.9%, and 13.6%, respectively, compared with those of the commercial DGEBA/PETMP thermoset. This work shows that TPI has the potential to be a powerful alternative to commercial latent catalysts, and EP/TPI thermosets with high performance have broad application prospects.