Synthesis and characteristics of novel 2-hydroxy-3-mercaptopropyl terminated polyoxypropylene glyceryl ether as an epoxy hardener of epoxy-based adhesives

Two epoxy hardeners with different molecular weights of 2-hydroxy-3-mercaptopropyl terminated polypropylene glyceryl ether (HMP-t-PGE) were synthesized as crosslinkers of bisphenol-A epoxy resin via the thiol-epoxy click reaction. After analyzing the curing characteristics and thermal behaviors of the formulated curing mixture with/without an amine-type curing catalyst, it was found that the molecular weight of the HMP-t-PGE hardener affected the curing kinetics and final properties of the epoxy networks. The thiol group of the low-molecular-weight (low-MW) HMP-t-PGE hardener was more reactive with the epoxy group than both the high-molecular-weight (high-MW) HMP-t-PGE hardener and the mercaptopropanoate-terminated polypropylene glyceryl ether (MP-t-PGE) hardener. When the high-MW HMP-t-PGE hardener was applied as the adhesive on mild steel substrate, the apparent adhesion strength was enhanced by up to ∼400% from that of MP-t-PGE hardener (assessed in single lap joint shear tests). By contrast, the low-MW HMP-t-PGE hardener enhanced the adhesion strength by only ∼230%. The high reactivity and excellent adhesion achieved by the thiol-epoxy click reaction at room temperature can be exploited in two-package epoxy adhesive applications such as quick repairs, tooling, and general bonding.