Epoxy-Rich Systems with Preference for Etherification over Amine-Epoxy Reactions for Tertiary Amine Accelerators
Björn Erik Fristrup Ekbrant, Anne Ladegaard Skov, Anders Egede Daugaard
Abstract
Epoxies are often cured using primary and secondary amines in an efficient reaction catalyzed by tertiary amines. Through coordination effects, tertiary amines lower the transition-state energy enough to allow the reaction to take place under ambient conditions. In this paper, we demonstrate how, under ambient conditions, tertiary amines bound in close intramolecular proximity to a primary amine are able to catalyze amine-epoxy reactions as well as promote etherifications in epoxy-rich systems. 13C nuclear magnetic resonance (NMR) was used to identify the sequence of curing reactions in a model reaction of benzyl glycidyl ether (BGE) with the amines 3-diethylaminopropylamine (DEAPA), 3-dimethylaminopropylamine (DMAPA), and hexylamine, respectively. The results clearly show that DMAPA favors etherification over amine-epoxy reactions, while DEAPA displays the expected trend toward amine-epoxy reactions. This experiment was repeated for a curing epoxy system based on bisphenol A diglycidylether (BADGE) with the same three amines, and differential scanning calorimetry (DSC) confirmed this preference of reactions via quantification of the curing exotherms. Finally, a novel DMA setup corroborated the observed rate of reaction for each catalyst by measuring gelation time and vitrification during isothermal curing.