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.
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