Influence of hydroxyl concentration on the cure kinetics of diglycidyl ether of bisphenol A (DGEBA) epoxy in the presence of a tertiary amine

DGEBA epoxy is reacted with hardener mixtures of diethanolamine (DEA, HN(CH₂CH₂OH)₂) and dibutylamine (DBA, HN(CH₂CH₂CH₂CH₃)₂). Both DEA and DBA have a single secondary amine that reacts rapidly with epoxide forming a non-crosslinked adduct at 70°C. The subsequent crosslinking reaction is slower, taking 24 hrs. at 70°C to near completion for the standard DGEBA/DEA mix. The mixtures studied keep the mole ratio of epoxide to amine constant at 4.8:1 and vary the epoxide to hydroxyl ratio from 2.4:1 for a hardener of pure DEA to nearly 1:0 for pure DBA. Proton transfer is expected to play a major role in the reaction at high hydroxyl concentrations (high DEA/DBA ratio). It is also expected that a high OH concentration will result in a high reaction rate while low OH concentration (low DEA/DBA ratio) will result in a negligible reaction rate. We show that these expectations are overly simplistic. At 70°C, the maximum reaction rate occurs at 25% DBA indicating an optimal hydroxyl concentration.