Gelation of DGEBA epoxy in the presence of a tertiary amine for temperatures above and below the ceiling temperature

DGEBA epoxy is reacted with the curative diethanolamine (DEA). DEA has a single secondary amine that reacts rapidly with epoxide forming a non-crosslinked adduct in about 30 minutes under normal, 70°C, cure conditions. The subsequent crosslinking reaction is much slower, taking 24 hrs. at 70°C to near completion for the standard DGEBA/DEA mix. Cures at elevated temperature (above ~80°C) display a slowing of the overall rate of reaction in a "ceiling temperature" effect which results from a series of reactions associated with the zwitterion driven addition reaction. Here we measure the gel point across a wide range of temperature. At low temperatures, a simple bubble viscometer is used, while traditional rheology is employed for high temperature tests. We find that the extent of cure at gelation is 42±3% for all cases. We show that our measurements are in good agreement with predictions of Carothers and Flory-Stockmayer Theories of gelation.