Identification of the epoxy curing mechanism under isothermal conditions by thermal analysis and infrared spectroscopy

A curing reaction of bisphenol A diglycidyl ether epoxy resin with 4,4′-diaminodicyclohexyl methane hardener was investigated by means of modulated differential scanning calorimetry (MDSC), thermal scanning rheometer (TSR), near-infrared (NIR) and mid-infrared (MIR) spectroscopy. The relation between change in the physical properties and molecular structures during the isothermal curing reaction were studied. MDSC and NIR results corroborated vitrification with the secondary to tertiary amine conversion; the process afforded a three-dimensional cross-linking structure. TSR estimation of the gelation point was corroborated with the NIR-determined maximum concentration of the generated secondary amine. Two-dimensional correlation spectroscopy confirmed that reaction between the primary amine and epoxy occurred more rapidly than any other functional group reaction. The ether groups were generated at the early stage of the curing reaction, and their formation occurred immediately with the generation of hydroxyl groups.