BUILDUP OF EPOXYCYCLOALIPHATIC AMINE NETWORKS - KINETICS, VITRIFICATION, AND GELATION

The curing of an epoxy prepolymer based on Bisphenol A diglycidyl ether (DGEBA) with 4,4’-diamino-3,3’-dimethyldicyclohexylmethane (3DCM) was analyzed using size-exclusion chromatography (SEC) and differential scanning calorimetry (DSC) as main techniques. It is shown that SEC may be used to determine overall conversion x, vs time in the pregel stage, by making x = 1 - (h/ho)0,5, where h/ho is the ratio of the peak height of DGEBA (M= 340), at any time, with respect to its initial value. Results, both from SEC and DSC, show that kinetics follows an autocatalytic mechanism (catalysis by OH groups), with an apparent activation energy, E = 57.5 kJ/mol (13.7 kcal/mol). The reactivity ratio of secondary to primary amine hydrogens, k2/k1 is close to 0.4, as revealed by the product distribution obtained in a model reaction with phenyl glycidyl ether (PGE) and by the correct fitting with the kinetic model. The evolution of number and weight average molecular weights is a unique function of conversion for every curing temperature, including the temperature region at which vitrification sets in well before gelation. This implies that segmental mobility decreases in the same way for every molecular size; i.e., vitrification does not alter the statistics of network formation. © 1990, American Chemical Society. All rights reserved.