Reaction kinetics and TTT cure diagrams for off‐stoichiometric ratios of a high‐T_g epoxy/amine system

The cure behavior of two off‐stoichiometric ratios, one amine‐rich and the other epoxy‐rich, of a high‐T_g aromatic difunctional epoxy/aromatic tetrafunctional diamine system are studied using differential scanning calorimetry (DSC). The curing reactions are presumed to consist of competing epoxy/amine reactions and a subsequent reaction of epoxy with hydroxyl, termed etherification, that is significant in the epoxy‐rich system after depletion of amino hydrogens. A one‐to‐one relationship between conversion and T_g independent of cure temperature is found for each of the off‐stoichiometric systems in spite of the competing reactions. Since it is uniquely related to fractional conversion, sensitive, and easily measured, T_g is used to monitor the curing reactions. Kinetically controlled master curves for isothermal cure are obtained for each system by shifting T_g vs. In time data to an arbitrary reference temperature; apparent activation energies for the epoxy/amine and the etherification reaction regimes appear to be identical at 15.5 kcal/mol. Experimental DSC data are satisfactorily described for both systems by a kinetic model of the competitive epoxy/amine and etherification reactions. The ratio of the rate constants for the reactions of epoxy with the secondary amine group and epoxy with the primary amine group, α = k₂/k₁, is found to be approximately 0.5 (i.e., equal reactivity of amino hydrogens), whereas the ratio of the rate constants for the reactions of epoxy with hydroxyl and epoxy with the primary amine group, β = k₃/k₁, is found to be 0.001. Diffusion control is incorporated in the model by the use of a free‐volume theory. Vitrification and iso‐T_g contours in the time–temperature–transformation (TTT) isothermal‐cure diagram are calculated for both systems from the kinetic model. © 1992 John Wiley & Sons, Inc.