RHEOKINETICS OF CURING OF EPOXY-RESINS NEAR THE GLASS-TRANSITION

The cure of four epoxy-amine systems was studied by viscometry, dynami c mechanical analysis, and DSC. The investigation was carried on at te mperatures below and above the glass transition temperature [T-g] of c ompletely cured products. Viscosity growth in the initial stage of cur e is described by an exponential-type equation, and on approach to the gel-point by a power law, but the exponent did not coincide with the universal theoretical scaling-law value. A universal viscosity master curve can be constructed if one reduces viscosity and time by their ch aracteristic values. Gel-times and activation energies of curing were found by different methods. It was established that the product of the initial rate of a reaction and the gel-time is constant at various co nditions of cure. Time dependences of the degree of conversion (calcul ated from calorimetric and dynamic mechanical data) in the full range of conversions are described by first-and second-order kinetics, corre cted by the factor reflecting self-accelerating character of the react ion. Kinetic constants found from both methods are the same. If a reac tion leads to a transition to the glassy state over the course of curi ng, a self-acceleration equation is valid in the initial stage of the process only. The complete kinetic curve can be described by the DiBen edetto method relating shift of the T-g with the degree of conversion. The rate constant for vitrificating systems is presented as a sum of reciprocal values of chemical and diffusion constants. Time and temper ature dependences of kinetic and diffusion constants were calculated.