Rheokinetics of curing of epoxy resins near the glass transition

Abstract

AbstractThe cure of four epoxy‐amine systems was studied by viscometry, dynamic mechanical analysis, and DSC. The investigation was carried on at temperatures below and above the glass transition temperature (Tg) of completely cured products. Viscosity growth in the initial stage of cure 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 characteristic 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 conditions of cure. Time dependences of the degree of conversion (calculated from calorimetric and dynamic mechanical data) in the full range of conversions are described by first‐ and second‐order kinetics, corrected by the factor reflecting self‐accelerating character of the reaction. Kinetic constants found from both methods are the same. If a reaction leads to a transition to the glassy state over the course of curing, a self‐acceleration equation is valid in the initial stage of the process only. The complete kinetic curve can be described by the DiBenedetto method relating shift of the Tg 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 temperature dependences of kinetic and diffusion constants were calculated.