CHEMORHEOLOGY OF AN EPOXY-RESIN SYSTEM UNDER ISOTHERMAL CURING

The chemorheological properties of a commercial Hercules 3501‐6 resin system were studied under isothermal curing conditions between 375 and 435 K. For cure temperatures ≥ 385 K, the storage modulus curing curves, G′(t), exhibited abrupt changes in slope which occurred at various times depending on the curing temperatures. These slope changes were attributed to the onset of gelation. Crossover points between G′(t) and G″(t) curves were observed for curing temperatures ≥ 400 K. The gelation and the crossover points obtained from the chemorheological measurements, therefore, defined two characteristic resin states during cure. Although gelation theory did not predict them correctly, the degree of cure in each of these states was approximately the same for every cure temperature. The temperature dependency of the viscosities in the characteristic resin states, and the rate constants of increase in moduli at different stages of curing were analyzed. Various G′(t) and G″(t) isothermal curing curves were also shown to be capable of being superimposed to one another by the principle of time‐temperature superposition. The resultant shift factors at(T) and aη(T) were shown to follow Arrhenius type relationships. Values of the activation energy suggested that the reaction kinetics, instead of the diffusion mechanism, was the limiting step in the overall resin advancement for cure at temperatures ≥ 385 K. Copyright © 1990 John Wiley & Sons, Inc.