BUILDUP OF STRUCTURE AND VISCOELASTIC PROPERTIES IN EPOXY AND ACRYLATE RESINS CURED BELOW THEIR ULTIMATE GLASS-TRANSITION TEMPERATURE

The build-up of structure and viscoelastic properties with conversion during cure below the ultimate glass transition temperature of epoxy a nd acrylate resins has been investigated. Using a torsional dynamic me chanical analyser, dynamic shear modulus and change in sample thicknes s was monitored simultaneously, thus giving information on both the ph ysical properties (stiffness) and the progress of the reaction (shrink age) in one experiment. Two step-wise curing epoxy systems and two cha in-wise curing acrylate systems with different crosslink densities wer e studied and compared. The results showed that in the epoxies vitrifi cation was a distinct event, occurring separately from gelation and en ding with the end of the cure reaction. In the acrylates vitrification began immediately after gelation, the two events being indistinguisha ble, and lasted until the end of the reaction, leaving the sample in i ts transition zone. Scaling of modulus-cure time data obtained at diff erent frequencies showed that the data for each system followed one si ngle curve, independent of frequency over five decades. This made it p ossible to estimate the modulus development at low frequencies early i n the reaction, which is difficult to measure directly. From the shrin kage and storage moduli approximate values of the relaxation modulus a s a function of chemical conversion were calculated. The relaxation mo dulus curves at different conversions were then shifted along the time axis to provide a relaxation master curve. The data and understanding gained in this work provide the basis for analysing the time-dependen t mechanical behaviour during cure, e.g. build-up and relaxation of re sidual stresses. Copyright (C) 1996 Elsevier Science Ltd.