Build-up of structure and viscoelastic properties in epoxy and acrylate resins cured below their ultimate glass transition temperature

Abstract

The build-up of structure and viscoelastic properties with conversion during cure below the ultimate glass transition temperature of epoxy and acrylate resins has been investigated. Using a torsional dynamic mechanical analyser, dynamic shear modulus and change in sample thickness was monitored simultaneously, thus giving information on both the physical properties (stiffness) and the progress of the reaction (shrinbdkage) in one experiment. Two step-wise curing epoxy systems and two chain-wise curing acrylate systems with different crosslink densities were studied and compared. The results showed that in the epoxies vitrification was a distinct event, occurring separately from gelation and ending with the end of the cure reaction. In the acrylates vitrification began immediately after gelation, the two events being indistinguishable, and lasted until the end of the reaction, leaving the sample in its transition zone. Scaling of modulus—cure time data obtained at different frequencies showed that the data for each system followed one single curve, independent of frequency over five decades. This made it possible to estimate the modulus development at low frequencies early in the reaction, which is difficult to measure directly. From the shrinkage and storage moduli approximate values of the relaxation modulus as a function of chemical conversion were calculated. The relaxation modulus 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-dependent mechanical behaviour during cure, e.g. build-up and relaxation of residual stresses.