RESIDUAL-STRESS BUILDUP IN THERMOSET FILMS CURED ABOVE THEIR ULTIMATE GLASS-TRANSITION TEMPERATURE

The build-up of in-plane stress over time during curing and on cooling of thermoset films is investigated. The stress in a thin contracting film on a rigid substrate is analysed, and obtained as an integral of essentially the shear modulus and thickness of the curing film. Films are cured and cooled between parallel plates in a dynamic torsional rheometer, which allows dynamic shear modulus and film thickness to be monitored simultaneously. Stress predictions are compared with independent stress observations, obtained using a bilayer beam bending technique. A conventional epoxy system and a low molar mass difunctional acrylate are studied and compared. The epoxy, in agreement with the literature, exhibits no detectable stress during the curing reaction, nor during cooling to the glass transition temperature (T-g), but develops stress on cooling below T-g. The acrylate, by contrast, generates considerable stress throughout the reaction and cooling, with the major part of the stress originating above T-g. The observed stress build-up agrees well with the theoretical calculations based on the time-evolution shear modulus and film thickness. Finally, approximate formulae for the estimation of residual stress are given. It is shown how the overall residual stress, as well as the contributions from the different parts of the cure process in a wide variety of systems, can be estimated from the mechanical and thermal properties of the polymer and the substrate.