RESIDUAL-STRESS BUILDUP IN THERMOSET FILMS CURED ABOVE THEIR ULTIMATEGLASS-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 rh eometer, which allows dynamic shear modulus and film thickness to be m onitored simultaneously. Stress predictions are compared with independ ent stress observations, obtained using a bilayer beam bending techniq ue. A conventional epoxy system and a low molar mass difunctional acry late are studied and compared. The epoxy, in agreement with the litera ture, exhibits no detectable stress during the curing reaction, nor du ring cooling to the glass transition temperature (T-g), but develops s tress on cooling below T-g. The acrylate, by contrast, generates consi derable stress throughout the reaction and cooling, with the major par t of the stress originating above T-g. The observed stress build-up ag rees well with the theoretical calculations based on the time-evolutio n shear modulus and film thickness. Finally, approximate formulae for the estimation of residual stress are given. It is shown how the overa ll residual stress, as well as the contributions from the different pa rts of the cure process in a wide variety of systems, can be estimated from the mechanical and thermal properties of the polymer and the sub strate.