RESIDUAL-STRESS BUILDUP IN THERMOSET FILMS CURED BELOW THEIR ULTIMATEGLASS-TRANSITION TEMPERATURE

The stress build-up during isothermal cure below the ultimate glass tr ansition temperature of epoxy and acrylate films is investigated in de tail. Four systems are studied; two acrylates and two epoxies, with di fferent crosslink densities. Relaxation modulus and film shrinkage are measured simultaneously during cure. The stress build-up is measured independently using a bi-layer beam bending technique. A model for the build-up of cure stresses is proposed, in which stresses are generate d by the cure shrinkage and decay by viscoelastic relaxation. The rela xation is described by a simple, modified Maxwell model. Owing to the absence of memory in the Maxwell model, the resulting equation is simp le and numerical stress computation straight-forward. The stress build -up over time is thus simulated for the four model systems based on th e relaxation and shrinkage data, and the simulations compared with the experimentally observed stress build-up. The model successfully predi cts the cure stresses where more standard elastic methods fail. It is found that the amount of stress build-up during cure varies greatly be tween the different systems. In general, a higher crosslink density re sults in higher stress build-up. The stress on cure ranged from less t han 1% of the total stress on cure and cool-down in a lightly crosslin ked epoxy to more than 30% of the total stress in densely crosslinked epoxies and acrylates. Finally simple approximations for estimating th e stress levels after cure and cool-down from basic material propertie s, e.g. modulus and cure shrinkage, are proposed. (C) 1997 Elsevier Sc ience Ltd.