ABSTRACT
Cure shrinkage is important because it can be responsible for changes in shape of the manufactured part geometry after cure is complete and because differential shrinkage can produce residual stresses in the material that can contribute to failure. Shrinkage in thermosetting polymer matrices is made up of two components: (a) shrinkage due to volumetric reduction resulting from cross link formation and (b) thermal contraction. While these phenomena occur together during the thermal curing process, it is the state of the polymer that determines the contribution of each in the development of residual stresses within the composite. Prior to gelation, the polymer offers little resistance to shrinkage, and in the rubbery state, the stiffness is significantly less than that after vitrification. Further, the viscous nature of the polymer prior to vitrification may also allow residual stresses to relax out over time. Thus, it is necessary to follow both the polymer shrinkage and its effective stiffness to determine the magnitude of residual stresses. To separate the chemical shrinkage from the thermal expansion behavior of the polymer, it is useful to examine a single thermal cycle of heating the initially uncured polymer above its glass transition temperature and cooling it to the initial temperature (see Figure 5.1).
