SHRINKAGE STRESSES IN THE COURSE OF ISOCH ORIC CURING

The shrinkage stresses developing in the course of isothermal curing o f epoxyamine polymer under quasi-isochoric conditions are studied over a wide temperature range both above and below the glass transition te mperature at the gel point. The plot of stress versus logarithm of tim e is S-shaped and shows a pronounced almost linear segment after the f irst relaxation transition and then grows slower in the diffusion-cont roled region. The stress amounts to several MPa and varies slightly in the studied temperature range. However, because of the different magn itudes of the shrinkage stress and the strength of the material cured at different temperatures, at some temperatures, failure of the materi al occurs, whereas at other temperatures no flaws develop. A theoretic al model for buildup of the shrinkage stress in the course of curing u nder quasi-isochoric conditions is proposed. By taking into account th e changes in bulk modulus upon vitrification good agreement between th e model and experimental data is achieved.