The irreversibility of dimensional changes in epoxy adhesives undergoing uptake and expulsion of water

Abstract

An optical interference method, developed to measure swelling inhomogeneities during water uptake by epoxy-based adhesive films 1, has now been used to study the extent of dimensional recovery during subsequent removal of the water responsible for swelling. A microscope cover slip is employed as marker to evaluate displacements normal to a resin film that is sandwiched between it and a rigid substrate. By placing an optical flat close to the free surface of the cover slip, a cavity is created within which optical interference can occur between light incident upon and light reflected from the specimen. Normal displacements in the resin cause similar displacements in the cover slip, i.e. the geometry of the cavity is altered, and this produces changes in the pattern of interference fringes. It is found that repeated exposure of the specimen to both wet and dry environments (distilled water at 62°C and dry air at 62°C) leads to reversible changes in the displacement field normal to the adhesive film when the exposure is relatively modest (∼ 1 day at 62°C), but that prolonged exposure (> 2 days at 62°C) produces irreversible changes.