Ultrasonic measurements related to evolution of structure in curing epoxy resins

The tracking of the cure of epoxy adhesives and the assessment of the cure state of adhesive bondlines joining engineering components are important for quality assurance during manufacture and for the safe functioning of manufactured assemblies in the field. Ultrasound can be used to give estimates of the compression modulus of curing and cured materials and thereby provide a means to assess non-destructively the cure state of adhered joints during manufacture and in service. These techniques are at present difficult to apply and are predominantly empirical in that little is known about the relationships between the measured ultrasound data and the evolving molecular structure of the adhesive as it cures. The present paper describes the application of a group of physical techniques that can be used to characterise the polymer structure during cure, with the aim of relating these to phenomena measurable by ultrasound. Wide angle X-ray scattering (WAXS) provides a basic measure of polymer chain formation, which is seen to correlate closely with the compression modulus as it develops during cure. Low resolution nuclear magnetic resonance (NMR) provides a means to observe the mobility of bound hydrogen nuclei and thereby to track the change in state of a resin-hardener system from a viscous liquid to a crosslinked solid. The NMR data obtained during cure correlated well with compression modulus development. Ultrasonic shear wave spectrometry indicates when a curing material can first support shear motions and this agrees well with NMR data and with specifications of gel point given by manufacturers. Ultrasonic compression wave absorption data provide frequency dependent patterns that change during the cure cycle and that can be explained on the basis of the results of the WAXS, NMR, and shear wave experiments. These changing patterns have potential for tracking cure using low cost ultrasonic techniques, the results of which can be related to phenomena taking place on a molecular scale. PRC/1575.