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 fo r quality assurance during manufacture and for the safe functioning of manu factured assemblies in the field. Ultrasound can be used to give estimates of the compression modulus of curing and cured materials and thereby provid e a means to assess non-destructively the cure state of adhered joints duri ng manufacture and in service. These techniques are at present difficult to apply and are predominantly empirical in that little is known about the re lationships between the measured ultrasound data and the evolving molecular structure of the adhesive as it cures. The present paper describes the app lication of a group of physical techniques that can be used to characterise the polymer structure during cure, with the aim of relating these to pheno mena measurable by ultrasound. Wide angle X-ray scattering (WAXS) provides a basic measure of polymer chain formation, which is seen to correlate clos ely 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 resi n-hardener system from a viscous liquid to a crosslinked solid. The NMR dat a obtained during cure correlated well with compression modulus development . Ultrasonic shear wave spectrometry indicates when a curing material can f irst support shear motions and this agrees well with NMR data and with spec ifications of gel point given by manufacturers. Ultrasonic compression wave absorption data provide frequency dependent patterns that change during th e cure cycle and that can be explained on the basis of the results of the W AXS, NMR, and shear wave experiments. These changing patterns have potentia l for tracking cure using low cost ultrasonic techniques, the results of wh ich can be related to phenomena taking place on a molecular scale. PRC/1575 .