The development of shear and compression elastic moduli in curing epoxy adhesives measured using non-contact ultrasonic transducers

Abstract

Thin epoxy resin adhesive samples were ultrasonically measured during cure using normal incidence radially polarized shear wave electromagnetic acoustic transducers (EMATs). Although the EMATs used predominantly generated shear waves (SH) they were designed so that they also generated/detected compression waves allowing the simultaneous measurement of shear and compression wave propagation through a curing polymer in a non-contact regime. The adhesive thickness examined in the experiments was approximately 1 mm, which was optimal for experimental measurement using our apparatus. The epoxy resin systems (rapid cure and a standard cure) described in this paper were supplied in a two-part cartridge form, mixed by injection through a mixing nozzle. Five different samples have been investigated at this stage, and there appears to be a fundamental difference in the way that the elastic moduli develop in the rapid cure and longer cure time epoxies. This can possibly be explained in terms of the reaction kinetics and the development of the structure on a microscopic scale. The rapid cure systems initially develop a shear elastic modulus at a faster rate which suddenly decreases at approximately the same time that the temperature of the adhesive reaches its maximum value during the exothermic reaction. This is consistent with an initial rapid reaction rate which is then greatly reduced as the remaining un-reacted polymer chains require a finite time to move into a position where they can react and join the network.