Study of ageing of adhesive bonds with various surface treatments: Part VI. Dicyandiamide-cured epoxy joints aged at 70°C in a water bath

Adhesive-bonded aluminium/dicyandiamide-cured epoxy joints prepared using two different surface treatments, a silica/siloxane and an etched only process, were exposed at 70°C to moisture and their ageing behaviour was studied using broadband dielectric spectroscopy and destructive mechanical tests. The dielectric measurements allow the uptake of moisture and changes in the nature of the surface oxide and pre-treatment to be monitored. Both sets of joints exhibited almost similar changes in their dielectric spectra as a function of exposure time to moisture; small differences being attributed to the influence of the pre-treatment on the moisture absorption behaviour. Changes in the mechanical properties as a result of exposure to moisture were monitored via lap-shear tests. Detailed electron microscopic examination of the surfaces indicated that after prolonged exposure to moisture changes in the physical state of the pre-treatment and oxide layer were observed. Small differences were observed in the dielectric data between the two pre-treatments and are a consequence of the differences in the extent and nature of the oxide to hydroxide conversion and hydration of the surface treatment which occurs on exposure to moisture. Further evidence for the nature of the change that occurred was obtained using X-ray photoelectron spectroscopy of the fracture surfaces. One side of the fractured joint was predominantly resin irrespective of the exposure conditions. The other fracture surface had a more metallic appearance but was covered by a nitrogen-containing organic material with high oxygen content. Predominantly, the initial changes observed in the mechanical strength of the joints are consistent with the plasticization of the adhesive in the joint. The effects observed after prolonged period of exposure to moisture are consistent with an increased contribution from inter-facial failure to the loss of the mechanical properties. The losses in mechanical properties are less dramatic than would be anticipated considering the mild nature of the surface treatments used and the severity of the exposure conditions.