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.

Ageing of adhesive-bonded aluminium-dicyandiamide cured epoxy alumina-filled joints prepared using a variety of different surface treatments and exposed to elevated temperatures and high humidity are reported. The uptake of moisture was followed using broadband dielectric spectroscopy, and attempts are made to correlate these changes with observations of variations in the mechanical properties and surface structure monitored by electron microscopy. It was found that the absorption of moisture, as indicated by the dielectric measurements, is similar for all the joints. Small differences observed may be ascribed to the influence of the pretreatment on the absorption behaviour. There was no evidence of changes in the oxide layer of the substrate. Detailed electron microscopic examination of the surfaces did indicate, after prolonged exposure, that change in the pretreatment is consistent with small differences in the dielectric data. Predominantly, the changes that were observed in the mechanical strength of the joints are consistent with the plasticization of adhesive in the joint rather than failure at the substrate interface. Ageing at 70 degrees C did, however, indicate that there were changes in the interfacial layer and these can be correlated with the change in the failure mechanism. It was also observed that the titanium/zirconium (Ti/Zr) pretreatment showed signs of being less durable than the others used in this study.

Ageing studies on adhesive-bonded aluminium-dicyandiamide cured epoxy joints prepared using a variety of different surface treatments were carried out using 100% humidity and cycling the joints between 42-48-42 degrees C every hour. The pretreatments include a chromic acid, silica/siloxane pretreatment, nonrinse chrome pretreatment, titanium/zirconium (Ti/Zr) pretreatment, and anodised substrate with no pretreatment. Dielectric spectroscopy measurements were used to characterise the rate of the water uptake and monitor the conversion of the surface oxide to hydroxide. These data were correlated with the changes that occurred in the mechanical strength of the bonds. The changes in the surface structure were observed using electron microscopy and elemental analysis conducted using X-ray photoelectron spectroscopy. The dielectric permittivity changes observed were similar for all the different pretreatments, indicating that the predominant process was water absorption. However, small differences were observed that reflect the different surface treatments used. Ageing at an elevated temperature of 70 degrees C provided definite evidence of hydration of the surface oxide layer. Electron microscopy of the fracture surface indicated oxide-to-hydroxide conversion and was reinforced by X-ray photoelectron spectroscopy. Predominantly, the changes in the mechanical strength observed at low temperature are consistent with the plasticization of the adhesive. However, at the elevated temperature of 70 degrees C, evidence for weakening of the interfacial layer by hydration becomes evident as a reduction in the mechanical strength. The dielectric measurements allowed the changes in the bond to be followed nondestructively. The mechanical strength of the etched-only pretreatment aged surprisingly well and at the lower temperature of ageing was comparable with the no-rinse chrome and titanium/zirconium pretreatments. The least durable was the PT2Cr-free treatment; however, all treatments showed a significant level of durability at low temperature.

ABSTRACT Ageing of adhesive-bonded aluminium–diglycidyl ether of bisphenol A based aliphatic amine–cured acrylic modified epoxy resin reinforced with fused silica exposed to 100% humidity and cycled between 42–48–42 °C every hour is reported. Moisture uptake was followed using broadband dielectric measurements. It was found that the adhesive absorbs moisture very rapidly, the adhesive undergoes significant swelling, and there is a consequent reduction in strength consistent with plasticization of the matrix. The dielectric measurements parallel closely the changes occurring in the adhesive bond line and provide a nondestructive method for monitoring the changes in bond strength. The changes that occur are dominated by the effects of moisture on the adhesive, various pretreatments have little effect on the ageing process, and interfacial failure in this case does not make a significant contribution to the failure of these joints.

Broadband dielectric measurements (10 −2 to 3 GHz) are reported on the effects of exposure of thick film adhesive-bonded structures to moisture. Measurement of the dielectric properties over a broad frequency range allows identification of water both in voids and as a molecular dispersion in the matrix. Changes in the low frequency region of the dielectric spectrum can be attributed to a combination of processes associated with plasticisation of the adhesive, interfacial polarisation effects, and hydration of the surface oxide layer. The data obtained are complemented by mechanical testing and failure analysis of the bond structure measured as a function of the time of exposure. This study indicates that for thick film adhesives the ageing characteristics are apparently independent of the surface treatment. In one of the joints studied an additional feature is identified which appears to correlate with the premature aging of the joint structure.