Investigation of the interphasial behaviour of adhesive joints
The theme of this report is the examination of the interphase. Three different aspects of the interphase have been examined. Firstly, the detrimental effects of contamination upon the interphase were examined. Substrates were contaminated with quantified levels of PDMS (polydimethylsiloxane) in an attempt to create weak boundary layers, and the resulting performance of joints manufactured from these substrates was evaluated. A notable reduction in bond strength was observed with specimens prepared from two different toughened adhesive systems whilst a similar system formulated without any toughener remained comparatively unaffected, demonstrating the capability of the epoxide adhesive to accommodate several monolayers of this contaminant. The second aspect of the interphasial region studied was the chemical heterogeneity within the bondlines of adhesive joints. An excess of unreacted curing agent as discovered in the lower interphasial regions of tensile test specimens coupled with a tendency for near interfacial failure within this region. The curing reagent was found to be settling out of the adhesive during cure. Significant differences were also observed in the interphasial polymer compared to the bulk on samples taken from annular joints that had undergone premature failure. Thirdly, the effects of the deliberate creation of an interphase for the enhancement of the bondabiIity on steel substrates was examined. In this aspect of the work, a new adhesive and novel surface treatments have been identified as providing potential improvements in performllnce over the current bonding system. Of particular importance was the performance of a phosphate-based treatment Bonderite 9Ol/Pyrene 8-90, which gave demonstrable improvements over the existing strip laminate bonding system. The improved performance was attributed to a very stable phosphate layer deposited on the steel substrate from the chemical treatment,providing a wettable, uniform, micro-rough, corrosion resistant surface ideal for adhesive bonding.