Water based adhesive primers on aluminum substrates
The use of water based adhesive primers in enhancing the durability of aluminum adhesively bonded with epoxy was studied. The effects of the thickness of both the aluminum oxide surface layer and the primer layer on bond durability were established. 5052 aluminum alloy was oxidized by phosphoric acid anodization prior to primer coating. The oxide layer thickness was controlled mainly by changing anodization voltage and directly measured using a HR-SEM (high resolution scanning electron microscope). The oxide layer was also examined by AES (Auger electron spectroscopy) and XPS (X ray photoelectron spectroscopy). The surface oxide layer was porous and the thickness varied from 0.2 to 2.2 μm. The anodized aluminum substrates contained 3 atomic % of phosphorus and were coated with several solid concentrations of water based adhesive primers. These primers contained non-reactive polyurethane emulsions, and were dried at room temperature. The primer thicknesses were measured by ellipsometry. The primer penetrated into the aluminum oxide pores, confirmed by observing the primer surface after the aluminum oxide was removed by dissolving in dilute NaOH aqueous solution. All adhesive bonding was done using the wedge test configuration with a structural epoxy adhesive, Naviloc® XY 0210, which was cured for 30 min. at 180°C. The wedge samples were immersed in 80°C water and crack lengths were monitored as a function of time. Both the initial crack length and crack propagation were minimized when the primer thickness was a little less than the aluminum oxide thickness. In a separate study, the aluminum oxide surfaces were aged in hot water or hot humid environment before bonding. The crack lengths of the aged samples for both 60°C water immersion and 70°C ≥95 %R.H. conditions were similar to those of non-aged sample. On the other hand, when the samples were aged in 80°C water, the oxide was hydrated and the crack length increased. The locus of failure also changed from cohesive failure in the adhesive for the lower temperature aged samples to a mixed mode of cohesive failure and aluminum oxide-primer interfacial failure for the 80°C aged samples.