Direct observation of oil-surface adhesion via sum frequency generation spectroscopy

ABSTRACT Adhesive technology can joint dissimilar lightweight materials that are used in the automotive industry to improve fuel efficiency. However, the strength of the adhesive bonding is often reduced by oil contaminants on the adherend surfaces. Adhesives should repel the oil layer and diffuse onto the adherend surface, but there is no general design for reliable adhesion on contaminated adherends because of the poor molecular-level understanding of oil-surface adhesion. Here, we report the direct observation of oil-surface adhesion at a two-part epoxy adhesive/aluminum oxide (AlOx) interface via sum frequency generation (SFG) spectroscopy, a method that can probe buried interfaces. The AlOx surface is intentionally contaminated with commercial silicone oil. The SFG spectra of the cured epoxy adhesive/oil-contaminated AlOx interface suggest that the silicone oil remains after curing, explaining the reduction in adhesion strength. Triethylenetetramine, a hardener used in this study, can repel the oil layer, whereas bisphenol A epoxy resin is much less effective at removing the oil layer upon contact. Thus, the migration of both adhesive components onto the AlOx surface is key to securing sufficient oil-surface adhesion. We also applied SFG to second-generation acryl adhesive (SGAA)/AlOx interface and found that SGAA completely repels the oil from the aluminum interface.