Coupling agent studies: reaction of thiol-containing esters with aluminum

Double cantilever beam experiments performed by Joseph et al. showed increases in strain energy release rate and cohesive failure for mercaptoester-treated aluminum substrates as compared to untreated controls. This paper presents evaluation of the mercaptoester–aluminum reactivity as a first step toward assessing the effectiveness of incorporating the mercaptoester functionality into a polymeric coupling agent for an epoxy-aluminum bond system. Grazing-angle infrared spectroscopy and XPS of mercaptoester-treated solid aluminum substrates did not show evidence of a mercaptoester/aluminum reaction. The reaction was also attempted in solution where potential spatial limitations associated with the solid surface were eliminated. IR and NMR showed no evidence of reactivity between the mercaptoester and aluminum ions in solution. Theoretically, the Hard–Soft Acid–Base Theory, in conjunction with the Mison equation, does not support a mercaptoester–aluminum reaction. Thus incorporating a mercaptoester into a polymeric coupling agent for an epoxy–aluminum bond system would not result in chemical bonding. In addition, no significant reaction between thiols and aluminum in general, in the presence of water and moist air, is expected.