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

Double cantilever beam experiments performed by Joseph et al. showed increa ses in strain energy release rate and cohesive failure for mercaptoester-tr eated aluminum substrates as compared to untreated controls. This paper pre sents evaluation of the mercaptoester-aluminum reactivity as a first step t oward assessing the effectiveness of incorporating the mercaptoester functi onality 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 react ion. The reaction was also attempted in solution where potential spatial li mitations associated with the solid surface were eliminated. IR and NMR sho wed no evidence of reactivity between the mercaptoester and aluminum ions i n solution. Theoretically, the Hard-Soft Acid-Base Theory, in conjunction w ith 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 additio n, no significant reaction between thiols and aluminum in general, in the p resence of water and moist air, is expected. O 2000 Elsevier Science Ltd. A ll rights reserved.