Polymeric coupling agents for enhancing the adhesion of epoxy to aluminum

Date of Completion

January 1998


Chemistry, Polymer|Environmental Sciences




Increasing environmental regulations have prompted industries using aluminum to seek alternatives to chrome pretreatment processes for adhesion enhancement. This research evaluated the influence of mercaptoester and $\beta$-diketone functionalized polymeric coupling agents on the adhesion within an epoxy/coupling agent/aluminum bond system.^ It was demonstrated that under the reaction conditions described in this dissertation that the mercaptoester functionality, as found in pentearythritol 3-mercaptopropionate and trimethylolpropane tris (2-mercaptoacetate), did not react with aluminum ions in solution or with aluminum substrates. Solution infrared spectroscopy, X-ray photoelectron spectroscopy, and solution NMR data did not provide any evidence that the mercaptoester/aluminum reaction was occurring under atmospheric conditions, in methanol. The lack of reactivity with aluminum precluded further evaluation of the mercaptoester functionality as a reactive group for incorporation into a polymeric coupling agent for the aluminum systems.^ Infrared spectroscopy was utilized to verify the reactivity of the $\beta$-diketone functionality with aluminum ions both in solution and with aluminum surfaces. Surface reactivity toward the $\beta$-diketone monomer was determined to be a function of surface pretreatment. However, grazing angle infrared spectroscopy of the $\beta$-diketone-containing polymeric coupling agent deposited onto aluminum substrates did not reveal evidence of significant reaction.^ Deposition studies of $\beta$-diketone containing polymeric coupling agents revealed that the thickness of the resulting PCA layer following solvent rinse was not a function solution concentration, temperature, the time of immersion or aluminum surface preparation; a constant 20-30A thickness was found in all cases. In addition, solvent refluxing argon sputtered aluminum thin films that had been coated with the $\beta$-diketone containing PCA resulted in an apparent reduction in the thickness of the deposited coating, only 9A, in the range of atmospheric contaminant values.^ The limited coating thickness, instability of the coating during reflux, and the lack of evidence of chelation suggested that the primary adsorption process for the polymers presented in this research was physisortion as opposed to the desired chemisortion.^ Joint strength and durability measurements showed that the 30A PCA coatings did not influence adhesion positively or negatively. Failure analysis of peel samples that were treated with a 1% solution of $\beta$-diketone containing polymeric coupling agent showed primarily interfacial failure. ^