Effects of Polyacrylic Acid Primers on Adhesion and Durability of Fpl-Etched Aluminum/Polyurethane Systems

It has been determined that when a polyacrylic acid [p(AA)] macromolecule primer is used in polyurethane (PU)/p(AA)/FPL-etched aluminum joint systems, it interacts preferentially both with the hydrated Al₂O₃ or aluminum hydroxide adherend surface through the formation of hydrogen bonds and with the isocyanate groups in the PU adhesive to yield polymer-to-polymer chemical bonding. This chemical crosslinking between the hydrated adherend and the polymeric adhesive acts significantly to promote interfacial adhesive bonds. It appears that a near monolayer of p(AA) is enough to occupy all the available functional groups at the PU and aluminum surface sites. This arrangement plays a key role in improving the adhesion durability of the joint system upon exposure to a hot NaOH solution. It was also determined that the presence of excessive carboxylic acid groups resulting from the use of a thick p(AA) layer caused gel-formation-induced adhesion failures; namely, the locus of failure of the joint after exposure was clearly identified to be cohesive failure in the primer.