Polyimide Adhesives and Aluminum

This study examines the effect of thioether sulfur in the polyimide backbone, polyimide T_g, and adherend surface pre-treatment on aluminum bond strengths as determined with both peel and wedge specimens. Surface pretreatment and T_g had more of an effect on peel strength than the presence of sulfur in the polyimide backbone. NaOH etching and comparatively low T_g polyimides combined to produce the highest peel strengths. Together, these factors combined the removal of surface oxide from the adherend with a flexible polyimide which could better relieve stress during testing. Little difference was observed between the peel strengths of sulfur and non-sulfur containing polyimides, and no oxidation of sulfur was observed in the peel samples. NaOH etching also caused both wedge and peel specimens to fail more within the polyimide than in the oxide layer of the adherend. Thus, the NaOH etch appeared to increase interfacial adhesion between the aluminum and the polyimide. The low T_g polyimides performed better than the high T_g polyimides in the wedge test, with the polyimide derived from 4,4'-bis(3,4-dicarboxyphenoxy) diphenyl sulfide dianhydride and 4,4'-diaminodiphenyl ether (BDSDA/ODA) performing the best. This observation could be due to a metal-sulfur interaction since oxidized sulfur was surprisingly observed on the failed surfaces of these bonds regardless of the environment or surface pretreatment. A metal component in the aluminum alloy rather the aluminum is believed to promote the sulfur oxidation.