Metal/polymer interactions in polyimide adhesives

Abstract

Due to their superior thermal and chemical stability, polyimides are often used as adhesives in hazardous environments. This study examines the effect of thioether sulfur in the polyimide backbone on bond strength. X-ray photoelecton spectroscopy (XPS) and reflectance infrared spectroscopy indicated that certain metals catalyze the oxidation of the thioether sulfur. It was believed that this oxidation could lead to direct metal-oxygen-sulfur bonds across the polymer/metal interface which would serve to enhance interfacial strength. Bonds were made using substrates that were believed to catalyze the oxidation strongly (steel) and minimally (aluminum). In addition, non-sulfur containing polyimides with similar Tg were also studied for comparison. The polymer/metal interface was studied using both the T-peel and wedge tests. In some cases, oxidized sulfur was detected on the failed surfaces via XPS. No apparent effect was observed in the T-peel test, where the T-peel strengths of non-sulfur and sulfur containing polyimides were similar. In the wedge test, however, the sulfur containing BDSDA/ODA bonded to steel had an initial crack length of 34 mm. Even after eleven days the crack length was only 47 rnm, which was the initial crack length for the next best polyimide. Thus, metal-catalyzed oxidation of sulfur did take place, but not to an extent to have a noticeable effect on peel strength.