Surface and interphase studies of the adhesion of a siloxane- modified-polyimide coating on metals

Abstract

This research focused on the surface modification and interfacial profile studies of a siloxane modified polyimide, BDS [BTDA (3,3\4.4'~ benzophenonetetracarboxylic dianhydride) ~ DDS (3.3'~diaminodiphenyl sulfone) - PSX (polydimethylsiloxane) copolymer], used as a coating material. The BDS coating surface can be modified by pretreatment in an alkaline solution. This surface pretreatment etched away the top siloxane surface layer, activated the surface by exposing and creating polar functional groups, particularly carboxylic acid groups, and roughened the surface. These changes on the coating surface significantly improved the wettability and the strength of the bond between the coating surface and a polar adhesive. Interfacial composition profiles were obtained from angular dependent X-ray photoelectron spectroscopy and Auger electron spectroscopy. Two kinds of interphases were found near the bond line of the coating/metal substrate. One was a component gradient interphase which was formed by component segregation of the BDS copolymer. The component gradient was different on different metal oxide surfaces with the siloxane interfacial excess in the order of AI > Ti > Zn. The relative acidities of the metal oxide surfaces were characterized by poly(vinyl chloride) adsorption tests which were quantified by XPS measurements. The relative acidities were found in the order of AI > Ti > Zn. Therefore, the cause for the BDS component segregation was suggested to be the influence of acid·base interactions between components of the BDS copolymer and the metal surface oxides. The other kind of interphase was a polymer-metal oxide mixture interphase which formed on penetration of the BDS copolymer solution into porous aluminum surfaces.