Effects of anodized aluminium surface parameters on the long-term adhesion of silicone structural glazing sealants

Abstract

The quality of silicone sealant adhesion to anodized aluminium varies widely. Key parameters that affect adhesion are the degree of surface sealing, the pigmentation of the anodization layer, the nature of the cleaner, and the time period allowed between cleaning the substrate and sealant application. While the cleaners display different effectiveness in removing organic contamination from the anodized aluminium surface, this effectiveness does not correlate with adhesion quality. It is hypothesized that by absorbing on the anodized aluminium substrate the cleaning solvents ‘condition’ the surface for improved silicone sealant adhesion. This conditioning effect diminishes, as the cleaning solvents desorb over time. For a given cleaning solvent, optimum adhesion is observed for a certain degree of surface sealing. Adhesion to unpigmented anodized aluminium substrates can be predicted by measuring the degree of surface sealing according to the ISO 2143 acid etch method. For pigmented anodized aluminium substrates, electrical phase shift measurements, made in accordance with the ISO 2931 test standard, can be used to predict adhesion quality. A quality control method for anodized aluminium surfaces is proposed that allows successful correlation of the electrical impedance measured at various frequencies with the long-term adhesion of silicone sealants to this surface.