Gelling induced autophobicity effects in multilayer sol-gel coating systems

Dewetting phenomena in silica dioxide multilayer sol-gel systems were studied. A second layer of coating was found to dewet and form domains on top of the first coating rather than forming a continuous film. To promote wetting of the first coating by the second, the application of three surface treatment techniques (thermal treatment, post-surface condensation in high pH solution, and surface oxidation by oxygen plasma) to the first coating was investigated. These treatments result in high surface energy hydrophilic surfaces onto which subsequent smooth and continuous coatings can be applied. However, the time interval between treatment of the first coating and application of the second coating (the `aging' time) was found to affect the wettability of the first applied layer. With increasing aging time, the surface energy decreases, resulting in a more hydrophobic surface. A detailed study of water contact angles with aging time after the three surface treatments supports the hypothesis that the observed `autophobicity' is due to migration of the residual hydrophobic ethoxy chains from the bulk of the coating to the surface, which consequently results in a more hydrophobic surface.