Gelling induced autophobicity effects in multilayer sol-gel coating systems

Dewetting phenomena in silica dioxide multilayer sol-gel systems were studi ed. 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 wettin g of the first coating by the second, the application of three surface trea tment techniques (thermal treatment, post-surface condensation in high pH s olution, and surface oxidation by oxygen plasma) to the first coating was i nvestigated. These treatments result in high surface energy hydrophilic sur faces onto which subsequent smooth and continuous coatings can be applied. However, the time interval between treatment of the first coating and appli cation of the second coating (the 'aging' time) wa found to affect the wett ability of the first applied layer. with increasing aging time, the surface energy decreases, resulting a more hydrophobic surface. A detailed study o f water contact angles with aging time after the three surface treatments s upports the hypothesis that the observed 'autophobicity' is due to migratio n of the residual hydrophobic ethoxy chains from the bulk of the coating to the surface, which consequently results in a more hydrophobic surface. (C) 2000 IoM Communications Ltd.