Increasing hydrophobicity of sol-gel hard coatings by chemical and morphological modifications

Sol-gel technology enables us to make scratch resistant hard coatings on plastic or non-ferrous metal substrates for industrial applications. Increasing hydrophobicity without decreasing the mechanical properties of these coatings is a challenge. We study the influence of chemical and morphological modifications on the hydrophobicity of a silica based sol-gel hard coating. To modify the chemical properties of the coating, a long side-chain alkyltrialkoxysilane is incorporated into the coating solution. To modify the surface morphology of the coating, a silica filler with selected particle size distribution is added. Varying the silica content resulted in different surface roughness and different morphology distributions. We observed a gradual increase in water contact angle with increasing roughness, and a rapid increase when the morphology changed to a lotus-leaf-like structure. We propose a new roughness parameter, the peak-to-valley ratio, to characterize morphologies with high hydrophobicity. This paper describes a single-step modification process that produces a material with microstructure and hydrophobic properties analogous to those of the lotus leaf while maintaining the coating hardness necessary for industrial applications.