Formation of superhydrophobic alumina coating films with high transparency on polymer substrates by the sol-gel method

Transparent, superhydrophobic coating films have been prepared on polymer substrates at low temperatures through the sol-gel method. Al2O3 gel films were prepared on poly( ethylene terephthalate) substrates from Al(O-sec-C4H9)(3) chemically modified with ethyl acetoacetate. A small roughness of about 20-50 nm was found to form on the surface of the Al2O3 gel films dried at room temperature and then immersed in hot water at 60degreesC. The electron diffraction measurements have shown that this roughened surface consists of pseudoboehmite nanocrystals. The coating of hydrolyzed fluoroalkyltrimethoxysilane on the Al2O3 gel films with the small roughness produced transparent, superhydrophobic films with contact angle for water larger than 150degrees.