(1) Field of the Invention
The present invention relates to a composition capable of forming a superhydrophobic coating on a surface, a substrate having a superhydrophobic coating formed of such a composition, as well as to a method of production of such a superhydrophobic coating. Preferably the coatings are transparent.
(2) Description of the Related Art
Surfaces with particular wetting characteristics, i.e. water repellent surfaces, are widely used and of great interest to various industries, such as the textile industry, construction industry, e.g. corrosion or masonry protection, the automotive industry, in medical technologies as well as for sanitary products. Likewise, the use of surface modification techniques to impart such properties to surfaces of various substrates, natural or artificial, such as metal, glass, wood, ceramics, paper, polymers, fabrics, building materials, such as stone, concrete, marble, bricks, tiles, etc, to achieve the desired characteristics is a widely researched field.
The hydrophobicity of a material, i.e. its tendency to repel water, may be determined by the contact angle of a water droplet to the surface. In general, hydrophobicity is achieved by lowering the surface energy. Thus, non-hydrophobic materials may be rendered hydrophobic by applying a surface coating of low surface energy material. Chemically this may be done for example by incorporating apolar moieties, such as methyl or trifluoromethyl groups, into the surface. Superhydrophobic properties, typically referring to contact angles larger than about 150° and theoretically up to 180°, additionally require a high surface roughness. On a rough and hydrophobic surface air can be trapped underneath the water droplet which greatly reduces the actual liquid/solid contact area and thus the contact angle increases. With higher contact angles, e.g. contact angles of more than about 150°, other important effects like self-cleaning properties or enhanced water sliding behaviour can be obtained (Yoshimitsu, Z. et al, Langmuir 18, 5818 (2002)). Yet, if the surface roughness is too high and reaches the submicrometer scale light is scattered at the surface and the coating appears no longer transparent. Thus, to obtain optically neutral coatings the roughness has to be restricted to be well below the wavelength of visible light.
Many techniques of rendering surfaces superhydrophobic are described in the literature (Nakajima, A. et al, Monatsh. Chem. 132, 31 (2002); L. Feng et al., Adv. Mater. 14, 1857 (2002)). Most common are plasma polymerization or etching of apolar polymers like polypropylene or polytetrafluoroethylene, plasma enhanced chemical vapor deposition of methyl or fluorine containing silanes, solidification of molten polymers or waxes, sublimation material and paint or sprays containing hydrophobized microbeads or evaporation of volatile compounds (Miwa, M. et al, Langmuir 16, 5754 (2000)). In order to increase the roughness very often additional steps like mechanical treatment, chemical or plasma etching or anodic oxidation are necessary before or after the coating step. However, there are several disadvantages associated with these methods, such as complicated and thus time-consuming procedures, expensive starting materials like fluorinated silanes and/or extreme reaction conditions which restrict the applicability to few resistant materials. In addition only a few coatings are optically transparent.
Thus, having regard to the wide variety of materials in daily life where a transparent and water repellent coating is highly desirable there is clearly a need for superhydrophobic, transparent surfaces as well as to simple and economical methods of preparation of such surfaces, which are overcoming the disadvantages mentioned hereinabove.