ABSTRACT
Cleaning of textiles as well as preventing the soiling of a textile product are amongst the most important topics in textile research. No matter whether the focus of application is, e.g., on clothing, home textile products, filter materials or textile architecture, one of the demands is that the product should stay clean as long as possible and it should be easy to clean when dirty. The conventional route to achieve this effect is to finish it with a hydrophobic or oleophobic material. Finishing with fluorocarbon formulations is well established in textile industry, yielding excellent water-and oil-repellence [1]. The fluorocarbons are relatively expensive finishing
agents but since no other materials offer a comparable effect they have been irreplaceable until now, at least for certain applications. In combination with nanoand micro-structured surfaces these materials show even super-repellent properties, according to a Cassie-Baxter wetting scenario. Today this is commonly known as the “Lotus effect” since such an effect can be observed in nature, e.g., on the leaves of the lotus plant (even if the nature does not produce fluorocarbons). Commonly used fluorocarbons are based on perfluorooctanoic acid (PFOA) which is known to be persistent, which means it accumulates in the environment. For this reason a replacement of the fluorocarbon chemistry has been discussed in many countries and a ban on products based on POFA is expected in the future. The substitution of these materials is actually difficult since potential alternatives are either less effective or more expensive. One novel approach is the preparation of so-called photocatalytic surfaces by applying titania, mostly in anatase modification. This approach is somehow contrary to the idea of preparing highly-repellent surfaces, since such materials become super-hydrophilic during irradiation with ultraviolet radiation. Super-hydrophilic surfaces are completely wetted by water and oil because of the photocatalytic effect. But this is more a side-effect of the photocatalytic effect; the photocatalytic activity is based on the semiconductor properties of the anatase. Anatase is able to absorb electromagnetic radiation with energy of more than 3.2 eV, which corresponds with the band gap of the crystalline titanium dioxide. By irradiating anatase with ultraviolet light of 387 nm and below, an electron of the valence band will be promoted into the conduction band, leaving a positive hole (cf. Fig. 1). The energy level of the valence band allows an electron transfer from a hydroxyl ion [2, 3]. The oxidation of the hydroxyl ion leads to the formation of a hydroxyl radical which is known to be a very aggressive species. Besides this oxidation, a number of different reactions may occur such as, e.g., a transfer of electrons from the conduction band to oxygen leading to other reactive species. Molecules such as the hydroxyl radicals have a strong oxidation potential and will, therefore, be able to oxidize and, consequently, decompose most organic molecules. A direct oxidation of organics by electron transfer to the anatase is also described [4].
