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A new chance to refine textiles is the coating with silica compounds. There are many methods to prepare inorganic metal oxide layers but most of those methods contain an application from the gaseous phase and need therefore vacuum devices on a high technical standard. A simple alternative for preparation of thin metal oxide coatings is the sol-gel technique. This technique allows the preparation of coatings at room temperature and normal pressure. Also ordinary processes like dip-coating or spraying could be used together with classic devices for textile refining.

The coating is performed by treatment of textile substrates with a solution containing silicon oxide or other metal oxide nanosols. The nanosols normally contain 3 to 20 weight percent of metal oxide and these metal oxide is present in solution as nanoparticles with diameters smaller than 10 nm. The quite large ratio of surface to volume of such nanoparticles leads to condensation and aggregation of the particles during the coating process. The result of these aggregation processes is a three dimensional network of former nanoparticles. After coating first a solvent containing lyogel layer is formed on the textile surface. In a second step this lyogel is dried and heated, so the solvent will removed from the coating. In the end, a xerogel layer is obtained, which is free of solvent molecules and contains a porous structure. The resulted xerogel layers are of high interest for functionalisation of textiles. The reason is the nanosols can be easily modified chemically or physically, so the properties of textiles can be modified in a wide range by using different nanosol coatings.

The chemical modification is performed with additives, which are able to form covalent bonds to the metal oxide particles during the coating or the drying process. Chemical modifications could be on the one hand the reaction of different types of metal alkoxides with each other, for example tetraethoxysilane Si(OC2H5)4 (TEOS) with other metal alkoxides Me(OR)n. In this case, different usual metals like silicon, aluminium, titanium, zinc and zirconium should be mentioned. On the other hand chemical modifications could be performed using trialkoxysilanes R-Si(OR)3 containing an organic rest R. The trialkoxysilanes are covalent connected to the silicon oxide matrix via hydrolysis and cocondensation, so the rest R is chemically bonded to the xerogel layer. Via the incorporated rest R the modification of the nanosol coating can be performed in a wide range, for example hydrophobic or oelophobic properties can be reached with alkyl or perfluoroalkyl containing trialkoxysilanes.

The physical modification of nanosols is performed with additives which are homogeneously incorporated and immobilized into the metal oxide matrix without any formation of covalent bonds. These additives are usually bigger molecules like polymers, dyes or biomolecules. The incorporation of such additives containing different properties allows the preparation of nanosol coatings containing new material properties (Figure 1).

In the simplest case the treatment of textiles with inorganic nanosols leads to an enhancement of solidity due to the decreased abrasion caused by the inorganic coating. For example the coating of polyester sieves with an alkylmodified silica nanosols can improved the abrasion properties of those materials, which are used for paper production (Figure 2).

Of special interest are the modification of textiles for variation of adhesion properties. By coating with modified nanosols it is possible to vary the textile between hydrophilic and hydrophobic properties. It is also possible to increase hydrophobicity and oelophobicity of fibres by coating with silica nanosols containing perfluoroalkyltriethoxysilanes. The use of special alkylsilane compounds without fluorine portion can also lead to excellent water repellent properties.

For preparation of coloured textile coatings nanosols containing dyes or pigments are usable. One advantage is that cheap and water soluble dyes can be used for immobilisation into the nanosol matrix and in such kind for textile dying. By modification of nanosols with crosslinkable silane compounds the leaching and the bleaching fastness of those dyes can be improved.

The sol-gel process is a suitable method to incorporate homogeneously bioactive compounds, biomolecules and also whole living cells into metal oxide matrices. Metal oxide matrices contain significant advantages compared to polymers, e.g. controllable porosity, high mechanical and biological stability. With variation of the layer composition the immobilisation of enebbed bioactive substances can be controlled.

A permanent incorporation of biological compounds into nanosol coatings could lead to an increased biocompatibility and therefore also to a better skin wholesomeness of textiles. Incorporation experiments were performed with collagen, gelatine, chitosan or hyaluronacid. While large biomolecules like enzymes could be fully incorporated into the metal oxide coating, smaller molecules from liquid or gaseous phase could diffuse into and out of the coating nearly without any barrier. In such cases a quite similar enzymatic activity as in solution was obtained by incorporated enzymes, so high effective biocatalysed reactions can be performed inside a sol-gel layer. An application could be the impregnation of bandaging materials to perform an enzymatic cleaning of the wound.

Also the controlled release of bioactive substances from metal oxide coatings can be performed. In this case, a therapeutical use could be realized with biofunctional textiles. First investigations on applications in textile area are approached in various areas. Antibacterial coatings can be designed by the incorporation and controlled release of silver compounds or organic biocides. Analogously also the release of liquid active or aromatic substances can be obtained.

Even high viscous or oily substances like vitamin E or dexpanthenol can be incorporated into nanosols coatings with the result of dry and non-sticky materials. Similar applications were obtained for perfumes like citronellol in carpets or for insects repellents like diethyltoluamid for tropical textiles.