Direct dyeing process
The direct dyeing process describes a dyeing method in which the water-soluble dyes used adhere directly to the dyed material through intermolecular forces without the addition of further chemicals . Wool and silk are particularly suitable for this process , as both have functional groups that react with the corresponding groups of the dyes. A distinction can be made here according to the type of binding:
- In substantive coloring , the dye molecules are attracted to the fiber by intermolecular dipole or van der Waals forces or by hydrogen bonds . They adhere to the surface or form aggregates in submicroscopic cavities of natural or synthetic cellulose fibers such as cotton or viscose , but also of wool . The binding of these relatively large dye molecules to the fiber is not very strong, so that the dye is only moderately washable. Important dyes for this process are polyazo dyes with one or more sulfo groups (e.g. Congo red ). The first direct dye was Congo red, invented by Paul Böttiger in 1883 .
- In direct dyeing in the narrower sense , colored anions from acidic (anionic) dyes are bound to the fiber by electrostatic ionic forces. The bond takes place via a salt bond, in which the free amino groups of wool or silk form a connection with the acid dyes (with one or more sulfo groups). The acid dyes mainly include various azo, anthraquinone and, to a lesser extent, azine dyes (e.g. acidol , isolan or nigrosine ). On leather, the basic skin substances react with the sulfo groups of the acid dyes and form lightfast salts. In the case of cationic (basic) dyes, the amino groups contained are bound by free carboxylation (e.g. mauvein ). Cationic dyes only produce colors that are moderately washable on natural fibers, but brilliant, extremely lightfast colors can be achieved on polyacrylonitrile fibers .
See also
literature
- Norbert Welsch and Claus Chr. Liebmann; Colors - nature, technology, art. Spectrum Academic Publishing House, ISBN 3-8274-1563-2