Why is the wet fastness and rubbing fastness poor when the reactive dyes dye the dark color of cotton? How to deal with it?
(1) Reasons for poor fastness.
Unlike direct dyes dyeing cotton, neutral (or acid) dyes dyeing nylon, disperse dyes dyeing polyester, reactive dyes dyeing cotton are not combined with hydrogen bond, van der Waals, or Coulomb attraction. Instead, they are combined by chemical bonds (covalent bonds). Once the dye is fixed, it becomes part of the cellulose macromolecular chain. Therefore, in terms of mechanism, the wet treatment fastness of reactive dyes on cellulose fibers should be the best. However, in actual production, the wet fastness and rubbing fastness of reactive dyes to dark colors are not satisfactory. The reason is that the absorption rate (E value) of reactive dyes is higher, and the fixation rate (lower value) is lower. The floating rate (E value-F value) of domestic medium temperature reactive dyes is 20% on average. That is, inside and outside the fiber (or fabric), there are more crystals and aggregates formed by unfixed dyes. Among them are hydrolyzed dyes, semi-hydrolyzed dyes, and unhydrolyzed and unfixed dyes. These floating dyes have weak adhesion in fibers (or fabrics), and are easy to melt and fall in the wet processing fastness (washing, soaping, perspiration) and rubbing fastness tests, causing serious staining of white cloth. The fastness is low.
Obviously, the key to improving the fastness of reactive dyes is to reduce the dye floating rate on the fiber (or fabric).
1. The mercerizing process should be strengthened. In recent years, many printing and dyeing enterprises have adopted the short-process pre-treatment process of "alkali oxygen one bath" or "enzyme oxygen one bath" cold-rolling pile. The scouring and bleaching quality is usually poor in transparency and wool effect. Lower. In addition, low-concentration (below 220g/L) caustic soda mercerizing is used for energy saving. Therefore, the mercerizing effect is often poor, and semi-mererized or surface mercerizing is more common. This will directly lead to the occurrence of "surface staining" problems, which will undoubtedly seriously harm the color fastness. Therefore, the quality of scouring and bleaching before mercerizing must be uniform and the hair effect must be good. Be sure to use 220~260g/L of caustic soda for full mercerization before dyeing.
2. The dyeing process should be optimized.According to the dyeing performance of different dyes, different dyeing methods are selected. Because it is compatible with the specific properties of the dye, it is very effective in increasing the fixation rate of the dye, reducing the floating rate, and improving the color fastness.
The alkaline agent should be applied slowly. This can effectively reduce the degree of aggregation and rapid dyeing of the dye. It plays an important role in improving the fixation rate of dyes and reducing the floating rate.
3. Pay attention to the soaping process. The soaping effect after dyeing is the key to improving the fastness of reactive dyes. Therefore, the bad habit of heavy dyeing and light soaping must be changed. If necessary, after soaping and washing, the fixing agent can be used for fixing treatment. This can significantly improve the wet processing fastness (the rubbing fastness is not significantly improved). However, this will bring trouble to the later "color repair".