Vat dyes can be used for dyeing by dip-dyeing, roll-dyeing or pad dyeing. In general, most yarns and knitted fabrics are used for dip dyeing, while most woven fabrics are used for roll dyeing and pad dyeing, which generally include the following four basic processes.
1) the reduction process of VAT dyes, that is, the generation process of VAT dye cryptochromes, is generally carried out in an alkaline medium. In the molecular structure of VAT dyes, there are at least two carbonyl groups (=C=O), which are reduced to hydroxyl groups (-oh) under the action of strong reductant sodium disulfite (commonly known as insurance powder). The chemical property of the powder is very active, even if the temperature is very low under alkaline conditions, it can also produce a strong reduction effect, so that the VAT dye is reduced to cryptochromic acid.
The hydroxyl compound produced in the reaction is the cryptochromic acid of the dye, which is also insoluble in water like the dye, but soluble in the alkaline medium and becomes the sodium salt solution of cryptochromite. Because the sodium salt of cryptochromatid does not often present the original color of dye, it is called cryptochromatid.
In the use of insurance powder-caustic soda method for dye reduction and dissolution, the amount of caustic soda, insurance powder and reduction temperature should be well mastered, in order to make the dye normal reduction, otherwise the dye will produce excessive reduction or hydrolysis and molecular rearrangement and other abnormal reactions, resulting in the destruction of the dye, color fade, color fastness reduced.
2) VAT dyes dyed on cryptochromes are reduced and dissolved into sodium salts of cryptochromes by means of insurance powder and caustic soda. That is to say, they have a direct effect on cellulose fibers, which are first absorbed on the surface of the fibers and then diffused to the interior of the fibers to finish the dyeing. Due to the large amount of electrolyte in the dye solution, it is more direct to the fiber, so the rate of dyeing the fiber is fast, the dye transfer dyeing performance is poor, often not easy to dye evenly. Therefore, in the process of dyeing can be added to a small amount of retarder, such as pingping plus 0, 0 cattle hide glue. 1-0. About 5%.
3) oxidation of cryptochromes the cryptochromes dyed on the fiber need to be oxidized by air or oxidants, which will change into the original insoluble VAT dye and restore the original color.
Due to the different types of VAT dyes have different oxidation properties, it should be appropriate to choose oxidation conditions, such as air oxidation, or the use of hydrogen peroxide, sodium perborate oxidation.
4) after soaping, the processed soaping can remove the oxidized floating color adsorbed on the fiber surface, and make the dyed fabric have bright color and good rubbing fastness. At the same time, in the process of soaping, dye molecules will accumulate on the surface of the fiber and form microcrystals, which can further improve the fastness of the dyed fabric.
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