The method of vat dye dyeing process, after vat dye leuco body and then into dyeing insoluble dye. First, it becomes soluble leuco sodium salt through reduction in lye, which is adsorbed by cellulose fiber, and then it is oxidized to return to the original insoluble dye. Generally, it has high fastness to washing and light. For example, Shilin Blue and so on. It is mainly used for printing and dyeing cotton and polyester-cotton blended fabrics. Vat dyes are insoluble in water. During dyeing, they must be reduced and dissolved in an alkaline strong reducing solution to become leuco sodium salt to dye fiber. After oxidation, it will return to insoluble dye color. Deposited and fixed on the fiber.
Vat dyes are dyes with excellent properties in all dyes. According to its main chemical structure, it can be divided into two categories: indigo and anthraquinone. Its color spectrum is relatively complete, its color is bright, and its soaping and light fastnesses are relatively high. However, due to its high price, some yellow and orange colors have photosensitive fragility, which limits their application.
1. Vat dyeing process When dyeing with vat dyes, dip dyeing, jig dyeing or pad dyeing can be used. Generally, yarns and knitted fabrics are mostly dip-dyed, and woven fabrics are mostly jig-dyed and pad-dyed, which generally include the following four basic processes.
1) The reduction process of vat dye, that is, the production process of vat dye leuco, is generally carried out in alkaline medium. The molecular structure of vat dyes contains at least two carbonyl groups (=C=O). They are reduced to hydroxyl groups (-OH) under the action of the strong reducing agent sodium hydrosulfite (commonly known as sodium sulfite). The chemical properties of sodium sulfonate are very active, and even at very low temperature under alkaline conditions, it can produce a strong reduction effect, so that the vat dye is reduced to leuco acid.
The hydroxy compound generated in the reaction is the leuco acid of the dye. It is also insoluble in water like the dye, but it is soluble in alkaline medium and becomes a leuco sodium salt solution. Since the sodium salt of leuco body does not often show the original color of the dye, it is called leuco body:
When using the soluene-caustic soda method for dye reduction and dissolution, you should master the amount of caustic soda and solubilizer and the reduction temperature in order to restore the dye normally, otherwise it will cause abnormal reactions such as excessive reduction or hydrolysis and molecular rearrangement of the dye. , Resulting in damage to the dye solution, dull color and reduced color fastness.
2) The vat dye dyed on the leuco body is reduced and dissolved into leuco body sodium salt by sodium hydroxide and caustic soda, which produces directness to the cellulose fiber, first adsorbs on the fiber surface, and then diffuses into the fiber to complete the fiber Dyed. Because the dyeing liquor contains a lot of electrolytes, it is more direct to the fiber, so the dyeing rate of the fiber is faster, the dye migration performance is poor, and it is often not easy to dye evenly. Therefore, a small amount of retarding agent can be added in the dyeing process, such as adding 0 for flat and 0.1-0.5% for cowhide rubber.
3) The leuco body oxidizes and dyes the fiber. The leuco body needs to be oxidized by air or an oxidant to transform into the original insoluble vat dye and restore the original color. The oxidation reaction is as follows:
Since different types of vat dyes have different oxidation properties, the oxidation conditions should be appropriately selected, such as oxidation with air, or oxidation with hydrogen peroxide or sodium perborate.
4) Post-soaping treatment. Soaping can remove the oxidized floating color adsorbed on the fiber surface, so that the dyed fabric has bright color and better rubbing fastness. At the same time, during the soaping process, dye molecules will aggregate on the surface of the fiber to form microcrystals, which can further improve the fastness of the dyed fabric.
2. Vat dye dyeing method Vat dye can be used for dyeing cotton and polyester-cotton and dimensional cotton blended fabric. According to different dyeing methods, it can be divided into leuco dyeing method and suspension pad dyeing method. There is also a leuco acid dyeing method, which is not commonly used, so it is not discussed here.
Leuco dyeing method Leuco dyeing is a dyeing method in which the dye is reduced to leuco in advance, adsorbed by the fiber in the dye bath, and then oxidized and soaped. It can be divided into dip dyeing, jig dyeing and pad dyeing, etc. Currently it is mostly used for dip dyeing and jig dyeing. Leuco dyeing is suitable for yarn dyeing. The jig dyeing method has poor degree of core penetration and white core. Depending on the nature of the dye, different reduction methods can be adopted.
(1) Dry vat method: During dip dyeing, some dyes have a slower reduction speed, and more severe conditions must be used to increase the dye reduction rate. This reduction method is called the dry vat method.
Dry vat reduction is to first mix the dye into a thin slurry with a small amount of water and Taikoo oil or powder, and then add an appropriate amount of warm water and a specified amount of caustic soda 10-15 g/l, and sodium hydroxide 15-18 g/l. Reduce at temperature for 10-15 minutes. After the dye is fully reduced, add it to the dye vat containing caustic soda and sodium hydroxide solution by filtration, and dilute to the required concentration for dyeing. Since the water consumption is very small during the reduction, it is equivalent to increasing the concentration of caustic soda and sodium hydroxide, which accelerates the reduction rate of the dye and accelerates the completion of the reduction reaction.
(2) Full bath method: This reduction method is suitable for vat dyes with a faster reduction rate. The method is to directly put the dye into the dyeing tank, add a prescribed amount of caustic soda and sodium hydroxide to reduce the dye, and dye it after 10-15 minutes. If more severe reduction conditions are used for this type of dye, it will easily cause the dye to be hydrolyzed or excessively reduced, which will affect the quality of the finished product.
The operation of vat dyes is more complicated, and the requirements for the grasp of the process are relatively high, and rich work experience is required. In addition, due to the large amount of caustic soda required in the dyeing process, the treatment of dyeing wastewater with sodium hydroxide is difficult, the environment is polluted, and the price of dyes is relatively high. In response, the market share has been declining in recent years.