What is the reason for the appearance of white stars when reactively dyeing dark colors?

- May 23, 2020-

 When dyeing dark fabrics with reactive dyes, dandruff-like white stars often appear. This white star phenomenon often makes the dyeing plant unpredictable. In fact, the white star phenomenon is also regular.

 When dyeing dark colors workwear fabrics such as active black, dark blue, army green, and brown, sometimes white stars such as dandruff appear on the fabric. After washing, the white stars have not improved significantly, and still have a great impact on the appearance of the fabric. In the end, it can only be solved by changing the color and re-dying. During the production process, we neither know how the white stars came from, nor how they disappeared. Such white stars come without a shadow and go without a trace. 

After observation, the white star is not without color, but the color is very light. This phenomenon often appears on the high special yarn (Tex is1000 meters yarn, the weight in grams when the public moisture regain) There are many, but rarely appear on low special yarn. Once the white star phenomenon appears on the fabric, almost all batches exist, even several consecutive batches appear at the same time. When the same batch of semi-products are dyed with vat dyes, there are no white stars, and the dyeing activity is light No white star defects were found at all, and there was no white star defects in the dark or light colors of the brushed fabric, and the white star phenomenon appeared only in the active dark fabric that was not polished.

The white star does not exist in the shape of a small cotton ball, but is entangled with the fiber to form a yarn. Therefore, the method of beading and multi-mercerization cannot effectively remove the white star. 

Analysis of the reason: when Baixing appeared, because the cotton blending grade of the cotton spinning plant was too low, there were many dead cottons in the blending cotton, and the pre-treatment process design of the dyeing plant was not mature. There is not enough time, there are certain defects in the coordination of the concentration of the auxiliary agent during the scouring, the scouring time and the scouring temperature. When the defects of multiple factors are superimposed together, the main reason for the appearance of white stars on the fabric.

  Pectin blocks some of the hydroxyl groups on the cellulose, making it less wettable. The pectin content of mature cotton fiber is 0.9% -1%, while the pectin content of unripe cotton fiber is as high as about 6%, and pectin is concentrated in the outermost layer of the fiber, which is compared with normal cotton fiber. It is said that the pectin content of the outer layer of dead cotton fiber is very high, and its wettability is very low. 

When dyeing with vat dyes, the concentration of caustic soda and insurance powder is higher during dyeing and steaming. During the reduction process of vat dyes, the pectin on dead cotton fibers is decomposed at the same time, and the leuco vat of vat dyes can smoothly enter the interior of dead cotton fibers. And coloring, as for the difference in the shade of the color ratio will not affect the appearance of the fabric. 

Reactive dyes and cotton fibers are combined in the form of chemical bonds. Reactive dyes are dyed at low alkalinity. When steaming, this alkali concentration cannot decompose the pectin wrapped in the outer layer of dead cotton fibers. Therefore, During the dyeing process, the dead cotton fiber is very hydrophobic, and reactive dyes cannot be dyed normally. Only a few reactive dyes can be dyed, which causes the phenomenon of white stars. 

Under such circumstances, it is unlikely that the greige factory is expected to increase the cotton blending. The key is that the printing and dyeing factory must find a better way in the pre-treatment process.