The Causes and Corresponding Measures of White Dyeing of Reactive Dyes

Causes of contamination in white areas
White soiling is an important issue in printing quality. The main cause is the dye itself and the printing process.
Reactive dyes are mainly composed of three parts: mother dye, active group and linker. The mother dye is the main part of the color body, which basically determines the color of the dye and the directness to the fiber. The active group mainly determines the reaction performance of the dye, and also determines the fixation rate. And there is a direct relationship with the fastness of the bond of the dye; the density of the electron cloud of the atom or group on the linker affects the reactivity of the active group. Reactive dyes react with fibers in a covalent bond. Due to the different stability of the dyes, they cause different degrees of hydrolysis and hydrolysis of the dye-fiber bonds under certain conditions. The hydrolyzed dyes have similar affinities to the reactive dyes. In the post-treatment, the fibers are dyed to cause white soiling.
2. Printing process
Reactive dye printing time-phase method and two-phase method, the fixation of dye and fiber is completed in alkaline medium. In alkaline conditions. The dyes will undergo different degrees of hydrolysis. If the process is not properly controlled, the amount of hydrolyzed dyes and unfixed dyes will increase. White soil contamination is also more serious.
1) The steaming time is too short and the dye does not diffuse into the interior of the fiber; if the time is too long, the bond between the dye and the fiber breaks, the fixation rate is low, and the amount of unfixed dye increases.
2) When the dyestuff is used to prepare the coloring paste. If baking temperature is high, baking soda is added, so that a large amount of bubbles will be generated in the printing paste, and the pH of the printing paste will increase, the stability will decrease, and the dye will be easily hydrolyzed.
3) Dyestuffs with high direct dye selectivity have high dye reactivity, hydrolysis dyes, and are not easily washed by post-treatment and can easily contaminate white soil.
4) Post-treatment The hydrolyzed dyes and unfixed dyes are dissolved in the wash solution during washing. When the wash solution concentration is high, fibers are adsorbed and contaminate the fabric.
How to prevent white soiling

To reduce the contamination of white areas. The key lies in reducing the hydrolysis dyes and increasing the fixing rate of the dyes.

1. The choice of dye
1) Try to use dyes with low directivity. Even if hydrolysis occurs, hydrolyzed dyes can be easily washed off after post-treatment, and the dye fixation rate is high. Such as: K, M type, this can reduce the amount of unfixed dye.
2) Select reactive cross-linked dyes. This type of dye is used together with a specific fixing agent. The reactive group contained in the fixing agent is a reactive group such as NH 2 or 0H, which can form a covalent bond with the fiber and react with the dye molecule to form a cross-linking effect. There is no active group in cross-linked dye molecules, so there is no problem of dye hydrolysis and white stain.
3) Liquid reactive dyes. The application is convenient, the fixing rate is high. In the production, the metering device controlled by the computer program can be used to add alkali, so that the alkali process can be automated to ensure the best fixing rate, and is particularly suitable for polyester-cotton blend fabric printing.
2. Select the appropriate printing method
Strictly control the DH value, time, temperature. Appropriately increase the amount of urea, improve the fixation rate. With the two-phase method or sodium trichloroacetate method, its 1:1 aqueous solution, pH = 5 ~ 6, the color paste is more stable, when the steam swallow decomposition of baking soda to fix the dye.

3 fixation
The printed fabrics were treated with a solid colorant to reduce the breakage of the dye-fiber and increase the fixation rate. Such as: cationic fixing agent, active fixing agent, organic silicon fixing agent, film fixing agent, etc.
4 strengthen the washing effect
Always wash unfixed dye before soaping
1) Use high-efficiency washing machine, also can use a lot of cold water rinse, wash liquid quickly discharged, and then hot wash, soaping, washing.
2) Add anti-staining agent in the hot water tank of the original equipment. Such as: anti-staining agent s. Has a higher net wash capacity and anti-staining ability. It works better after use.
5. Borrow other finishing effects
After reactive dye printing, hydrolyzed dyes and unfixed dyes are re-dyed onto fabrics during the washing process using water as the medium. Causes white soiling. If the color of the reactive dye is printed on the anti-printing, the water-repellent finishing agent will be added to the anti-whitening paste so that the contact surface between the water and the fabric is reduced, thus limiting the dyeing onto the fabric. Water repellent finishing is that the water repellent finishing agent forms a film by baking, and in the process of resisting dyeing of the reactive dye, it is usually baked (or steamed) to meet the requirements of the finishing agent, and the finishing agent is the same as the anti-white pulp. Are all acidic.
Although reactive dye printing has certain disadvantages, with the development of dye production technologies and application processes, the printing of reactive dyes will inevitably reach a new level.

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