Main features and technology of discharge printing

Discharge printing is also called engraving. It uses the reducing agent and oxidant to destroy the background color of the colored fabric (that is, it is printed on the colored fabric with the discharge agent to destroy the dyed part of the pigment) to obtain partial achromatic or colored. Printing methods for various patterns and patterns. Due to the more detailed and realistic pattern of the dyeing and printing, the pattern is rich in layers, rich and colorful, the color contrast is strong, the outline is clear and detailed, and the background color is full and more and more people pay attention to it.

The discharge printing process can be roughly divided into two steps, that is, the fabric is first dyed in a single color, and then the pattern is printed on the fabric. In pattern printing, the printing paste contains chemicals that can destroy the ground color dye (such as strong bleach, also known as discharge agent), which will destroy the background color during steaming, so it can be produced by this method. The blue-and-white dot pattern of the flower cloth, this process is called white printing. When the bleach is mixed with the dye which does not react with it in the same color paste (the vat dye belongs to this type), color printing can be performed. That is, while the white color is broken, the dye which is stable to the discharge agent is added to the anti-staining slurry, and the dye is dyed at the same time to obtain a colored pattern different from the ground color, that is, the color drawing effect. Thus, when a suitable yellow dye (such as a vat dye) is mixed with a colored bleach, a yellow dot pattern can be printed on the blue fabric. The white print and the color pull print can be applied to one pattern at the same time, collectively called discharge printing.

Since the background color of the discharge printing is mostly dyed by the piece dyeing method, if the same color is printed on the floor printing, the color of the background color is richer and deeper. This is the main purpose of using discharge printing. The discharge printed fabric can be printed by roller printing and screen printing, but cannot be printed by thermal transfer printing. Since the production cost of the printed fabric is high compared to the direct printing, the reducing agent required for the use must be carefully and accurately controlled. Fabrics printed in this way are sold better and at a higher price. Sometimes, the reducing agent used in this process can cause damage or damage to the fabric at the print pattern. If the color of the background of the front and back sides of the fabric is the same (because it is a piece of dye), and the pattern is white or not the same color as the background color, it can be confirmed that the printed fabric is discharged; carefully check the reverse side of the pattern, The trace of the original color is revealed (the reason for this phenomenon is that the chemical that destroys the dye cannot penetrate completely to the opposite side of the fabric).

The discharge agent is the main solvent used in the discharge printing, and the discharge agent is a chemical reducing agent, which has the function of decolorizing the dyed bottom. Such as: hanging white block, stannous chloride and so on. This method can be divided into white (white) and colored (printed). The latter dyeing method makes the fabric pattern fine, but the process is complicated and the cost is high. Due to the different conditions, regions and types of printing of various printing and dyeing factories, the additives and processes used in the printing and dyeing are also very different. For example, alkali chemicals such as caustic soda, soda ash, and potassium carbonate which are commonly used in dyeing auxiliaries have different processes and objects due to different characteristics. Caustic soda is generally suitable for indigo and vat dyeing printing with slow reduction rate and coarse particles. If the sulphate blue color uses caustic soda as an alkali agent, its discharge effect is better. The amount of caustic soda is about 10% of the weight of the color paste. Since the caustic soda will reduce the stability of the white powder in the color paste, the color paste is not suitable for storage for a long time. Soda ash can be applied to most vat dyes. However, its solubility is small, up to 6%. When pure alkali is used as the alkali agent, the humidity during steaming is large to ensure the full decomposition of the white powder, which is beneficial to the dyeing and fixing of the vat dye. The basicity of potassium carbonate is the same as that of soda ash, but the solubility and hygroscopicity are good, especially when it is steamed, it can maintain the wet state, which is beneficial to fiber puffing and dye penetration, which can make the flower color bright and full and can increase the color amount. The general dosage is 9% to 10%, and the ratio of the amount of soda ash to anhydrous potassium carbonate is 78:100. Further, the alkali agent used is also a mixed base, that is, sodium hydroxide and potassium carbonate are used in combination. The amount of the vat dye base agent for the ruthenium structure is high, and the amount of the anaerobic dye base which is easy to reduce is less, and only potassium carbonate 4% to 6% can be used.
In discharge printing, the selected discharge agents are also different for different fabrics. For example, in general, rayon fabrics and reactive dyes generally use carved white blocks (hanging white blocks) or Degulin as discharge agents, and the white blocks are white blocks or crystalline powders. The scientific name is “formaldehyde hydrogen sulfate. "(NaHSO2·CH2O·2H2O). Silk fabrics are usually selected from stannous chloride, while polyester fabrics are selected from Decolin as a discharge agent. In addition, sodium hypochlorite can be used as a discharge agent for low whiteness requirements; stannous chloride can be used as a discharge agent for acid dyes and disperse dyes and cationic dyes; vulcanization and indigo dyes can be used as oxidants - potassium permanganate or sodium hypochlorite Discharge agent, but need to choose the dye used, generally use those easy to pull azo dye as the background color. Therefore, attention should be paid to the combination of the discharge agent and the dye, such as the use of acid red C, which is not resistant to stannous chloride, so it produces a decolorizing effect. Chai Linyan Blue 6B is resistant to stannous chloride, and the binder is usually made of acrylate.


In the discharge printing, more applications are now used for whitening and coloring of reactive dyes. Woven fabrics are generally used for whitening more, and knitted fabrics are more hand-painted in whitening and coloring. The whitening agent selected is generally carved white powder and Degulin (carved white zinc), but the whiteness is not high, and only the shallower can also be printed with sodium hypochlorite. The discharge agent is determined, so what kind of paste is used to meet the requirements of the print is the key. The printing paste must have good water holding capacity and strong resistance to reducing agents, and requires no reaction with the reducing agent. Only in this way can the outline and fullness of the printed product be guaranteed. The main component of DP-1 and DP-2 is polyurethane and acrylate copolymer. It has good water holding capacity, and it is not diluted with whitening agent. It has excellent effect of whitening and printing, and is easy to use. It can be used for both whitening and printing. It is suitable for rotary screen printing. It can also be used for manual plate printing. It can be processed by steaming, pressing, high temperature and baking after printing. Discharge effect. If white is removed, it can be printed by adding appropriate amount of whitening agent to DP-2. If color is selected, DP-1 can be added with appropriate amount of whitening agent and paint paste. Stir well and use. The process of reactive dye discharge printing is to select the reactive dyeing cloth which is easy to discharge before discharge. After printing, it is best to steam or press on the same day to avoid the exposed surface of the printing surface for a long time. Discharge and dyeing failure; before steaming and pressing, do not press the dyed and printed fabrics on each other to avoid sticking of the dyeing paste. Even after steaming and pressing, the printed fabric should be cooled and fully After drying, they can be stacked on each other; sometimes the main color fades after washing, mainly because the selected dyes and color pastes are improper. If the paint paste is used, the amount of color paste and the drying time should be increased.


Among the factors affecting the discharge effect, the effects of different fabrics are also different, which should also be noted in the discharge printing. For example, the phenol, the chromophore structure and the directness of the cotton fiber have a great influence on the discharge. When the coloration product of the lake is darker and the affinity for the cotton fiber is large, it is difficult to obtain a good whitening effect. The whitening effect is generally determined by the phenol, and the relative stability of the azo component to the reducing agent is different. Although it also affects the whitening effect, it is subordinate to the former. When dyeing, the floating color will affect the discharge effect, and the floating color will be poorly discharged. In order to reduce the floating color, it is important to control the suitable coupling ratio of the phenol and the chromophore and the appropriate color development conditions, such as controlling the appropriate pH value and adding an appropriate penetrant. In addition, the post-treatment should be fully washed, but it should not be vigorously soaped. The oxidant is eaten after dyeing and the fabric is immersed in an oxidizing agent before printing. In the discharge printing, due to the influence of the reducing agent, the ground color may be damaged to some extent during the printing and steaming process. This phenomenon is called relief. Especially the old flower tube, the surface finish is reduced, the color is not easy to scratch, and even the ground color is severely dark or white hair is produced. This embossing phenomenon can be prevented by eating an oxidizing agent.

In addition, pay attention to the following points in the discharge printing:

(1) The paste selected for discharge printing must have certain acid resistance, additive resistance, good storage stability and easy release. Moreover, it requires good permeability, uniform printing and clear outline; after the fabric is printed with the dyeing pulp, it should be fully dried to prevent color matching.

(2) The printed fabric should not be put on for a long time and should be steamed in time. If the steaming is insufficient, it can be steamed once again to effectively improve the dyeing effect.
(3) Dyeing and dyeing printing should use dyes with better whitening effect. If necessary, some dyes with moderate whitening effect can be used to adjust the color light. Dyes with poor whitening effect should not be used.
(4) The dyed printed fabric should be fully washed with water and soaped after steaming to improve the printing effect of the discharge printing. This is an important factor affecting the whiteness of discharge.
(5) The design of the original should be designed in consideration of the characteristics of the discharge printing. Generally speaking, the pattern suitable for the discharge printing has a large area of ​​deep color printing, fine white flowers and some complicated patterns.