Discharge printing concept
On fabrics that have been dyed, printing contains reducing agents Or the slurry of oxidant will destroy the color and partially expose white ground or colored patterns. Usually the former is called white drawing, and the latter is called color drawing.
There are many ground color dyes that can be used as discharge dyes, such as insoluble azo dyes, reactive dyes, direct dyes, etc. Since vat dyes themselves are in a strong alkali reducing agent medium, they are most suitable as discharge dyes in ground color discharge dyeing of these dyes. The discharge printing process is complicated, prone to defects, and costly. However, discharge dyeing has a better effect than resist dyeing because the pattern is more delicate, the outline is clear and the edges are not exposed. Because this printing method mostly uses carved white powder, it is also called carved printing. In order to master the system, the editor has compiled a directory to facilitate learning.
Pre-treatment and discharge auxiliaries
1. Pre-press Processing
Prepress processing includes ground color dyeing and pad oxidizer. The ground color dyeing process is basically the same as the commonly used ice dye dyeing process, but with slight differences.
First of all, we must pay attention to generating as little floating color as possible. Too much floating color will affect the whitening effect, resulting in a large amount of discharge agent being consumed on the floating color, which will affect the interaction with the ground color. Therefore It is necessary to strictly control the coupling process conditions; secondly, it cannot be soaped after color development, because the dye particles will aggregate and increase during the soaping process, making it difficult to decompose the reducing agent and make it difficult to discharge the dye.
In discharge printing, the printing paste on the surface of the tube is difficult to scrape off, causing the parts without flower shapes to also Stained with a small amount of carving white powder, the ground color will be destroyed and the relief or pattern will bleed.
In order to prevent these defects, the dyed ground color can be padded with oxidizing agent first and then dried before printing. Most of the oxidants use anti-dyeing salt S, which can consume the white powder. The amount of anti-dying salt S should not be too large, otherwise it will affect the dye discharge effect. The dosage is generally 2-3g/L, and for easy-to-pull out or large-area patterns, increase it to 5-6g/L.
2. Dye-discharge agent
The main dye-discharge agent is the strong reducing agent ivory powder , in addition to pulling aids and alkali agents. The molecular formula of caraway powder is: HCHO·NaHSO2·2H2O. It has no odor under normal circumstances. If it decomposes and deteriorates, it will smell like garlic. Eagle white powder does not show its strong reducing properties at normal temperature. After exceeding 60℃, it gradually begins to decompose and shows its strong reducing properties. It decomposes quickly under alkaline conditions.
The dosage of ivory powder should generally be selected based on the following principles, that is, the stability of the insoluble azo dye to the reducing agent, the amount of oxalin required for the dyeing discharge Shilin dye The number, as well as the depth of the flower tube carving and the arrangement of the flower tubes, etc.
In order to improve the discharge effect, discharge aids are often added to the discharge printing paste. Generally, anthraquinone and whitening agent W are used, among which anthraquinone is widely used.
Anthraquinone is also called a hydrogen-conducting agent, and its pulling-out mechanism is:
When steaming, anthraquinone is removed by the reducing agent Reduced to hydroanthraquinone, hydroanthraquinone has a certain reducing ability, which can reduce the dye and turn itself into anthraquinone again. This reaction continues until the dye is sufficiently reduced, so the anthraquinone acts as a catalyst. Adding anthraquinone can improve the whiteness and make the whiteness more stable in the air.
An alkali agent must be added to the discharge dyeing slurry, because the decomposition product of the insoluble azo dye after reduction must become a sodium salt under the action of an alkali agent before it can be dissolved in water and washed. remove. The alkali agent can also neutralize the acid produced by the decomposition of caraway powder, giving it a strong reducing effect. In addition, the colorant vat dye can become a leuco body with dyeing ability in an alkali agent. Commonly used alkali agents include caustic soda, soda ash, baking soda, potassium carbonate, etc. Caustic soda is generally used to remove white pulp. The dosage of alkali agent is generally about 4%.
Post-printing treatment
1. Drying and printing after printing Post-drying requires fast and thorough drying. Because gallbladder powder is easy to decompose under humid air conditions, the decomposition of gallbladder powder will be affected by the drying temperature. The best drying temperature is around 110°C. After drying, it should be ventilated and cooled before dropping the cloth, and steamed as soon as possible.
2. The steaming of printed cloth and the steaming of printed cloth are carried out in the reduction steaming machine. This process decomposes the white powder, gives full play to its reducing effect, decomposes and destroys the ground color, and dissolves and diffuses the colored dye into the fiber. The steaming temperature is 102~105℃, the time is 7~8 minutes, it is best to operate in isolation from air, and the air content is less than 0.3%. In order to maintain the humidity in the steamer, water is generally stored at the bottom of the steamer for evaporation.
3. Oxidation and oxidation treatment are performed to fix the vat dye in the pattern. Generally, dyes with small flowers and easily oxidized are oxidized by water padding and ventilation, and dyes with large pattern areas or those that are difficult to oxidize are oxidized with sodium perborate. For details, please refer to the chapter on vat dye direct printing.
4. Water soaping. When performing water soaping, it is necessary to fully remove the ground color decomposition products in the pattern, so that the white flowers are white and the flowers are brightly colored. At the same time, large areas of ground color can be Increase the brightness during soaping and further remove floating colors. Soaping conditions: soap 3~5g/L, soda ash 2g/L, temperature above 95°C, wash for 3~5 minutes, and then wash thoroughly with water.
White-out printing
White-out printing can be divided into neutral The difference between whitening and alkaline whitening processes is that they are used in printing paste.�Add alkali. Neutral whitening is better than alkaline whitening for some ground colors, but sometimes this is not the case and must be determined based on ground color testing.
When dissolving the gallbladder powder, you can heat it appropriately (about 60℃) and fully control the amount of water to prevent exceeding the total amount. Add caraway powder into the paste and mix well, then add caustic soda, and then add the whitening agent dissolved in warm water. Before printing, add anthraquinone to the whitening paste.
Color discharge printing
The colorants used in color discharge printing are generally Vat dyes. The alkali agent and azo powder in the insoluble azo dye discharge printing paste not only destroy the ground color, but also reduce the vat dye to form a leuco body, which can be dyed on the fiber. Therefore, vat dye discharge can be used to dye the insoluble azo dye. Since the preparation of vat dye printing paste needs to reduce the vat dye to form a leuco body first, and the stability of vat dye paste is good, the basic slurry with a higher concentration is often prepared in advance and stored for later use. When preparing the color paste, weigh a certain weight of the basic paste according to the process requirements, then add the diluted paste that does not contain dye and only contains additives. After dilution, it can be printed.
The preparation of reduced basic slurry can be divided into pre-reduction method and non-pre-reduction method according to the performance of the dye. If potassium carbonate or soda ash is used as an alkali agent in the basic color paste, the caustic soda should also be changed to the corresponding alkali agent in the diluted paste.
Drying, steaming and post-processing
After fabric printing It should be dried in time, the drying time should be short, and ventilated and cooled after drying to avoid thermal decomposition of the white powder and its failure.
The steaming temperature is 102~104℃ and the time is 7~10min. For oxidation, please refer to the dyeing chapter. Post-processing is the same as the general process.
In addition to insoluble azo dye ground-colored fabrics that can be used for discharge printing, direct copper salt dyes, azo reactive dyes, indigo vat dyes, and azo disperse dyes Colored fabrics can be used for pad printing, but their applications are not as extensive as insoluble azo dyes. </p
