Printing a slurry containing a reducing agent or an oxidizing agent on the fabric that has been dyed to destroy the ground color and partially expose white or colored ground pattern. 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.
Prepress and discharge agents
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 ground. Therefore, the coupling process conditions must be strictly controlled; secondly, it cannot be soaped after color development, because the dye particles will aggregate and increase during the soaping process, making it difficult for the reducing agent to decompose and make it difficult to discharge the dye.
at In discharge printing, the printing paste on the surface of the flower tube is difficult to scrape off, causing the parts without flower shapes to be stained with a small amount of carving white powder, thus destroying the ground color and causing relief or pattern bleeding.
In order to prevent these defects, the dyed ground color can be padded with oxidants 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. Discharge agent
Pull out The dyeing agent is mainly the strong reducing agent cartilage powder. In addition, it also includes extraction 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 dark brown powder should generally be selected based on the following principles, that is, the stability of the insoluble azo dye to the reducing agent, the color discharge Shilin dye The amount of carving white powder required, as well as the depth of the flower tube carving and the arrangement of the flower tube, 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 steamed, anthraquinone Reduced by reducing agent to hydroanthraquinone, hydrogen>Drying, steaming and post-processing
Fabric should be dried in time after printing Dry, the drying time should be short, and ventilate and cool after drying to avoid white powder.
Failure due to thermal decomposition. 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 color fabrics that can be used for discharge printing, direct copper salt dyes, azo reactive dyes, indigo vat dyes, and azo disperse dye ground color fabrics can all be used for discharge printing, but their applications are not as good as A wide range of insoluble azo dyes.
The influence of fabrics on discharge dyeing
at Among the factors that affect the discharge effect, different fabrics have different effects, which should also be paid attention to in discharge printing.
For example, the color phenol, color base structure and directness to cotton fiber have a greater impact on discharge dyeing. When the color of the lake decomposition product is darker, it will When the affinity of cotton fiber is high, it is difficult to obtain a good whitening effect. The whitening effect is generally determined by phenol. 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. Floating colors during dyeing will affect the dye discharge effect. When there is a lot of floating colors, the dye discharge effect will be poor. In order to reduce floating color, it is very important to control the appropriate coupling ratio of phenol and color base and the appropriate color development conditions, such as controlling the appropriate pH value and adding appropriate penetrant.
In addition, post-processing should be fully washed, but violent soaping should not be done. The oxidizing agent is used to pad and dry the fabric with oxidizing agent after dyeing and before printing. During discharge printing, due to the influence of reducing agents, the ground color may be damaged to a certain extent during the printing and steaming processes. This phenomenon is called relief. Especially for old flower tubes, the surface smoothness is reduced, the color is easy to be scratched and the color is unclear, and the ground color may even become seriously dull or produce white hair. This relief phenomenon can be prevented by taking oxidants.
Pay attention to the following points when printing discharge dye
(1) The paste used for discharge printing must have certain properties Excellent acid resistance, additive resistance, good storage stability and easy depasting, etc. It also requires good permeability, uniform printing, and clear outlines; after the fabric is printed with discharge paste, it should be fully dried to prevent color matching.
(2) The printed fabric should not be left for a long time and should be steamed in time. If one steaming is not enough, you can steam it again to effectively improve the discharge effect.
(3) Discharge printing dyeing ground should try to use dyes with better whitening effect. If necessary, some dyes with medium whitening effect can be appropriately selected to adjust the color light. Dyes with poor whitening effect should not be used.
(4) Discharge printed fabrics should be fully washed and soaped after steaming to improve the printing effect of discharge printing. This is an important factor affecting discharge whiteness.
(5) When designing the original, the characteristics of discharge printing should be considered when designing the pattern. Generally speaking, the pattern suitable for discharge printing has a large area of dark ground color. Prints, fine white flowers and some intricate patterns.
