Sulfur dye yarn dyeing
Dark varieties of yarn-dyed cotton such as denim, Yuan Most of the colored yarns used by tributes and others are dyed with sulfur dyes. Sulfur dyes are cheap, have rich colors after dyeing, and have good washing fastness. They are one of the commonly used dyes for yarn dyeing. Sulfur dyes contain a large amount of sulfur in their molecular structure and are insoluble in water. They must be reduced and dissolved with sodium sulfide during dyeing. In recent years, water-soluble sulfur dyes and anti-brittle sulfur black have emerged, giving new prospects for the application of sulfur dyes.
Key points of dyeing process
— The pre-dyeing treatment of yarn is the key to ensure the quality of yarn dyeing. The dyeing of sulfur dyes can be divided into matte and mercerized. Some yarns dyed with light colors need to be bleached.
If the scouring is poor, the wool effect of the yarn will be poor, or the concentration of mercerizing alkali solution will be different, or the bleached yarn will contain more chlorine, etc., which will directly affect the dyeing quality. Therefore, the white yarn must be carefully inspected before dyeing, and the whiteness must be uniform and free of yellow spots, black spots, alkali spots, rust spots, stains, and chlorine, etc. The dehydrated white yarn must be covered with a damp cloth to prevent the surface from drying. White yarn that has been stacked for too long must be washed again before dyeing, otherwise local white stripes or shallow color defects will occur after dyeing.
For matte yarn that has not been boiled with alkali but only boiled with water, because the cotton grease and wax grease have not been completely removed, the stacking time should not be too long, especially in high temperature seasons, it is easy to deteriorate. Before dyeing, it needs to be re-treated with 80-70℃ hot soda lye. In severe cases, it needs to be pickled. Otherwise, the dyed yarn is prone to light color and whitening.
Since sulfur dyes are dyed slowly, it is suitable to use high temperature dyeing, which is conducive to the continuous melting and thinning of the dye particles and their rapid penetration into the inner layer of the fiber. Penetration and diffusion can also make fiber yarns expand and loosen when heated, achieving better leveling effect. Generally, the dyeing temperature of sulfur dyes is approximately 75 to 80°C. It is inconvenient to operate at high temperatures by hand dyeing yarn, so the temperature is controlled at 60 to 70°C. If the temperature is too low, it will affect the dyeing process, make the color lighter, and cause the dye concentration in the residual liquid to gradually increase, resulting in color differences between vats. However, some dyes are suitable for low-temperature dyeing. If the temperature is too high, the color will be lighter. This is mainly because the directness of the dye to the fiber is reduced when the temperature rises.
Generally, light-colored yarns are dyed at low temperature. For example, the weft threads of labor cloth dyed with sulfur bleaching ash can be dyed at room temperature. In addition, the dyeing temperature is closely related to the mercerization, matte and dyeing machinery of cotton yarn. Generally, the dyeing temperature of matte yarn is high, while the dyeing temperature of mercerized yarn is low. High-temperature and high-pressure yarn dyeing has a closed device, and the dyeing temperature can be raised to above 100°C. However, if the temperature of the reciprocating dyeing machine is too high, the yarn will easily float, causing yarn chaos and yarn breakage, so the dyeing temperature is mostly between 70 and 80°C. .
When sulfur dyes are dyed in dark colors, due to the high dye concentration in the dye bath and the fast oxidation rate, after dyeing, the unfixed dye and air on the yarn Contact produces lakes and is adsorbed on the surface of colored yarns, which can easily cause defects such as red streaks and gold and copper colors, and reduce the dye fastness. In order to obtain better level dyeing effect and dye fastness, the general dyeing method is to immediately immerse the yarn taken out from the dye bath into another reducing solution for re-reduction, and gradually reduce the temperature of the dyed yarn itself and the adsorbed The concentration of the dye liquor can slow down the oxidation rate to achieve uniform color and stable quality, or remove part of the floating color on the surface of the colored yarn through the reducing liquid. This process is often called a stuffy vat.
When dyeing cotton yarn with sulfur dyes, leuco dyeing is commonly used, among which black and navy blue are the most common. The dye uptake rate is generally 25% to 30%, so the dye residue remains in the dye. There is a lot of dye in the bath, so the residual liquid after dyeing with sulfur dyes is usually used continuously. The ratio between the input amount of dye in the first cylinder and the replenishment amount of dye in the continuation cylinder is often called the bonus number of the dye in the first cylinder. If the bonus number is wrong, it will affect the color depth of the first cylinder and the continuation cylinder.
When dyeing light-colored yarn, the bonus number of the head cylinder can be appropriately reduced, while when dyeing dark-colored yarn, it should be increased. When the dye uptake rate is 25.4%, the bonus of the dye in the first tank is about 2.5 to 4 times that of the dye in the continuation tank. There are many factors that affect the dyeing of sulfur dyes. In addition to the amount of dye and alkali sulfide, the yarn’s cotton blending, scouring quality, water hardness, alkali sulfide content, etc. all directly affect the dye uptake rate of the yarn.
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