The colored yarns used in dark-dyed cotton-dyed fabrics, such as labor cloth and Yuangong, are mostly dyed with sulfur dyes. Sulfur dyes are cheap, dyed with full color and good fastness to washing, and are one of the common dyes for yarn dyeing. Sulfur dyes do not dissolve in water due to their sulfur content in the molecular structure, and must be reduced by sodium sulfide during dyeing . In recent years, there have been the appearance of water-soluble sulfur dyes and anti-fragile sulfur blacks, which has given new prospects for the application of sulphur dyes.
Dyeing process points
- Former yarn dyeing process is to ensure that the key to sulfur dyeing yarn dyeing quality of matt and silk points, the individual light-colored yarn dyed bleached needed.
If scouring is unsatisfactory, the yarns will have poor hair quality, or the concentration of mercerized lye will be different, and the bleached yarns containing more chlorine will directly affect the dyeing quality. Therefore, the white yarn must be inspected carefully before dyeing, and the whiteness is required to be uniform. There must be no yellow spots, dark spots, alkali spots, rust spots, stains, and chlorine. After dewatering, the white yarn must be covered with a damp cloth to prevent the surface from drying out. For the white yarn that has been piled up for too long, it must be washed once before dyeing. Otherwise, a white strip or a light stripe will be produced after dyeing.
For the rayless scouring that uses only water without simmering, the time for stacking should not be too long due to the fact that the cotton grease and wax have not been exhausted. Especially in the hot season, it is easy to degenerate. Before the dyeing, 80-70°C hot soda is needed again. Liquid treatment, serious still need to be pickled, otherwise the dyed yarns tend to appear pale and pale.
Due to the slow dyeing of sulphur dyes, it is suitable to use high-temperature dyeing, which is not only beneficial to the continuous dissolution and thinning of the dye particles, but also allows the fiber yarns to penetrate and diffuse rapidly into the fiber inner layer, and also allows the fiber yarns to be heated and expanded loosely to obtain a good leveling effect. The dyeing temperature of general sulphur dyes is approximately 75-80°C. Manual dyeing of high temperature is inconvenient and the temperature is controlled at 60-70°C. If the temperature is too low, dye uptake will be affected, the color will become lighter, and the dye concentration in the raffinate will gradually increase, resulting in a color difference between the cylinder and the cylinder. However, some dyes are suitable for low-temperature dyeing. The higher the temperature is, the lower the color is. This is mainly due to the fact that the dye directly reduces the directness of the fiber after the temperature rises.
Light-colored yarns are generally used for low-temperature dyeing, such as dyeing vulcanized bleaching ash on the weft of a work cloth and dyeing at room temperature. In addition, the dyeing temperature is closely related to the mercerizing, dullness, yarn dyeing machinery of cotton yarns. Generally, the dyeing temperature of the unglazed yarn is high and the mercerized yarn is low. High-temperature and high-pressure dyeing yarns have a closed device, and the dyeing temperature can be increased to above 100°C. However, if the temperature of the reciprocating yarn dyeing machine is too high, the yarn can easily float, causing chaos and yarn breakage. Therefore, the dyeing temperature is mostly 70 to 80°C. .
When the sulfur dye is dyed dark, due to the high concentration of dye in the dye bath, the oxidation rate is fast. After dyeing, the unfixed dye on the yarn is in contact with the air to produce a lake and is adsorbed on the surface of the colored yarn, which may easily cause red stripes and gold. Copper and other defects make the color fastness decreased. In order to obtain better leveling effect and fastness, the general dyeing method is to immediately re-reduce the yarn taken out of the dye bath into another reducing solution to reduce the temperature of the dyed yarn itself and be adsorbed. Dye concentration, delay the oxidation rate, achieve uniform color, stable quality, or remove the floating color on the surface of a part of the colored yarn through the reducing solution. This process is often referred to as boring cylinder.
In the dyeing of cotton yarns with sulphur dyes, leucochromatism is commonly used. Black and navy colors are the most common. The dye uptake is generally 25% to 30% . Therefore, more dye remains in the dyeing bath, so the sulphur dyes are dyed. After the residue is usually used continuously. The ratio between the dye input volume of the head cylinder and the refill volume of the cylinder-up dye is often referred to as the number of head cylinder dye addition. If the number of additions is incorrect, it will affect the depth of color development of the head cylinder and the continuous cylinder.
When dyeing light-colored yarns, the number of head cylinder additions can be appropriately reduced, and the dyeing of darker yarns should be increased. When the dye uptake rate was 25.4%, the number of dyes added to the head tank was approximately 2.5 to 4 times that of the group of the continuous dyes. There are many factors that affect the dyeing of sulfur dyes. In addition to the amount of dyes and sulfurized alkali, the yarn blending, scouring quality, water hardness, and sulfide content directly affect the yarn dyeing rate.
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