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How do sulfur dyes dye yarn?

Date:Nov 23, 2020

Most of the yarns used in dyed cotton fabrics, such as labor cloth and Yuangong, are dyed with sulfur dyes. Sulfur dyes are cheap, with full color after dyeing and good washing fastness. They are one of the commonly used dyes for yarn dyeing.

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Sulfur dyes are insoluble in water due to their molecular structure containing more sulfur, so they must be reduced by sodium sulfide during dyeing. Dissolve. In recent years, water-soluble sulphur dyes and anti-brittle damage sulphur black have appeared, giving new prospects for the application of sulphur dyes.


Dyeing process points


(1) Control the quality of pretreatment

The pre-dyeing treatment of yarn is the key to guarantee the quality of yarn dyeing. The dyeing of sulfur dyes can be divided into matt and mercerized, and some light-colored yarns need to be bleached. If the scouring is not good, the yarn wool effect will be poor, or the concentration of mercerizing lye will be different, and 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 is required to be uniform, and there should be no yellow spots, dark spots, alkali spots, rust spots, stains, and chlorine. The dehydrated white yarn must be covered with a damp cloth to prevent the surface from drying out. For the white yarn that has been stacked for a long time, it must be washed again before dyeing, otherwise local white or light color defects will occur after dyeing.


For the matt yarn that is not boiled in alkali but only boiled in clean water, because the cotton grease and wax have not been exhausted, the stacking time must not be too long, especially in the high temperature season, it is easy to deteriorate, and 60~70) hot soda ash needs to be reused before dyeing For liquid treatment, pickling is needed in serious cases, otherwise the dyed yarn is prone to light color and whitening.


(2) Control the dyeing temperature

Because sulfur dyes are dyed slowly, high temperature dyeing is suitable, which not only facilitates the continuous melting and fineness of dye particles, and quickly penetrates and diffuses into the inner layer of the fiber, but also makes the fiber yarn puffed and loosened by heating to obtain a better leveling effect. Generally, the dyeing temperature of sulfur dyes is 75~80℃, and manual dyeing is inconvenient to operate at high temperature. The temperature is controlled at 60~70℃. If the temperature is too low, it will affect the dye uptake, the color will become lighter, and the dye concentration in the residual liquid will gradually increase, resulting in a color difference between the tank and the tank. However, some dyes are suitable for low-temperature dyeing, and the color is lighter when the temperature is too high. This is mainly because the directness of the dye to the fiber decreases after the temperature rises. Generally, light-colored yarns are dyed at low temperature. For example, the weft of denim yarn can be dyed at room temperature by dyeing vulcanized dark gray. In addition, the dyeing temperature is closely related to the mercerizing, matting, and dyeing machinery of cotton yarn. Generally, the dyeing temperature of matte yarn is high, while mercerized yarn is lower. High-temperature and high-pressure yarn dyeing has a closed device, and the dyeing temperature can be increased to above 100°C. If the temperature of the reciprocating yarn dyeing machine is too high, it will easily cause the yarn to float up, causing yarn disorder and yarn breakage, so the dyeing temperature is mostly 70-80.


(3) Control the concentration of dye bath

When sulphur dyes are dyed in dark colors, due to the high dye concentration in the dyeing bath and the fast oxidation rate, after dyeing, the unfixed dyes on the yarn come into contact with air to produce lakes and are adsorbed on the surface of the dyed yarns, which can easily cause red bars and gold Defects such as copper color reduce the color fastness. In order to obtain a better level dyeing effect and dye fastness, the general dyeing method is to immediately immerse the yarn taken out of the dyeing bath into another reducing solution to restore it again, and gradually reduce the temperature and absorption of the yarn itself. The concentration of the dye solution can delay the oxidation rate, achieve uniform color and stable quality, or remove part of the floating color on the surface of the colored yarn through a reducing solution. This process is often called a stuffy cylinder.


When dyeing cotton yarn with sulfur dyes, leuco dyeing is commonly used. Among them, black and navy blue are the most common. The dye uptake is generally 25% to 30%. Therefore, there are more dyes remaining in the dyeing bath, so sulfur dyes are dyed. The remaining liquid is usually used continuously. The ratio between the amount of dye in the head tank and the replenishment amount of dye in the next tank is often called the addition number of the first tank dye. If the addition number is not correct, it will affect the color depth of the first tank and the second tank. When dyeing light-colored yarns, the head tank bonus can be appropriately reduced. It should be increased when dyeing dark yarns. When the dye uptake rate is 25.4%, the addition number of the first tank dye is about 2.5 to 4 times that of the secondary tank dye.


There are many factors that affect the dyeing of sulfur dyes. In addition to the amount of dye and alkali sulfide, the cotton blending, scouring amount, water hardness, and alkali sulfide content of the yarn directly affect the dye uptake of the yarn.


Example: Mercerized yarn dyed with sulfur black on reciprocating yarn dyeing machine, yarn weight 25kg, bath ratio 1:40.


The process flow is as follows:

Water scouring (100℃, 3h) → mercerizing (caustic soda 21.55%) → neutralization (98% sulfuric acid 2.5~3g/L) → warm water (40~50℃) → dehydration → stretch yarn (stretch loose, stretch straight) → Skewer → dyeing (70~75℃, 20min) → warm vat (35~40℃, 5min) → water washing (cold wash, flowing) → drain → anti-brittle treatment (25~30℃, 10min) → discharge vat → Wrapping → Dehydration → Stretching