Dye characteristics
Direct dyes contain water-soluble groups such as - SO3Na and - COONa, and their solubility significantly increases with increasing temperature. For direct dyes with poor solubility, pure alkali can be added to assist in dissolution.
Direct dyes are not resistant to hard water and can mostly combine with calcium and magnesium ions to form insoluble precipitates, causing color spots on dyed fabrics. Therefore, direct dyes must be dissolved in soft water. If the hardness of dyeing water in production is on the high side, soda ash or Sodium hexametaphosphate can be added, which is not only conducive to the dissolution of dyes, but also has the function of softening water.
Direct dyes have higher directness towards cellulose fibers than other dyes. This is mainly due to the large molecular weight of direct dyes, linear molecular structure, good symmetry, long Conjugated system, good homoplanarity, and large Van der Waals force force between dyes and fiber molecules. At the same time, direct dye molecules contain groups such as amino, hydroxyl, and azo groups, which can form hydrogen bonds with hydroxyl groups in cellulose fibers and amino groups in protein fibers, further improving the directness of the dye.
When dyeing cellulose fibers with direct dyes, salt plays a dyeing promoting role. The dyeing promotion mechanism is that direct dyes dissociate into pigment anions in solution and dye cellulose fibers. Cellulose fibers also carry negative charges in water, and there is a charge repulsion force between the dyes and fibers. Adding salt to the dyeing solution can reduce the charge repulsion force, increase the dyeing rate and percentage. The dyeing promotion effect of different direct dye salts varies. The salt effect of direct dyes with more sulfonic acid groups in the molecule is significant. When promoting dyeing, the salt should be added in batches to ensure uniform dye uptake. Direct dyes with low dye uptake require the addition of more salt, and the specific amount can be determined based on the type of dye and dyeing depth. Light colored products with high requirements for levelness should reduce the amount of salt appropriately to avoid local uneven dyeing and dyeing defects such as color spots.
Temperature effect
The effect of temperature on the dyeing performance of different dyes varies. For direct dyes with high dye uptake rate and good diffusion performance, the deepest color is obtained at 60-70 ℃, while the dye uptake rate decreases above 90 ℃. When dyeing with this type of dye, in order to shorten the dyeing time, the dyeing temperature is set at 80-90 ℃. After a period of dyeing, the temperature of the dye solution gradually decreases, and the dye in the dye solution continues to dye the fibers to improve the dye uptake rate. For direct dyes with high aggregation degree, slow dye uptake rate, and poor diffusion performance, increasing temperature can accelerate dye diffusion, increase dye uptake rate, and promote dye absorption in the dye solution, thereby increasing the dye uptake percentage. The temperature at which the highest percentage of dye uptake is obtained during conventional dyeing time is called the highest dye uptake temperature. According to the different highest dyeing temperatures, direct dyes are often divided into low-temperature dyes with the highest dyeing temperature below 70 ℃, medium temperature dyes with the highest dyeing temperature between 70-80 ℃, and high-temperature dyes with the highest dyeing temperature between 90-100 ℃ in production. In production practice, cotton and viscose fiber knitted fabrics are usually dyed at around 95 ℃, while silk knitted fabrics have a lower dyeing temperature because excessive temperature can damage the fiber luster. The optimal dyeing temperature is 60-90 ℃. Reducing the dyeing temperature appropriately and extending the dyeing time is beneficial for production.
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