Publish Time: 2021-01-29 Origin: Site
Direct dyes are dyes that can be heated and boiled in neutral and weak bond phase media without the help of mordants. Direct dyes are formed by the hydrogen bonding van der Waals force between the direct dye and cotton fiber. Mainly used in fiber, silk, cotton spinning, leather and other industries, but also in papermaking and other industries. Direct dyes can be divided into azo and stilbene types according to their structure. According to its application classification, there are mainly ordinary direct dyes, direct light fast dyes and direct azo dyes.
Direct dyes have water-soluble groups such as sulfonic acid group (-SO3H) or carboxyl group (-COOH) arranged in linear molecular structure. The aromatic ring structure is in the same plane, so the direct dye has a greater affinity for cellulose fibers. It can be dyed directly in a neutral medium. As long as the dye is dissolved in dry water, it can be dyed. The dye is absorbed by the fiber to the surface in the solution, and then continuously diffuses to the amorphous area of the fiber, forming a hydrogen bond and a combination of van der Waals force with the fiber macromolecule. Its derived dyes include direct lightfast dyes and direct copper salt dyes.
Direct dyes contain water-soluble groups such as -SO3Na and -COONa, and their solubility increases significantly with the rise of temperature. For direct dyes with poor solubility, soda ash can be added to aid solubility. Direct dyes are not resistant to hard water, and most of them can combine with calcium and magnesium ions to form insolubility.
Precipitation causes stains on dyed fabrics, so direct dyes must be dissolved in soft water. If the dyeing water has a high hardness in the production, soda ash or sodium hexametaphosphate can be added, which is beneficial to the dissolution of the dye and has the effect of softening the water.
Direct dyes are more direct to cellulose fibers than other dyes. This is mainly due to the large molecular weight of direct dyes, linear molecular structure, good symmetry, long conjugate system, good coplanarity, and large van der Waals forces between dye and fiber molecules. At the same time, the direct dye molecule contains amino groups, hydroxyl groups, azo groups and other groups, which can form hydrogen bonds with the hydroxyl groups in cellulose fibers and the hydroxyl groups and amino groups in protein fibers to further improve the directness of the dye.
When direct dyes are used to dye cellulose fibers, salt plays a role in promoting dyeing. The dye-promoting mechanism is that the direct dye dissociates into the dye anion and dyes the cellulose fiber. The cellulose fiber is also negatively charged in water. There is a charge repulsion between the dye and the fiber. Adding salt to the dyeing solution can reduce the charge. Repulsion, increase the dyeing rate and the dyeing percentage. Different direct dye salts have different accelerating effects. The salt effect direct dyes with more sulfonic acid groups in the molecule, the salt has a significant effect of promoting dyeing, and the salt should be added in batches to ensure even dyeing of the dye. Direct dyes with a low percentage of dyeing require more salt, and the specific dosage can be determined according to the type of dye and the depth of dyeing. For light-colored products with high level dyeing requirements, the amount of salt should be appropriately reduced to avoid local uneven dyeing and staining defects.
The influence of temperature on the dyeing performance of different dyes is different. For direct dyes with high dyeing rate and good diffusion properties, the darkest color will be obtained at 60-70℃, but the dyeing rate will decrease above 90℃. When this kind of dye is dyed, in order to shorten the dyeing time, the dyeing temperature is 80-90℃. After dyeing for a period of time, the temperature of the dye liquor will gradually decrease, and the dye in the dye liquor will continue to dye the fiber to increase the dye uptake percentage. For direct dyes with a high degree of aggregation, slow dyeing rate, and poor diffusion performance, increasing the temperature can speed up the diffusion of the dye, increase the dyeing rate, and promote the exhaustion of the dye in the dye liquor, and increase the dyeing percentage. During the regular dyeing time, the temperature at which the highest percentage of dye uptake is obtained is called the highest dye uptake temperature. According to the difference of the highest dyeing temperature, direct dyes are often divided into low-temperature dyes with the highest dyeing temperature below 70℃, medium-temperature dyes with the highest dyeing temperature of 70-80℃ and those with the highest dyeing temperature of 90-100℃. High temperature dyes. In production practice, cotton and viscose fiber knitted fabrics are usually dyed at around 95°C, while the dyeing temperature of silk knitted fabrics is relatively low, because too high temperature will damage the luster of the fiber, and the best dyeing temperature is 60-90°C. Appropriately lowering the dyeing temperature and extending the dyeing time are beneficial to production.