Direct dyes refer to dyes that can be directly dissolved in water, have high directness to cellulose fibers, and do not need to use relevant chemical methods to color fibers and other materials.
The principle of direct dyes
Direct dyes have water-soluble groups such as sulfonic acid group (-SOH) or carboxyl group (-COOH) arranged in a linear molecular structure. The aromatic ring structure is in the same plane, so the direct dye has a greater affinity for cellulose fibers, and it can be dyed directly in a neutral medium as long as the dye is dissolved in dry water.
The dyes are adsorbed to the surface by the fibers in the solution, and then continuously diffuse to the amorphous region of the fibers, forming hydrogen bonds and van der Waals forces with the macromolecules of the fibers.
Its derived dyes include direct lightfast dyes and direct copper salt dyes.
The characteristics of direct dyes
Direct dyes contain water-soluble groups such as -SO Na and -COONa, and the solubility increases significantly with the increase of temperature. For direct dyes with poor solubility, soda ash can be added to help dissolve them. Direct dyes are not resistant to hard water, and most of them can combine with calcium and magnesium ions to form insoluble precipitates, resulting in stains on dyed fabrics, so direct dyes must be dissolved in soft water. If the hardness of the dyeing water in production is high, soda ash or sodium hexametaphosphate can be added, which is not only conducive to the dissolution of dyes, but also softens 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 conjugated system, good coplanarity, and large van der Waals forces between dyes and fiber molecules. At the same time, the direct dye molecules contain groups such as amino groups, hydroxyl groups, azo groups, etc., which can form hydrogen bonds with the hydroxyl groups in cellulose fibers, and the hydroxyl groups and amino groups in protein fibers, which further improves the directness of the dyes.
When direct dyes are used to dye cellulose fibers, salts play a role in promoting dyeing. The dye-promoting mechanism is that the direct dye dissociates into pigment anions in the solution and dyes the cellulose fiber. The cellulose fiber is also negatively charged in water, and there is a charge repulsion between the dye and the fiber. Adding salt to the dye solution can reduce the charge. Repulsion, improve the dyeing rate and dyeing percentage. The dyeing effect of different direct dye salts is different. For direct dyes containing more sulfonic acid groups in the molecule, the salt has a significant effect of promoting dyeing. When promoting dyeing, the salt should be added in batches to ensure uniform dyeing of the dye. Direct dyes with a low dyeing percentage need to add more salt, and the specific amount 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 uneven dyeing locally and dyeing defects such as colored flowers.