Dyeing Mechanism of Direct Dyes | Working Procedure of Dyeing with Direct Dyes
Direct dyes are mainly applied on Cellulosic fibers. It is Anionic dyes, it is also called substantive dye. Direct dyes are one of the most versatile classes of dyestuff applicable to cellulose, wool, nylon fibers. Although direct dyes have deficiency in wet colorfastness it exhibits a wide range of fastness properties as shown by their use in apparel, upholstery, lining, and automotive fabrics. They can be directly died from simple solutions in water. That’s why these dyes are called direct dyes. They have an affinity for cellulose so can also be known as substantive dyes. Congo red was the first direct dye which was discovered in 1894. Introduction of reactive dyes was welcomed because its was not expensive to implement. Direct dyes are marketed under different brand names by different dyestuff manufactures.
Dyeing Mechanism of Direct Dyes:
The dyeing mechanism for the application of direct dyes to cellulose fibre involves the adsorption, diffusion, and migration over fiber. Several factors persuade the dyeing mechanism; however, most important is the cellulose fibre structure, morphology and the use of electrolytes. When the cellulosic fibre is immersed into the water the amorphous regions of the fibre swell to produce small pores in the order of 20-100A units- the smaller size dye molecules diffuse into the fibre structure through these pores. The addition of electrolytes (e.g. Sodium chloride, sodium sulfate) assists the diffusion and exhaustion of direct dye anionic by neutralizing the negative surface charge of cellulosic fibers. Then the dye anions become affixed to the cellulosic fibre through hydrogen bonding and van der Waals forces.
Typical Recipe of Direct Dyeing:
|pH||Neutral to alkaline|
Working Procedure of Dyeing with Direct Dyes:
Set the dye bath with substrate at room temperature. Add dye solution with auxiliaries and raise the temperature to 90°C. Run the bath for 15-20minutes and add salt step by step according to dye bath concentration, higher the depth of shade need more salt concentration. This is better to add this salt after reaching the temperature to boiling point since at this period the maximum penetration is achieved all over the substrate. Run the dyebath for 30-50minutes at 90-95°C for complete the dyeing cycle. Cool down the dyebath temperature to 60-70°C. Drop the bath and rinse. Carry on the aftertreatment process to improve wet fastness.