It can apply directly to the fiber as they have a strong affinity and so they are called direct dyes. Mainly these types of dyes are sodium salts of sulphonic acid or carboxylic acid, and their leading chromophoric group is azo. Direct dyes are called substantive dyes also.
Properties of Direct Dyes:
1 . The dyeing process requires electrolytes and alkaline conditions.
2. They are attached to the fiber with weak hydrogen bonds and Van der Waals’ force of attraction.
3. These types of dyes are water-soluble and anionic.
4. Generally, cellulosic fiber is dyed but protein fiber can also be used.
5. These dyes are cheap compared to other dyes.
6. Fastness properties are average.
7. Mainly, the wet fastness is poor. For improved fastness, after-treatment is required.
Mechanism of Direct Dyes:
The dyeing involves three stages: adsorption, diffusion, and migration.
In water, the cellulosic fiber’s surface becomes anionic. As the dye is also anionic, neutralizing the surface to remove the repulsion electrolyte is necessary.
Due to swelling in water, the pores of cellulosic fiber’s amorphous region get opened, and the dyes get diffused inside the pores. Dyes are fixed in the fiber with weak H-bond and Van der Waals force. So the fastness properties are not good enough.
Applications: Cotton and Viscose