Direct dyes | Textile dyes | Textile chemistry

Introduction: 

Direct dyes. The name itself indicating direct dyes means, the dyes are applied directly on the textile fabrics. What are those textile fabrics? Fabrics made from Cellulosic fibers. Examples of cellulose containing fibers are cotton, viscose, rayon, cuprammonium, acetate,.

Dyebath requirement:

          The solution required for direct dye is water. The simple water solution is enough to dye direct dyes. The direct dyes are water soluble dyes. There is no need of converting from water insoluble into water soluble to prepare dye solutions like Vat dyes or Acid dyes. In case of acid dyes and vat dyes, they are water insoluble dyes and are converted into water soluble while preparing dye solution. Again they are reconverted into water soluble dyes, when they are absorbed by the fibers into their structure. But in the case of direct dyes, because of they are water soluble, the affinity between the cotton fiber or viscose fiber or rayon fiber or cuprammonium fiber, whichever the cellulosic fibers will be more. Because of this high affinity, the dye takes place.

          Another important point here is to consider that, the direct dyes are more affinity towards cellulosic fibers than affinity towards the water. Because of that, the dye go and penetrate into the surface of the cotton fibers.

Dyeing Mechanism:

          What mechanism is there in dyeing of cellulosic material with direct dyes?.Direct dyes,when dissolved in water,it ionizes.How? like sodium chloride.Dyes ionizes as anions.Sodium ionizes as cations.After ionization, the affinity takes place between the dye and the fiber.This is the principle of dyeing of direct dyes involved.This leads to the manufacturing of the sulphonic acids.Sulphonic acid is manufactured by the dyers.These sulphonic acids are generally converted into the solutions,dye solutions.Sulphonic acids are less soluble than their sodium salts.And also,that have less affinity to towards the cellulosic fiber than sodium salts.Sulphonic acids are produced as a direct dyes by the dyers.

          These sulphonic dyes when involved in the application of textile materials,that is cellulosic materials,they ionizes as sodium salts,their corresponding sodium salts.

Dye exhaustion:

          when the complete dyeing takes place ?,that is the total amount of dyes penetrate into the fabric completely.The amount means material to liquor ratio for dyeable textile material weight.How much amount of dye is taken by the textile material.This is the question.Many times the dyes are not completely absorbed by the fibers or penetrate into the fibers.That is,the dyeing is not taken place properly due to the less affinity between dye and material.It depends upon the different sulphonic acids used to dye different fiber materials.They produce their corresponding sodium salts in dye solutions.To increase the affinity,that is to increase the dyeability of the dyes,soda ash is added into the solution.Soda ash means sodium carbonate is added.This sodium carbonate will increase the affinity between the fibers and the dyes.That is increase in the dye absorption.

          If the exhaustion of the dyeing of fiber is not completely takes place, what will happen? Most of the dye remaining's will be left in the dye bath as residues. Due to this, incomplete dyeing will take place, shade variation will occur on the dyed material. And also waste of the dye leads to the loss in dye cost. To avoid these drawbacks, sodium carbonate is added into the solution to complete the dyeing performance of the direct dyes.

 

Note: This article will be updated as and when required.