Textile Dyes and Dyeing

Dyes:
Dyes are coloring materials used in coloration of textiles. Most of the dyes are organic compound. They may be soluble or insoluble in water. It can also be defined as colored, unsaturated organic chemical compounds capable of giving color to substrate (textile) i.e. coloring or dyeing it.

Dye molecules are organic molecules, which can be classified as:
  1. Anionic- in which the color is caused by the anionic part of the dye molecule.
  2. Cationic- in which the color is caused by the cationic part of the dye molecule.
  3. Disperse- in which the color is caused by the whole molecule.
The first two dye molecule types are applied from an aqueous solution. The third is applied from an aqueous dispersion.
Textile dyes and dyeing
Fig: Textile dyes and dyeing
Classification of dyes:
 
Dyestuffs can be classified by many methods as follows:

  • By their ionic (e.g. cationic, anionic and nonionic)
  • By their fiber dye abilities (e.g. on cellulosic, protein, polyester, polyamide etc.)
  • By their names (e.g. Reactive dye, vat dye, acid dye, direct dye, disperse dye, basic dyes etc.)
  • By their chemical structures (e.g. Azo, anthraquinone, stilbene etc.)
  • By their origins (e.g. Natural, synthetic)
  • By their colors (e.g. Red, green, blue, black etc.)
Classification of dyes according to their names:

A. Readymade dyes:

  1. Water soluble dyes:
  • Direct dyes
  • Acid dyes
  • Basic dyes
  • Reactive dyes
  • Optical whiteners
  2. Water insoluble dyes:
  • Vat dyes
  • Sulphur dyes
  • Disperse dyes
B. Ingrain dyes:
  • Azoic colors
  • Oxidation colors
  • Mineral colors
Dye molecules is constituted of Chromophore, Chromogen, and Auxochrome.

Chromophore:
The name is derived from Greek word “chroma” meaning “color” & “phore” from protein meaning “to bear”.

The color bearing unsaturated group of a dye molecule is called chromophore group. Examples are,

Ethylene: = C = C = , Azo: - N = N - , Keto: = C = O , Nitroso: - N = O , Thioketo: = C = S

Chromogen:
A compound of a dye structure which contains both chromophore and auxochrome is called chromophore.

Auxochrome:
The Greek word “auxein” meaning “to increase” & “chroma” meaning “color”. The auxochrome (color helping group) by itself does not produce any color but depends the color of a chromogen. Examples are –OH, -NH2, -COOH, -SO3, NHR etc. There are mainly two types of auxochromes:
  • Acidic: -COOH, -OH, -SO3H
  • Basic: -NHR, -NR2, -NH2
Pigment:
Pigments are insoluble colorants with very little attraction or affinity for textile fibers. Therefore they are applied to fabric with resin or binder.

Dyeing:
The process of coloration of textiles is termed as dyeing. In this process different types of dyes are used to produce different shades at different types of fabrics. Dyeing is a distribution process in which the dye or pigment is distributed at the textiles.

Theory of dyeing:
The general theory of dyeing explains the interaction between dye, fiber, water and dye auxiliary. The dyeing process is essentially a distribution process. The dye is distributed over at least two phase systems, the dye bath and textile materials.

The dyeing process consists of three stages which are:
  1. Migration of the dye form the solution to the interface accompanied by adsorption on the surface of the fiber.
  2. Diffusion of the dye from the surface towards the center of the fiber.
  3. The anchoring of the dye molecules by covalent, ionic, van-der-walls or hydrogen bonds or other forces of a physical nature between fiber and dye molecules.
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