Author: Dr Indu Chopra
Dyes use is very old and has been used by man for many thousands of years. Dyes are colored ionizing organic compounds that go into solution during application process by selective absorption of light. As these compounds are relatively stable to light and washing so these are used to color wide range of substrates including paper, drugs, hair, plastics, rubber, cosmetics, fibers and other materials.
Dyeing is a process in which color is imparted uniformly to a material in such a way that it becomes part of the material. For dyeing, three factors considered important which should be kept in mind before carrying out the procedure. These factors include:
- The fiber that has to be dyed, which can be natural or synthetic.
- Dyestuff to be used for the purpose, which is determined by the specific fiber
- The medium in which the dyeing is to be done.
Generally it depends upon the specific fiber and desirable fastness properties to decide the type of dyes to be used for a particular dyestuff. In principle, there should be some affinity between the fiber and the dyestuff as the same dye can react differently with different fibers. The affinity depends upon the chemical and physical nature of the fiber and dyes. Depending on different parameters, textile dyes can be classified on the basis of class, chemical structure, their application, fastness properties, application etc.
Though the topic deals with the classification of the textile dyes, we are classifying the dyes on the basis of class only.
Types of dyes according to the class include the following:
- Direct Dyes or substantive Dyes : these dyes use ionic and/ or electrolyte as fixative. These dyes color the fabric directly in a hot dye bath to give bright colors. They bond to fibers by electrostatic force and hence are not fast to light or washing. These dyes are used to color cotton and other cellulosic fibers.
- Acid dyes : these dyes are water soluble anionic dyes. As these dyes are used for nitrogenous fibers of fabrics in organic/ inorganic acid solutions therefore called as acid dyes. These dyes are used for dyeing wool, silk, synthetic fibers, leather which have fair light fastness.. depending on the method of application, the acid dyes can further be classified into ( Baumann and Fletcher, 1965):
- Strong acid dyes in which strong acid is required to exhaust on wool.
- Weak acid dyes in which weak acid is required for the purpose.
- Chrome dyes require treatment with chromium salts.
- Acid premetalized dyes are fast dyes in which strong acid is required to exhaust on wool.
- Neutral premetalized dyes are used without acid
- Basic Dyes : These are the dyes which are water soluble but used with a mordant most of the times. With mordant, these dyes are used to cotton, linen, acetate, nylon, polyesters. These dyes are generally used as after treatment to the fabric already dyed with acid dyes.
- Disperse or Ingrain Dyes : These are colloidal dyes, derived from aryl amides, having low ionic character with very low water solubility. Because of it, these dyes are applied by direct colloidal absorption. These dyes are used for polyester, nylon, acetate and triacetate fibers.
- Mordant Dyes : Some dyes need mordant for dyeing the fabrics. The mordant plays the role of binding agent between the dye and the fabric. These dyes are mainly used for dyeing cotton and wool.
- Vat Dyes : Vat dyes, based on quinine structure, are insoluble dyes but are the fastest for cellulose fibers. The dye is solubilized with alkali and with the help of reducing agent it dyes the fabric by removing all the oxygen from the liquid. The true color is achieved by oxidation of dye in air. Eg indigo
- Sulfur Dyes: these dyes are used for dyeing cotton, linen and rayon as these are resistant to washing. These dyes provide very deep shades and are low in cost. They have good fastness to light and acid but they weaken the fabric structure. For e.g. sulphur red 7
- Azo Dyes : these are water insoluble azo compounds which are used for bright shades. These are widely used in Asia and Australia for batik printing.
- Nitro Dyes : these dyes are used to dye wool and are mainly used to dye wool and are mainly polynitro derivatives of phenols containing atleast one nitro group either ortho or para to the hydroxyl group. E.g. Martius yellow
- Reactive Dyes : these dyes are most popular dyes which can be used from wide range of fibres (cotton, silk, wool) to screen printing, polychromatic printing and piece dyeing. These dyes form covalent bond with the cellulosic fibre leading to high wash and light fastness properties. E.g. Reactive blue 7
1. Baumann, H.P. and Fletcher, J.M. (1965). Textile Dyeing. International Correspondance Schools, Scranton, PAC.
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About Author / Additional Info:
Working as Scientist at IARI, New Delhi