Classification of Dyes
A dye is a natural or synthetic coloring material employed for dyeing or printing of textiles, paper, leather and other materials. The color of the dyed or printed materials is anticipated not to be faded by washing, heat, light, or in other external conditions to which the material is frequently exposed. The dyes are normally applied from aqueous solution and are capable of sticking onto compatible surfaces by forming solution, a covalent bond or in situ complexes with salts or metals through physical adsorption or mechanical retention (Babatunde et al., 2018). Dos Santos et al. (2007) and Choudhury (2018) stated that classification of dyes is based on either their application methods or chemical structures whereby a group of atoms in dye molecules known as chromophores such as azo, anthraquinone, methine, nitro, arylmethane, carbonyl are responsible for the color of dye while substituents called auxochromes, such as amine, carboxyl, sulphonate and hydroxyl, withdraw and/or donate electrons hence producing or intensifying the colour of the chromophores. Dye classes are as follows: Reactive dyes, Disperse dyes, Direct dyes, Acid dyes including metal complex, Basic or Cationic Dyes, Chrome mordant dyes, Azoic dyes, Sulphur dyes, Vat dyes, Solubilized vat dyes, and Pigment colours etc (Benkhaya et al., 2017; El Harfi & El Harfi, 2017; Venkataraman, 1952; Walsh, 1979; Waring & Hallas, 2013).
Reactive Dyes
Reactive dyes are made from synthetic petrochemicals, quite expensive compared to conventional dyes and currently makeup 50% of the world’s total dye consumption. Reactive dyes are known to form a complex with textile fibres via covalent bonds ultimately existing as single chemical entities within the fibre. They are not only easy to apply but are also available in a wide range of colour. Although Reactive dyes require good amounts of salt and alkali they create less wastewater problems and are applied at lower temperatures (30-70ºC) than direct dyes (around 100ºC) therefore saving energy. The fastness properties of reactive dyes do not depend on the structure and molecular size of dye, unlike in other dye classes (Benkhaya et al., 2017; El Harfi& El Harfi, 2017; Venkataraman, 1952; Walsh, 1979; Waring & Hallas, 2013).
Sulphur Dyes
Sulphur dyes have been applied to both a chemical class of dyes and a dying class. These dyes used to dye cotton with dark colours. Sulfur black is the largest selling dye by volume. The first sulfur dyes were made from thionation of various organic materials with either sulfur or sodium polysulphide and they were all applied to cotton from sodium sulfide solution (Benkhaya et al., 2017; El Harfi& El Harfi, 2017; Venkataraman, 1952; Walsh, 1979; Waring & Hallas, 2013).
Disperse Dyes
Dyes generally used for dying cellulose acetate, nylon and other hydrophobic fibers are called disperse dyes. They are also known as acetate dyes. Sulphoricin oleic acid (SOA) is used as the dispersing agent. Dispersal and celliton are important dispersing agents (Benkhaya, El Harfi, & El Harfi, 2017; Venkataraman, 1952; Walsh, 1979; Waring & Hallas, 2013).
Direct Dyes
These dyes are used for dyeing animal or vegetable fibers directly from a solution of the dye in water. All dyes of this class are anionic dyes having an affinity for cellulose fibers. Certain direct dyes are extensively employed in the dyeing of paper, leather, baste fibers and other substrates (Benkhaya et al., 2017; Venkataraman, 1952; Walsh, 1979; Waring & Hallas, 2013).
Acid Dyes
The common structural feature of this class of dyes is the chromophoric structure with a sulphonic or analogous group, which gives the dye an anionic character. Acid dyes are mainly used for dyeing wool, silk, nylon etc. These are applied from an acid dye bath (Benkhaya et al., 2017; Venkataraman, 1952; Walsh, 1979; Waring & Hallas, 2013).
Basic or Cationic Dyes
Basic dyes are water-soluble cationic dyes that are mainly applied to acrylic fibers, but find some use for wool and silk. Usually, acetic acid is added to the dye bath to help the uptake of the dye onto the fiber. Basic dyes are also used in the colouration of paper (Benkhaya et al., 2017; Venkataraman, 1952; Walsh, 1979; Waring & Hallas, 2013).
Vat Dyes
Vat dyes are essentially insoluble in water and incapable of dyeing fibers directly. However, reduction in alkaline liquor produces the water-soluble alkali metal salt of the dye, which, in this leuco form, has an affinity for the textile fiber. Subsequent oxidation reforms the original insoluble dye. The colour of denim is due to indigo, the original vat dye (Benkhaya et al., 2017; Venkataraman, 1952; Walsh, 1979; Waring & Hallas, 2013). The classification of dyes is summarized in Figure 1 below.
Azoic Dyes
Azoic dyes are produced by reacting two components named coupling method and diazo component contains insoluble azo group. It should be noted here that, azoic dyes are different from other dyes and the application process is also different from others. They are also known as ice colours or magic colours because they require very low temperatures during dyeing (Benkhaya et al., 2017; Venkataraman, 1952; Walsh, 1979; Waring & Hallas, 2013).