Saminathan Ratnapandian

Figure 1.1 Classification of synthetic dyes
Direct dyes
These are relatively cheap and easy to apply. They provide a wide range of colours primarily for dyeing cotton. Starting from Congo Red (CI Direct Red 29), the first direct dye, these dyes can colour cotton directly from a dye-bath containing sodium chloride without the assistance of a mordant [20]. Mordant, a metallic salt, was considered essential for dyeing cotton using natural dyes [21]. This class of dye is sub- divided based on chromophore, fastness properties and application methods. The main drawback of these dyes is their moderate wet fastness.


Vat dyes
These dyes are usually insoluble in water. The typical presence of two carbonyl (C=O) groups permits the formation of a corresponding soluble leuco-compound by reduction under alkaline conditions. Cotton can be dyed under these conditions and in the subsequent oxidation step the dye is regenerated inside the fibre. This results in high light and wash fastness [11].


Sulfur dyes
These have complex structures obtained by the thionation of various aromatic intermediaries. Deep, muted shades with good wet fastness and moderate to good light fastness can be obtained using sulfur dyes. The Colour Index identifies different classes of sulfur dyes based on their solubility [11].


Azoic dyes
These are dyes formed in situ in the fibre by the interaction of a diazonium compound with a coupling compound. Bright shades especially in the orange-scarlet-red regions can be obtained by the use of these dyes [12].


Reactive dyes
These dyes are formed by either Michael’s addition reaction involving vinyl sulfone groups or nucleophilic substitution reaction of halogen-o-heterocyclic groups [22]. If only one reactive group is present the dye is a mono-functional reactive dye, and if two groups are evident, then it becomes a bi-functional reactive dye. A bi-functional dye having two similar reactive groups is homo-bi-functional and if they are different then it is hetero-bi-functional. Dye hydrolysis, that is, the reduction in dye effectiveness by its reaction with water, a major drawback of reactive dyes, is minimized in the bi- functional class. These dyes react with the fibre, resulting in high substantivity [14].