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Reactive dyes are a class of highly coloured organic substances primarily used for tinting textiles. Reactive dyes form covalent bonds with cotton through nucleophilic substitution or nucleophilic addition mechanism and the dyes are familiar as substitutive and additive dyes respectively.
Due to the presence of strong dye–fibre interaction, fastness properties are remarkably good except wash fastness which is poor to moderate due to the hydrolytic nature of dyes. Pre-Dissolved dye is applied on cotton; salt is added for better exhaustion followed by fixation with alkali. Subsequent soaping and washing remove all superficial and hydrolysed dyes.
Applications of Reactive Dyes are in Textile Industry, Paper Industry, Leather Industry, Paints and Coatings, Plastic Industry
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Direct dyes are water-soluble anionic dyes and retain sulphonic acid groups in structure. Though light fastness seems to be good in most of the cases, wash fastness suffers due to its water solubility. Most of the direct dyes belong to azo class and are suitable for cellulosic, where the product faces no domestic washes, like cheaper carpets. Dyes are retained by the fibre through hydrogen bonds and van der Waals force. Wash Fastness may be improved by suitable after-treatment technique. Due to presence of acid groups, direct dyes react with protein fibres forming washfast shades but lack brilliancy.
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Acid dyes are mostly sulphuric or carboxylic acid salts and are essentially applied from an acidic bath and hence the name ‘acid dye’. The dye anion is the active-coloured component in these dyes; invariably synthesised as sodium salts as free dye acids are more difficult to isolate. Dyes possess affinity for protein fibres and are used in dyeing of wool, silk and nylon; ionic bond is formed to retain dye with fibre.
Fastness properties vary with chemical constitution as well as application class of dyes. From an application point of view, acid dyes are of three types, viz. (i) strong or levelling or equalising acid dyes, (ii) milling or weak acid dyes and (iii) very weak or super milling or neutral or aggregated acid dyes.
It is useful in various sectors –
■ It is useful in textiles, fertilisers and leather.
■ Acid dyes are also used in detergents and metal anodization.
■ It acts like the coolants for engines.
■ It is used in wool for resulting in the ionic or salt link between the dye molecule and fibre polymer.
■ It is also useful in silk with various colours. Silk exerts affinity for acid dyes at low temperature and dyeing is commenced at 40 degrees Celsius.
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