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Introduction
Garments made of cellulosic fibres are mostly dyed with reactive dyes. These dyes are much brighter, longer-lasting, and easier-to-use. Unlike other dyes, they actually form a covalent bond with the cellulose molecule. No wonder one can safely wash a garment that has been dyed in bright fibre reactive colours with white clothing, a hundred times, without endangering the white.
Salt performs two key functions during dyeing of cotton garments with reactive dyes. Sodium chloride or sodium sulphate is fully dissociated in water to loose ion pairs of sodium (Na+ ) and chloride (Cl- ) or Sulphate (SO4-2 ) ions. The positive sodium ion has the capability of traveling to the fiber/water interface and effectively negating the zeta potential, destroying the barrier to the initial dye/fiber interaction.
Picturing the water as a lattice structure that has only limited sites for accommodating charged species, and considering the fact that dissociated sodium salt is more compatible with the water lattice than a large molecular weight, dye molecule with a few sulphonate groups hanging off, the salt is the preferred species to occupy the limited lattice sites. Dye is thus effectively "salted out" of the bath, with the distribution coefficient of the dye shifted from solution towards the fiber. Both these actions of the salt lead to the reactive dye exhaustions achieved during dyeing of cotton with reactive dyes.
In the absence of adequate salt concentration, reactive dye bath exhaustion is poor. Therefore, the major part of the dye remains in the bath and subsequently gets hydrolysed in the dye bath in the presence of alkali. Part of these hydrolysed dye gets exhausted on the fabric, but remains unfixed. To remove the hydrolysed/unfixed dye, time consuming, energy intensive and expensive washing off procedures are required.
A number of processes have therefore been proposed from early 1930s, till date to improve the substantivity of anionic dyes towards cellulosic fibres by introducing cationic sites on the fibres. [1] Schlack (1938) was the first to report improved affinity of acid dyes towards cellulose modified through the introduction of aminated epoxy groups. [2] Rupin (1976) and [3] Rupin et al. (1970) studied the dyeability of cellulose towards direct and reactive dyes after pretreatment with glycidyl trimethyl ammonium...