Reactive dyes dyeing process

There is no doubt that the major weakness of the reactive dyeing process is the hydrolysis reaction and the consequent need for a wash-off" process. The extent to which dye hydrolysis takes place in competition with dye-fibre reaction varies quite markedly within the range 10 40% depending upon the system in question. A considerable amount of research has therefore been devoted to the search for reactive dyes with improved fixation. The most successful approach to addressing this issue has involved the development of dyes with more than one fibre-reactive group in the molecule, which statistically improves the chances of dye fibre bond formation. Examples of products of this type are the Procion H-E... 

Reactive dyes in general are not unusually sensitive to hard water. Nevertheless, the alkali used in most reactive dyeing processes may precipitate calcium or magnesium hydroxide on... 

Nigam P, Banat IM, Singh D, Marchant R (1996) Microbial process for the decolorization of textile effluent containing azo, diazo and reactive dyes. Process Biochem 31 435-442... 

In principle, for separation of salt from concentrated brine solutions, desahnation techniques common for seawater are suitable. Koyuncu et al. (2004) reported studies on reactive dye/sodium chloride separation using nanofiltration membranes made from polysulfone-polyamide. The emphasis of this research, however, was more directed towards the quality of the purified water than on reclamation of salt or dye. Wenzel et al. (1996) suggested re-use of the bath of a reactive dyeing process containing all auxiliaries after the hydrolyzed reactive dye has been removed by adsorption on activated carbon. The dyes were degraded in an anaerobic digester. 

Research has focused on both the salt requironents of these dyes and the dyeing process itself Low-salt reactive dyes with enhanced application characteristics (Sewekow, 1993 Shukla, 1999) have been developed and are commercially available. Modifications of the reactive dyeing process have also been suggested. Scheyer et al. (2000) proposed a two-bath process with the first bath being neutral and containing dye and salt, and the second bath providing the alkaline conditions essential for the chemical reaction to take place. Such a process sequence allows for the first bath to be re-used and dye hydrolysis to be drastically diminished. 

Many aminonaphthalenesulfonic acids are important in the manufacture of azo dyes (qv) or are used to make intermediates for azo acid dyes, direct, and fiber-reactive dyes (see Dyes, reactive). Usually, the aminonaphthalenesulfonic acids are made by either the sulfonation of naphthalenamines, the nitration—reduction of naphthalenesulfonic acids, the Bucherer-type amination of naphtholsulfonic acids, or the desulfonation of an aminonaphthalenedi-or ttisulfonic acid. Most of these processes produce by-products or mixtures which often are separated in subsequent purification steps. A summary of commercially important aminonaphthalenesulfonic acids is given in Table 4. 

From an appHcations point of view, the sulfur dyes are between vat, direct, and fiber-reactive dyes. They give good to moderate lightfastness and good wetfastness at low cost and rapid processing (see Dyes, application and evaluation). 

Eig. 3. Amounts and forms of fiber-reactive dye on the fiber as a function of time for a low affinity dye, where X represents the reactive group. Point A represents the amount of dye exhausted in neutral conditions B is the total amount of dye exhausted at the end of the dyeing process, ie, [dye—OH] +... 

Alkali is usually added in a second stage. However, with low reactivity high affinity dyes it is possible to add the alkah at the beginning of the dyeing process and control the rate of uptake and chemical reaction by temperature control. With high affinity dyes the exhaustion takes place at low temperature rapidly before the chemical reaction becomes significant. If dyes are carefully selected or synthesized to have identical dye uptake it is possible to include all the electrolyte from the beginning and operate an "ah-in" technique. 

The traditional use of dyes is in the coloration of textiles, a topic covered in considerable depth in Chapters 7 and 8. Dyes are almost invariably applied to the textile materials from an aqueous medium, so that they are generally required to dissolve in water. Frequently, as is the case for example with acid dyes, direct dyes, cationic dyes and reactive dyes, they dissolve completely and very readily in water. This is not true, however, of every application class of textile dye. Disperse dyes for polyester fibres, for example, are only sparingly soluble in water and are applied as a fine aqueous dispersion. Vat dyes, an important application class of dyes for cellulosic fibres, are completely insoluble materials but they are converted by a chemical reduction process into a water-soluble form that may then be applied to the fibre. There is also a wide range of non-textile applications of dyes, many of which have emerged in recent years as a result of developments in the electronic and reprographic... 

The great majority of coloration processes demand some control over the treatment pH, which varies from strongly alkaline in the case of vat, sulphur or reactive dyes, to strongly acidic for levelling acid dyes. The concept of pH is a familiar one its theoretical derivation can be found in all standard physical chemistry textbooks and has been particularly well explained in relation to coloration processes [6,7] both in theory and in practice. We are concerned here essentially with the chemistry of the products used to control pH and their mode of action. It has been stated [7] that Unfortunately, pH control appears simple and easy to carry out. Add acid and the pH decreases add base (alkali) and the pH increases. However, pH is the most difficult control feature in any industry . 

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