Saturday, March 26, 2011

Informative articles on Introduction to Reactive Dye


Definition of Reactive dye:
Reactive dyes are anionic soluble dyes. Those dyes are applied by means of chemical reaction, the mechanism of which is difficult. Reactive dyes react with the fibre to form covalent bond. They possess in their dye molecule, a reactive group of Reactive dyes reacts with the hydroxyl, groups of the cellulose (cotton, flex , jute) to form a stable chemical linkage. The dyestuff of Reactive dyes thus becomes a part of the fibre substance.

The development of reactive dyes:
Dyeing of cotton fibre with direct dyes has rather poor textile washing fastness because only weak polar and dispersion forces bind the dye molecules to the cellulose fibres polymer chains. Direct dye molecules can therefore easily diffuse out of the cotton polymer chain during washing. The best fastness to washing of a perfect textile materials requires precipitating an insoluble pigment and mechanically trapping it within the cotton fibres. This type of dyeing process with vat and azoic dyes is, however, much more complicated than direct dyeing.
The idea of immobilizing a dye molecule by covalent bond formation with reactive groups in a fibre originated in the early 1900s.Various chemicals were found that reactive group of reactive dyes reacted with the hydroxyl groups of cellulose and eventually converted into colored cellulose derivatives specially cotton.
A number of dyes now known to be capable of covalent bond formation with groups in wool and cotton were not initially considered as fibre-reactive dyes, despite the good fastness to washing of their dyeing.
In 1955, Ratte and Stephen, working for ICI in England, developed a procedure for dyeing cotton with fibre reactive dyes containing dichlorotriazine groups. They established that dyeing cotton with these dyes under mild alkaline conditions resulted in a reactive chlorine atom on the triazine ring being substituted by an oxygen atom from a cellulose hydroxyl group. The role of the alkali is to cause acidic dissociation of some of the hydroxyl groups
in the cellulose, and it is the cellulosate ion (Cell-O ) that reacts with the dye.
Cell-OH + HO-             Cell-O- + H20
Cell-O + Dye – CI         Cell - O - Dye + Cl-
Where Cell-OH is the cellulose with a reactive hydroxyl group, Dye-Cl is the dye with its reactive chlorine atom, and Cell-O-Dye the dye linked to the cellulose by a covalent bond.


Market situation of reactive dye:


In the US cotton represents approximately one third of the fibre used in textile processing. Reactive dyes are the most important class of dyes for dyeing and printing cotton and other cellulosic blends. They represent about 45% (by value) of colorants used for coloring cellulosic fibres. Reasons for the success of reactive dyes include,
flexible application methods,
wide range of shades available (including brilliant shades),
their fastness properties,
and Cost-efficiency.
The demands made on reactive dyes are wide-ranging. Requirements depend to a large extent on the specifications for particular textile articles and on operating conditions (equipment available legal constrains, etc.). Economic considerations, quality requirements and ecological issues are major factors in the debate concerning the ideal reactive dyeing process and optimum dyestuff selection. Over 60% of reactive dyes are applied by the exhaust dyeing method. The remainder is applied by padding method of dyeing. 

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