Reactive dyes are highly coloured organic substances that are used for colouring textiles. The term ‘reactive’ itself refers to the chemical reaction that takes place when a reactive dye is applied to fibre. The molecule of the dye forms a covalent bond with the fibre – this is one of the strongest types of chemical reactions, and ensures that the colouring is permanent.
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Reactive dyes are mainly used for dyeing cellulose fibers such as cotton and viscose, but they are also increasingly gaining importance for wool and polyamide. The range of available reactive dyes is wide and enables a large number of dyeing techniques to be used. In dyeing cellulose fibers with reactive dyes the following chemicals and auxiliaries are used:
1-Alkali (sodium carbonate, bicarbonate and caustic soda)
2-Salt (mainly sodium chloride and sulphate
3-Urea may be added to the padding liquor in continuous processes
4-Sodium silicate may be added in the cold pad-batch method.
Poor dye fixation has been a long-standing problem with reactive dyes in particular in batch dyeing of cellulose fibers, where a significant amount of salt is normally added to improve dye exhaustion (and therefore also dye fixation). Thus color and salt in the effluent are major environmental issues in reactive dyes. Because both unfixed reactive dye and its hydrolyzed form are water soluble. Many reactive dyes contain halogens. However, since it is not attached to the chromosphere,
Heavy metals can be present both as impurities from the production process and as an integral part of chromospheres. The latter concerns phthalocyanine dyes which are still widely used specifically for blues and turquoise shades.
Reactive dyes form a new chemical compound when they come into contact with a fiber molecule. Reactive dyes are applied either from a solution with high pH or from neutral solutions that are later alkalized through a separate process. Sometimes different shades are brought out by applying heat to the dyed textile.
Reactive dyes react with cellulose under alkaline conditions to form covalent bonds between fiber and dye. There are various classes of reactive dyes, monochlorotriazine (MCT), vinyl sulphone etc., and these require different strengths of alkali for optimum fixation.
Reactive dyes consist of four parts:
1-The chromogen or the chromophoric part, which contributes color to the dye.
2-The reactive system, which enables the dye to react with the substrate. This part can also react with water molecules present in the dye bath, a phenomenon called hydrolysis of the reactive dyes which is not a desirable reaction during dyeing.
3-A bridging unit that joins the reactive system to the chromophoric part.
4-The solubilizing group(s) attached to the chromophoric grouping confers water solubility to the dye.
Properties of Reactive dyes:
In general, textile materials coloured with reactive dyes have very good light-fastness, the light-fastness rating being about six. The very stable electronic arrangement of these dyes provides good resistance to the degrading effect of sunlight. There are, however, some reactive dyes with only fair light-fastness. Reactive dyes with azo chromophore show lower light-fastness. Metal complex type azo reactive dyes have higher light-fastness. Textile materials coloured with reactive dyes have good wash-fastness; their wash-fastness rating is about four to five. This is attributed to the very stable covalent bond that exists between the dye molecule and the cellulosic fibre. Perspiration and atmospheric pollution, which are both acidic in nature, may affect textile materials that are dyed or printed with reactive dyes and result in some fading. It is found that textiles dyed with bactericidal monochlorotriazine reactive dyes containing hexachlorophene possess a wide spectrum of bactericidal action (Kalontarov and Kalandarov, 1993). This effect is reported to depend on the dye functionality based on phenol OH-groups. With monofunctional dyes the effect is there to eight times larger than observed with dyes containing disubstituted hexachlorophene residue.
Principles of reactive dye application:
The methods by which reactive dyes can be applied are as follows:
1-Exhaust method (on jigger/winch)