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Characteristics:
These are also called fibre reactive dyes containing a reactive group or agent, capable of forming a covalent bond with fibre.
Reactive dyes are so called because their molecules react chemically with the fibre polymers of some fibres to form a covalent bond between the dye molecule and fibre polymer.
The first practical dyes of this type were introduced in 1956.
Advantages of reactive dyes are their excellent fastness to light and washing and their brilliant shades, which are rivaled only by acid dyes on silk.
An important use of reactive dyes is it can be combined with disperse dyes for dyeing on blends of cotton.
They offer bright colours with very good colour fastness particularly good wash fastness. Most of them have excellent colour fastness to light.
One of the few major problems with reactive dyes is that some of them are susceptible to damage from chlorine bleaches.
Suitability to textiles:
The fibres most readily coloured with reactive dyes are the manmade and natural cellulose fibres, synthetic nylon and natural protein fibres.
These dyes were first designed for cellulosic fibres; types are now available for wool, silk, nylon, acrylics and blends of these fibres.
Primarily used for cotton apparel, decorative fabrics for bright colours, where good all-round fastness is important. Applicable to stock, yarn, piece goods and printing.
Method of application:
The process of applying reactive dyes is considered below for cellulosic, protein and nylon fibres.
Manmade and natural cellulosic fibres: the dye liquor is prepared in a manner similar to that for applying direct dyes. The reactive dye is first dissolved in water in to which an electrolyte is added which helps to assist exhaustion of the dye. The textile material is then introduced in to the dye liquor and the dye is exhausted on to the fibres. Alkali must be added to enhance the reaction between dye and fibre. With some reactive dyes reaction with alkali can be carried out at room temperature. However, with most reactive dyes, the temperature of the dye liquor must be increased, in some cases to the boil to effect the reaction between the dye molecule and the polymer system of the fibre. Reactive dyes have specific temperatures at which reaction between dye and fibre are optimum. The formation of covalent bond requires an addition of an alkali.
Nylon fibres: the dye liquor is made slightly acidic for nylon fibres. Heat is applied to assist the exhaustion of the dye.
Protein fibres: reactive dyes are applied to protein fibres under slightly acidic conditions. As the temperature increases the formation of covalent link between the dye molecule and fibre polymer will take place. The application of heat to the dye liquor serves to increase exhaustion as well as to effect covalent bond formation between dye and fibre.
Fastness properties:
- Textile materials coloured with reactive dyes have very good light fastness rating about 6. However, there are some reactive dyes with only fair light fastness.
- Textiles coloured with reactive dyes have very good wash fastness rating about 4-5.
- Textile materials which are coloured with reactive dyes have to be thoroughly rinsed and scoured. If the molecules of the dye are not removed, poor rub fastness may result.
- Perspiration and atmospheric pollution which are slightly acidic may affect the textile materials coloured with reactive dyes and result in some fading.
- When reactive dyes were first introduced it was found that some of them were adversely affected by bleaches which contained chlorine.
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