Dye
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A dye can generally be described as a coloured substance that has an affinity to the substrate to which it is being applied. The dye is usually used as an aqueous solution, and may require a mordant to improve the fastness of the dye on the fibre. In contrast, a pigment generally has no affinity for the substrate, and is insoluble.
Archaeological evidence shows that, particularly in India and the Middle East, dyeing has been carried out for over 5000 years. The dyes were obtained from either animal, vegetable or mineral origin, with no or very little processing. By far the greatest source of dyes has been from the plant kingdom, notably roots, berries, bark, leaves and wood, but only a few have ever been used on a commercial scale.
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Organic dyes
The first man-made organic dye, mauveine, was discovered by William Henry Perkin in 1856. Many thousands of dyes have since been prepared and, because of vastly improved properties imparted upon the dyed materials, quickly replaced the traditional natural dyes. Dyes are now classified according to how they are used in the dyeing process.
Acid dyes are water-soluble anionic dyes that are applied to fibres such as silk, wool, nylon and modified acrylic fibres using neutral to acid dyebaths. Attachment to the fibre is attributed, at least partly, to salt formation between anionic groups in the dyes and cationic groups in the fibre. Acid dyes are not substantive to cellulosic fibres.
Basic dyes are water-soluble cationic dyes that are mainly applied to acrylic fibres, but find some use for wool and silk. Usually acetic acid is added to the dyebath to help the uptake of the dye onto the fibre. Basic dyes are also used in the coloration of paper.
Direct or substantive dyeing is normally carried out in a neutral or slightly alkaline dyebath, at or near boiling point, with the addition of either sodium chloride (NaCl) or sodium sulphate (Na2SO4). Direct dyes are used on cotton, paper, leather, wool, silk and nylon. They are also used as pH indicators and as biological stains.
Mordant dyes require a mordant, which improves the fastness of the dye against water, light and perspiration. The choice of mordant is very important as different mordants can change the final colour significantly. Most natural dyes are mordant dyes and there is therefore a large literature base describing dyeing techniques. The most important mordant dyes are the synthetic mordant dyes, or chrome dyes, used for wool; these comprise some 30% of dyes used for wool, and are especially useful for black and navy shades. The mordant, potassium dichromate, is applied as an after-treatment.
Vat dyes are essentially insoluble in water and incapable of dyeing fibres directly. However, reduction in alkaline liquor produces the water soluble alkali metal salt of the dye, which, in this leuco form, has an affinity for the textile fibre. Subsequent oxidation reforms the original insoluble dye.
Reactive dye utilize a chromophore containing a substituent that is actived and allowed to directly react to the surface of the substrate.
Disperse dyes were originally developed for the dyeing of cellulose acetate, and are substantially water insoluble. The dyes are finely ground in the presence of a dispersing agent and then sold as a paste, or spray-dried and sold as a powder. They can also be used to dye nylon, triacetate, polyester and acrylic fibres. In some cases, a dyeing temperature of 130 ?C is required, and a pressurised dyebath is used. The very fine particle size gives a large surface area that aids dissolution to allow uptake by the fibre. The dyeing rate can be significantly influenced by the choice of dispersing agent used during the grinding.
Azoic dyeing is a technique in which an insoluble azoic dye is produced directly onto or within the fibre. This is achieved by treating a fibre with both diazoic and coupling components. With suitable adjustment of dyebath conditions the two components react to produce the required insoluble azoic dye. This technique of dyeing is unique, in that the final colour is controlled by the choice of the diazoic and coupling components.
Natural dyes
Animal origin
These include tyrian purple (vat dye), kermes and cochineal (mordant dyes) and techelet.
Vegetable origin
Substantive dyes include safflower and turmeric, while indigo and woad are vat dyes. Mordant dyes include alizarin (madder), dyer's broom, brazilwood, quercitron bark, weld and old fustic. Cudbear is unclassified.
Inorganic dyes
These include eosin and iron buff.
Food dyes
One other class which describes the role of dyes, rather than their mode of use, is food dyes. This is a special class of dye, and have characteristically very high purity. These include direct, mordant and vat dyes, and the use thereof is strictly controlled by legislation. Many are azoic dyes, although anthraquinone and triphenylmethane compounds are used for colours such as green and blue. Some naturally-occurring dyes are also used.
Other
A number of other classes have also been established, including:
- Oxidation bases, for mainly hair and fur
- Sulfur dyes, for textile fibres
- Leather dyes, for leather
- Fluorescent brighteners, for textile fibres and paper
- Solvent dyes, for wood staining and producing coloured lacquers, solvent inks, colouring oils, waxes and fats.