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.
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.
Water soluble anionic dyes that are applied to fibres such as silk, wool, nylon and modified acrylic fibres from 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.
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 take up of the dye onto the fibre. Basic dyes are also used in the coloration of paper.
Direct (Substantive) dye
Dyeing is normally carried out in a neutral or slightly alkaline dyebath, at or near the boil, 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.
- As the name suggests these dyes require a mordant. This improves the fastness of the dye on the fibre such as water, light and perspiration fastness. 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 (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 used is potassium dichromate applied as an after-treatment.
- These 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. In this leuco form these dyes have an affinity for the textile fibre. Subsequent oxidation reforms the original insoluble dye.
Originally developed for the dyeing of cellulose acetate. They are substantially water insoluble. The dyes are finely ground in the presence of a dispersing agent 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 deg 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.
- A dyeing technique in which an insoluble azo dye is produced directly onto or within the fibre. This is achieved by treating a fibre with a diazo component and a coupling component. With suitable adjustment of dyebath conditions the two components react to produce the required insoluble azo dye. This technique of dyeing is unique in that the final colour is controlled by the choice of the diazo and coupling components.
One other class which describes the role dyes have rather than their mode of use is food dyes .
This is a special class of dyes of very high purity. They include direct, mordant and vat dyes. Their use is strictly controlled by legislation. Many are azo dyes but anthraquinone and triphenylmethane compounds are used for colours such as green and blue. Some naturally occurring dyes are also used.
A number of other classes have also been established and these include:
Oxidation bases mainly hair and fur
Sulfur dyes textile fibres
Leather dyes leather
Fluorescent brighteners textile fibres and paper
Solvent dyes wood staining,
producing coloured lacquers,
producing solvent inks,
colouring oils, waxes and fats.