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dye

[dahy]

dye, any substance, natural or synthetic, used to color various materials, especially textiles, leather, and food. Natural dyes are so called because they are obtained from plants (e.g., alizarin, catechu, indigo, and logwood), from animals (e.g., cochineal, kermes, and Tyrian purple), and from certain naturally occurring minerals (e.g., ocher and Prussian blue). They have been almost entirely replaced in modern dyeing by synthetic dyes. Most of these are prepared from coal tar, being formed from an aromatic hydrocarbon such as benzene, from which indigo is derived (see also aniline), or anthracene, which yields alizarin. Although some materials, e.g., silk and wool, can be colored simply by being dipped in the dye (the dyes so used are consequently called direct dyes), others, including cotton, commonly require the use of a mordant (see also lake). Alizarin is a mordant dye and the color it gives depends upon the mordant used. Dyes are classified also as acidic or basic according to the medium required in the dyeing process. A vat dye, e.g., indigo, is so called from the method of its application; it is first treated chemically so that it becomes soluble and is then used for coloring materials bathed in a vat. When the materials become impregnated with the dye, they are removed and dried in air, the indigo reverting to its original, insoluble form. The process by which a dye becomes "attached" to the material it colors is not definitely known. One theory holds that a chemical reaction takes place between the dye and the treated fiber; another proposes that the dye is absorbed by the fiber. Dyeing is an ancient industry. The Chinese, Persians, and Indians used natural dyes many centuries ago, including indigo, probably the oldest dye in use, and Tyrian purple, derived from a species of snail. The Egyptians prepared some brilliant colors. In the 13th and 14th cent. dyeing assumed importance in Italy; the methods employed were carried to other parts of Europe and, as new dyes became known, the dyeing industry flourished and grew. Cochineal was introduced from Mexico. Finally, in the 19th cent. the work of W. H. Perkin and Adolf von Baeyer produced the first synthetic dyes.

See S. Robinson, The History of Dyed Textiles (1970); H. Zollinger, Color Chemistry: Syntheses, Properties, and Applications of Organic Dyes and Pigments (1987); D. R. Waring and G. Hallas, ed., The Chemistry and Application of Dyes (1989).

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Licensed from Columbia University Press

dye

Any of a class of intensely coloured complex organic compounds used to colour textiles, leather, paper, and other materials. Dyes known to the ancients came from plants such as indigo and madder (see madder family) or from the shells of mollusks; today most dyes are made from coal tar and petrochemicals. The chemical structure of dyes is relatively easy to modify, so many new colours and types of dyes have been synthesized. Dye molecules are deposited from solution onto materials in such a way that they cannot be removed by the original solvent. Fibre-reactive dyes form a covalent bond with the fibre. Other dyes require prior application of a mordant, an inorganic material that causes the dye to precipitate as an insoluble salt. Another technique is vat dyeing, in which a soluble colourless compound is absorbed by the fibres, then oxidized (see oxidation-reduction) to the insoluble coloured compound, making it remarkably resistant to the fading effects of washing, light, and chemicals. Seealso azo dye.

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azo dye

Any of a large class of synthetic organic compounds, dyes containing two nitrogen atoms joined by a double bond (see bonding, covalent bond) in the form singlehorzbondNdoublehorzbondNsinglehorzbond as part of their molecular structure. More than half the dyes in commercial use are azo dyes. They are usually classed according to the fibres for which they are useful or the methods by which they are applied; direct dyes are absorbed from solution by the fibres, but others require a second solution (a mordant) or a second step before the colour is fast.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Dye

A dye can generally be described as a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and may require a mordant to improve the fastness of the dye on the fiber.

Both dyes and pigments appear to be colored because they absorb some wavelengths of light preferentially. In contrast with a dye, a pigment generally is insoluble, and has no affinity for the substrate. Some dyes can be precipitated with an inert salt to produce a lake pigment, and based on the salt used they could be aluminum lake, calcium lake or barium lake pigments.

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 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.

Organic dyes

The first human-made (synthetic) organic dye, mauveine, was discovered by William Henry Perkin in 1856. Many thousands of synthetic dyes have since been prepared.

Synthetic dyes quickly replaced the traditional natural dyes. They cost less, they offered a vast range of new colors, and they imparted better properties upon the dyed materials. 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 fibers such as silk, wool, nylon and modified acrylic fibers using neutral to acid dyebaths. Attachment to the fiber is attributed, at least partly, to salt formation between anionic groups in the dyes and cationic groups in the fiber. Acid dyes are not substantive to cellulosic fibers. Most synthetic food colors fall in this category.

Basic dyes are water-soluble cationic dyes that are mainly applied to acrylic fibers, 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 fiber. 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 sulfate (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 color 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. It is important to note that many mordants, particularly those in the heavy metal category, can be hazardous to health and extreme care must be taken in using them.

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. The color of denim is due to indigo, the original vat dye.

Reactive dyes utilize a chromophore attached to a substituent that is capable of directly reacting with the fibre substrate. The covalent bonds that attach reactive dye to natural fibers make them among the most permanent of dyes. "Cold" reactive dyes, such as Procion MX, Cibacron F, and Drimarene K, are very easy to use because the dye can be applied at room temperature. Reactive dyes are by far the best choice for dyeing cotton and other cellulose fibers at home or in the art studio.

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. Their main use is to dye polyester but they can also be used to dye nylon, cellulose triacetate, 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.

Azo 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 azo dye. This technique of dyeing is unique, in that the final color is controlled by the choice of the diazoic and coupling components.

Sulfur dyes are two part "developed" dyes used to dye cotton with dark colors. The initial bath imparts a yellow or pale chartreuse color. This is aftertreated with a sulfur compound in place to produce the dark black we are familiar with in socks for instance. Sulfur Black 1 is the largest selling dye by volume.

Food dyes

One other class which describes the role of dyes, rather than their mode of use, is the food dye. Because food dyes are classed as food additives, they are manufactured to a higher standard than some industrial dyes. Food dyes can be direct, mordant and vat dyes, and their use is strictly controlled by legislation. Many are azoic dyes, although anthraquinone and triphenylmethane compounds are used for colors such as green and blue. Some naturally-occurring dyes are also used.

Other important dyes

A number of other classes have also been established, including:

Oxidation bases, for mainly hair and fur
Leather dyes, for leather
Fluorescent brighteners, for textile fibres and paper
Solvent dyes, for wood staining and producing colored lacquers, solvent inks, coloring oils, waxes.
Carbene dyes, a recently developed method for coloring multiple substrates