Dyes and Pigments
Dyes and pigments are substances that are used to impart color to liquid solutions and solid materials. Dyes are generally soluble (or partly soluble) organic compounds used in the textile industry to color fabrics, textile fibers and clothing. In additional to their application in textiles, dyes are used on paper and leather. Dyes are also added, under government regulation, to foods, drugs and cosmetics. Pigments, on the other hand, are insoluble inorganic and organic chemicals that are added to color paints, glazes and printing inks.
Natural plant materials have been used to dye wool since ancient times. Tyrian purple, from the shellfish murex, was a dye reserved for the emperor; this dye was worth several times its weight in gold. Woad, used since the time of the ancient Celts, produces a blue dye. Dyer's broom, or greenweed, yields a yellow dye, often added to woad-dyed cloth to turn it green. The roots of lady's bedstraw, a roadside weed in the northeastern United States, produce a red dye on wool yarn, as does the root of the madder plant, a perennial originating in the Mediterranean regions. Dyer's chamomile makes a yellow textile dye or hair coloring, and dyer's coreopsis provides a bright gold dye. Alkanet root or dyer's bugloss, yields a red dye. Indigo, the parent compound of the indigoid class of dyes, has been in use as a vat due since before recorded history. Natural indigo is obtained from the plant indigofera. It is no longer commercially important since synthetic indigo is produced inexpensively.
In order to get the color to stick well on the fiber, the wool was usually pretreated with a metal compound called a mordant; common mordants, still used in craft dying, are compounds of aluminum, iron, copper, tin, and chromium. The mordant reacts with the dye to form an insoluble complex called a "lake." An example of a mordant is aluminum hydroxide precipitated into cotton fiber. This mordant reacts with the dye alizarin to form a read lake, exactly as it does in a test tube in the typical analytical test for aluminum. Mordant dyes are no longer used commercially, since equal or better results can be obtained less expensively. They are still popular with home dyers; the vegetable dye madder yields a substance that decomposes into alizarin, requiring an aluminum hydroxide mordant to result in the color known as turkey red. A tin mordant produces an orange color with madder. Carmine red can be formed from cochineal, an insect dye, with an aluminum ion mordant.
The first synthetic dye, mauve, was prepared in 1856 by the English chemist William Henry Perkin. In 1876, Otto Witt described the structural features of organic molecules that cause them to be colored. The color of a dye is due to its ability to absorb light in the visible region of the spectrum, an ability enhanced by unsaturation (presence of double bonds in the molecule) and resonance. The main structural unit of an organic dye, which is always unsaturated, is called the chromophore. Dyes are classified according to their chromophores. Today we recognize azo dyes, one of the most important classes of dyes because they can be used on synthetic fibers, which contain one or more azo (-N=N-) groups. Polyazo dyes can have four or more azo linkages. Some other dye systems are based on the chromophores triphenylmethane, xanthene, anthroquinone, indigo, and phthalocyanine.
The raw materials for today's dyes are mainly aromatic hydrocarbons: benzene, toluene, naphthalene, anthracene, pyrene, and others. These compounds once came from the distillation of coal tar, and the dyes are still known as coal-tar dyes. The most commonly used, benzene and toluene, are now produced from petroleum and natural gas. Indigo is now derived from naphthalene, and anthracene yields alizarin, the dye formerly obtained from madder root.
The process of dyeing can be carried out in batches or on a continuous basis. Either the yarn or the finished fabric can be dyed. A single dye or a mixture of up to a dozen dyes can be used. Vat dyes are insoluble, but a soluble form with an affinity for cotton is formed when the dye is reduced. This reduction was originally carried out in wooden vats, giving rise to the name "vat dye."
Pigments are also used to impart desired color to solid substances. Important pigments include inorganic oxides and insoluble salts, which are mechanically mixed in a coating material. The pigment is deposited on the substrate as the coating dries. Several hundred pigments have been used commercially. Iron oxides of varying composition (i.e., Spanish red, Indian red and Venetian red), cadmium selenide and select organic toluidines are the most popular red pigments. Basic lead chromate and molybdate orange (lead chromate-molybdate) give a yellow color. Blue pigments include Prussian blue (ferric ferrocyanide) and phthalocyanine blue.Titanium dioxide and zinc oxide are added to coating materials to produce a white color. The vast majority of black pigments consist of finely divided particles of carbon-carbon black or lampblack. Finely, flakes of bronze, copper, lead, nickel, aluminum or silver can be used to give a metallic appearance.
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