Dyes, Synthetic | Research & Encyclopedia Articles

Dyes, Synthetic

A dye is a material used to impart color to a substrate such as fabric or hair. Natural dyes have been used for this purpose for thousands of years. Egyptian clothing dating from 3000 b.c. was dyed with indigo, yellow, red, and green colorants. These dyes were derived from from plant and occasionally animal sources. Dyes from plants included blue dye from the indigo plant; blue dye from woad; red and brown dyes from the madder plant; yellow, orange, brown, and black dyes from various trees; orange or red dye from henna; and yellow from safflower and weld. Animal dyes included red dyes from the cochineal insect and Tyrian purple from shellfish. Dye baths were made by mixing the dye with an alkali, perhaps wood ash, and stale urine, a process noteworthy for the odors it produced. (Despite the smell, urine was a convenient source of ammonia for both dyeing and disinfecting for numerous ancient and medieval cultures.)

Dyes were dissolved and applied to the cloth and the color change would take place either because the dye molecules became bonded to the fiber molecules or because the dye was unable to pass out of the fibers' fine capillaries once it is inside of them. Dyes that do not wash out easily are called fast dyes. Some dyes are not easily absorbed by fibers or are easily washed out. These dyes require the addition of a mordant which breaks down the fibers and helps the dye molecules to be absorbed more easily. For example, in 1844 John Mercer (1791-1866), the inventor of mercerized cotton, realized that wool dipped in chlorine was more easily dyed.

The synthetic dye industry began in Britain when August Wilhelm von Hofmann, a German chemist, was invited by Prince Albert (1819-1861) to become the director of the Royal College of Chemistry. Hofmann had received his Ph.D. for his work with coal tar and aniline. Hofmann, unfortunately, was somewhat clumsy in the laboratory and so hired good laboratory assistants, among them William Henry Perkin. While trying to synthesize quinine, Perkin created the first synthetic dye in 1856 when he added potassium dichromate and alcohol to aniline. Perkin and his family founded a dye factory to produce aniline purple dye for the silk industry. The color became known as mauve, from the French word for the plant that had previously been used to produce violet. Perkin followed up these inventions with aniline red (1859) and aniline black (1863) dyes. At first, Hofmann disapproved of Perkin's commercial activities, but he soon followed the path blazed by his student. Hofmann began researching dyes and eventually reacted carbon tetrachloride and aniline to form a dye called rosaniline. The reaction would not have been possible if the aniline had been pure. The presence of the impurities orthotoluidine and paratoluidine were necessary. In 1863 Hofmann succeeded in replacing the hydrogen in rosaniline with aniline to form aniline blue.

The next breakthrough in dye technology came when Heinrich Caro, building on the work of Peter Greiss (1829-1888), produced induline, and later Bismarck brown and Martius yellow; the two latter compounds were derived with the help of Martius. There followed a flurry of discoveries in the industry. Carl Graebe (1841-1927), a German chemist, developed aizarine, a red dye, in 1869. Adolf von Baeyer synthesized indigotin in 1880. He distilled isatin over hot zinc dust, a process that had a great impact on the development of synthetic dyes. Baeyer left the commercial development of indigotin to Karl Huemann, who is credited with the production of indigotin on an industrial scale. Huemann's process was improved by accident when a chemist named Sapper broke a thermometer while heating the naphthalene dictated by Huemann's method. The mercury combined with the sulfuric acid used in the process and formed mercury sulfate. The mercury sulfate converted the naphthalene to phthalic anhydride, which in turn was converted to indigo.

The first vat dye was developed by Rene Bohn in 1901. Bohn had received his doctorate from the University of Zurich with Huemann. He then went to work for BASF Corporation. Bohn tried to formulate a dye that would have the best traits of both alizarin and indigo. Instead he produced indanthrone, a blue dye. He soon discovered that when the reaction was run at a high temperature, it resulted in flavanthrone, a yellow dye. Today more than 200 dyes have been developed from indanthrone or are related to it chemically. These synthetic dyes are brighter than their natural counterparts but tend to wash out or fade when used in cotton or linen. Thus these fabrics require a mordant to set the dye. Johann Peter Griess (1829-1888) discovered that compounds could be bonded together using nitrogen compounds to form a chemical bond called azo bond. Numerous dyes contain this bond. Some, called acid dyes, dissolve in water and react easily with animal derived fibers, wool, and silk. Metal complex dyes are azo dyes that contain a metal molecule. Azo dyes used on cotton or cellulose fibers, such as rayon, must be formed inside the fiber itself. Many cellulose fabrics are dyed using vat dyes. Vat dyes and sulfur dyes are similar in that they are both embedded in the cellulose fiber rather than bonding to the fiber molecules. Reactive dyes bond with the fiber molecules and are used on cellulose fibers.