Dye
A dye is a colored substance that is soluble in some solvent; by contrast pigments are insoluble or nearly so in all solvents. Because of their solubility, dyes can chemically bind to the material they color. Dye is generally applied in an aqueous solution and may require a mordant to improve the fastness of the dye on the fiber.
The majority of natural dyes are derived from non-animal sources such as roots, berries, bark, leaves, wood, fungi and lichens. However, due to large-scale demand and technological improvements, most dyes used in the modern world are synthetically produced from substances such as petrochemicals. Some are extracted from insects and/or minerals.
Synthetic dyes are produced from various chemicals. The great majority of dyes are obtained in this way because of their superior cost, optical properties, and resilience. Both dyes and pigments are colored, because they absorb only some wavelengths of visible light. Dyes are usually soluble in some solvent, whereas pigments are insoluble. Some dyes can be rendered insoluble with the addition of salt to produce a lake pigment.
History
dyeing dates back to the Neolithic period. Throughout history, people have dyed their textiles using common, locally available materials. Scarce dyestuffs that produced brilliant and permanent colors such as the natural invertebrate dyes Tyrian purple and crimson kermes were highly prized luxury items in the ancient and medieval world. Plant-based dyes such as woad, indigo, saffron, and madder were important trade goods in the economies of Asia and Europe. Across Asia and Africa, patterned fabrics were produced using resist dyeing techniques to control the absorption of color in piece-dyed cloth. Dyes from the New World such as cochineal and logwood were brought to Europe by the Spanish treasure fleets, and the dyestuffs of Europe were carried by colonists to America.Dyed flax fibers have been found in the Republic of Georgia in a prehistoric cave dated to 36,000 BP. Archaeological evidence shows that, particularly in India and Phoenicia, dyeing has been widely carried out for over 5,000 years. Early dyes were obtained from animal, vegetable or mineral sources, with no to very little processing. By far the greatest source of dyes has been from the plant kingdom, notably roots, berries, bark, leaves and wood, only few of which are used on a commercial scale.
Early industrialization was conducted by J. Pullar and Sons in Scotland. The first synthetic dye, mauve, was discovered serendipitously by William Henry Perkin in 1856. The discovery of mauveine started a surge in synthetic dyes and in organic chemistry in general. Other aniline dyes followed, such as fuchsine, safranine, and induline. Many thousands of synthetic dyes have since been prepared.
The discovery of mauveine in 1856 led to the development of a synthetic dyestuff industry. In Manchester, England, a number of people set up dyestuff manufacturing plant including Ivan Levinstein, Levinstein Ltd, Charles Dreyfus, Clayton Aniline Company, William Claus, Claus & co.
The discovery of mauve also led to developments within immunology and chemotherapy. In 1863 the forerunner to Bayer AG was formed in what became Wuppertal, Germany. In 1891, Paul Ehrlich discovered that certain cells or organisms took up certain dyes selectively. He then reasoned that a sufficiently large dose could be injected to kill pathogenic microorganisms, if the dye did not affect other cells. Ehrlich went on to use a compound to target syphilis, the first time a chemical was used in order to selectively kill bacteria in the body. He also used methylene blue to target the plasmodium responsible for malaria.
Classification of dyes
The color of a dye derives from the absorption of light within the visible region of the electromagnetic spectrum. The chemical structure determines the light absorption and is therefore the basis for many classification schemes.Classification according to chemical structure
Anthraquinone dyes
The basic structure of this group of dyes is anthraquinone. By varying the substituents, almost all colors from yellow to red and from blue to green can be obtained, with red and blue anthraquinone dyes being particularly important. Through reduction, the quinone can be converted into the corresponding water-soluble hydroquinone, allowing anthraquinone dyes to be used as [|vat dyes]. With appropriate substituents, anthraquinone dyes can also be used as [|disperse dyes] for dyeing synthetic fibers. Water-soluble anthraquinone dyes containing sulfonic [|acid] groups are used as acid or [|reactive dyes].Azo dyes
Azo dyes contain an azo group substituted with an aryl group or alkenyl group as their basic structural element. Azo dyes containing multiple azo groups are referred to as bisazo, trisazo, tetrakisazo, and polyazo dyes. Aryl substituents are usually benzene or naphthalene derivatives, but may also include heteroaromatic systems such as pyrazoles or pyridones. Enolizable aliphatic groups, for example substituted anilides of acetoacetic acid, are used as alkenyl substituents.The dyes are synthesized by diazotization of aromatic amines followed by azo coupling of the diazonium salts with electron-rich aromatics or β-dicarbonyl compounds. Azo dyes are by far the most important and extensive class of dyes and are represented in almost all application-related dye categories. No naturally occurring azo dyes are known. With the exception of turquoise and a brilliant green, almost all colors can be achieved using azo dyes. The azo group is sensitive to reducing agents; it is cleaved, resulting in discoloration of the dye. Some examples of different types of azo dyes :
Dioxazine dyes
Dioxazine dyes, also known as triphendioxazine dyes, contain triphendioxazine as their basic structure. These intensely colored, brilliant dyes exhibit good color fastness and thus combine advantages of both azo and anthraquinone dyes. Dioxazine dyes are commercially available as direct and reactive dyes.Indigoid dyes
Indigoid dyes belong to the carbonyl dyes and are used as vat dyes. The most important representative is indigo, which was extracted from plants as a natural dye in ancient times and is still produced industrially in large quantities, particularly for dyeing jeans. Another natural dye is the ancient purple.Metal complex dyes
Metal complex dyes consist of complex compounds formed from a metal and one or more dye ligands containing electron donors. Copper and chromium compounds predominate, although cobalt, nickel, and iron complexes are also used to a lesser extent. The ligands are often azo dyes, methine dyes, formazans, or phthalocyanines. Metal complex dyes are characterized by excellent fastness properties.Formazan dyes
Formazan dyes are structurally related to azo dyes. Their basic structure is triphenylformazan. They form chelate complexes with transition metals such as copper, nickel, or cobalt. Depending on the substituents, non-complexed formazans are orange to deep red, whereas metal-complex formazans are violet, blue, or green. They are synthesized by azo coupling of diazonium salts with hydrazones. Of particular commercial importance are blue tetradentate copper chelate complexes of various formazans, which are used mainly as reactive dyes for cotton:Phthalocyanine dyes
Phthalocyanine dyes are copper or nickel metal complexes based on the phthalocyanine structure. They are structurally related to porphyrins and share the annulene element. By introducing water-soluble substituents—primarily via sulfochlorination—turquoise to brilliant green dyes can be obtained. Phthalocyanine dyes are distinguished by outstanding light fastness.Methine dyes
Methine or polymethine dyes possess conjugated double bonds as their chromophoric system, with two terminal groups acting as an electron acceptor A and an electron donor D. These terminal groups, which usually contain nitrogen or oxygen atoms, may be part of a heterocycle, and the double bonds may be part of an aromatic system. If one or more methine groups are replaced by nitrogen atoms, the dyes are referred to as aza-analog methine dyes. This gives rise to different subclasses:Cyanine dyes, in which the conjugated double bonds are flanked by a tertiary amino group and a quaternary ammonium compounds.
If two methine groups are replaced by nitrogen atoms and one terminal group is part of a heterocycle while the other is open-chain, the important diazahemicyanine dyes are formed. Example: Basic Red 22.
Styryl dyes: by insertion of a phenyl ring into the polyene backbone, these dyes contain a styrene structural element. Example: Disperse Yellow 31.
Triarylmethine dyes, also referred to in older literature as triphenylmethane dyes because they are derived from triphenylmethane, in which at least two of the aromatic rings carry electron-donating substituents. Example: Basic Green 4.
Nitro and nitroso dyes
In nitro dyes, a nitro group is located on an aromatic ring in the ortho position relative to an electron donor, either a hydroxy or an amino group. The oldest representative of this dye class is picric acid. Hydroxynitro dyes are no longer of commercial importance. This is a relatively small but historically significant dye class, whose representatives are characterized by high light fastness and simple production. Nitro dyes exhibit yellow to brown hues. Owing to their relatively small molecular size, an important application as disperse dyes is the dyeing of polyester fibers. They are also used as acid and pigment dyes.The rare nitroso dyes are aromatic compounds containing a nitroso group. Nitroso dyes with a hydroxy group in the ortho position to the nitroso group are used exclusively as metal complexes. A typical representative is naphthol green B.