in a hairdresser shop The color of a dye derives from the absorption of light within the visible region of the electromagnetic spectrum (380–750 nm). The chemical structure determines the light absorption and is therefore the basis for many classification schemes. Direct Blue 106.svg|
Direct Blue 106 Reactive Blue 204.svg|
Reactive Blue 204 Indigoid dyes , the blue coloration of blue jeans. Although once extracted from plants, indigo dye is now almost exclusively synthesized industrially. 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 (
C.I. Natural Violet 1 /
Dibromindigo). Indigo skeletal.svg|C.I. Vat Blue 1 (Indigo) Tyrian-Purple.svg|C.I. Natural Violet 1 Indirubin.svg|
Indirubin 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: Reactive Blue 160.svg|C.I. Reactive Blue 160 Reactive Blue 235.svg|C.I.
Reactive Blue 235 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. Phtalocyanine Aza(18)annulene.svg|Phthalocyanine (Aza[18]annulene) Reactive Blue 7.svg|C.I. Reactive Blue 7
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 (malachite green). Basic Red 22.svg|C.I. Basic Red 22 Disperse Yellow 31.svg|C.I. Disperse Yellow 31 Malachite green Structural Formulae V.1.svg|C.I. Basic Green 4 (malachite green)
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 (–OH) or an amino group (–NH2). The oldest representative of this dye class is
picric acid (2,4,6-trinitrophenol). 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 (C.I. Acid Green 1). Pikrinsäure.svg|Picric acid C.I. Acid Orange 3.svg|
Acid Orange 3 Disperse Yellow 42.svg|
Disperse Yellow 42 Naphthol Green B.svg|C.I. Acid Green 1
Sulfur dyes Sulfur dyes (sulfin dyes) are water-insoluble, macromolecular dyes that contain disulfide bridges or oligosulfide bonds between aromatic residues. They are produced by melting
benzene,
naphthalene, or
anthracene derivatives with
sulfur or
polysulfides and have an ill-defined
constitution. They are particularly suitable for dyeing
cotton fiber. Similar to vat dyes, they are reduced to a water-soluble form (
leuco compound) using
caustic soda and
dithionites or
sodium sulfide, applied to the fiber, and then fixed in an insoluble form by
oxidation. For toxicological and ecological reasons, oxidation with
chromates is increasingly being replaced by low-sulfide sulfur dyes and sulfide-free reducing agents. Owing to their low production costs, sulfur dyes continue to play an important role in terms of volume. They are characterized by good wash and light fastness, although the colors are generally muted.
Classification according to application technology While the color shade of a dye is essentially determined by its chromophore, dye properties can be modified by incorporating suitable chemical groups to enable dyeing of different substrates. This leads to a classification of dyes according to the dyeing process. This classification is also used by the
Colour Index, an important standard reference in dye chemistry. The Colour Index (C.I.) indicates the dye class, color, and chemical identity. It lists more than 10,000 dyes, over 50% of which are azo dyes.
Mordant dyes The term derives from
mordant dyeing, in which suitable acid dyes are applied to
mordanted fabrics, primarily wool and silk. Prior to dyeing, the fibers are treated with [chromium] , [iron] , or
aluminum salts. During subsequent steaming, metal hydroxides form on the fiber. During dyeing, these
hydroxides react with the (usually specialized) acid dye to form a
metal complex dye. The process on the fiber corresponds to
varnishing. When chromium salts are used, the dyes are referred to as chromium dyes. Depending on the dye type, the chromium salt—usually
chromates or dichromates—may be added before, during, or after dyeing. Accordingly, pre-mordanting, post-mordanting, and single-bath chromium dyeing processes are distinguished. Chromium dyes are noted for their excellent wet fastness. However, heavy metal contamination of fibers and dyeing wastewater is a significant ecological concern. Mordant dyes are designated as "C.I. Mordant Dyes" in the Colour Index. Examples: Mordant Black 9.svg|C.I. Mordant Black 9 Mordant Yellow 8.svg|C.I. Mordant Yellow 8 Mordant Black 7.svg|C.I. Mordant Black 7 Mordant Red 60.svg|C.I. Mordant Red 60 Mordant Blue 9.svg|C.I. Mordant Blue 9 Historically, in addition to chromium, iron, and aluminum salts, mordants based on
ammonium vanadate,
tannic acid,
aluminum oxide,
antimony,
barium,
lead,
cobalt,
copper,
manganese,
nickel,
tin, and
Turkish red oil were also used. Various antimony salts such as
potassium antimony tartrate or
antimony(III) chloride, as well as
sodium silicate and
sodium phosphate, and even
cow dung, were employed as fixing agents.
Direct dyes Direct dyes (or
substantive dyes) are absorbed directly from aqueous solution onto the fiber due to their high
substantivity. They are particularly suitable for
cellulose fibers. Binding to the fiber occurs through physical interactions, mainly
Van der Waals forces. Most direct dyes belong to the azo dye group, especially polyazo dyes. In the Colour Index, they are designated as
C.I. Direct Dyes. Examples: Direct Blue 8 (free acid).svg|C.I. Direct Blue 8 Direct Orange 26 (free acid).svg|C.I. Direct Orange 26 Direct Yellow 9 (free acid).svg|C.I. Direct Yellow 9
Disperse dyes Disperse dyes, which are almost insoluble in water, are primarily used for dyeing hydrophobic polyester and
cellulose acetate. They are finely ground together with
dispersing agents, enabling the molecularly dissolved dye to diffuse into the fiber during dyeing, where it forms a solid solution. This results in dyes with good wash and light fastness. The vast majority of disperse dyes belong to the azo dye class. Disperse dyes represent a highly important group, particularly due to the widespread use and mechanical performance of polyester fibers. In 1999, the total sales volume in Western Europe amounted to 98 million euros. According to the Colour Index, they are designated as "C.I. Disperse Dyes". Examples: Disperse Orange 44.svg|C.I. Disperse Orange 44 Disperse Blue 79.svg|C.I. Disperse Blue 797 Disperse Red 177.svg|C.I. Disperse Red 177
Development or coupling dyes In developing dyes, a practically water-insoluble dye is formed directly on the fiber by the reaction of a water-soluble coupling component (
C.I. Azoic Coupling Component) with a water-soluble diazo component (
C.I. Azoic Diazo Component). This dye class is mainly used for cellulose fibers and is characterized by very good wet fastness. The most important coupling component in developing dyes is
Naphthol AS. Azoic Coupling Component 2.svg|C.I. Azoic Coupling Component 2 (naphthol AS) Azoic Coupling Component 35.svg|C.I. Azoic Coupling Component 35 (naphthol AS-LG) Azoic Diazo Component 3.svg|C.I. Azoic Diazo Component 3 (Echt scarlet salt GG) Azoic Diazo Component 35.svg|C.I. Azoic Diazo Component 35 (Variamin blue salt B)
Cationic dyes Cationic dyes are
cationic compounds that produce brilliant and lightfast colors, particularly on
polyacrylonitrile (PAN) fibers and anionically modified
polyester fibers. They form ionic bonds with negatively charged groups on the fiber. Various chromophores can be used in cationic dyes; in methine dyes, the positive charge is delocalized, in contrast to other chromophoric systems. Although cationic dyes are designated as "C.I. Basic Dyes" in the Colour Index, the term "basic dyes" is no longer commonly used for this dye class in recent literature. Water-insoluble
sulfur dyes exhibit similar behavior. The most important vat dye is
indigo.
Indanthrene dyes are also of major importance. Vat dyes are designated as "C.I. Vat Dyes" in the Colour Index. Examples: Vat Green 11.svg|C.I. Vat Green 11 Vat Orange 7.svg|C.I. Vat Orange 7 Vat Red 23.svg|C.I. Vat Red 23 Flavanthron.svg|
Flavanthron Yellow Food colorants / food dyes Food colorants are used as
food additives to compensate for color changes caused by processing or to meet consumer expectations. Both naturally occurring and synthetically produced colorants are employed. The use of food colorants is strictly regulated by law—within the
EU by Regulation (EC) No. 1333/2008 of December 16, 2008, on food additives. Only approved additives bearing an E number may be marketed, and these must be declared on the product. Food colorants are designated as "C.I. Food Dyes" in the Colour Index. 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
azo dyes, although
anthraquinone and
triphenylmethane compounds are used for colors such as
green and
blue. Some naturally occurring dyes are also used.
Solvent dyes Solvent dyes, designated as "Solvent Dyes" in the Colour Index, are water-insoluble dyes that are soluble in various organic solvents such as alcohols, esters, or hydrocarbons. As a rule, solvent dye structures do not contain sulfonic acid or carboxyl groups. Exceptions include cationic dyes with an intramolecular sulfonate or carboxylate group acting as the counterion. Solvent dyes occur across various dye classes, including azo dyes, anthraquinone dyes, metal complex dyes, and phthalocyanines. They are used in lacquers (e.g.,
Zapon dyes for
Zapon lacquers), for coloring mineral oil products (
Sudan dyes),
wax,
inks, and transparent plastics. According to the Colour Index, they are designated as
C.I. Solvent Dyes. Examples: Solvent Yellow 124.svg|
Solvent Yellow 124 Oil Red O.svg|
Solvent Red 27 Oil Blue 35 Structural Formula V1.svg|
Solvent Blue 35 Solvent Yellow 32.svg|C.I. Solvent Yellow 32 Sudan Black B.svg|
Sudan Black B Solvent Red 8.svg|C.I. Solvent Red 8
Reactive dyes During the dyeing process, reactive dyes form a
covalent bond with functional groups of the fiber, resulting in dyes with high wet fastness. They constitute the largest group of dyes used for cellulose fibers, but are also employed for wool and polyamide in deep shades. Chemically, reactive dyes consist of two components: a chromophore and one or more reactive groups, also referred to as reactive anchors. Two major reactive anchor systems are used: • Heterocyclic compounds, such as halogen-substituted
triazines or
pyrimidines. During dyeing, these react with hydroxyl groups of the fiber, eliminating
halogen hydrides and forming stable covalent
ether bonds: • So-called
vinylsulfones, which react with
nucleophilic groups of the fiber during dyeing via a
Michael addition. Here as well, stable ether bonds are formed. In many vinyl sulfone dyes, the vinyl sulfone group is initially present in a protected form as a sulfuric acid semiester. Only under alkaline dyeing conditions is the vinyl sulfone group generated by elimination of sulfuric acid. Both types of reactive anchors may be present simultaneously in a single reactive dye. Azo dyes are by far the most common chromophores used in reactive dyes. However, other chromophoric systems, such as anthraquinone, formazan, and phthalocyanine dyes, are also important. Reactive dyes are designated as "C.I. Reactive Dyes" in the Colour Index. Examples: Reactive Orange 107.svg|C.I.
Reactive Orange 107 Cibacron Blue F 3GA.svg|C.I.
Reactive Blue 2 Reactive Blue 21.svg|C.I. Reactive Blue 21 Reactive Red 227.svg|C.I. Reactive Red 227 Procionbrilliantorange GS.svg|C.I. Reactive Orange 1
Acid dyes Acid dyes are
hydrophilic dyes containing
anionic substituents, usually sulfonic acid groups. Most acid dyes belong to the azo dye class, although other chromophores also occur. They are mainly used for dyeing wool, silk, and polyamide, with dyeing carried out in the pH range 2–6. When small dye molecules are used, uniform dyeing is achieved, with dye molecules forming primarily salt-like bonds with ammonium groups of the fiber. The wash fastness of such dyes is relatively moderate. With increasing molecular size, dye–fiber binding is enhanced through adsorption forces between the hydrophobic parts of the dye molecule and the fiber. This improves wet fastness, but often at the expense of dyeing uniformity. Acid dyes are designated as "C.I. Acid Dyes" in the Colour Index. Examples: Acid Black 1.svg|C.I. Acid Black 1 Acid Yellow 36.svg|C.I. Acid Yellow 36 Acid Blue 117.svg|C.I. Acid Blue 117 Acid Orange 19.svg|
Acid Orange 19 Patent blue V.svg|C.I. Acid Blue 3 (
Patent Blue V)
Functional dyes While conventional dyes are used to modify the appearance of textiles, leather, and paper, functional dyes generally serve non-aesthetic purposes. Typical applications include
indicator dyes or
voltage-dependent dyes. Special dyes can • absorb light at a specific wavelength and convert it into heat (e.g., in chemical and biochemical analysis), From an economic perspective, functional dyes are particularly important in the manufacture of CDs and DVDs. The dye molecules are embedded in the polycarbonate of the disc. The laser beam of the burner causes the dye molecules to absorb light energy and convert it into heat, leading to localized melting of the polycarbonate. This slightly altered surface structure is then detected during the reading process.
Laser dyes are for example
rhodamine 6G and
coumarin dyes.
Vital dyes A "vital dye" or stain is a dye capable of penetrating living cells or tissues without causing immediate visible degenerative changes. Such dyes are useful in medical and pathological fields in order to selectively color certain structures (such as cells) in order to distinguish them from surrounding tissue and thus make them more visible for study (for instance, under a microscope). As the visibility is meant to allow study of the cells or tissues, it is usually important that the dye not have other effects on the structure or function of the tissue that might impair objective observation. A distinction is drawn between dyes that are meant to be used on cells that have been removed from the organism prior to study (supravital staining) and dyes that are used within a living body - administered by injection or other means (intravital staining) - as the latter is (for instance) subject to higher safety standards, and must typically be a chemical known to avoid causing adverse effects on any biochemistry (until cleared from the tissue), not just to the tissue being studied, or in the short term. The term "vital stain" is occasionally used interchangeably with both intravital and supravital stains, the underlying concept in either case being that the cells examined are still alive. In a stricter sense, the term "vital staining" means the polar opposite of "supravital staining." If living cells absorb the stain during supravital staining, they exclude it during "vital staining"; for example, they color negatively while only dead cells color positively, and thus viability can be determined by counting the percentage of total cells that stain negatively. Because the dye determines whether the staining is supravital or intravital, a combination of supravital and vital dyes can be used to more accurately classify cells into various groups (e.g., viable, dead, dying).
Other important dyes A number of other classes have also been established, including: •
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. • Contrast dyes, injected for magnetic resonance imaging, are essentially the same as clothing dye except they are coupled to an agent that has strong paramagnetic properties. • Mayhems dye, used in water cooling for looks, often rebranded RIT dye == Pollution ==