Dyes and Pigments
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Wikipedia-Article "Dyes"
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 usually used as an aqueous solution, and may require a mordant to improve the fastness of the dye on the fiber. In contrast, a pigment generally has no affinity for the substrate, and is insoluble.
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 either 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.
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Organic dyes
The first man-made organic dye, mauveine, was discovered by William Henry Perkin in 1856. Many thousands of dyes have since been prepared and, because of vastly improved properties imparted upon the dyed materials, quickly replaced the traditional natural dyes. 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.
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 colour 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.
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.
Reactive dyes utilize a chromophore containing a substituent that is capable of directly reacting with the fibre substrate. The covalent bonds that attach reactive dye to natural fibers make it 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 dye is 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. They can also be used to dye nylon, triacetate, polyester 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 colour is controlled by the choice of the diazoic and coupling components.
Natural dyes
Animal origin
These include tyrian purple (vat dye), kermes and cochineal (mordant dyes) and techelet.
Vegetable origin
Substantive dyes include walnut hulls, safflower and turmeric, while indigo and woad are vat dyes. Mordant dyes include alizarin (madder), dyer's broom, brazilwood, quercitron bark, weld and old fustic. Cudbear is unclassified.
Inorganic dyes
These include eosin and iron buff.
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 colours such as green and blue. Some naturally-occurring dyes are also used.
Other
A number of other classes have also been established, including:
- Oxidation bases, for mainly hair and fur
- Sulfur dyes, for textile fibres
- Leather dyes, for leather
- Fluorescent brighteners, for textile fibres and paper
- Solvent dyes, for wood staining and producing coloured lacquers, solvent inks, colouring oils, waxes.
- Carbene dyes, a recently developed method for colouring multiple substrates
Chemical classification
By the nature of their chromophore, dyes are divided to: [1]
- Category:Acridine dyes, derivates of acridine
- Category:Anthraquinone dyes, derivates of anthraquinone
- Arylmethane dyes
- Category:Diaryl methane dyes, based on diphenyl methane
- Category:Triarylmethane dyes, derivates of triphenyl methane
- Category:Azo dyes, based on -N=N- azo structure
- Category:Cyanine dyes, derivates of phthalocyanine
- Category:Diazonium dyes, based on diazonium salts
- Category:Nitro dyes, based on a -NO2 nitro functional group
- Category:Nitroso dyes, based on a -N=O nitroso functional group
- Category:Phthalocyanine dyes, derivates of phthalocyanine
- Category:Quinone-imine dyes, derivates of quinone
- Category:Azin dyes
- Category:Eurhodin dyes
- Category:Safranin dyes, derivates of safranin
- Indamins
- Indophenols
- Category:Oxazin dyes, derivates of oxazin
- Category:Oxazone dyes, derivates of oxazone
- Category:Thiazin dyes, derivates of thiazin
- Category:Azin dyes
- Category:Thiazole dyes, derivates of thiazole
- Xanthene dyes, derived from xanthene
- Fluorene dyes, derivates of fluorene
- Category:Pyronin dyes
- Category:Rhodamine dyes, derivates of rhodamine
- Category:Fluorone dyes, based on fluorone
- Fluorene dyes, derivates of fluorene
Also
External links
Wikipedia-Article "Pigments"
- For the drug referred to as "pigment," see black tar heroin.
In biology, pigment is any material resulting in colour in plant or animal cells which is the result of selective absorption. Some biological material has so-called structural colour, which is the result of selective reflection or iridescence, usually done with multilayer structures. Unlike structural colour, pigment color is the same for all viewing angles. Nearly all types of cells, such as skin, eyes, fur and hair contain pigment. Butterfly wings typically contain structural colour, although many of them contain pigment as well. Creatures that have deficient pigmentation are called albinos.
Because pigment colour is the result of selective absorption, there is no such thing as white pigment. A white object is simply a diffuse reflecting object which does not contain any pigment.
In the coloring of paint, ink, plastic, fabric and other material, a pigment is a dry colorant, usually an insoluble powder. There are both natural and synthetic pigments, both organic and inorganic ones. Pigments work by selectively absorbing some parts of the visible spectrum (see light) whilst reflecting others.
A distinction is usually made between a pigment, which is insoluble, and a dye, which is either a liquid, or is soluble. There is a well-defined dividing line between pigments and dyes: a pigment is not soluble in the vehicle while a dye is. From this follows that a certain colourant can be both a pigment and a dye depending on in which vehicle it is used. In some cases, a pigment will be made by precipitating a soluble dye with a metallic salt. The resulting pigment is called a "lake".
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List of pigments
Heme/Porphyrin based
Light emitting
Lipochromes
- Carotenoids
- Alpha and Beta Carotene
- Cyanins
- Lycopene
- Rhodopsin
- Xanthophylls
Photosynthetic
Other
- Hematochrome
- Melanin - Which causes human skin coloration
- Phthalocyanine blue
- Urochrome
- polyene enolates are a class of red pigments unique to parrots.
Painting pigments
- Alizarin (Alizarin Crimson)
- Bone black (also known as bone char)
- Cadmium pigments (Cadmium Green, Cadmium Red, Cadmium Yellow, Cadmium Orange)
- Caput Mortuum
- Carbon black
- Cerulean blue
- Chromium pigments (Chrome Green, Chrome Yellow)
- Cobalt pigments (Cobalt Blue)
- Crimson
- Fugitive pigments
- Gamboge
- Indian Yellow
- Indigo
- Ivory black
- Vine black
- Lamp black
- Mars black
- Lead pigments (Lead white, Naples Yellow, Cremnitz White, Foundation White, Red Lead)
- Paris Green
- Phthalocyanine (Phthalo Green, Phthalo Blue)
- Prussian blue
- Quinacridone (Quinacridone Magenta)
- Oxide Red
- Red ochre
- Sanguine
- Sienna (Raw Sienna, Burnt Sienna)
- Titanium dioxide (Titanium White)
- Ultramarine (Ultramarine Green Shade, French Ultramarine)
- Umber (Raw Umber, Burnt Umber)
- Van Dyke brown
- Venetian Red
- Verdigris
- Vermilion
- Viridian
- Yellow ochre
- Zinc white