Everything about Dyes totally explained
A
dye can generally be described as a
colored substance that has an
affinity to the to which it's being applied. The dye is generally applied in an
aqueous solution, and may require a
mordant to improve the fastness of the dye on the fiber.
Both dyes and pigments appear to be colored because they absorb some wavelengths of
light preferentially. In contrast with a dye, a
pigment generally is insoluble, and has no affinity for the substrate. Some dyes can be
precipitated with an inert salt to produce a
lake pigment, and based on the salt used they could be aluminum lake, calcium lake or barium lake pigments.
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
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.
Organic dyes
The first human-made (synthetic)
organic dye,
mauveine, was discovered by
William Henry Perkin in 1856. Many thousands of synthetic dyes have since been prepared.
Synthetic dyes quickly replaced the traditional natural dyes. They cost less, they offered a vast range of new colors, and they imparted better properties upon the dyed materials. Dyes are now classified according to how they're 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. Most synthetic food colors fall in this category.
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 (Na
2SO
4). 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 color significantly. Most natural dyes are mordant dyes and there's 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. It is important to note that many mordants, particularly those in the heavy metal category, can be hazardous to health and extreme care must be taken in using them.
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. The color of denim is due to indigo, the original vat dye.
Reactive dyes utilize a
chromophore attached to a
substituent that's capable of directly
reacting with the fibre substrate. The
covalent bonds that attach reactive dye to natural fibers make them 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 dyes are 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. Their main use is to dye
polyester but they can also be used to dye nylon,
cellulose triacetate, 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 color is controlled by the choice of the diazoic and coupling components.
Sulfur dyes are two part "developed" dyes used to dye cotton with dark colors. The initial bath imparts a yellow or pale
chartreuse color. This is aftertreated with a sulfur compound in place to produce the dark black we're familiar with in socks for instance. Sulfur Black 1 is the largest selling dye by volume.
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're 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 colors such as
green and
blue. Some naturally-occurring dyes are also used.
Other important dyes
A number of other classes have also been established, including:
- Oxidation bases, for mainly hair and fur
- 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.
- Carbene dyes, a recently developed method for coloring multiple substrates
Chemical classification
By the nature of their
chromophore, dyes are divided into:
(External Link
)
, derivates of acridine
, derivates of anthraquinone
Arylmethane dyes
, based on -N=N- azo structure
Cyanine dyes, derivates of phthalocyanine
Diazonium dyes, based on diazonium salts
Nitro dyes, based on a -NO2 nitro functional group
Nitroso dyes, based on a -N=O nitroso functional group
Phthalocyanine dyes, derivates of phthalocyanine
Quinone-imine dyes, derivates of quinone
-
- Indamins
- , derivates of indophenol
- , derivates of oxazin
- Oxazone dyes, derivates of oxazone
- , derivates of thiazin
, derivates of thiazole
Xanthene dyes, derived from xanthene
Further Information
Get more info on 'Dyes'.
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