Love for color is a natural instinct. Every individual has his own
choice and liking for color. Whether it is icy appearance of Himalayan
ranges or evergreen forests or fields of agricultural produce or trees
ladden with colorful ripe fruits or butterfly moving from one flower to
another, generations after generations are being attracted. A beautiful
fascinating color is anybody's choice selection.
Many natural dyes have been known for a long time. They were obtained from
animal and vegetable sources. Today however practically all dyes are
synthetic. They are prepared from aromatic compounds, for which then the
only available source was coal tar. Hence the name coal tar dyes. Organic
dyes are colored. Organic compounds have an ability to impart their
color to the material to be dyed, in an aqueous medium.
Until the middle of nineteenth century, all dyes available to man came
from natural sources. Most of these were vegetable extracts and a few
were animal products. The range of colors was limited as was the utility
of dyes. If a natural dye did not have the chemical groupings necessary
to react with the chemical groupings of a particular fabric, the fabric
could not be dyed with that material.
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The history of natural dyes is very interesting. Egyptian mummies have been found wrapped in cloth dyed from the madder plant. |
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Alexander the great is supposed to have deceived the Persians into thinking that his army was wounded, by sprinkling his soldiers with a red dye, probably madder juice, which contains the dye alizarin. |
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The dark blue indigo dye has been known for over 4000 years. When Romans invaded England, they found that the country was inhabited by the ancient people called Picts. They both tattooed and painted themselves with indigo. Latin meaning of Briton is painted men. |
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A legend recorded on coins attests that Hercules, the god of strength, discovered Tyrian purple. His dog bit a snail which stained his jaws purple. |
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Mark Antony's flight from the crucial naval battle of Actium was especially conspicious because he fled in Cleopatra's barge, which was carrying snails, known for their color royal purple. |
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In 1771 Woulfe prepared picric acid by the action of nitric acid
on indigo and showed that it dyed silk in bright yellow shades. Laurent
in 1842 converted phenol into picric acid and Perkin in 1856 discovered
Mauve, the first synthetic dye to be manufactured and used for practical
dyeing. Faraday discovered benzene in 1825 and Hofmann isolated it from
coaltar in 1845. By 1869 Kekule established the structure of benzene.
It paved the way for the systematic study of aromatic compounds. During
the last 125 years synthetic dyes have been prepared in bewildering number
and variety. Also the possiblities of further synthesis are unlimited.
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Direct or subtantive dyes |
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Mordant dyes and chrome dyes |
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Azoic dyes |
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Sulphur dyes |
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Vat dyes |
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Disperse dyes |
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Fibre reactive dyes |
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Oil and spirit soluble dyes |
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Flourescent brightening and optical whitening agents |
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Food, drug and cosmetic color |
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Cellulose acetate dyes |
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Acetate rayon dyes |
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Nylon dyes |
Dyes can be classified according to their chemical constitution or on the
basis of application to fibres. There is common ground between the two
methods of classification. Certain groups of dyes with their specific
chemical character and the methods of dyeing are dependent on each other.
Azo Dyes
It is the largest group of dyes, with-N=N- as a chromophore, in an aromatic
system. Depending upon the number of azo-groups present they are called as
monazo, disazo, trisazo, tetrakisazo and polyazo dyes. Diazotisation of a
primary amine, in presence of Hcl + NaNo2 at freezing temperature, produces
a diazonium salt which in turn coupled with aromatic compounds, producing
an azo-dye. All types of azo-dyes amount to over one thousand, commercially
most important class of synthetic coloring compound. It has wide variety
of applications.
Anthraquinone Dyes
Anthraquinone is the basic unit of this class of dyes. It is faint yellow
in color which is sufficient to use it as a dye but it cannot be classified
as a dye. Introduction of hydroxyl and amino groups in anthraquinone a wide
range of colors are available. Dyes containing anthraquinone unit belong
to mordant, disperse and vat dyes. Its quinonoid system acts as a chromophore.
Anthraquinone dyes have excellent fastness properties.
Disperse dyes
Dyes generally use to dyecellulose acetate, nylon and other hydrophobic
fibres are called disperse dyes. They are also known as acetate dyes.
Sulphoricin oleic acid (SAR) is used as the dispersing agent. Dispersal
and cellitoin are the important dispersing agents.
Reactive dyes
They are mainly of three types :
Procion dyes
Remazol - sulphone dyes
Disperse dyes
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