Natural Indigo
Indigo dye is an important dyestuff with a distinctive blue color. The natural dye comes from several species of plants.A variety of plants have provided indigo throughout history, but most natural indigo is obtained from plants in the genus Indigofera, which are native to the tropics. In temperate climates indigo can be obtained from woad (Isatis tinctoria) and dyer's knotweed (Polygonum tinctorum), although the Indigofera species yield more dye. The primary commercial indigo species in Asia was true indigo (Indigofera tinctoria, also known as Indigofera sumatrana). In Central and South America the two species Indigofera suffructicosa and Indigofera arrecta (Natal indigo) were the most important.
Properties of Indigo
Indigo is an insoluble dye ,a dark blue crystalline powder, which has to be reduced with suitable reducing agents to make it soluble in water,Indigo works by a chemical reaction called oxidation reduction.Indigo does not dissolve in water. It must be reduced — i.e. the oxygen must be removed— in the presence of alkali by a reducing agent such as thiourea dioxide (thiox), sodium hydrosulfite, Zinc, or bacteria. Upon reduction, indigo becomes colorless and water soluble. In this state, indigo has a high affinity for cellulosic fibers and enters the open spaces of the fiber. The dyed fibers are then exposed to air, which oxidizes the dye molecule back to its insoluble form. The insoluble dye particles are trapped inside the fiber, coloring them permanently blue. Unlike most dyes, indigo forms a mechanical, not chemical, bond.
A.Indigo is a dark blue crystalline powder that melts at 390°–392°C.
B.It is insoluble in water, alcohol, or ether but soluble in chloroform, nitrobenzene, or concentrated sulfuric acid.
C.The chemical structure of indigo corresponds to the formula C16H10N2O2.
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D.The naturally occurring substance is indican, which is colorless and soluble in water. Indican can easily be hydrolyzed to glucose and indoxyl.
E.Mild oxidation, such as exposure to air, converts indoxyl to indigo.
F.Indigo is a frequency range of visible light, from 440 to 420 nanometers in wavelength, placing it between blue and violet.
Indigo dye
Indigo has despite many other blue dyestuffs kept his popularity. This by no doubt is achieved by the fact that Indigo has a number of properties which have in this combination not yet been achieved by other single dyestuffs.
The main properties are:
• Pleasant colour shade.
• Possibility to achieve by simple repeated dipping, deep marine blue shade.
• Possibility to dye cotton in cold dye bath.
• Competitive in price.
• Possibility to achieve an acceptable colour fastness and the exceptional advantage by repeated washing of fading colour, to keep the colour shade that always a clear, pleasant blue shade result.
Indigo Application
Three basic steps for indigo application are .
1.Dissolving the dye by reduction involving vatting and loosening up.
2.Dyeing from the vat.
3.Oxidising in the air.
Since indigo has a low affinity for cotton, deep blue dyeings are only obtained when dyeing and oxidising are repeated several times.
Dyeing Methods
Five major Indigo dyeing methods for the basic denim are
1- Indigo rope dyeing process
2- Indigo one sheet dye slashing
3- Indigo double sheet dyeing
4- Loop dye 1 for 6 (continuous dye slashing)
5- Loop dye 1for 6 with dyemer (continuous mercerization dyeing and sizing).
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