Technology of Denim Production: Part- II (Warping & Indigo Dyeing Techniques for Denim)
Indigo dyestuff originally extracted from the varieties of plants, including wood plants. Indigo belongs to the legume family and over three hundred species have been identified. Most natural indigo is extracted throughout the ancient world from the genus Indigofera. In Asia, the primary commercial indigo species was Indigofera tinctoria (true Indigo) which was also known as Indigofera sumatrana. In Central and South America the commercial indigo species were Indigofera suffructicosa and Indigofera arrecta (Natal indigo).
Indigo is popular from the ancient time due to its brilliant blue hue to fabric. Indigo partially penetrates into the fibers and impart unique surface color. The inner layers remain uncolored when it is indigo dyed. The indigo having the unique characteristic of fading during rubbing or daily usage of wear and repeated washing. This gives a worn look and for this reason it is commonly used to color denim.
Until about 1900, natural indigo was the only source of the dye. As the demand for indigo increased during the industrial revolution, the natural extraction process of indigo could not fulfill the huge demand of indigo. Hence there is a need of alternate source of indigo. All over the world, chemists began researching for synthetic methods of producing the dye.
In 1883 Adolf von Baeyer (of Baeyer aspirin fame) developed the chemical structure of indigo. He found that oxindole can be produced by treating omega-bromoacetanilide with an alkali (a substance that is high in pH). Based on his observation, K. Heumann identified a synthesis pathway to produce indigo.
Indigo is a crystalline powder that melts at 390392°C. It is dark blue in colour, insoluble in water, alcohol, or ether but soluble in chloroform, nitrobenzene, or concentrated sulfuric acid. Its chemical structure of indigo corresponds to the formula C16H10N2O2.
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The author is Manager (TQM) at Shri Lakshmi Cotsyn Limited, UPSIDC Industrial Area, Malwan, Dist. Fatehpur, UP