Indigo ( indigotin, ~2. 2 ' - biindoline-3,3'dione) is one of the oldest colouring agents known to man, having been found in the wrappings of Egyptian mummies. Its natural sources are 'dyer's woad' (lsatis tinetoria), a plant indigenous to Europe, and the indigo-plant (lndigofera tinctoria or sumatrana), native to India and Java. Immediately after harvesting these plants, their leaves and stalks are steeped in water. Once fermentation sets in, the indican (2) is hydrolysed to glucose and indoxyl (3). The resulting aqueous solution is decanted and aerated to oxidise indoxyl to indigo.
CONVERSION OF INDIGO PRECURSOR INTO INDIGO
Excavation in the Indus valley indicates that Indigo dye has been used in India since its ancient eras. The association of Indigo with India is reflected in the Greek word for the dye, Indikón (coming from India). The Romans latinised the term to Indicum, which went into the Italian dialect and eventually into English as the word ‘Indigo’. The dye was also known to ancient civilisations in Mesopotamia, Egypt, Greece, Rome, Britain, Peru, Iran and Africa. Indigo was also the foundation of centuries old textile traditions throughout West Africa (Kriger and Connah, 2006). Thus indigo is one of the oldest dyes used by mankind.
The traditional reducing agent used is sodium dithionite (Na2S2O4) because of its effective reduction for indigo dyes. It is also called sodium hydrosulphite or hydrose. Typically, the amount of Na2S2O4 that is industrially used in the reduction reaction of indigo dyeing is more than the stoichiometric amount, resulting in a large amount of by-products from the process e.g. sulfite (SO32-) and sulfate (SO42-) ions as shown in below equation.
These ions can cause ecological problems. For example, SO32- can be easily oxidised to SO42-and the high concentration of SO42- can cause corrosion in concrete pipes. Researchers highlight the use of copperas’ method, followed by the zinc–lime method for indigo reduction. This is the metal-based chemical technique.
These methods were not satisfactory due to heavy precipitates of ferrous and zinc hydroxide. Also highlighted is the use of eco-friendly reducing agents such as Alpha-hydroxy ketones (more expensive product), Glucose (need high temperature) etc. Hence, the search for natural reducing agents & natural alkalis is inevitable. Michel’s technique, which is based on the traditional Indigo vats of Morocco, India and Provence, relies on the chemical reactions between a mineral alkali and a natural reducing agent to remove excess oxygen (a chemical process called reduction) , which liberates the Indigo dye molecule, allowing it to attach to fibers and bond. These natural reducing agents include dried and fresh fruits, minerals, and flavanoids.
Keeping all of this in mind and with continuous R&D, we found new methods for the reduction of natural indigo dyes using natural reducing agents (different from all other researchers & dyers) and also eco-friendly alkalis which were never before used for the Indigo dye reduction purpose.