Dyeing textiles by traditional water-based methods demands large amounts of water. Roughly to dye 1 kg of textile 100 1 of clean water are used; annually this requires 9 trillion liters of water. At the end of the dyeing process, same equivalent amount of wastewater is generated that needs to be treated before discharge. The dye bath effluents contain salts, bases and hydrolyzed dye molecules, which make water purification expensive. For the last 25 years, supercritical carbon dioxide (scCO_2) has been developed as an alternative for water as dyeing medium to eliminate and reduce the usage of water and wastewater production in the conventional textile dyeing industry. The physical properties of scCO_2 with regards to high diffusion rates and low mass transfer resistance facilitate the penetration of the dye into the textile decreasing dyeing times. Since no water is use, the drying of the textile is not necessary, saving a large amount of energy. Dyes are dissolved in the scCO_2 and can be reused as is the CO_2 Nowadays, supercritical dyeing of Polyester is commercially available. Several years of equipment engineering and process development have been crystallized in industrial supercritical dyeing machines. This development has opened up a sustainable way of textile dyeing that will be followed soon by natural textiles such as wool and cotton on a commercial scale.
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