Indigo, the unique colorant for ubiquitous blue jeans, is one of the oldest dyes known. The indigo dye product itself is highly sustainable; it is safe enough to be widely used as a food colorant. However, the indigo dyeing process is criticized for poor sustainability, primarily because sulfur reducing compounds and large amounts of problematic wastewater are still required with current dyeing methods.
Indigo is insoluble until it undergoes a chemical change called reduction – the chemistry opposite of oxidation – which makes it soluble and able to penetrate into cotton. Once inside the cotton, it must be returned to an insoluble state, which is accomplished by exposing the dyed cotton to oxygen. Simply exposing it to the air will accomplish this within seconds, resulting in the intensely blue indigo that is so dominant in blue jeans and other textiles.
This project is to be understood as applied development of a known technology, foam application of dye. Foam dyeing is a water-saving, environmentally friendly technology that is increasingly used around the world. However, its use for indigo dyeing of denim yarns has been stymied by the inability to use air for creating the foam medium for the indigo. To overcome this limitation, a unique, patented design maintains oxygen-free conditions until yarns are ready for exposure to either open air or to an oxidation chamber.
The system developed can convert raw cotton yarns to indigo-dyed yarns without pretreatment of the yarns. Speed of the dyeing process can be multiplied, dye uptake and dye fastness greatly improved, water and energy use greatly reduced, floor space required for dyeing dramatically reduced, and all without the use of the sulfur compounds. The problem of treating effluents in the water from dyeing becomes insignificant. Moreover, existing indigo dyeing technology allows substantial dye liquid to oxidize on the yarn surfaces; this must be washed off as waste because this dye is subject to rubbing off and has a duller color than the purer blue held in the interior of the fibers. This new technology prevents this from occurring, which results in superior dye uptake, dye fastness, and appearance. In summary, substantial environmental benefits come simultaneously with substantial cost reductions and quality improvements, giving this technology the potential to have a revolutionary impact on denim manufacturing.