In situ fabric coloration with indigo synthesised in flow

Indigo (CI Vat Blue 1) is a water-insoluble pigment exhibiting no affinity for fibres and must be chemically reduced in basic solution to form the water-soluble, alkaline leucoindigo (CI Reduced Vat Blue 1), in order to exhibit substantivity for fibres. Typical vat dyeing processes are time- and resource-intensive, and hazardous by-products are formed, primarily through the use of reducing agents. We describe a method for synthesising indigo in a flow reactor that allows for application of dye precursors to fibres, milliseconds before the reaction completes. The soluble precursors soak into the cotton fabric just prior to the precipitation of the insoluble indigo, effectively providing in situ coloration, without the need for a traditional redox dyebath. The reaction may be coupled with a propellant, an adaptation that allows for a sprayable form of indigo. In situ coloration with Tyrian purple (6,6′-dibromoindigo; CI Natural Violet 1) was also demonstrated using the flow chemistry method. This research provides compelling proof of concept, but we acknowledge that the process is in its infancy and needs further development to reach a stage where it can compete commercially with current technology.