Cleaner dyeing based on mass transfer enhancement: Pad-steam dyeing of cotton fabric pretreated with deep eutectic solvent

High consumption of salt and alkali is a prominent characteristic of the conventional reactive dyeing process for cotton fabrics. The wastewater from dyeing processes contains substantial amounts of sodium ions and unused dyes, which can cause severe environmental pollution. Therefore, this study developed a novel and clean dyeing method based on mass transfer enhancement in micro dyebath. This was achieved by pretreating cotton fabric with deep eutectic solvent (DES) in combination with “pad-steam” water-saving dyeing process. Under identical conditions, this method achieved a fixation rate of 93.41% for Reactive Black 5(RB5), whereas the conventional method achieved only 72.12%. When achieving optimal dyeing results, this method reduces the consumption of salt and alkali by half compared to conventional methods. Moreover, DES could be recovered and reused, and the dyeing effect remained stable over five cycles. Research indicates that the novel “Urea-Glycerol-K 2 CO 3 (UGK)” ternary DES, leveraging its hydrogen bond disruption capacity and distinctive ionic characteristics, can alter the original hydrogen bond network within cotton fibers, appropriately enlarge amorphous regions, expand dye diffusion pathways, and simultaneously regulate the surface chemical properties of the fibers. These effects collectively enhance the mass transfer of dyes into the fiber during dyeing, improve the fixation rate, reduce the consumption of salt and alkali, and shorten the steaming time, thereby achieving the goal of cleaner dyeing. This study presents a green and sustainable approach for a cleaner dyeing process of cotton fabrics.