Single box dyeing of denim yarns under nitrogen protection with high indigo concentration

In the era of "double carbon," green short process preparation of denim textiles will become more and more popular, and dyeing is an essential step in this process. In order to improve dye utilization and energy efficiency, a high-concentration indigo single box dyeing method based on nitrogen (N-2) protection has been studied. The optimal dyeing conditions were obtained. The dyeing mechanism was revealed by evaluating the stability of the dyeing solution and dye distribution in the dyed yarns. Various dyeing properties, chemicals, energy consumption, and dyeing wastewater were measured and compared with conventional slasher dyeing. It has been shown that N-2 protection effectively avoids the oxidation of soluble leuco-indigo and improves the stability of the leuco dyeing bath. In addition, the dyes diffused more deeply into the fiber in the dyeing box under a micro-positive pressure environment, enhancing the dyeing efficiency and depth. The proposed technique exhibited better dye utilization and excellent color fastness. Dyeing wastewater was more chemical and biodegradable, relieving wastewater treatment. Generally, cleaner dyeing not only reduces the consumption of energy, water, and chemicals, but also saves 46.8% of the economic cost compared with traditional slasher dyeing, indicating that the proposed technology is interesting for industrial application.