Cleaner Dyeing Technology for Denim Fabrics with Excellent Utilization of Indigo Based on Inert Gas Protection

Traditional indigo denim dyeing has a remarkable impact on the environment as it consumes large amounts of energy, water, and chemicals. This study examined the feasibility of using nitrogen (N2) protection to overcome dyeing difficulties caused by leuco-indigo early oxidation in cotton fabrics dyed with indigo reduced by sodium dithionite (Na2S2O4). The effect of N2 protection on the molecular electrostatic potential (ESP) of indigo and leuco-indigo was investigated by density functional theory calculations, and it was shown that the stability of indigo and leuco-indigo increased upon the injection of N2. Furthermore, it was discovered that the continuous injection of lower amounts of N2 during reduction and dyeing with indigo prevented dye agglomeration in a dye bath and produced deeper shades and more level dyeing. Besides, the dyeing kinetic model and thermodynamic parameters showed that the dyeing processes fitted well with the Freundlich model and the adsorption processes were exothermic and spontaneous. Furthermore, orthogonal experiments and gray clustering analysis were used to optimize the dyeing process, and it could increase the K/S and S values of the dyed denim fabric by 10.63 and 54.72%, respectively, compared to the traditional dyeing process. It is evident that the eco-friendly dyeing strategy presented here could significantly enhance denim production’s sustainability in the areas of economics, energy efficiency, and environmental protection.