Abstract:

Objective Although powdered natural indigo is convenient for storage and transportation, it readily forms soft agglomerates in aqueous media due to electrostatic adsorption during dyeing. These aggregates are difficult to disperse by stirring, leading to low dye utilization, dust generation, and excessive wastewater in industrial production. Preparing liquid indigo dyes offers a promising solution by reducing particle size, enhancing reduction efficiency and dye uptake, and eliminating dust at its source.

Method A dispersion was prepared by mixing 30 g of vegetable indigo, 3 g of abrasive MF-2000A, 0.9 g of viscosity modifier Z5, and 63.1 g of water, followed by stirring at 600-700 r/min for 15 min. The mixture was transferred into a custom grinder, combined with 100-150 g of zirconium beads, and ground at 1 700-1 900 r/min. Particle size was monitored every 30 min with adjustments in grinding speed, and grinding was terminated once particle size plateaued. The resulting suspension was filtered through silk crepe de chine to achieve a 30% liquid indigo dye. The dipping process for dyeing cotton fabrics with 10 g/L liquid indigo was then optimized via single-factor experiments at a fixed dyeing temperature of 60 ℃ and subsequent steaming at 102 ℃ for 30 s. Finally, cotton fabrics dyed with 5-25 g/L liquid or powdered indigo were evaluated and compared in terms of K/S, L^*, a^*, b^* values, rubbing and soaping fastness, as well as UV protection and antibacterial performance.

Results After grinding, the liquid indigo dye exhibited an average particle size of 543 nm with a polydispersity index of 0.01, which only increased to 656 nm after six months of storage, indicating excellent stability. Upon 50 000-fold dilution and 24 h storage, the absorbance change was merely 3.2%. MF-2000A facilitated ultrafine dispersion by preventing particle re-aggregation, while Z5 coated suspended particles to reduce interparticle friction and inhibiting sedimentation. The viscosity of the liquid dye decreased rapidly with increasing shear rate or temperature, consistent with pseudoplastic fluid behavior. In optimized conditions (10 g/L liquid indigo, 3 g/L NaOH, 12 g/L sodium hydrosulfite, 15 min reduction, six dips), the K/S value reached 15.61. The dyed fabrics achieved rubbing fastness of more than 3, soaping fastness of more than 3, UVA transmittance of 1.07, UVB transmittance of 1.15, and an ultraviolet protection factor of 90.19. Additionally, the antibacterial activity against Escherichia coli increased to 70%. Compared with powdered indigo, liquid indigo improved K/S values of cotton fabrics by 6.6%-49.3% across 5-25 g/L dye concentrations. This improvement arises from the finer particle size and absence of agglomeration, which enhance dye reduction and fabric wettability, while powdered dyes form agglomerates through van der Waals and hydrogen bonding interactions, reducing their effective surface area and diminishing reduction efficiency.

Conclusion The prepared liquid indigo dye demonstrated excellent storage stability, pseudoplastic rheology, and superior dyeing performance compared with powdered indigo. The optimal process parameters were a dye/NaOH/sodium hydrosulfite ratio of 1∶1∶4, a 15 min pre-reduction time, and six dipping cycles. At 10 g/L, liquid indigo increased the K/S value of dyed cotton fabrics by 49.3% relative to powdered indigo, while also imparting strong UV protection and moderate antibacterial activity.

Key words: liquid dye, plant indigo, cotton fabric, rheological property, dyeing property, UV protection, antibacterial property