Abstract:

Objective Conventional dyeing with indigo dyes would cause great damage to wool fibers and high alkalinity of dye wastewater. Aiming to achieve industrial scale production of denim-imitating wool fabrics, wool dyeing in micro-water system with indigo dyes is studied by using thymol-menthol hydrophobic deep eutectic solvent (HDES) as the dyeing medium, expecting to achieve high efficiency of indigo dyeing with minimal strength loss.

Method The influences of dyeing temperature, dye dosage and bath ratio on dyeing effect of HDES were investigated by using K/S and dyeing rate as indexes. The quality of wool indigo dyeing in HDES was analyzed through evaluations of K/S value, uptake rate, color fastness and dye transmittance. The influence of thymol-menthol on the physical and chemical properties of wool fabrics was investigated using attenuated total reflectance fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy. The mechanical properties and alkali solubility of the dyed wools were explored. The reusability of the HDES was investigated and the medium residue on the dyed wool fabric was measured.

Results Menthol and thymol became homogeneous liquids when being heated and could form hydrophobic deep eutectic solvents. As the dyeing temperature, dye dosage and bath ratio were increased, the K/S value of wool fabrics using the thymol-menthol system also increases. The dyeing temperature of 80 ℃, the dye dosage of 4%, and the bath ratio of 1∶20 were selected as the optimum dyeing conditions. Under the optimum dyeing conditions, the K/S value of indigo dye-dyed wool fabrics reached 18.23. The wool fabrics were dyed thoroughly and evenly to the degree of dyeing required by denim fabrics. The color fastness test of the dyed wool fabrics showed that the color fastness to soaping reached grade 4-5, the color fastness to dry and wet rubbing were grade 3-4 and grade 3, respectively, and the color fastness to light was grade 3-4, which are comparable to the effect of conventional water-phase dyeing. Comparison of the physical and chemical structure of wool fabrics before and after dyeing revealed that the protein macromolecule structure and crystal structure of wool were not seriously affected. Although the breaking strength of dyed wool became lower than that of the original wool, the strength loss was within the acceptable range, and the breaking elongation was similar to that of the original wool fabric. The mechanical properties of dyed wool under the thymol-menthol eutectic solvent system were better than those of dyed wool under the traditional water system. Through a simple separation process, most of the hydrophobic deep eutectic solvent was recovered from the dyeing waste liquid and reused for recycled dyeing. In addition, the residue of thymol-menthol on the wool fabrics after soaping was found extremely low, confirmed by UV-absorption spectroscopy analysis of the ethanol extract.

Conclusion In this study, K/S value and uptake rate were adopted as indexes to investigate the influences of dyeing temperature, dye dosage and bath ratio on dyeing effect of the system. The dyeing quality of wool indigo dyeing in HDES was analyzed, and the influences of thymol-menthol on physical and chemical properties of wool fabric were investigated. The following conclusions were made. 1) The dyeing effect of wool is better when the dyeing temperature is 80 ℃, the dye dosage is 4% (o.w.f) and the bath ratio is 1∶20 in HDES. 2) Under the optimum dyeing conditions, the K/S value of wool dyed with indigo dye can reach 18.23, which have a certain dyeability and can achieve the dyeing degree required by denim clothing. The color fastness to soaping, the dry and wet rubbing fastness, and the fastness to sunlight are good. 3) The influence of thymol-menthol medium dyeing on the macromolecular structure, crystal structure, morphology and mechanical characteristics of wool protein are all within an acceptable range. 4) Almost no residue of thymol-menthol medium on wool fabric was found after dyeing and soap washing. After simple separation of dyeing waste liquid, the thymol-menthol medium can be reused.

Key words: wool, indigo, minimal water dyeing, deep eutectic solvent, denim-like style, thymol, menthol