Abstract:

Objective To address the issues of low production efficiency, high water and energy consumption, excessive salt usage, and significant wastewater discharge associated with the traditional exhausting dyeing method for loose cotton fibers with reactive dyes, the development of a complete set of continuous dyeing equipment for loose fibers could effectively solve this problem. In order to enhance the dyeing performance of this equipment, this study investigates the continuous dyeing technology for loose fibers under laboratory conditions, aiming to explore the optimal process parameters.

Method A laboratory padder dyeing process were employed to simulate the continuous dyeing technology. Taking the color fastness, evenness, fixation rate and color depth as the evaluation indexes, experiments were carried out, and the optimized process were obtained. The experiment on the equipment of XCYL-15 was conducted in a factory. The color parameters and color fastnesses of fibers dyed with the continuous method were compared with those of fibers dyed with the conventional exhausting method. In addition, the economic and environmental benefits were analyzed.

Results The optimized process parameters were identified through laboratory experiments, which are 50 g/L for the concentration of Reactive Black 5, 20 g/L for the concentration of mixed alkali (1∶4 of the mass ratio for NaOH against Na₂CO₃), 0 sodium sulfate addition, and 16 h stacking time at room temperature. With this optimal condition, the factory experiment was carried out with the XCYL-15 equipment. For the cotton fibers dyed with the continuous dyeing method, the K/S value was 39.86, the color difference ΔE₁ between the left and the middle of the cotton fiber layer was 0.49, and the color difference ΔE₂ between the right and the middle was 0.36, and the color difference ΔE₃ between the inner and outer was 0.27, and the color difference ΔE₄ between the front and rear was 0.62, indicating even dyeing of the cotton fiber layer and satisfaction of requirements. The color fastness of cotton fibers dyed via the continuous method was comparable to that of cotton fibers dyed using the traditional exhaust dyeing method. Specifically, the dry rubbing fastness and wet rubbing fastness of cotton fibers dyed by the continuous method were 4-5 and 3-4, respectively. Additionally, the K/S values of cotton fibers dyed by the continuous method and the traditional exhaust dyeing method were 39.86 and 36.57, respectively. However, the dye fixation of the continuous dyeing method reaches 86.5%, much higher than that of the traditional exhausting dyeing method, which is only 70.5%, resulting in high discharge of dyes into the dyeing wastewater. Compared with the traditional exhausting dyeing method, the continuous dyeing method would save 72% of water, 31.25% of dyes, 48% of electricity, 38% of steam, and reduce the salt usage by 100%.

Conclusion By adopting the continuous dyeing method for loose cotton fibers, the dye concentration on the fiber surface is 3.75 times that of traditional salt-containing exhausting dyeing method. According to the Fick's second law, when the dye diffuses into the fiber, the higher the concentration, the greater the concentration gradient along the fiber radial direction. Consequently, the diffusion rate of the dye into the fiber interior is also higher, which is beneficial to the level dyeing. Therefore, the dyeing and fixation of the cotton fibers by the high concentration dye can be completed without the necessity of adding additional salt. This technology will revolutionize the exhausting dyeing process in the cotton color-spinning industry and is of great social, economic, and environmental significance.

Key words: loose cotton fiber, continuous dyeing, reactive dye, cold pad-batch, water saving, energy conservation and emission reduction