There are two kinds of electrochemical reduction dyeing: direct method and indirect method. The direct method is to make the dye get electrons directly from the cathode, become colorless and dissolve in alkaline solution. The indirect method is that the medium reacts with the electrode to generate reducing medium, and then the dye is reduced to colorless body to dye the fiber. Because the dye matrix is insoluble in water, it is difficult to be directly reduced by the electrode, so the indirect method is usually efficient.
Many compounds can be used as media, including inorganic compounds and organic compounds. Inorganic compounds are mostly metal double salts, especially some Fe2+ complexes. There are many different ligands, especially some complexes that are stable in alkaline solution, which are easily reduced to Fe2+ by Fe3+ (the redox potential in alkaline solution is at least below -600mv, preferably below-1000 mv). In addition, the speed of transferring electrons from the cathode to the oxidation medium (Fe3+ complex) should be high, and the speed of transferring electrons from the reduction medium (Fe2+ complex) to the dye matrix should be fast, so that the medium can be reused.
The mechanism of indirect electrochemical reduction is as follows (taking Fe2+ triethanolamine complex as an example);
The first reduction step is the reduction of Fe3+ complex at the cathode;
Fe3+L+e= Fe2+L(L is triethanolamine ligand)
The reduced complex diffuses from the electrode into the dye matrix and reduces it.
The dyed dye leuco is finally oxidized into insoluble dye matrix and fixed on the fiber;
For vat dyes with complex structure, there are many carbonyl groups and the reduction reaction is complicated. Like the conventional reduction method, there will be problems such as over-reduction. Therefore, different electrochemical reduction conditions should be selected for different dyes, and the specific process needs further study. It is worth developing because it is easy to control without or with little reducing agent. At present, it is especially suitable for stable products dyed with a single dye, such as indigo dyed denim, which is said to have been applied abroad. In addition, it is also suitable for batik products produced in China, because such products use a single dye and have low requirements for levelness. In addition, in order to stabilize the process, it is also feasible to reduce the dosage of sodium hydrosulfite by electrochemical reduction in conventional dyeing.
There are many new dyeing processes for other dyes, including disperse dyes and acid dyes, which are introduced in many articles, so I won't repeat them here. Their characteristics are developing in the direction of high efficiency, high speed, clean environment and friendliness, which will be introduced later in this paper.
1.4 washing process after dyeing
Reactive dyes should be fully washed after fixation, especially for dark products in recent years. If the washing is insufficient, it will not only affect the color fastness of the product, but also accelerate the bond breaking of the dyed product in application, making the color of the product unstable. In the dyeing wet fastness, besides soaping fastness, wet rubbing fastness often becomes the most important technical index of products, which is closely related to washing after dyeing.
In recent years, a lot of research has been done on washing after dyeing, on the one hand, to improve the fastness of products, on the other hand, to save water and reduce pollution. In order to improve the cleaning efficiency, the cleaning principle and evaluation standard are deeply studied, and the theoretical relationship between intermittent water change and continuous water change is put forward. According to the different properties of substances removed in different stages of water washing, the technical parameters of water washing in different stages are designed, and it is considered that water washing mainly consists of three steps, namely:
(1) dilution exchange stage, at this time, some substances that do not directly act on the fiber, such as salts and floating colors, are mainly diluted by water exchange.
(2) Accelerate the diffusion of hydrolyzed dyes from fibers. In order to accelerate the diffusion speed, the temperature must be raised to near boiling (soaping).
(3) Wash away the diffused hydrolyzed dye, and then dilute and exchange it with hot water and cold water.
Accord to that above washing proces, the notorious washing process was designed and the was equipment was improved, among which the most important development was as follows:
(1) Change the large-flow cold water washing after dyeing to hot water washing (60-70℃). In some processes, the intermittent water change is changed to continuous water change, but the water flow is accurately controlled, which greatly reduces:
(2) The washing with large bath ratio is constantly changing to the washing with small bath ratio or ultra-small bath ratio;
(3) Slow running water washing, constantly changing to fast liquid-liquid washing.
(4) changing the experience control water washing into the control water washing;
(5) Replacing the conventional builder with high-efficiency dispersed chelating agent to help washing.
Especially after controlled washing, the process is greatly shortened, water consumption is greatly reduced, and sewage is also greatly reduced. The processing cost is also greatly reduced and the efficiency is greatly improved. The new dyeing machine has small bath ratio and can be controlled by washing.