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What Is The Mechanism Of The Vat Dye?
Apr 24, 2018

1. Structure and classification of vat dyes

The vat dyes can be divided into two major classes of yttrium dyes and anthraquinone dyes. The indigo dyestuffs of the Yi dyestuffs are the dyes with the longest history of application. The indigoid dyes also include the indigo derivatives such as halogens, isomers, and the sulfonium derivatives of sulfonium skeletons; The system is a pair of electron-donor acceptors that are connected by the same vinyl group. This cross-conjugated chromophoric system is called H-type chromophore. There are many types of hydrazine vat dyes, of which the simplest is an acylamino hydrazine derivative. The yttrium itself is light yellow, and after the introduction of the amino group, it forms an electron-accepting electron-accepting color system, resulting in a dark-color effect, such as reducing red 5GK. The structure of polycyclic ketone dyes in hydrazine-based vat dyes is relatively complex. There are roughly two types of fluorene-based fused ring dyes (two or more fluorene rings fused via a nitrogen heterocycle, for example, yield. Largest reduced blue RSN) and fluorenone fused products (eg, reduced gold orange G with fluorenone structure, reduced yellow G with fluorenone structure, reduced brilliant green FFB with purpurin skeleton, isopurpurone skeleton Restored purple 3B, etc.). The reduced polycyclic ketone dye is a cyclic polyene color body, and the carbonyl is reduced to generate a leuco chromogen. For many dyes, the conjugated bond length of the color body is not affected, and some dyes are generated due to the power supply effect of hydroxyl groups. The color effect, for example, the leucosome solution of the reduced yellow G is blue, the leuco solution of the reduced gold orange G is blue red, the reduced blue RSN dye is blue red, and the leuco basic solution is blue.

2. Dyeing characteristics and mechanism

The vat dye molecules do not contain water-soluble groups, so they are insoluble in water. The dyes have a large molecular structure, good coplanarity of the aromatic rings, and a high affinity for cellulose fibers. Therefore, the initial dyeing rate is high, and the dye migration is poor. The phenomenon of uneven dyeing; mainly used for the dyeing of cotton, viscose and other fibers, dyeing methods are mainly leucochromatism (dip dyeing) and suspension dyeing (plumbing); dyeing fabric wet processing fastness Very good, most of the dyes have high fastness to sunlight, and they are widely used high-grade dyes in various commercial dyes. At least two carbonyl groups in the conjugated system are contained in the vat dye molecules, and the carbonyl group is reduced to an enolase structure with an enol structure under the action of a reducing agent. The leuco acid is soluble in the alkaline solution and adsorbed by the fiber; adsorption The leuco form on the fiber (the soluble sodium salt of the dye) is restored to the original water-insoluble carbonyl (steroid or ketone) state by the action of the acid and the oxidizing agent, and is fixed inside the fiber. The vat dye leuco bodies up-stained cellulose fibers are anionically adsorbed on the surface of the fibers by van der Waals forces and hydrogen bonds between the fibers, and then diffuse into the interior of the fibers.

3. Restore conditions

The vat dye is reduced under the effect of the reducing agent, and the leucosome is generated when the solution is basic; and the leuco acid is generated when the solution is acidic. The solubility of leuco acid in water is very low, so that the production of leuco acid is not conducive to dyeing, so the reduction must be carried out under alkaline conditions. Reducing dyes commonly used in the reduction agent is the insurance powder, insurance powder in the caustic (NaOH) conditions have a strong reduction capacity, almost all vat dyes can be used to restore the insurance powder. Caustic soda produces a leucosome after reduction of the dye, and neutralizes the acidic substances that are formed during the reduction of the protective powder.