Specific description of sulphur dyes

Sulphur dyes are a class of sulfur-containing dyes with complex molecular structures. Generally, it is made by heating some organic compounds such as aromatic amines and aminophenols with sulfur or sodium polysulfide, that is, sulfurization.
Most Sulphur dyes are insoluble in water. When dyeing, they need to be dissolved in sodium sulfide or alkaline hydrosulfite solution to reduce to leuco, and then oxidized to show color after absorption by fiber.

An overview of Sulphur dyes
Sulphur dyes have a history of more than 100 years since they were first manufactured for cellulose fiber dyeing in 1873. Its production process is relatively simple, usually by mixing aromatic amines or phenolic compounds with sulfur or sodium polysulfide. Made by heating. Low cost, easy to use, non-carcinogenic, good washing fastness and light fastness, is a kind of popular dye. But because it is a kind of water-insoluble dye, when dyeing, they are reduced to soluble leuco sodium salt in sulfide alkali solution. After dyeing human fiber, it becomes insoluble state and fixed on the fiber after dyeing, so the dyeing process is complicated. , and cannot be used for protein fibers such as wool and silk under strong alkaline conditions.
Therefore, Sulphur dyes are mostly used in the dyeing of cellulose fibers, especially in the dyeing of dark products of cotton fabrics, among which black and blue are the most widely used.

Second, the dyeing mechanism of Sulphur dyes
The Sulphur dyes is reduced and dissolved to form a dye solution, and the formed dyed leuco is adsorbed by the cellulose fiber, and is treated by air oxidation to make the cellulose fiber show the desired color. Its chemical reaction formula is
D-S-SO3Na + Na2S→D-SNa + Na2S2O3
Sulphur dyesprecursor has no affinity for fiber, its structure contains sulfur bond (-S-), disulfide bond (-S-S) or polysulfide bond (-Sx-), which is reduced to sulfur under the action of sodium sulfide reducing agent Hydrogen group (-SNa), becomes a water-soluble leuco sodium salt. The reason why leuco has a good affinity for cellulose fibers is that the molecules of the dyes are larger, which in turn produces larger van der Waals and hydrogen bonding forces with the fibers.

Third, the classification of Sulphur dyes
The development of sulfur dyes can be divided into four categories:
1. Powdered Sulphur dyes
The general structure of the dyestuff is: D-S-S-D. Generally, it needs to be boiled with sodium sulfide and applied after dissolving.

2. HydrolyzableSulphur dyes
The general structure of dyestuff is: D-SSO3Na. This type of dyestuff is made by treating traditional Sulphur dyes with sodium sulfite or sodium bisulfite. The dyes contain water-soluble groups, so they are water-soluble, but the dyes do not contain reducing agents and have no effect on fibers. Affinity is generally applied to the fabric by suspension pad dyeing.

3. Liquid Sulphur dyes
The general structural formula of the dye is: D-SNa, which contains a certain amount of reducing agent to pre-reduce the dye to a soluble leuco.
Before 1936,Sulphur dyes were in the form of powdered formulations. When using, powdered sulfur dyes were heated with sulfuric and soda ash aqueous solutions to boil to dissolve them. In 1936, John Le Crist in the United States made a pre-reduced, fairly stable concentrated solution of sulfur dyes and obtained a patent, which is now known as liquid sulfur dyes.

4. Environmentally friendly Sulphur dyes
In the production process, it is refined into dye leuco, but the sulfur content and polysulfide content are much lower than ordinary sulfur dyes. The dye has high purity, stable reduction degree and good permeability. At the same time, the binary reducing agent of glucose and hydrosulfite is used in the dye bath, which can not only reduce the Sulphur dyes but also play an environmental protection role.

Fourth, the dyeing process of Sulphur dyes
The Sulphur dyeing process can be divided into the following four steps:
1. Reduction of dyes
It is easier to reduce and dissolve Sulphur dyes, and sodium sulfide is often used as a reducing agent, which also acts as an alkali agent. In order to prevent the leuco from being hydrolyzed, soda ash and other substances can be added appropriately, but the alkalinity of the reducing bath should not be too strong, otherwise the reduction rate of the dye will slow down.

2. The dye leucobody in the dye liquor is adsorbed by the fiber
The leuco body of Sulphur dyes exists in an anionic state in the dye liquor, which has directness to cellulose fibers, can be adsorbed on the surface of the fibers and diffuse into the interior of the fibers. The leuco of Sulphur dyes has low directness to cellulose fibers. Generally, a small bath ratio is used, and appropriate electrolytes are added at the same time. At higher temperatures, the dyeing rate can be increased, and the levelness and dye penetration can be improved.

3. Oxidation treatment
After the Sulphur dyes  leucobody is dyed on the fiber, it must be oxidized to show the desired color. Oxidation is an important step after dyeing with Sulphur dyes . After dyeing, the oxidizable Sulphur dyes  can be oxidized by air after being washed with water and ventilated, that is, the air oxidation method; for some Sulphur dyes  that are not easily oxidized, they are treated with oxidizing agents to promote oxidation.

4. Post-processing
Post-processing includes washing, oiling, anti-brittleness and color fixing. Sulphur dyes  must be fully washed after dyeing to reduce the residual sulfur on the fabric and prevent the fabric from being brittle, because the sulfur in the dye and the sulfur in the sulfide alkali are easily oxidized in the air to form sulfuric acid, which will cause acid hydrolysis to cellulose fibers. Decreased strength, making fibers brittle. Therefore, it can be treated with anti-brittle agents, such as: urea, trisodium phosphate, bone glue, sodium acetate, etc. In order to improve the light and soaping fastness of sulfur dyes, it can be fixed after dyeing. There are two methods of color-fixing treatment: metal salt treatment (such as potassium dichromate, copper sulfate, copper acetate and mixtures of these salts) and cationic color-fixing agent treatment.