Dye Basics: Sulfur Dyes

Dye Basics: Sulfur Dyes
Introduction
Sulfur dyes are a class of sulfur-containing dyes with a complex molecular structure. Generally made by certain aromatic amines, aminophenols and other organic compounds with sulfur or sodium polysulfide together with heating, that is, vulcanization.
Most of the sulfide dyes are insoluble in water, in the dyeing, need to be dissolved in sodium sulfide or alkaline insurance powder solution reduced to cryptochrome, and then oxidized by the fiber absorption and color.

I. Overview of sulfide dyes
Sulfur dyes since 1873 for the first time since the manufacture of cellulosic fiber dyeing, has a history of more than a hundred years, its production process is relatively simple, generally made of aromatic amines or phenolic compounds mixed with sulfur or sodium sulfide heating. It is low cost, easy to use, non-cancerous, has good washing fastness and sunlight fastness, and is a class of popular dyes. But because it is a class of water insoluble dyes, dyeing, they are reduced to soluble sodium salt of cryptochrome in sulfide alkali solution, after dyeing fibers, after oxidation to become insoluble state fixed on the fiber, so the dyeing process is complicated, and in strong alkaline conditions can not be used for wool, silk and other protein fibers.
Therefore, sulfide dyes are mostly used for dyeing cellulose fibers, especially the dyeing of cotton textiles dark-colored products, which are most widely used in black and blue colors.
Second, the dyeing mechanism of sulfide dyes
Sulfide dyes reduced dissolved into a dye solution, the formation of dyeing cryptochrome adsorbed by cellulose fibers, air oxidation treatment, so that the cellulose fibers show the desired color. Its chemical reaction formula is

D-S-SO3Na + Na2S→D-SNa + Na2S2O3
The parent body of sulfide dyes has no affinity for fibers, and its structure contains sulfur bonds (one S one), disulfide bonds (one s-S) or polysulfide bonds (one Sx one), which are reduced to sulfur hydroxyl group (one SNa) under the action of sodium sulfide reducing agent, and turn into sodium salt of cryptochrome which is soluble in water. Cryptochrome has a good affinity for cellulose fibers because the molecules of the dye are larger, which in turn generates a larger van der Waals force and hydrogen bonding force with the fibers.
Third, the classification of sulfide dyes
The development of sulfide dyes can be divided into four major categories:
1、Powder sulfide dyes
Dye structure general formula: D-S-S-D, generally need to be boiled with sodium sulfide, dissolved after application.
2、Hydrolyzable sulfide dyes
Dye structure formula: D-SSO3Na, this kind of dyes is with sodium sulfite or sodium bisulfite treatment of traditional sulfur dyes and become, the dyes contain water-soluble groups, so it is water-soluble, but the dyes do not contain reducing agent, no affinity for the fiber, generally take the suspension body rolling dyeing mode applied on the fabric.
3、Liquid sulfide dyes
The general formula of the structure of the dye is: D-SNa, containing a certain amount of reducing agent, the dye will be pre-reduced to soluble cryptochrome.
Before 1936, the sulfide dyes are in the form of powder as its commercial form, the use of powdered sulfide dyes and sulfur, soda ash aqueous solution of common heat to boiling so that it dissolves. 1936, the United States of John Le Clijsters produced a pre-reduced, stable sulfide dyes concentrated solution, and patented, which is now well known as liquid sulfide dyes.
4、Environmentally friendly sulfur dyes
It is refined into dye cryptochrome in the production process, but the amount of sulfur content and polysulfide is much lower than that of common sulfide dyes. The dyes have high purity, stable reduction and good permeability, meanwhile, the binary reductant of glucose and insurance powder is used in the dyeing bath, which can not only reduce the sulfide dyes but also play the role of environmental protection.

Fourth, the dyeing process of sulfide dyes
The process of sulfide dyeing can be divided into the following four steps:
1, the reduction of dyes
Reduction of dissolved sulfide dyes is relatively easy, commonly used sodium sulfide as a reducing agent, it also plays the role of alkali agent. In order to prevent the cryptochrome from being hydrolyzed, substances such as soda ash can be added appropriately, but the alkalinity of the reduction bath should not be too strong, otherwise the rate of dye reduction slows down.
2、The dye cryptochrome in the dye solution is adsorbed by fiber.
The cryptochrome of sulfide dyes exists in anionic state in dyeing solution, it has directness to cellulose fiber, it can be adsorbed on the surface of fiber and diffuse into the inner fiber. Sulfide dyes cryptochrome on cellulose fiber directness is low, generally using a small bath ratio, while adding the appropriate electrolyte, at higher temperatures can make the dyeing rate increase, even dyeing and dye penetration is improved.
3、Oxidizing treatment
Sulfide dyes cryptochrome dyed on the fiber, must be oxidized to show the desired color. Oxidation is an important step after sulfide dyeing. For easy oxidation of sulfide dyes after dyeing by washing and air ventilation can be air oxidation, that is, air oxidation method; for some not easy to oxidize sulfide dyes, oxidizing agent is used to promote oxidation.
4、After-treatment
Post-treatment includes net washing, oiling, antifragile and color fixing. Sulfur dyes must be washed after dyeing, in order to reduce the residual sulfur on the fabric, to prevent the fabric from brittle damage, because the sulfur in the dyes and the sulfur in the sulfur alkali is easy to be oxidized in the air to generate sulfuric acid, which causes acid hydrolysis of cellulosic fibers and reduces the strength of the fibers, making the fibers brittle. Therefore, it can be treated with anti-brittle loss agent, such as: urea, trisodium phosphate, bone gum, sodium acetate, etc.. In order to improve the fastness of sulfide dyes to sunlight and soaping, can be solidified after dyeing. There are two methods of color-fixing treatment: metal salt treatment (eg: potassium dichromate, copper sulfate, copper acetate and a mixture of these salts) and cationic color-fixing agent treatment.