Sulfur Dyes Basic Knowledge

Sulfur dyes are a class of sulfur-containing dyes with a relatively complex molecular structure. They are typically synthesized by heating organic compounds such as aromatic amines and amino phenols with sulfur or sodium polysulfide, through a process called sulfuration.

Most sulfur dyes are water-insoluble and, during the dyeing process, they need to be dissolved and reduced in a sodium sulfide or alkaline insurance powder solution to form a leuco compound, which is then absorbed by the fibers and subsequently oxidized to develop the color.

I.Overview of Sulfur Dyes

Since their first production in 1873 for dyeing cellulosic fibers, sulfur dyes have a history of more than 150 years. The manufacturing process of sulfur dyes is relatively straightforward, normally involving the heating of aromatic amines or phenol compounds with sulfur or sodium polysulfide. They are cost-effective, convenient to use, non-carcinogenic, and exhibit good wash and light fastness, making them popular dyes. However, being water-insoluble, during dyeing, they are reduced to soluble leuco sodium salts in a sulfide alkaline solution and, upon application to fibers, are fixed to the fibers in an insoluble state through oxidation. Consequently, the dyeing process is complex, and due to the strong alkaline conditions, they cannot be used for protein fibers like wool and silk.

Therefore, sulfur dyes are mostly used for dyeing cellulosic fibers, particularly for dark-colored cotton textiles, with black and blue as the most widely used colors.

II. Dyeing Mechanism of Sulfur Dyes

The dyeing solution of sulfur dyes is made by reduction and dissolution, forming a dyeing leuco compound that is adsorbed by cellulosic fibers. Upon air oxidation treatment, the desired color manifests on the cellulose fibers. The chemical reaction is as follows:

D-S-SO?Na + Na?S → D-SNa + Na?S?O?

Sulfur dye precursors have no affinity for fibers; they contain sulfide (-S-), disulfide (-S-S-), or polysulfide (-Sx-) linkages, which are reduced to thiols (-SNa) in the presence of a sodium sulfide reducing agent, becoming water-soluble leuco sodium salts. The reason why these leuco compounds have a good affinity for cellulosic fibers is due to the large molecular size of the dyes, which leads to significant van der Waals forces and hydrogen bonding with the fibers.

III. Classification of Sulfur Dyes

Developed to date, sulfur dyes can be categorized into four main types:

1. Powdered Sulfur Dyes:

The general structure of these dyes is D-S-S-D, typically requiring boiling with sodium sulfide to dissolve before application.

2. Hydrolyzed Sulfur Dyes:

These dyes have the general formula D-SSO?Na and are produced by treating traditional sulfur dyes with sodium sulfite or sodium bisulfite. These dyes contain water-soluble groups, hence are water-soluble. However, they do not contain reducing agents and have no affinity for fibers; typically, a pad-dry method is used for application.

3. Liquid Sulfur Dyes:

With the general formula D-SNa, these dyes contain a certain amount of reducing agent and are pre-reduced to soluble leuco compounds.

Before 1936, sulfur dyes were available in powdered form. They were dissolved by boiling the powdered sulfur dye with sulfide and soda ash solution. In 1936, John L. Christ in the United States produced a pre-reduced, relatively stable concentrated solution of sulfur dyes, which was patented and is now well-known as liquid sulfur dyes.

4. Eco-friendly Sulfur Dyes:

These dyes are refined to leuco compounds in the production process with significantly lower sulfur and polysulfide contents than conventional sulfur dyes. They have high purity, stable reduction, good penetration, and utilize a binary reducing system with glucose and insurance powder in the dyebath, reducing dyes and providing environmental benefits.

Ⅳ.The Dyeing Process of Sulfur Dyes

The sulfur dyeing process can be divided into the following four steps:

1. Reduction of the Dye:

Reducing and dissolving sulfur dyes is relatively easy using sodium sulfide, which also acts as an alkalinizing agent. To prevent hydrolysis of the leuco compound, soda ash can be added appropriately. However, the alkali strength of the reduction bath should not be too strong; otherwise, the reduction rate of the dyes is slowed.

2、Adsorption of the Dye Leuco Compound by the Fiber

The sulfur dye leuco compounds exist as anions in the dye bath, displaying directness to cellulosic fibers, enabling them to adsorb on the fiber surface and diffuse into the interior. Although the directness of sulfur dye leuco compounds to cellulosic fibers is rather low, a low liquor ratio is usually employed, complemented by the addition of suitable electrolytes. At elevated temperatures, this can increase the rate of dye uptake, thereby enhancing the evenness and penetration of the dye.

3. Adsorption of the Dye Leuco Compound by the Fiber:

Sulfur dye leuco compounds in the dye bath are in an anionic state and exhibit directness to cellulosic fibers, allowing them to be adsorbed onto the fiber surface and diffuse into the interior. The directness of sulfur dye leuco to cellulosic fibers is comparatively low. A low liquor ratio is typically employed in combination with the addition of appropriate electrolytes; this, coupled with higher temperatures can enhance the rate of dye uptake by the fibers, resulting in improved dye evenness and penetration.

4. Oxidation Process:

Post adsorption, the sulfur dye leuco compounds must undergo oxidation to reveal the intended color. Oxidation represents a crucial phase subsequent to the dyeing process. Easily oxidized sulfur dyes can achieve oxidation through simple washing and air exposure, known as air oxidation method. For sulfur dyes less susceptible to oxidation, an oxidizing agent may facilitate the oxidation process.

5. Post-treatment:

Subsequent processes comprise thorough washing, oiling, anti-tenderizing, and color-fixing treatments. Extensive washing post-dyeing is imperative to minimize residual sulfur on the fabric to avert fabric brittleness. Since sulfur within the dye and alkali can oxidize to form sulfuric acid, which prompts acid hydrolysis of cellulosic fibers, hence diminishing strength and inducing brittleness, agents like urea, trisodium phosphate, gelatin, and sodium acetate can be utilized for anti-tenderizing treatment.

To augment the lightfastness and soapfastness of sulfur dyes, color fixation post-dyeing is advisable. Two prevalent methods for color fixation exist: one utilizing metal salts such as potassium dichromate, copper sulfate, copper acetate, and their mixtures; the other leveraging cationic color fixative.