Introduction

Sulfur dyes are a class of sulfur-containing dyes with relatively complex molecular structures. They are typically produced by heating certain organic compounds, such as aromatic amines and amino phenols, together with elemental sulfur or sodium polysulfide, a process known as sulfuration.

Sulfur dyes are mostly insoluble in water; during dyeing, they must be reduced to leuco forms by dissolution in sodium sulfide or an alkaline hydrosulfite solution, after which they are absorbed by the fibers and then oxidized to develop color.

I. Overview of Sulfur Dyes

Sulfur dyes have a history of more than 150 years, dating back to their first synthesis in 1873 for the dyeing of cellulosic fibers. Their production process is relatively simple: they are typically prepared by heating mixtures of aromatic amine or phenolic compounds with sulfur or sodium polysulfide. Characterized by low cost, ease of use, non-carcinogenicity, and excellent wash and light fastness, sulfur dyes are a widely used class of dyestuffs. However, because they are water-insoluble, during dyeing they are reduced in an alkaline sulfide bath to soluble leuco sodium salts; after being absorbed by the fiber, these leuco forms are oxidized to their insoluble, fixed state on the fiber. Consequently, the dyeing process is complex, and under strongly alkaline conditions sulfur dyes cannot be used on protein fibers such as wool and silk.

Therefore, sulfur dyes are predominantly used for dyeing cellulosic fibers, particularly for producing deep-colored cotton textiles, with black and blue being the most widely applied shades.

II. Dyeing Mechanism of Sulfur Dyes

Sulfur dyes are reduced and dissolved to form a dye bath; the resulting leuco form of the dye is adsorbed onto cellulose fibers, and subsequent air oxidation develops the desired color on the fibers. The corresponding chemical reaction equation is:

D-S-SO3Na + Na2S → D-SNa + Na2S2O3

The parent structure of sulfur dyes exhibits no affinity for fibers; it contains sulfur–sulfur bonds (—S—), disulfide bonds (—S—S—), or polysulfide bonds (—Sx—). Under the reducing action of sodium sulfide, these bonds are cleaved to form thiol groups (—SH), yielding water-soluble leuco sodium salts. The reason why these leuco forms display excellent affinity for cellulosic fibers is that the dye molecules are relatively large, resulting in strong van der Waals forces and hydrogen bonding interactions with the fiber.

III. Classification of Sulfur Dyes

To date, sulfur dyes can be classified into four main categories:

1. Powdered sulfur dyes

The general structural formula for dyes is D–S–S–D; they are typically boiled in sodium sulfide to dissolve before use.

2. Hydrolyzable Sulfur Dyes

The general structural formula for these dyes is D–SSO3Na. They are produced by treating conventional sulfur dyes with sodium sulfite or sodium bisulfite, introducing water-soluble groups into the dye structure and thereby rendering it water-soluble. However, since these dyes do not contain reducing agents, they exhibit no affinity for fibers and are typically applied to fabrics via suspension padding.

3. Liquid sulfur dyes

The general structural formula of the dye is D–SNa, and it contains a certain amount of reducing agent to pre-reduce the dye into a soluble leuco form.

Prior to 1936, sulfur dyes were marketed in powdered form; to use them, the powder was heated to boiling together with a solution of sulfide and soda ash until it dissolved. In 1936, John LeClerc in the United States developed a highly stable, pre-reduced concentrated solution of sulfur dye and obtained a patent for it—this is what is now widely known as liquid sulfur dye.

4. Environmentally Friendly Sulfur Dyes

The dye leuco form is refined during the manufacturing process. However, the sulfur content and the amount of polysulfides are significantly lower than those of conventional sulfur dyes. This dye boasts high purity, stable reduction performance, and excellent penetrability. Moreover, the use of a binary reducing agent consisting of glucose and sodium hydrosulfite in the dye bath not only effectively reduces sulfur dyes but also contributes to environmental protection.

IV. Dyeing Process with Sulfur Dyes

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

1. Reduction of Dyes

Reducing dissolved sulfur dyes is relatively straightforward; sodium sulfide is commonly used as the reducing agent, and it also functions as an alkalizing agent. To prevent hydrolysis of the leuco form, small amounts of soda ash or similar substances may be added; however, the alkalinity of the reduction bath should not be too high, as excessive alkalinity will slow down the dye reduction rate.

2. The leuco form of the dye in the dye bath is adsorbed by the fibers.

The leuco form of sulfur dyes exists in the dye bath as an anion; it exhibits directness toward cellulosic fibers, adsorbing onto the fiber surface and diffusing into the fiber interior. Due to its relatively low directness on cellulosic fibers, sulfur dye leuco forms are typically applied using a low liquor ratio, with the addition of an appropriate electrolyte, and at elevated temperatures, which can enhance the dyeing rate and improve dyeing uniformity and penetration.

3. Oxidation Treatment

After the leuco form of sulfur dyes is applied to fibers, it must undergo oxidation to develop the desired color. Oxidation is a crucial step in sulfur dyeing. For sulfur dyes that are readily oxidizable, simple water washing and exposure to air after dyeing are sufficient to achieve oxidation—this is known as air oxidation. For certain sulfur dyes that are less prone to oxidation, an oxidizing agent is used to accelerate the oxidation process.

4. Post-processing

Post‑treatment processes include scouring, oiling, anti‑brittleness treatment, and color fixation. After sulfur dyeing, thorough water washing is essential to minimize residual sulfur on the fabric and prevent brittleness and strength loss, since the sulfur in the dye and in the sulfuring alkali readily oxidizes in air to form sulfuric acid, which can induce acidic hydrolysis of cellulosic fibers, leading to reduced tenacity and increased brittleness. Therefore, anti‑brittleness agents such as urea, trisodium phosphate, bone glue, and sodium acetate may be used for treatment.

To enhance the lightfastness and soap-fastness of sulfur dyes, a fixing treatment can be applied after dyeing. There are two main methods for such treatment: treatment with metal salts (e.g., potassium dichromate, copper sulfate, copper acetate, and mixtures thereof) and treatment with cationic fixing agents.

(Source: Encyclopedia of Dyeing and Finishing)