Water-Soluble Sulphur Black: Properties, Applications, And Key Considerations

1. Product Overview

Water-Soluble Sulphur Black (C.I. Sulphur Black 1, soluble grade) is a chemically modified derivative of conventional Sulphur Black. Unlike traditional sulphur dyes, which are insoluble in water and require reduction with sodium sulfide before use, water-soluble sulphur black contains hydrophilic groups (such as -SSO₃Na) introduced into its molecular structure. This modification allows it to dissolve rapidly in water at room temperature, making it significantly more convenient to handle.

Typical product specifications include a black uniform powder or granular appearance, solubility of at least 160 grams per liter at 25 degrees Celsius, moisture content not exceeding 7.0 percent, and insoluble impurities limited to 0.5 percent or less.

2. Chemical Structure and Characteristics

Water-soluble sulphur black is typically produced by reacting conventional sulphur black with sodium sulfite or sodium bisulfite, forming a thiosulfonate derivative. Its key characteristics include excellent water solubility allowing rapid dissolution at room temperature without insoluble residues, good thermal stability under typical dyeing process temperatures, and a unique dyeing mechanism.

Although the dye itself is water-soluble, it has no direct affinity for cellulose fibers. During dyeing, sodium sulfide must be added to convert the dye into its leuco (reduced) form via nucleophilic and reduction reactions. This leuco form then exhibits high affinity for cellulose fibers. The dyeing process consists of four steps: reduction dissolution of the dye, adsorption of the leuco form onto fibers, oxidation and color development, followed by after-treatment including washing and anti-brittleness treatment.

3. Major Applications

Water-soluble sulphur black is widely used in several industrial areas. In textile dyeing, it is applied in pad dyeing, exhaustion dyeing, and jigger dyeing processes for cotton, linen, viscose, and other cellulose fibers. In leather processing, it is suitable for chrome-tanned leather. For paper manufacturing, it is used in the production of black paper and black-core paperboard such as playing cards and mounting boards. Additionally, it is employed in viscose fiber dope dyeing, where it is added directly during viscose fiber production for mass coloration.

4. Critical Considerations

Fiber Brittleness Risk

The molecular structure of sulphur black contains unstable disulfide and polysulfide bonds. Under humid and warm conditions, these bonds can be oxidized by atmospheric oxygen to form sulfuric acid, which hydrolyzes and degrades cellulose fibers, leading to brittleness. To prevent or minimize this risk, several measures are essential. Operators should control dye dosage to avoid excessive application. Thorough washing after dyeing is necessary to remove unfixed dye and residual sulfides. Anti-brittleness treatment should be applied using agents such as urea, sodium carbonate, or sodium acetate. The material must be dried promptly after dyeing and allowed to cool to room temperature before packaging. As an alternative, one may consider using anti-brittle sulphur black formulations that already contain formaldehyde and chloroacetic acid additives.

Eco-Friendly Liquid Sulphur Black

In recent years, environmentally friendly liquid sulphur black has gained widespread popularity. Compared to traditional powder products, liquid sulphur black offers several notable advantages. It requires no sodium sulfide for pre-dissolution and can be used similarly to direct dyes. With near-zero impurity content, it provides stable color shade and minimal batch-to-batch variation. Dry and wet rubbing fastness is improved by approximately half a grade compared to powder forms. The product generates lower odor and reduced wastewater, making it more environmentally friendly. It does not undergo oxidation or degradation during transport and storage. Finally, it allows convenient on-demand dispensing, which minimizes waste. In Western industrialized countries, eco-friendly liquid sulphur black has become the mainstream product in the market.

Quality Standards

Water-soluble sulphur black complies with standards such as GB/T 25809-2010. Key quality indicators include appearance as a black uniform powder or granules, color strength between 95 and 105 percent of the standard, water-insoluble impurities not exceeding 0.5 percent, moisture content no higher than 7.0 percent, and solubility at 60 degrees Celsius of at least 60 grams per liter.

5. Packaging and Storage

Water-soluble sulphur black is highly hygroscopic and must be protected from moisture. Typical packaging consists of iron drums lined with plastic bags, with a net weight of 25 kilograms per drum. Storage should be in a cool, dry, well-ventilated area, away from direct sunlight and heat sources.

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