Water-Soluble Sulfur Black (C.I. Solubilized Sulfur Black 1)

Water-soluble sulfur black represents a significant technological advancement over traditional sulfur dyes. Unlike conventional sulfur black, which requires reduction with sodium sulfide (Na₂S) before dyeing, this modified version dissolves directly in hot water, offering a cleaner, safer, and more consistent dyeing process. Its standard designation is governed by the Chinese national standard GB/T 25809-2010 for "Sulfur Black S-BR," with the Colour Index number C.I. Solubilized Sulfur Black 1 (Index 53186, CAS 1326-83-6).

Physically, the product is a uniform black granule or powder. For dyehouses, the most critical quality control parameter is its strength, which must be between 97% and 103% of the standard reference sample. To ensure reliable processing, the moisture content is strictly limited to a maximum of 7.0% to prevent caking and ensure stability. High purity is another key feature; the insoluble matter in water must not exceed 0.5%, as impurities can lead to spots on the final fabric. A standard commercial batch should also achieve a minimum solubility of 60 grams per liter at 60 degrees Celsius, guaranteeing a homogeneous dye solution.

Regarding performance, water-soluble sulfur black is primarily used for dyeing cotton, linen, viscose, and other cellulosic fibers, as well as leather. It offers excellent levelness and deep penetration. However, different fastness properties must be considered. While it performs well under light exposure (rated Grade 6 on the wool scale) and washing at 95°C (color change Grade 3, staining Grade 4-5), its wet rubbing fastness remains a known weakness, typically rating only Grade 1 to 2. Users must account for this in end-use requirements.

A major commercial advantage is its reduced risk of tendering (fiber brittleness). Traditional sulfur black often requires a post-dyeing anti-tendering treatment. While not always mandatory for the water-soluble variant, a final treatment with a cationic fixing agent or mild oxidizing rinse is still strongly recommended to neutralize any residual acidity and enhance the fabric's long-term tensile strength. The product must be stored in sealed containers in a cool, dry place due to its high affinity for atmospheric moisture. Recent innovations have also introduced liquid forms of this dye, which contain fewer impurities and offer even greater ease of use in automated dosing systems for sustainable textile processing.

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