Sulphur Red 6
[CAS# 1327-85-1]

Identification

• Classification: Dyes and pigments >> Dyes >> Sulfur dye
• Name: Sulphur Red 6
• Synonyms: Acco Sulfur Bordeaux A-CF; Acco Sulfur Red Brown B-CF; Acco Sulfur Red Brown R-CF; Atul Sulfur Red Brown 3B; C.I. 53720
• CAS Registry Number: 1327-85-1
• EC Number: 215-503-2

Discovory and Applicatios

Sulphur Red 6 is a member of the sulfur dye class, a group of colorants produced through the reaction of aromatic amines or phenols with sulfur or sulfur donors under elevated temperature. Sulfur dyes became industrially important in the early twentieth century as manufacturers sought inexpensive, water-insoluble colorants that could deliver deep shades on cellulosic fibers. Sulphur Red 6 emerged within this development as one of the comparatively rare red shades in a dye class dominated by browns, olives, and blacks. Its introduction provided textile producers with a more diverse color palette while maintaining the good wash fastness and low production cost characteristic of sulfur dyes.

The discovery of sulfur dyes relied on the observation that heating organic compounds with sulfur produced polymeric chromophores capable of strongly dyeing cotton when subsequently reduced. Early patents and industrial records describe the evolution of the so-called sulfur melt process, in which aromatic raw materials are condensed in molten sulfur or alkali polysulfide to yield complex, high-molecular-weight products. Sulphur Red 6 was developed within this context as dye chemists experimented with substituted aromatic precursors to obtain brighter or more saturated hues. Like other sulfur dyes, its chromophore is not a single identifiable molecule but a mixture of sulfur-linked aromatic species formed during the melt reaction.

Application of Sulphur Red 6 follows the established sulfur-dyeing procedure. The dye, originally insoluble in water, is reduced to a soluble leuco form using sodium sulfide or related reducing agents. This reduced dye has affinity for the cellulose matrix of cotton fibers. Once the fiber is saturated, the material is oxidized with air or aqueous oxidants, regenerating the insoluble colored form within the fabric. The result is a durable shade embedded in the fiber structure, offering good wash fastness and acceptable resistance to light for an economically produced dye. These practical features led to widespread use of Sulphur Red 6 in cotton goods such as workwear, furnishing fabrics, and lower-cost colored textiles.

The pigmentary characteristics of Sulphur Red 6 also allowed it to be incorporated into some leather-dyeing processes, where sulfur dyes offered advantages in penetration and cost for certain substrates. Although not as commonly used in leather as in cellulosic textiles, its properties made it an option for specific industrial formulations. The dye’s moderate resistance to chlorine and bleaching agents limited some applications, but its performance remained sufficient for many mass-market goods in which cost efficiency was a major consideration.

Environmental reviews of sulfur dyes, including Sulphur Red 6, highlighted the need for appropriate effluent treatment due to the presence of sulfide in dye baths and the formation of oxidized sulfur species during processing. This led to improved wastewater management practices and gradual optimization of dye formulations and application procedures. The shift toward reactive dyes in the late twentieth century reduced the overall share of sulfur dyes in certain textile sectors, yet Sulphur Red 6 continued to hold relevance where economic factors or specific shade requirements dictated its use.

Research on sulfur dye chemistry has also used materials such as Sulphur Red 6 to study the structure of sulfur chromophores, reduction–oxidation behavior, and the mechanism of fiber bonding. Although the structural complexity of sulfur dyes makes full characterization challenging, analytical studies confirmed the presence of polysulfide linkages and aromatic condensation products typical of the class. These findings supported refinements in production methods that improved shade reproducibility and application consistency for Sulphur Red 6 and related dyes.

Today, Sulphur Red 6 maintains a defined niche in industrial dyeing. Its history reflects the broader development of sulfur dyes as robust, economical workhorse colorants for cotton, and its continued use underscores the relevance of classical dye technologies alongside more modern dye classes.

References

Hunger, K. (Ed.) (2003). Industrial Dyes: Chemistry, Properties, Applications. Wiley-VCH. DOI: 10.1002/3527602011