Direct Blue 6 is a synthetic azo dye known for its intense blue color. It is widely used in various industrial applications, particularly in the textile industry, due to its color and excellent dyeing properties.

Chemical Name

Tetrasodium 3,3'-[[1,1'-biphenyl]-4,4'-diylbis(azo)]bis[5-amino-4-hydroxynaphthalene-2,7-disulphonate]

Chemical Composition and Structure

Direct Blue 6 is an azo dye, meaning it contains one or more azo groups (-N=N-) in its molecular structure. The chemical formula for Direct Blue 6 is C32H20N6Na4O14S4. Its structure includes multiple aromatic rings linked by azo groups and sulfonate groups, which enhance its solubility in water and its ability to bind to fibers.

Physical Properties

Direct Blue 6 typically appears as a dark blue powder. It is highly soluble in water, producing a bright blue solution when dissolved. The dye exhibits excellent stability under various conditions, including resistance to light and moderate heat, making it suitable for a wide range of applications.

Production Process

The production of Direct Blue 6 involves the diazotization of aromatic amines followed by coupling with suitable aromatic compounds to form the azo dye. The process requires precise control of reaction conditions to ensure high purity and consistency in the final product. The sulfonation step enhances the dye's solubility and affinity for various substrates.

Chemical Industrial Synthesis Process

• Preparation of reagents. The main raw materials include benzidine, sulfuric acid (H₂SO₄), sodium nitrite (NaNO₂), a base such as sodium hydroxide (NaOH), and 2-naphthylamine-4,8-disulfonic acid.
• Diazotization. Benzidine is treated with sulfuric acid and sodium nitrite to form a diazonium intermediate. This reaction produces a diazonium salt.
• Coupling. The diazonium salt is then coupled with 2-naphthylamine-4,8-disulfonic acid in the presence of a base, such as sodium hydroxide, to form the azo compound intermediate.
• Formation of Direct Blue 6 dye. The azo compound intermediate is further processed and stabilized to form the Direct Blue 6 dye.
• Filtration. The resulting suspension is filtered to separate the solid precipitate from the aqueous solution.
• Washing. The precipitate is washed with deionized water to remove any soluble impurities.
• Drying. The washed precipitate is dried at controlled temperatures to remove residual moisture and obtain a dry powder.
• Grinding. The dried product is ground to obtain a fine and uniform powder.
• Classification. The dried powder is classified to ensure a uniform particle size. This step may involve sieving or the use of air classifiers.
• Stabilization. The Direct Blue 6 powder is stabilized to ensure its stability during transportation and storage, preventing aggregation and degradation.
• Quality control. The Direct Blue 6 undergoes rigorous quality testing to ensure it meets standards for purity, color intensity, and safety. These tests include chemical analysis, spectroscopy, and physical tests to determine particle size and rheological properties.

What it is used for and where

Textile Dyeing: Direct Blue 6 is widely used in the textile industry for dyeing cotton, wool, silk, and nylon. Its vibrant color and excellent fastness properties make it suitable for producing high-quality fabrics.

Paper and Inks: Direct Blue 6 is used to dye paper products and in the production of certain types of inks. Its bright blue color and water solubility make it suitable for these applications.

Leather: In the leather industry, Direct Blue 6 is used to dye leather products, providing a rich and durable blue color. It is valued for its ability to penetrate leather deeply and uniformly.

Biological Staining: While less common, Direct Blue 6 can also be used as a biological stain in certain research applications, providing contrast and clarity in microscopic studies.

Cosmetics. Restricted cosmetic substance as  II/988 a Relevant Item in the Annexes of the European Cosmetics Regulation 1223/2009. Substance or ingredient reported:

Tetrasodium 3,3'-[[1,1'-biphenyl]-4,4'-diylbis(azo)]bis[5-amino-4-hydroxynaphthalene-2,7-disulphonate]. Prohibited in cosmetic products.

Molecular Formula    C₃₂H₂₀N₆Na₄O₁₄S₄

Molecular Weight    932.8 g/mol

CAS     2602-46-2

UNII    21ZL85SYWM

EC Number  220-012-1

Synonyms:

Indigo Blue 2B

Direct Blue 2B

Direct Sky Blue K

References__________________________________________________________________________

(1) Chung KT, Stevens SE Jr, Cerniglia CE. The reduction of azo dyes by the intestinal microflora. Crit Rev Microbiol. 1992;18(3):175-90. doi: 10.3109/10408419209114557.

Abstract. Azo dyes are widely used in the textile, printing, paper manufacturing, pharmaceutical, and food industries and also in research laboratories. When these compounds either inadvertently or by design enter the body through ingestion, they are metabolized to aromatic amines by intestinal microorganisms. Reductive enzymes in the liver can also catalyze the reductive cleavage of the azo linkage to produce aromatic amines. However, evidence indicates that the intestinal microbial azoreductase may be more important than the liver enzymes in azo reduction. In this article, we examine the significance of the capacity of intestinal bacteria to reduce azo dyes and the conditions of azo reduction. Many azo dyes, such as Acid Yellow, Amaranth, Azodisalicylate, Chicago Sky Blue, Congo Red, Direct Black 38, Direct Blue 6, Direct Blue 15, Direct Brown 95, Fast Yellow, Lithol Red, Methyl Orange, Methyl Red, Methyl Yellow, Naphthalene Fast Orange 2G, Neoprontosil, New Coccine, Orange II, Phenylazo-2-naphthol, Ponceau 3R, Ponceau SX, Red 2G, Red 10B, Salicylazosulphapyridine, Sunset Yellow, Tartrazine, and Trypan Blue, are included in this article. A wide variety of anaerobic bacteria isolated from caecal or fecal contents from experimental animals and humans have the ability to cleave the azo linkage(s) to produce aromatic amines. Azoreductase(s) catalyze these reactions and have been found to be oxygen sensitive and to require flavins for optimal activity. The azoreductase activity in a variety of intestinal preparations was affected by various dietary factors such as cellulose, proteins, fibers, antibiotics, or supplementation with live cultures of lactobacilli.