Polyazo dyes

The direct dyes that are most suitable for leather have the following features predominantly long-chain dye molecules, sulfonate groups at the ends of the molecule, additional non-dooble-bonded groups, a balanced ratio of double bonds to solubilizing groups, and predominantly meta -substituted azo coupling components. These empirical observations have been used to enhance the technical properties of anionic polyazo leather dyes. 

While the development of direct dyes was essentially completed in the 1930s, it was reactivated in the 1960s and 1970s, when it was found that benzidine, an important direct dye intermediate at the time, is carcinogenic. Benzidine dyes are used for some yellows and reds and mostly for black. In view of the ensuing concern, the leading dye producers withdrew benzidine-based dyes preemptively to protect their workers, dyers, and consumers. Studies to find alternative dyes were undertaken, and this work led eventually to many of today s most important black dyes for leather, e.g,C.I. DirectBlack 168, 30410 [85631-88-5] (16). 

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Chemically, the azo class is subdivided according to the number of azo groups present into mono-, dis-, tris-, tetrakis-, etc. Mono- and disazo dyes are essentially equal in importance, trisazo dyes are less important, and tetrakisazo dyes, except for a few, are much less important. For this reason, substances with more than three azo linkages are generally included under the heading of polyazo dyes. The Colour Index]isX.s the azo dyes as follows ... 

TrisaZO and Polyazo Dyes. These are mostly direct dyes, the hues ate predominandy brown, black, or dark blue or green. Some are leather dyes. Benzidine, which used to be an important bisdiazo component, has been replaced by 4,4 -diaminobenzanilide [785-30-8] ... 

Chemists working with azo dyes use a shorthand designation based on the synthesis by azo coupling. It is characterized by an arrow from the amine used for diazotization (the so-called diazo component) to the coupling component, e. g., for 1.8 sulfanilic acid - 2-naphthol. This designation is especially useful for polyazo dyes (see Zollinger, 1991, p. 109). 

In Chemical Abstracts and in Beilstein specific azo compounds are found under the heading diazenes . As their systematic nomenclature may be very complex (polyazo dyes ), it is much easier to search for them with the aid of the Formula Index of Chemical Abstracts. 

Polyazo dyes, 9 363—361 Polybenzamide, 15 109-110 Poly(p-benzamide) (PBA), 10 212 19 714 crystal lattice parameters of, 19 728t laboratory synthesis of, 19 719 Polybenzimidazole (PBI), asbestos substitute, 3 3141 Polybenzimidazole (PBI) fibers,... 

It is in the direct dye class that the more complex polyazo dyes come into their own, and trisazo structures such as Cl Direct Blue 78 (4.64), Cl Direct Brown 222 (4-65) and Cl Direct Black 38 (4.66) are classic examples of this type of dye. The last-named dye is now known to be carcinogenic [72]. Generally, the A—>M2— E type (such as 4-64) afford reasonably bright blue shades whilst dyes prepared according to other sequences, such as the E<—D— Z<—A type (examples 4.65 and 4-66) yield drab shades. 

Vesuvin and Bismarck Brown products are obtained when rn -phenylenedia-mine, 2,4- and 2,6-diaminotoluene, or mixtures thereof are treated with nitrite in acidic solution, or when a mixture of the amines and nitrite is acidified. The final products consist of mixtures of monoazo, disazo, and polyazo dyes, with the dis-azo species probably predominating in commercial products. The dye is salted out immediately after its formation, and thereby the formation of polyazo dyes is prevented. 

Disazo and polyazo dyes containing sulfonic acid groups are also frequently used in the above applications. Many of them are also sold as substantive dyes for cotton because of their pronounced affinity toward cellulosic fibers (see Section 3.3). 

Direct dyes (see Section 3.3) that contain aromatic amino groups can be diazo-tized on the fiber after dyeing and then coupled with a developer (a phenol, naphthol, or aromatic amine). Wetfastness, in particular, is improved by such an enlargement of the molecule, and the shade also changes (see Section 2.2). Conversely, water-soluble, substantive azo dyes which bear amino or hydroxyl groups capable of coupling can also be used, followed by aftertreatment with a diazo-nium compound. The resulting polyazo dye shows excellent wetfastness (see also Section 4.8). 

The 1,6- and 1,7-naphthylaminesulfonic acids have long been very important as components for azo dyes, particularly as the middle component in polyazo dyes. They are used in the preparation of the important black cotton dyes of the type of Columbia black FF, and also of a whole series of developed dyes, such as the important naphthogene blue, Zambesi black V, and others. Sulfonic acids of this type are also frequently used in preparing dyes of the Sirius blue series (Bayer). 

It is customary, to use a small excess (3 to 5 per cent) of an end component so that the coupling reaction goes rapidly to completion. Middle components in disand polyazo dyes, however, must be used in exactly the theoretical amount in order to prevent the formation of by-products. 

Coupling reactions with diazo compounds which are very sensitive to alkali often are most successful when pyridine is used as an acidfixing agent. This is especially true in the preparation of secondary polyazo dyes with l-amino-2-naphthol ethers or their sulfonic acids as intermediates. ... 

Technical Observations. The pure 1,6- and 1,7-naphthylaminesulfonic acids give practically identical dyes. Frequently it is advantageous to use a mixture of the two acids because the resulting dyes are stronger, particularly in the case of black polyazo dyes (e.g., Columbia black or Zambesi black V). 

Polyazo-dyes. —Examples of these are obtained as follows ... 

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