Disperse Disazo Dyes

In the following survey, the most important disperse azo dyes are divided into mono- and disazo types then each of these classes is subdivided according to the diazo and the coupling components. The diazo component is further subdivided where appropriate into aromatic and heteroaromatic amines. 

Disazo Dyes. About 10 % of all disperse dyes are disazo compounds [9], Even the simplest hydroxy disazo dyes, such as 4-aminoazobenzene coupled to phenol and 4-aminoazobenzene coupled to o-ere sol, have a good affinity for polyester fibers and yield lightfast reddish yellow hues. However, these shades are frequently less bright than those obtained with mono azo dyes. 

The simplest route to disazo dyes is by using / -am inoazobcnzcnc [60-9-3], As p-aminoazobenzene has been classified in the EU as carcinogenic, the use of derived disazo dyes like Disperse Red 151, DisperseYellow 7,Disperse Yellow23, and DisperseYellow56 has diminished. 

Fig. 13.97. Disazo disperse black dye synthesis conducted inside cellulose acetate fibers.

All azo dyes contain one or more azo groups (-N=N-) as chromophore in the molecule on the basis of the number of azo groups in each molecule, they are named monoazo-, disazo-, trisazo-, etc. The azo groups are in general bound to a benzene or naphthalene ring, but they can also be attached to heterocyclic aromatic molecules or to enolizable aliphatic groups. On the basis of the characteristics of the processes in which they are applied, the molecule of the dye is modified to reach the best performances so they can be acid dyes, direct dyes, reactive dyes, disperse dyes, or others. 

Commercially viable systems for the decolorisation of spent dyebaths can be based on hydrogen peroxide treatment initiated by UV radiation. A representative selection of six disulphonated monoazo acid dyes and two disazo disulphonated types was exposed for various times in a pilot-scale photochemical reactor of this kind. The controlling parameters were dye structure, pH, peroxide dosage and UV light intensity [39]. In a wider survey of the response of various classes of dyes to the combination of UV radiation and hydrogen peroxide, viable candidates for further in-plant treatment trials were the water-soluble reactive, direct, acid and basic classes. On the other hand, water-insoluble colorants such as disperse and vat dyes did not appear to be viable [40]. 

Two commercial disazo disperse dyes of relatively simple structure were selected for a recent study of photolytic mechanisms [180]. Both dyes were found to undergo photoisomerism in dimethyl phthalate solution and in films cast from a mixture of dye and cellulose acetate. Light-induced isomerisation did not occur in polyester film dyed with the two products, however. The prolonged irradiation of Cl Disperse Yellow 23 (3.161 X = Y = H) either in solution or in the polymer matrix yielded azobenzene and various monosubstituted azobenzenes. Under similar conditions the important derivative Orange 29 (3.161 X = N02, Y = OCH3) was degraded to a mixture of p-nitroaniline and partially reduced disubstituted azobenzenes. 

The widespread adoption of high-temperature dyeing methods has also allowed the use of simple disazo structures, such as Cl Disperse Orange 13 (4.89) and Cl Disperse Orange 29 (4.90), as economic dyes giving chiefly yellow and orange hues. The latter dye is known to exist in the syn conformation in the crystal [95] the unsubstituted parent dye prefers the anti conformation. A few monoazo and disazo disperse dyes have absorption bands in the near infrared [96]. 

Examples of dyes made via an A —> M — E synthesis are shown in Fig. 13.98. Although most azo disperse dyes are based on monoazo structures, disazo structures such as 15 (C.I. 

Azo dyes are classified according to a color index system based on usage and chemical constituents. These are subdivided into monoazo, disazo, trisazo, and polyazo derivatives with a specific assigned range of color index number. Another classification system involves dividing dyes into dyeing classes such as acid, basic, disperse, direct, mordant, and reactive dyes. Azo compounds are used extensively as dyes to color varnishes, paper, fabrics, inks, paints, plastics, and cosmetics. They are used in color photography. 

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