Direct dye structures

Direct Blues. Direct Blue 86, a phthalocyanine direct dye, represents a small but important segment of the direct dye structure groups. The dyes are brilliant greenish blue or turquoise shades. Sales of neady 2 million were reported in the United States in 1988. Among the disazo blues, Direct Blue 80 (74) and Direct Blue 98 (75) were sold in 152 t and 107 t quantities at an average cost of 2.22 and 1.81/kg, respectively, in 1988. Table 7 and Figure 6 show some direct blues. 

These are defined as anionic dyes with substantivity for cellulosic fibres applied from an aqueous dyebath containing an electrolyte. The forces that operate between a direct dye and cellulose include hydrogen bonding, dipolar forces and non-specific hydrophobic interaction, depending on the chemical structure and polarity of the dye. Apparently multiple attachments are important, since linearity and coplanarity of molecular structure seem to be desirable features (section 3.2.1). The sorption process is reversible and numerous attempts have been made to minimise desorption by suitable aftertreatments (section 10.9.5). The two most significant non-textile outlets for direct dyes are the batchwise dyeing of leather and the continuous coloration of paper. 

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]. 

The significance of conjugation as a contributor to the substantivity of dyes for cellulose is not always easy to distinguish from the effect of the degree of linearity of the molecule. Almost all direct dye molecules possess flexible chains of aryl nuclei linked by azo or other unsaturated groups. Such structures can readily adopt a near-linear spatial conformation, as... 

A full interpretation of the relationships between direct or vat dye structure and substantivity for cellulose must take into account the contribution of multilayer adsorption of dye molecules within the pore structure of the fibre [71]. The great difference in substantivity between Cl Direct Red 28 (3.66) and the monoazo acid dye (3.67) that is the half-size analogue of this symmetrical disazo dye may be interpreted in terms of their relative tendencies to form multilayers within the fibre pores as a result of dye-dye aggregation. Saturation adsorption values of these two dyes on viscose fibres at pH 9 and 50 °C corresponded to monolayer coverage areas of approximately 90 and 11 m2/g of internal surface respectively [72]. In view of the smaller molecular area and greater mobility of the half-size acid dye, higher uptake than the direct dye would be anticipated if there were only a limited area of internal surface available for true monolayer adsorption. 

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. 

Many of the premetallised direct dyes are symmetrical structures in the form of bis-1 1 complexes with two copper(II) ions per disazo dye molecule. Scheme 5.12 illustrates conversion of the important unmetallised royal blue Cl Direct Blue 15 (5.43), derived from tetrazotised dianisidine coupled with two moles of H acid, to its much greener copper-complex Blue 218 (5.44) with demethylation of the methoxy groups as described above. Important symmetrical red disazo structures of high light fastness, such as Cl Direct Red 83 (5.45), contain two J acid residues linked via their imino groups. Unsymmetrical disazo blues derived from dianisidine often contain a J acid residue as one ligand and a different coupler as the other, such as Oxy Koch acid in Cl Direct Blue 77 (5.46), for example. 

In contrast to direct dyes, metal-complex azo reactive dyes are almost always monoazo chromogens coordinated to one copper(II) ion per molecule. The important structural types include phenylazo J acid reds (5.47), phenylazo H acid violets (5.48) and naphthylazo H acid blues (5.49), where Z represents the reactive system attached through the imino group in the coupling component. Less often the reactive system is located on the diazo component, as in Cl Reactive Violet 5 (5.50) and analogous red to blue members of various ranges. 

Dyeing of paper has been traditionally carried out with acid, direct and basic dyes, whilst newer systems are based on cationic direct dyes. These cationic direct dyes are related structurally to reactive dyes with the labile chlorines replaced by pendant cationic groups, e.g. (2.50). 

Direct Dyes. These are defined as anionic dyes, again containing sulfonic acid groups, with substamivily fur eellulosie libers. They are usually azo dyes and can be mono-, dis-. or polyazo. and are in general planar structures. They are applied 10 eellulosie fibers from neutral dyebuths. i.c., they have direct substamivily without the need of other gents. Salt is used to enhance dyebalh exhaustion. Some direct dyes can be applied to wool and polyamides under acidic conditions, but these are the exception,... 

Acid, basic, and direct dyes are all ionic in nature. Acid dyes contain free acid groups which are ionized in the aqueous application medium (dyebath). They generally used to dye polyamine, wool, or silk and are primarily azo, anthraquinone, or triarylmethane structures. 

Reactive Dyes. These dyes form a covalent bond with the fiber, usually cotton, although they are used to a small extent on wool and nylon. This class of dyes, first introduced commercially in 1956 by ICI, made it possible to achieve extremely high washfastness properties by relatively simple dyeing methods. A marked advantage of reactive dyes over direct dyes is that their chemical structures are much simpler, their absorption spectra show narrower absorption bands, and the dyeings are brighter. The principal chemical classes of reactive dyes are azo (including metallized azo), triphendioxazine, phthalocyanine, formazan, and anthraquinone (see Section 3.1). 

Azo dyes make up the major proportion of the direct dyes apart from these, only a few azine, phthalocyanine, and nonazo metal-complex dyes possess a certain significance. The following structural characteristics are necessary for high substantivity ... 

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