Direct Dyes with Aftertreatment

In spite of the reactive dyes with their outstanding wet-fastness, the direct dyes continue to hold a large share of the market for inexpensive cellulose and paper dyes. Apart from the new developments to replace the old established benzidine dyes, con- 

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The group of direct dyes with aftertreatment includes direct dyes that, after being applied to the fiber by the usual method, are subjected one of the following aftertreatments (1) Aftertreatment with cationic auxiliaries, (2) aftertreatment with formaldehyde, 3) diazotization of the dye on the fiber and coupling with suitable components (diazotization dyes), and (4) aftertreatment with metal salts. 

Sodium dichromate and various chromic salts are employed in the textile industry (195,196). The former is used as an oxidant and as a source of chromium, for example, to dye wool and synthetics with mordant acid dyes, oxidi2e vat dyes and indigosol dyes on wool, aftertreat direct dyes and sulfur dyes on cotton to improve washfastness, and oxidi2e dyed wool. Premera11i2ed dyes are also employed. These are hydroxya2o or a2omethine dyes in which chromium or other metals are combined in the dye (see Azine dyes DYES Azo dyes). 

It is not surprising, therefore, that chitosan and its basic derivatives will complex with anionic dyes. Giles et al. [68,69] researched the use of chitosan for the removal of dyes from effluent as long ago as 1958. The binding capacity of chitosan for anionic dyes is pH-dependent, but it has been reported [65] that in effluent treatment as much as 10 g dye per kg chitosan can be complexed at pH values above about 6.5. Similarly, chitosan has been used for the aftertreatment of direct dyeings on cotton to improve their fastness. 

The fact that the aftertreatment of direct dyes has a long history is not surprising since wet fastness within this class is not particularly good. Their prime advantages are ease of application and economy compared with dyes of higher fastness (reactive, sulphur or vat) -hence the continued search for highly effective aftertreatments that improve wet fastness... 

A third approach utilised copper salts, especially copper(II) sulphate, in conjunction with dyes containing chelatable groupings such as salicylic acid or o,o -dihydroxyazo moieties. Indeed, special ranges of copperable direct dyes, for which the treatment with copper(II) sulphate was really part of the dyeing process rather than an optional aftertreatment, were introduced. In the past the main use of this chelation treatment was to enhance light fastness, but it is little used for this purpose nowadays. 

The earliest polymeric cationic aftertreatments stemmed from the development of crease-resist finishes for cellulosic fibres. One such, promoted specifically for its colour fastness improvements when applied as an aftertreatment to direct dyeings, was a condensation product of formaldehyde with dicyandiamide (Scheme 10.82). Many similar compounds followed, such as condensation products of formaldehyde with melamine (10.212), polyethylene imine) with cyanuric chloride (10.213) and alkyl chlorides with polyethylene imine) (10.214 R = alkyl). 

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. 

In a later development [136,137], cyanuric chloride was proposed as an aftertreatment for cotton already dyed with direct dyes containing amino groups. This approach appears even less likely to succeed than in situ addition to the dyebath. Serious hazards are associated with the handling of cyanuric chloride under these conditions. This highly reactive compound is a primary skin irritant and is known to cause severe allergic reactions in certain individuals. Dye-agent reaction will be inefficient because of hydrolytic deactivation. Uptake of cyanuric chloride (or its hydrolysis products) by the dyed cotton will be poor. 

Direct Dyes. These water-soluble anionic dyes, when dyed from aqueous solution in the presence of electrolytes, are substantive to, i.e., have high affinity for, cellu-losic fibers. Their principal use is the dyeing of cotton and regenerated cellulose, paper, leather, and, to a lesser extent, nylon. Most of the dyes in this class are polyazo compounds, along with some stilbenes, phthalocyanines, and oxazines. Aftertreatments, frequently applied to the dyed material to improve washfastness properties, include chelation with salts of metals (usually copper or chromium), and treatment with formaldehyde or a cationic dye-complexing resin. 

Furthermore, direct dyes (see Section 3.3) are among the most important silk dyes. Their fastness properties can be improved by aftertreatment with metallic salts or formaldehyde. 

The Benzo Fast Copper (I.G. Farbenindustrie) and the Cuprophenyl (Geigy) dyes are converted to insoluble copper complexes on the fiber by treating the dyeings with a copper salt. Copper complex formation on the fiber has strongly lost technical significance due to the copper content in dyehouse effluents. The mostly copper-containing Indosol direct dyes recently launched by Sandoz are cross-linked on and with the fiber by aftertreatment with a special reactive agent [35],... 

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). 

Because many direct dyes do not have good washfastness and lightfastness, their dyeings on cotton are often treated with a chemical agent, in what is commonly known as an aftertreatment process. The most widely used aftertreatment methods involve (1) cationic fixatives, (2) copper sulfate, or (3) diazotiza-tion and coupling reactions. The first and third methods are designed to enhance wash-... 

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