Anionic direct dyes

40-diaminostilbene-2,2 0-disulfonic acid exhibits similar behavior to benzidine. For example, when it is coupled with phenol and the phenolic hydroxyl groups are etherified, chrysophenine, C.I. Direct Yellow 12, 24895 [2870-32-8] (2) is obtained, which is a highly substantive yellow paper dye. 

However chrysophenine is fairly difficult to dissolve and is no longer widely used for dyeing paper. 

With the increasing popularity of the continuous method of dyeing in the paper industry, new direct dyes were developed with particularly good cold-water solubility without any impairment to the substantivity. Improvements of this kind with regard to substantivity and solubility can be seen in the further development of the widely used, but only moderately soluble, cotton dye C.I. Direct Red 81, 28160 [2610-11-9] (4). 

If the benzoyl group is exchanged for 1,3,5 trichlorotriazine and the residual chlorine atoms are treated with, e.g., monoethanolamine, C.I. Direct Red 253, [142985-51-1] (5) is obtained, which has both higher substantivity and better solubility. 

Oxidation of dehydro tli i o-/ - to 1 u i dine su 1 fo nic acid with sodium hypochlorite [7] gives the yellow azo paper dye C.I. Direct Yellow 28, 19555 [8005-72-9] (10) without diazotization and coupling. 

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Gellulosic—Acrylic Fibers. Commonly this blend is used ia koitgoods, wovea fabrics for slacks, drapery, and upholstery fabrics. Siace anionic direct dyes are used for the ceUulosic fiber and cationic dyes for the acryHcs, a one-bath dyeiag process is only suitable for light to medium shades. Auxiliaries are needed to prevent precipitation of any dye complexes. 

Substantive anionic direct dyes can be repolarized by incorporating an excess of cationic groups [11] without impairing the substantive properties. Several dyes of this kind have been developed for the paper industry, e.g., C.I. Basic Red 111, [118658-98-3] (25). 

In general it can be said that the cationic direct dyes are the ideal complement to the anionic direct dyes. By combining selected elements from both dye groups it is even possible to achieve dyeing results which cannot be obtained with dyes from a single dye group. 

However, the characteristically large pi electron system of anionic direct dyes provides a strong attraction between dye and fibre, known as Yoshida or van der Waals forces. 

Anionic direct dyes are the most widely used colourant within the tissue, printing and writing paper sectors. Having a typical molecular weight of 550-2,000, they produce a wide range of shades, although certain bright shades cannot be achieved... 

Cationic direct dyes are similar in shape to anionic direct dyes (Fig. 10.5), but while anionic dyes contain negative groups which have repulsive forces and impart water solubility to the product, cationic direct dyes have positive groups attached which allow an attraction between fibres and dyestuff Accordingly, therefore, dye fixation with these dyes is excellent. This property does, however, lead to certain problems when these dyes are used either in conjunction with anionic dyes, or with furnishes which contain a degree of mechanical fibres or anionic trash (such as CTMP pulps). 

Colourants should be added at the maximum consistency that allows for thorough mixing of the furnish and colourant to take place. Preferential dyeing of fibres can take place if dye and a limited portion of the fibre are kept in contact for any period of time. By adding colourants, particularly anionic direct dyes and pigments, at a lower consistency, the Yoshida and van der Waals forces are less effective and dyeing... 

These retain the planar molecular structure of acid direct dyes, but cationic groups have been incorporated in the structure. This is to increase their affinity to paper fibers. They are moderately adsorbed on bleached Hgnin-containing stocks, and even in the case of deep shades, alum or a fixative is usually not required. These dyes produce fairly good bleed fastness. Combination dyeing with anionic direct dyes can be carried out to improve the fixation of the dyes on the paper machine... 

Direct dyes are defined as anionic dyes substantive to ceUulosic fibers (cotton, viscose, etc), when applied from an aqueous bath containing an electrolyte. Before the discovery of Congo Red in 1884, only mordanted cotton could be dyed. Congo Red [573-58-0] (62) (Cl Direct Red 28 Cl 22120) a primary symmetrical disazo dye, which is made readily from bisdiazotized benzidine and naphthionic acid [84-86-6] (4-arnino-l-naphthalenesulfonic acid), was the precursor of a most important line of dyes, including all shades, derived from benzidine and its homologues. Today, no benzidine dye is produced because benzidine is carcinogenic. 

Direct Dyes. These are defined as anionic dyes, again containing sulfonic acid groups, with substantivity for ceUulosic fibers. They are usually a2o dyes (qv) and can be mono-, dis-, or polya2o, and are ia general planar stmctures. They are appHed to ceUulosic fibers from neutral dyebaths, ie, they have direct substantivity without the need of other agents. Salt is used to enhance dyebath exhaustion. Some direct dyes can be appHed to wool and polyamides under acidic conditions, but these are the exception. 

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. 

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. 

Churchley, J.H. Greaves, A.J. Hutchings, M.G. Phillips, D.A.S. Taylor, J.A. A chemometric approach to understanding the bioelimination of anionic, water-soluble dyes by a biomass - Part 3 Direct dyes. J. Soc. Dyers Colour. 2000, 116, 279-284. 

For the present purpose anionic dyes are classified as those containing sulfonic acid groups or those capable of solubilization in alkali, such as vat dyes. Such dyes are usually applied to fibres with hydrophilic character such as cotton, which is still the most important single fibre from a point of view of world usage, nylon and wool. Azoic dyes and direct dyes, although economically important, will not be discussed. Pigments, which are sometimes used in cotton printing, are treated separately. 

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. 

In principle the anionic dyes also include direct dyes, but, because of their characteristic structures, these are used to dye cellulose-containing materials and are applied to the fiber from a neutral dye bath (see Section 3.3). 

Continuous dyeing of PAC-cotton plush with cationic and direct dyes by the pad steam process plays an important role. The choice of dyes must take into account liquor stability, reservation of PAC or CEL fiber, and solubility. Precipitation of cationic and anionic dyes present in the pad liquor at relatively high concentrations cannot be avoided solely by dye selection. Suitable auxiliary systems have been developed. Differently charged dyes are kept in solution separated from each other in two phases by the combination of anionic and nonionogenic surfactants. With the help of fixing accelerators, good penetration of PAC fibers can be achieved in 10-15 min with saturated steam at 98-100°C. 

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. 

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