Cationic direct dyes

If aromatic diamines are coupled to substituted 4-methyl-6-hydroxy-2-pyridi-nones [10] which contain a quaternary ammonium group (22) or primary, secondary, or tertiary amines (23), brilliant yellow to orange dyes are obtained. 

Usually it is necessary to exchange the chloride ions for organic acid anions to obtain adequate solubility. Dyes containing aliphatic amino groups (e.g., 23) are more soluble because they can be protonated with organic acids such as acetic, lactic, or formic acid. 

In polycationic dyes such as 24, the presence of a conjugated molecular structure is only of secondary importance. The positive charge is sufficient to enable complete exhaustion onto paper fibers. 

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). 

Attractive lightfast turquoise shades can be obtained witb Cu phthalocya-nines. Although these molecules are planar, they are insufficiently substantive in sulfited form so that the dyeings must be post treated with a fixative. By introducing cationic groups, e.g., by treatment with chlorosulfonic acid and subsequent amidation (26), very good affinity to paper is obtained, e.g., C.I. Basic Blue 140, [61724-62-7]. 

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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). 

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. 

Example Environmental Emission of a Cationic Direct Dye Applied to the Pulp in Tissue and Packaging Coloration... 

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). 

Since the early 1970s a new class of colorants called cationic direct dyes has become important for coloring paper. These dyes combine the molecular shape and geometry of direct... 

A reactive dye for ceUulose contains a chemical group that reacts with ionized hydroxyl ions in the ceUulose to form a covalent bond. When alkaH is added to a dyebath containing ceUulose and a reactive dye, ionization of ceUulose and the reaction between dye and fiber is initiated. As this destroys the equihbrium more dye is then absorbed by the fiber in order to re-estabUsh the equUibrium between active dye in the dyebath and fiber phases. At the same time the addition of extra cations, eg, Na+ from using Na2C02 as alkaH, has the same effect as adding extra salt to a direct dye. Thus the addition of alkaH produces a secondary exhaustion. 

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. 

The traditional use of dyes is in the coloration of textiles, a topic covered in considerable depth in Chapters 7 and 8. Dyes are almost invariably applied to the textile materials from an aqueous medium, so that they are generally required to dissolve in water. Frequently, as is the case for example with acid dyes, direct dyes, cationic dyes and reactive dyes, they dissolve completely and very readily in water. This is not true, however, of every application class of textile dye. Disperse dyes for polyester fibres, for example, are only sparingly soluble in water and are applied as a fine aqueous dispersion. Vat dyes, an important application class of dyes for cellulosic fibres, are completely insoluble materials but they are converted by a chemical reduction process into a water-soluble form that may then be applied to the fibre. There is also a wide range of non-textile applications of dyes, many of which have emerged in recent years as a result of developments in the electronic and reprographic... 

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). 

Brighteners are applied to cotton by methods similar to direct dyes. By far the most common are triazinyl derivatives of diaminostilbenedisulphonic acid (DAS) of general formula 11.5, where M is an alkali metal, ammonium or alkylammonium cation. Examples of groups Ilj and R2 are shown in Table 11.1. Most suppliers of FBAs market such compounds, often called DAST brighteners. Products in this class have sometimes been marketed because the supplier needed to offer something different for commercial reasons, or to avoid infringing a competitor s patent, rather than for any real technological necessity. 

Another terminal bidentate ligand that has been exploited occasionally in bright disazo direct dyes is the sulphated 8-hydroxyquinoline residue (5.15). On aftercoppering, fastness to light and wet tests is enhanced by hydrolysis catalysed by the copper(II) ion and formation of a bidentate 1 2 complex (Scheme 5.3). Apparently, electron withdrawal by sulphur facilitates removal of the sulphite grouping and approach of the copper(II) cation [10]. 

The thiazole ring system is found in many types of dye. Thiazole-containing sulphur dyes and primuline were considered in section 6.4-2. Quaternised dehydrothio-p-toluidine 6.133 is available as Cl Basic Yellow 1 (6.152). Other derivatives of this intermediate are used as direct dyes, such as Cl Direct Yellow 8 (4-58). The benzothiazole ring appears in various azo disperse dyes [14], quatemisation of which gives useful cationic dyes, an important example being Cl Basic Blue 41 (4.99). Another example containing a quaternised thiazole ring is Cl Basic Red 29 (4-102). 

Chromosorb T shows the direct correlation of capacity with the oc-tanol-water partition coefficient expected of hydrophobic adsorbents (6). Its overall affinity for hydrophobic water contaminants is similar to that of Amberlite XAD-8 (Rohm and Haas), but it has additional affinity for humic acid and for water-soluble, cationic, aromatic dyes. Chromosorb T is more easily cleaned than the XAD resins, and it is more inert, contributing essentially no contaminants to eluates. Table IV illustrates the adsorption and recovery of a series of hydrophobic test solutes at 50 ppb in 8 L of synthetic hard water on a 50-mL bed of Chromosorb T. 

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. 

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. 

A cyclic ammonium residue can either be linked directly to the aromatic ring or attached to the diazo component via an aliphatic residue. Cyclic ammonium residues linked via carbon bonds to the aromatic ring of the diazo component include 1,3-dialkylbenzimidazolium [118], benzothiazolium [119], and pyridinium [120] moieties, linked via nitrogen, as well as the benzotriazolium moiety [121], Upon reaction with pyridine, azo dyes that carry a chloroethylsulfonamido group in the diazo component form cationic azo dyes, e.g., 39 [33869-97-5], which color polyacrylonitrile in brilliant shades [122],... 

Printing with Direct Dyes is common only for inexpensive goods, because of the extended steaming time required and the limited wetfastness of the prints, even after treatment with cationic quaternary products [5, p. 514],... 

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