Direct dyes class

Certain water-soluble dyes are directly adsorbed onto cotton that has not been pretreated with a mordant (section 3.2.1). The first dye in which this phenomenon was observed was Congo red (4.57 Cl Direct Red 28), discovered in 1884 by Bottiger. The extreme pH sensitivity of this dye now restricts its use to that of an indicator, but it deserves mention as the forerunner of the direct dye class. 

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. 

Stilbene dyes ate classed as a subgroup of azo dyes having excellent colorfastness and typical direct dye wash fastness on cotton and ate arranged iato six categories by the Society of Dyers and Colourists (2), as described ia the foUowiag. 

Direct dyes are one of the most versatile classes of dyestuff. U.S. production in 1988 was nearly 18,900 t valued at 100 million. In worldwide usage for ceUulosic textiles, direct dyes are the second largest class of dyestuff. The AATCC Buyers Guide (July 1991) Usts over 180 different Cl categories for direct dyes representing nearly 850 commercially available products. U.S. production figures are not released for most of these dyes the important direct yeUows and oranges of revealed chemical composition are Usted in Table 5. 

Class B direct dyes have poor leveling power and exhaustion must be brought about by controlled salt addition. If these dyes are not taken up uniformly in the initial stages it is extremely difficult to correct the urdevelness. They are dyes that have medium—high affinity and poor diffusion. In their apphcation the cellulose is entered into a dyebath containing ordy dye. The salt is added gradually and portionwise as the temperature is increased and possibly the final additions made after the dyebath has come to the bod. 

Class C direct dyes are dyes of poor leveling power which exhaust well in the absence of salt and the only way of controlling the rate of exhaustion is by temperature control. These dyes have high neutral affinity where, resulting from the complexity of the molecules, the nonionic forces of attraction dominate. When dyeing with these dyes it is essential to start at a low temperature with no added electrolyte, and to bring the temperature up to the boil very slowly without any addition of electrolyte. Once at the bod the dyeing is continued for 45—60 min with portionwise addition of salt to complete exhaustion. 

Wetfastness. Class A direct dyes offer the most trouble-free process for dyeing cedulose. However, they do not always provide sufficient wetfastness. 

Practical Processes. With acid leveling dyes no real problems exist because the dyes show good migration, electrolyte is added from the beginning, and rather like Class A direct dyes level dyeing is achieved by prolonging the times at the boil. 

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 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... 

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. 

Table 1.4 Percentage distribution of chemical classes in direct dye hue sectors...

Chemical class Distribution in hue sector (%) % of all direct dyes... 

The simplest monoazo dyes fail to meet these requirements, but by choosing intermediates known to confer substantivity and by building up the molecule to provide the necessary length and coplanarity (section 3.2.1), direct dyes can be produced from this class. Thus the highly substantive character of the benzothiazole nucleus is exploited in Cl Direct Yellow 8 (4-58), as is the alignment of the azo, ureido and acylamino groups in the substituted J acid coupling component of Cl Direct Red 65 (4-59). 

Direct dyes have only modest fastness to washing, which may be improved by after-treatments such as metal-complex formation (section 5.5.3) or by diazotisation of the dye on the fibre and further coupling of the diazonium salt with an insoluble coupling component (section 1.6.14). In addition to their use on cotton and viscose, direct dyes are important in the dyeing of leather. The cheapest members of this class are also used in the coloration of paper, since for this purpose fastness properties are largely irrelevant and price is all-important. 

The nomenclature given for these componnds is a nniversally recognised system for the naming of dyestuffs devised by the Society of Dyers and Colourists as part of their Colour Index (Cl). The Cl Generic Name is made up of the application class, the hne and a nnmber. Acid dyes are nsed on wool and polyamide, direct dyes on cel-lulosic fibres, paper and leather, disperse dyes on polyester fibres, reactive dyes on cellnlosic fibres and basic dyes on polyacrylonitrile and paper. 

These chromophores have declined significantly in importance as textile dyes bnt have remained of interest becanse of their fluorescent behaviour, as discussed in Chapter 3, section 3.5.1.5. One exception is the triphenodioxazine ring system, which is used to produce valuable blue dyes in the Direct (2.19) and Reactive dye classes (2.20) as well as pigments (see section 2.4.1.7). The dyes from this chromogen have a very high molar absorption coefficient (ca. 80 000) versus typical anthraquinone dyes (ca. 15 000) and have therefore replaced some of the dyes from this latter chromogen in the reactive dyeing of cotton. ... 

Tt is the purpose of this paper to describe methods for determining and A interpreting dye spectra in aqueous dispersions of silver halides and other substrates. Such spectra can be utilized for the direct measurement of surface concentrations of dyes from which, in turn, the surface area of the substrate can be derived. The techniques involved are not limited to a specific dye class but will be illustrated in this paper by the behavior of cyanine dyes. 

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