Direct dyes dyeing process

As opposed to the use of acid-based dye baths, water-soluble direct dyeing processes utilize a more alkaline dye bath with sodium chloride or sodium sulfate added to promote dye wlckability or uptake. The mixture is heated to near 100 prior to introducing cellulosic, natural, and polymeric fiber structures. 

Table 8 fists some other direct blacks. Progress in the field of direct dyes is the development of reactant fixable dyes (indosol dyes). These dyes and the economics of dyeing processes have been reviewed (57,58). 

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. 

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... 

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. 

In the course of his studies of the dyeing process, he became deeply interested in the structure of natural fibers, and most of his efforts were directed toward this new field of research, with the help of able associates, among them R. Brill, M. Dunkel, G. von Susich, and E. Valkd. His investigation of various aspects of the problem utilized physical means (for example, x-ray diffraction, optical properties, and viscosity) and the purely chemical approach. A young scientist, H. Mark, who later became an authority in the field of high polymers, was appointed head of the physical chemistry laboratory. 

All azo dyes contain one or more azo groups (-N=N-) as chromophore in the molecule on the basis of the number of azo groups in each molecule, they are named monoazo-, disazo-, trisazo-, etc. The azo groups are in general bound to a benzene or naphthalene ring, but they can also be attached to heterocyclic aromatic molecules or to enolizable aliphatic groups. On the basis of the characteristics of the processes in which they are applied, the molecule of the dye is modified to reach the best performances so they can be acid dyes, direct dyes, reactive dyes, disperse dyes, or others. 

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. 

Attempts to examine the process of cellulose crystallization have frequently involved culturing Acetobacter in the presence of fluorescent brighteners, direct dyes, carboxy-methyl-cellulose, or other agents which compete for interchain hydrogen bond sites, thereby disrupting microfibril formation... 

Clarke-Othmer process Clark oxygen electrode Clary sage oil [8016-63-5] Class A direct dyes Classification of dyes Classified removal Classifiers Clathrate... 

Wool-Cellulasic Fibers. One of (he oldest fiber blends in (he textile market is (he combination of wool and cotton or wool and viscose. In a one-bath process, selected direct and acid dyes are applied at pH 4.5-5.0 at 98 ItXfC. A phenolsulfonie acid condensation prodact is added as a reserving agent, to prevent the direct dyes from dyeing the wool under acid conditions, if optimum wetfastness properties are required, fiber-reactive dyes can be applied to both fibers by use of a two-bath process. 

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