Direct Dyes Bonds

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. 

Amination A/-Methylolacrylamide in presence of Lewis acid catalyst. Further modifications possible by addition to double bond (Scheme 10.61) Amines with durable press resins Improved dyeability with dichlorotriazine dyes at pH 5 without salt, giving 99% fixation Some improvements in dyeability, especially with direct dyes, but light fastness can be a problem... 

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... 

Reactive Dyes. These dyes form a covalent bond with the fiber, usually cotton, although they are used to a small extent on wool and nylon. This class of dyes, first introduced commercially in 1956 by ICI, made it possible to achieve extremely high washfastness properties by relatively simple dyeing methods. A marked advantage of reactive dyes over direct dyes is that their chemical structures are much simpler, their absorption spectra show narrower absorption bands, and the dyeings are brighter. The principal chemical classes of reactive dyes are azo (including metallized azo), triphendioxazine, phthalocyanine, formazan, and anthraquinone (see Section 3.1). 

In general, dyeing with reactive dyes is very similar to the well-known process of direct dyeing (see Section 4.3). The major difference is that the reactive dye forms a chemical bond to the fiber. Important factors determining the dyeing properties of reactive dyes include affinity of the dye for the fiber (substantivity), its diffusibility, reactivity, and the stability of the dye-fiber bond. 

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. 

This is an important class of dyes for the dyeing of paper. Direct dyes are also called substantive dyes [4,5] because they tend to have a high affinity to cellulose fibers due to their linear molecular structure and a system of conjugated double bonds and usually also exhibit good wetfastness properties with the addition of a fixative. 

It can be prepared either by using appropriate direct dyes or reactive dyes. In the latter case, pure linters (the raw material for high-quality paper) are suspended in water, and the solution of the reactive dye is added. For example, the dye 15 reacts via the sulfonic acid group in the side chain. The reaction mixture is then made alkaline and the dye reacts with the linters. After completion of the reaction, the fiber pulp is centrifuged, washed electrolyte-free, and processed on a paper machine to form the pH paper. Paper produced in this way is mostly bonded onto a plastic material and used as pH test strip. 

Direct dyes are anionic colorants that have affinity for cellulosic fibers.17 They were the first dyes that could be used to dye cotton in the absence of a mordanting agent, giving rise to the term direct-cotton dyes. Like acid dyes, direct dyes contain one or more -SOjNa groups, making them water-soluble. Unlike acid dyes, they interact with cellulose (Cell-OH) chains via secondary valency forces (e.g., H-bonding and dipole-dipole interactions), as illustrated in... 

Fig. 13.6. H-bonding interactions between cellulose and polar groups in direct dye molecules.

With cotton (cellulose), the direct dyes usually contain o-hydroxy- or aminoazo compounds, in which chelation assists in the production of hydrogen bonds between the dye and the cotton (1) ... 

These early people used alum as an astringent and as a mordant. An astringent is a chemical that causes skin to pull together. Sprinkling alum over a cut causes the skin to close over the cut and start healing. A mordant is used in dyeing cloth. Few namral dyes stick directly to cloth. A mordant bonds to the cloth and the dye bonds to the mordant. 

The adsorption isotherm for Chrysophenine G, which is shown in Fig. 16.1 and is typical of that obtained with most direct dyes, corresponds with the Freundlich equilibrium. This indicates that with the direct dyes opportunity for hydrogen bonding and Van der Waals forces to bind the dye are, for practical purposes, almost unlimited. The limitation which is imposed is the available surface for attachment, but this does not terminate... 

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