Direct dyes

Direct or substantive dyes are colored compounds that are mainly used to dye materials made from natural or regenerated cellulose (e.g., cotton, jute, viscose, or paper) without employing mordants as auxiliaries. The essential requirement for classification of a dye in this group is its substantivity, i.e., its absorption from an aqueous salt-containing solution onto cellulosic materials. Absorption onto cotton takes place in a neutral to soda alkaline medium, and onto paper in a weakly acid to neutral medium. 

Substantivity was initially attributed to secondary valence bonding between fiber and dye. The fact that coplanar molecules are always more substantive than nonplanar ones later led to the coplanarity theory with its assumption that coplanar dyes are in contact with the cellulose molecule along their entire length. 

The presence of hydrogen bonds has also been expounded as a possible explanation for high affinity between fiber and dye [1] however, such bonds are probably prevented by a water layer between fiber and dye [2], 

The nature of substantivity has been convincingly explained [3], According to this explanation, single dye molecules are adsorbed by the intermicellary cavities of the cellulosic fibers and, unlike nonsubstantive dyes, they form aggregates in these cavities. Because of their size, these aggregates can no longer be directly washed out with water, but only after further solvation has taken place. 

Because direct dyes become aggregated in aqueous solutions at normal temperatures, substantivity often cannot take effect until the temperature has risen. 

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. 

Direct dyes are attracted to the textile, according to their substantivity, by intermolecular forces without the need of mordant. They are used to color cotton and paper leather, silk, and nylon, and are also used as pH indicators or as biological 

Reactive dyes contain substituent that, when activated, react with the -OH groups of cellulose (i.e., cotton) or with -NH2 and -SH groups of protein fibers (i.e., wool) forming covalent bonds, making them among the most permanent of dyes. 

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Textile dyes were, until the nineteenth century invention of aniline dyes, derived from biological sources plants or animals, eg, insects or, as in the case of the highly prized classical dyestuff Tyrian purple, a shellfish. Some of these natural dyes are so-caUed vat dyes, eg, indigo and Tyrian purple, in which a chemical modification after binding to the fiber results in the intended color. Some others are direct dyes, eg, walnut sheU and safflower, that can be apphed directly to the fiber. The majority, however, are mordant dyes a metal salt precipitated onto the fiber facUitates the binding of the dyestuff Aluminum, iron, and tin salts ate the most common historical mordants. The color of the dyed textile depends on the mordant used for example, cochineal is crimson when mordanted with aluminum, purple with iron, and scarlet with tin (see Dyes AND DYE INTERMEDIATES). 

Table 2. Comparison of Basic, Acid, and Direct Dyes...

Parameter Basic dyes Acid dyes Direct dyes... 

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. 

Stilbene dyes have generally been important as direct dyes and fluorescent brighteners for ceUulosic fibers (4). Most stilbene dyes are yeUow and orange, with some examples of reds and browns and even a few blues. Brown stilbene dyes have commercial value as leather dyes (4). 

TrisaZO and Polyazo Dyes. These are mostly direct dyes, the hues ate predominandy brown, black, or dark blue or green. Some are leather dyes. Benzidine, which used to be an important bisdiazo component, has been replaced by 4,4 -diaminobenzanilide [785-30-8] ... 

Chromium is the principal metal used with mordant dyes for wool, whereas both chromium and cobalt are used extensively ia premetallized types for wool and nylon. Copper(II) is employed almost exclusively as the chelating metal ion ia both metaUizable and premetallized direct dyes for cotton. 

DIRECT DYES The extent of direct dye production and sales in the United States can be judged by the following data suppHed by the USlTC (39) for domestic usage in 1988 ... 

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 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. 

Table 5. Yellow and Orange Shade Commercial Direct Dyes, U.S. 1988...

Direct Blues. Direct Blue 86, a phthalocyanine direct dye, represents a small but important segment of the direct dye stmcture groups. The dyes are brilliant greenish blue or turquoise shades. Sales of nearly 2 million were reported in the United States in 1988. Among the disazo blues. Direct Blue 80 (74) and Direct Blue 98 (75) were sold in 152 t and 107 t quantities at an average cost of 2.22 and 1.81/kg, respectively, in 1988. Table 7 and Figure 6 show some direct blues. 

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). 

Sodium dichromate and various chromic salts are employed in the textile industry (195,196). The former is used as an oxidant and as a source of chromium, for example, to dye wool and synthetics with mordant acid dyes, oxidi2e vat dyes and indigosol dyes on wool, aftertreat direct dyes and sulfur dyes on cotton to improve washfastness, and oxidi2e dyed wool. Premera11i2ed dyes are also employed. These are hydroxya2o or a2omethine dyes in which chromium or other metals are combined in the dye (see Azine dyes DYES Azo dyes). 

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. 

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. 

Because of the limitations of direct dyes and the abiHty to use simple acid dye chromophores to give bright washfast dyeings, fiber-reactive dyes have become a weU-estabHshed, popular way of dyeing cellulose. A market of 56,000 t of reactive dyes was forecast for cellulose fibers in 1989 (18), and the growth rate of reactive dye consumption of 3.9% per annum is four times the growth rate of other dyes for ceUulosic fibers (19). 

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. 

Basic Theory of Fiber-Reactive Dye Application. The previously described mechanisms of dyeing for direct dyes apply to the apphcation of reactive dyes in neutral dyebaths. In alkaline solutions important differences are found. The detailed theoretical treatments are described elsewhere (6) but it is important to consider some of the parameters and understand how they influence the apphcation of fiber-reactive dyes. 

Electrolyte therefore plays three important roles increasing absorption in the neutral state, preventing desorption/promoting secondary exhaustion, and increasing the amount of ioni2ed ceHulose. Thus the amounts of salt used in the apphcation of fiber-reactive dyes are larger than for direct dyes. 

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. 

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