Synthetic Mordant Dyes

The use of both natural and synthetic mordant dyes has long passed its zenith. Synthetic mordant dyes are still listed as leather dyes in the Colour Index. These are chiefly monoazo dyes, which are used especially for prechromed wool. For leather, the operations involved in the metallization step are too complex and time-consuming. Nowadays, modem tanneries prefer premetallized dyes. 

The first synthetic leather dyes were cationic. Mauveine, discovered by W. H. Perkin, marked the start of the synthetic dye industry. The cationic dyes were characterized by a previously unknown brightness. Nevertheless, the introduction of the new synthetic dyes was not a simple matter. The lightfastness properties were poor. However, the presence of Perkin at the first International Leather Congress in 1897 in London, undoubtedly ensured the breakthrough in the use of synthetic dyes for leather. 

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Mordant Dyes. This group includes many natural as well as synthetic dyes. They have no or low substantivity for textile fibers and are therefore appHed to ceUulosic or protein fibers that have been treated (mordanted) with metallic oxides to give points of attraction for the dye. The dye... 

Synthetic alizarin (5.1), Cl Mordant Red 5 (5.2) and Cl Mordant Orange 1 (5.3), the first azo dye capable of forming a metal complex, in this case via the salicylic acid residue, are examples of the simple mordant dyes in widespread use at the time when Werner propounded his theory. The first metal-complex dyes to be prepared in substance, rather than within the fibre, were discovered by Bohn of BASF in 1912 by treating hydroxyanthraquinonesulphonic acids with a warm solution of a chromium(III) salt. In the... 

A large number of naphthoquinone and benzoquinone dyes occur naturally as plant constituents, and in the past many of these have found use as colorants. The chemistry of synthetic analogues began with the discovery of naphthazarin (1) [475-38-7] by Roussin in 1861. Naphthazarin and its derivatives first found use as black chrome mordant dyes for wool and silk. In contrast to the naphthoquinones, synthetic benzoquinone dyes are few in number and are generally of the... 

Synthetic mordants are rarely applied. Reactive dyes for hair or suede and soluble sulfur dyes for suede are chosen only for special purposes. Finally organic... 

The group includes many natural and synthetic dyes, the latter usually being obtained from anthracene. They have no natural affinity for textiles but are applied to cellulosic or protein fibres which have been mordanted previously with a metallic oxide. The acid mordant dyes are a special class of dyes applied to wool or polyamide fibres as if the were acid dyes, and then given very high wet-fastness by subsequent mordanting. 

The mordant dyes are capable of combining with metallic ovides to form insoluble coloured lakes. Many of the old natural dyes siurli as. Mizarin, Logwood, Weld, and rustic were dyed on libres which had ]ire iously been impregnated, or mordanted, with hydroxides of chromium, tin, or aluminium. Some of these natural products v ere replaced by their synthetic equivalents and, after the structure of Alizarin had been determined, the range was increased by the addition of many analogous substances derived from anthraquinone. 

It is now made synthetically and dyes wool a bluish red on an aluminium mordant. 

There is a wide diversity of chemical structures of anthraquinone colorants.Many anthraquinone dyes are found in nature, perhaps the best known being alizarin, 1,2-dihydroxyanthraquinone, the principal constituent of madder (Chapter 1). Naturally-based anthraquinone dyes are of limited current commercial importance, although synthetic alizarin, as Cl Mordant Red 11, is used to an extent in the dyeing and printing of natural fibres. Many of the current commercial range of synthetic anthraquinone dyes are simply substituted derivatives of the anthraquinone system. For example, a number of the most important red and blue disperse dyes for application to polyester fibres are simple non-ionic anthraquinone molecules, containing substituents such as amino, hydroxyl and methoxy, and several sulfonated derivatives are commonly used as acid dyes for wool. 

When colorfastness against harsh external environmental exposures, such as UV light and acid moisture, is required, natural and synthetic (chrome) mordant dyes are employed. Synthetics are primarily used, whereby the dyed substrate is commonly overcoated with potassium dichromate as the mordant or dye-setting agent. 

Stannic chloride is also used widely as a catalyst in Eriedel-Crafts acylation, alkylation and cycHzation reactions, esterifications, halogenations, and curing and other polymerization reactions. Minor uses are as a stabilizer for colors in soap (19), as a mordant in the dyeing of silks, in the manufacture of blueprint and other sensitized paper, and as an antistatic agent for synthetic fibers (see Dyes, application and evaluation Antistatic agents). 

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). 

Logwood [8005-32-2] (Cl Natural Black 1 Cl 75290) is a modified ben2opyrone, and was the last of the natural dyes to survive after the appearance of synthetic dyes because of the desirable bluish-black hue it produced on chromemordanted fibers. However, for economic reasons and because it requires a mordant, it too is no longer used commercially. 

Many brilliantly coloured and tinctorially strong basic dyes for silk and tannin-mordanted cotton were developed in the early decades of the synthetic dye industry. Most of these belonged to the acridine, azine, oxazine, triarylmethane, xanthene and related chemical classes their molecules are usually characterised by one delocalised positive charge. Thus in crystal violet (1.29) the cationic charge is shared between the three equivalent methylated p-amino nitrogen atoms. A few of these traditional basic dyes are still of some interest in the dyeing of acrylic fibres, notably as components of cheap mixture navies and blacks, but many modified basic dyes were introduced from the 1950s onwards for acrylic and modacrylic fibres, as well as for basic-dyeable variants of nylon and polyester [44] ... 

The structure of kermisic acid is l,3,4,5-tetrahydroxy-7-carboxy-8-mcrhylanthraquinone. Carminic acid (Cl Natural Red 4 Cl 75470). is a red dye occurring as a glycoside in the body of the cochineal insect Dactylopius coccus of the order Homoptera. family Coecidae, Until the advent of synthetic dyes, the principal use for carminic acid was for dyeing tin-mordanted wool or silk. Its aluminum lake, carmine, finds use in Lhe coloring of foods. The structural formula of carminic acid is (2). 

The success of logwood, from the heartwood of the tree Haematoxylan campechianum L found in Central America, as a black dyestuff also depended on complex formation. The active constituent of logwood is the chroman hematein (4), which is itself red. When applied with a chromium mordant, however, logwood gave black shades and held the premier position for blacks and blues until the late 1890s, by which time synthetic dyes accounted for some 90% of the dyes used. 

Basic dyes of the azo class were among the earliest known synthetic dyes. They were used originally for dyeing cotton mordanted with tannin and potassium antimonyl tartrate and wool from neutral solution. They continue to have minor importance for dyeing leather, paper, plastics, and waxes, and as constituents of graphic arts colors. 

The fiber is first treated with metal salts (mordanted). Highly adhesive, basic metal compounds are formed on the fiber. These compounds are capable of producing insoluble colored complexes (lakes) with certain azo and anthraquinone derivatives. Alizarin is the best-known anthraquinone derivative for this process (see Section 2.3). It used to be isolated from the root of the madder plant but has now been replaced by the synthetic product. Suitable azo dyes contain, e.g., hydroxyl or carboxyl groups in the position ortho to the azo group on one or both of the aromatic nuclei. The shade of the dyeing depends on the type of metallic mordant used. Alizarin with aluminum or calcium salts produces the well-known Turkey red. 

Disperse and other synthetic dyes can only be taken up by the hair at elevated temperatures, and for this reason the skins must be chrome-tanned. The shrinking temperature of suede has to be at least 20° C higher then the dyeing temperature. No mordant is required, unlike with oxidation dyes, and killing need be far less intensive, too. In actual fact the operation is more hair cleaning than a killing process. The suede portion is not dyed and can be cleaned very easily with standard washing auxiliaries. 

Artificial dyes in the laboratory of the Lancashire calico-printer John Lightfoot included his own invention of aniline black he also pioneered new methods for mordanting and the use of vanadium in aniline black printing.104A general account has been given of the role of rosaniline in the development of the synthetic dye industry.105 A paper on quinones focuses chiefly on the case of anthraquinone and the synthesis of alizarin from anthracene.106... 

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