Dyes, Azo
Authors: Chudgar Rasik J. , Oakes John 
Published: July 2004

The term azo dyes is applied to those synthetic organic colorants that are characterized by the presence of the chromophoric azo group (\documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${{\relbar \kern-5pt{\relbar}\kern-7pt{\relbar}}{\rm{N}}{\raise1pt\hbox{$\Relbar \kern-4pt{\Relbar}$}}{\rm{N}}{\relbar \kern-5pt{\relbar}\kern-7pt{\relbar}}}$\end{document}). This divalent group is attached tosp2hybridized carbon atoms: on one side, to an aromatic or heterocyclic nucleus; on the other, it may be linked to an unsaturated molecule of the carbocyclic, heterocyclic, or aliphatic type. No natural dyes contain this chromophore. Commercially, the azo dyes are the largest and most versatile class of organic dyestuffs. There are more than 10,000 Colour Index (CI) generic names assigned to commercial colorants; approximately 4,500 are in use, and over 50% of these belong to the azo class. Synthetic dyes are derived in whole or in part from cyclic intermediates. Approximately two‐thirds of the dyes consumed in the United States are used by the textile industry to dye natural and synthetic fiber or fabrics, about one‐sixth are used for coloring paper, and the rest are used chiefly in the production of organic pigments and in the dyeing of leather and plastic. Dyes are sold as pastes, powders, and liquids; concentrations vary from 6–100%. The concentration, form, and purity of a dye are determined largely by the use for which it is intended. The most authoritative compilation covering the constitution, properties, preparations, manufacturers, and other coloring data is the publicationColour Index, which is edited jointly by the Society of Dyers and Colourists and the American Association of Textile Chemists and Colorists (AATCC). The coupling reaction between an aromatic diazo compound and a coupling component is the single most important synthetic route to azo dyes. Of the total dyes manufactured, about 60% are produced by this reaction. All coupling components used to prepare azo dyes have the common feature of an active hydrogen atom bound to a carbon atom. In addition to classification according to the number of azo groups, further subdivision is achieved, first according to whether the compound is water‐soluble and, secondly, according to the types of component used. Another system of classification is based on dyeing classes. In the disazo group of azo dyes, primary and secondary types are distinguished. Trisazo and polyazo dyes are mostly direct dyes, the hues are predominantly brown, black, or dark blue or green. A section of this article is a distillation of recent progress on the degradation of azo dyes. An outline of the key physical properties of dyes, followed by a brief description of physical characteristics of detergent bleaches, before focusing upon the kinetics and mechanism of oxidation are given.

Initially, the focus of attention of mechanistic work is on the oxidation of dyes by typical detergent bleaches, which occurs by oxygen atom transfer. This is followed by an outline of electron‐transfer mechanisms and radical mechanisms or by metal catalysis, involving high oxidation state species. Finally, attention is directed towards understanding photofading mechanisms which, although more complex, is facilitated by the foregoing discussion.

Of all classes of dyestuffs, azo dyes have attained the widest range of usage because variations in chemical structure are readily synthesized and methods of application are generally not complex. There are azo dyes for dyeing all natural substrates such as cotton, paper, silk, leather, and wool; and there are azo dyes for synthetics such as polyamides, polyesters, acrylics, polyolefins, viscose rayon, and cellulose acetate; for the coloring of paints, varnishes, plastics, printing inks, rubber, foods, drugs, and cosmetics; for staining polished and absorbed surfaces; and for use in diazo printing and color photography. The shades of azo dyes cover the whole spectrum. Commercial acid dyes contain one or more sulfonate groups, thereby providing solubility in aqueous media. There are three general classifications of acid dyes: acid dyes that dye directly from the dyebath, mordant dyes that are capable of forming metallic lakes on the fiber when aftertreated with metallic salts, and premetallized dyes. Direct dyes are defined as anionic dyes substantive to cellulosic fibers (cotton, viscose, etc), when applied from an aqueous bath containing an electrolyte. Direct dyes are one of the most versatile classes of dyestuff. In worldwide usage for cellulosic textiles, direct dyes are the second largest class of dyestuff. Direct yellows and oranges, direct reds, and direct blues are prominent direct dyes. Direct violets and greens are small‐volume products. Azoic dyes (known also as ice colors and ingrain colors) are water‐insoluble azo pigments. Disperse dyes are coloring substances having very low aqueous solubility which are applied to hydrophobic fibers from an aqueous system in which the dye is present in a highly dispersed state. The sharp increase in the importance of disperse dyes in the 1970s and 1980s can be attributed directly to the emergence of polyester and nylon as the principal synthetic fibers. Azo and anthraquinone compounds comprise the two principal structural types which are used as disperse dyes. Manufacturing procedures for producing dye dispersions are generally not disclosed. The principal dispersants in use include long‐chain alkyl sulfates, alkaryl sulfonates, fatty amine–ethylene oxide condensates, and others. Disperse dyes are classified as high energy or low energy types. The use of higher dyeing temperatures for polyester fibers has made possible the use of dyes of higher molecular weight, the so‐called high energy dyes. A use for disperse dyes which has undergone rapid growth since 1970 is in inks for the heat‐transfer printing of polyester. The oil‐soluble, water‐insoluble, azo dyes dissolve in oils, fats, waxes, etc. Generally, yellow, orange, red, and brown oil colors are azo structures and greens, blues, and violets are primarily anthraquinones. Spirit‐soluble azo dyes dissolve in polar solvents, such as alcohol and acetone, and find application in the coloring of lacquers, plastics, printing inks, and ball‐point pen inks. Basic dyes of the azo class are the simplest and oldest known synthetic dyes. Current cationic dyes are used for modified acrylics, modified nylons, modified polyesters, leather, unbleached papers, and inks. An important application is for conversion into pigments. Organic pigments are an important class of organic colorants. Expanding areas of usage include the mass coloration of synthetic fibers and textile printing in the textile field, and in the nontextile area, plastics. A pigment is insoluble in the medium in which it is used. The physical properties of pigments are of great significance since the coloring process does not involve solution of the colorant. Azo pigments can be grouped as metal toners, metal chelates, and metal‐free azo pigments.