Textile colorants impart color to a textile material, usually with a high degree of permanency, as a result of their chemical binding or physical entrapment within or around the textile material. The textile material may be in one of several forms such as fiber, yarn, fabric, garment, etc. Textile colorants are supplied in both solid and liquid forms, for example, as powders, granules, solutions, or dispersions. In certain instances, precursors are applied to textile materials to generate the colorant in situ within the textile.
Textile Dyes and Pigments
Both dyes and pigments are used in the coloration of textiles . The former substances are present in solution at some point during their application, whereas the latter colorants remain insoluble within any vehicle in which they are applied as well as within the textile material itself. Pigments must therefore either be incorporated into textile fibers during their construction (e.g. mass coloration of a polymer followed by its melt extrusion into fibers) or be printed onto a fabric as part of a formulation that contains a binder so that the colorant is physically held to the surface of the textile in a coating. Textile dyes are usually applied from solution although certain types may be present initially as a dispersion or applied from the vapor phase. The mechanism by which dyes remain within a textile depends on the particular colorant type. Retention may rely on intermolecular forces operating between dye and fiber following adsorption onto and/or dissolution within the polymer, formation of covalent bonds between the dye and the fiber, or entrapment of colorant particles within the textile by deposition of an insoluble form of the dye.
Natural Textile Colorants
Prior to the synthesis of picric acid in the eighteenth century as a yellow dye for silk, all textile colorants were obtained directly from natural sources, such as plants, insects, and shellfish [1, 2]. These natural colorants began to be superseded by synthetic dyes and pigments during the second half of the nineteenth century since the latter products offered a wider and brighter gamut of color as well as greater economy and fastness. While there has been a revival of interest recently in natural textile colorants driven by perceptions of renewable sourcing and low environmental impact, they are not suited to industrial use and offer only a limited color gamut and display moderate levels of fastness at best. In addition, natural dyes often require the use of a fixative, known as a mordant, to achieve satisfactory permanency; traditional metallic mordants are environmentally unfriendly. Textile dyes that were originally obtained from natural sources, such as Indigo, are more efficiently obtained by chemical synthesis .
Synthetic Textile Colorants
The vast majority of textile colorants are synthesized chemically on an industrial scale [3, 4]. Over a million tonnes are produced globally each year. Tens of thousands of colorants have been marketed since the first commercially successful synthetic textile dye, Mauveine, was manufactured in the late 1850s [1, 5]. The financial rewards from this particular colorant and its contemporaries spurred on much endeavor into making synthetic dyes. The genesis of the modern chemical industry, which dwarfs the colorant sector and now manufactures products as diverse as plastics, pharmaceuticals, and drugs, lies in the attempts of mid-nineteenth-century chemists to prepare textile dyes and other colorants. These efforts initially involved a trial-and-error approach since little was known about molecular structure, and dyes often reached the market as mixtures of different compounds with their discoverers having little idea of their composition. However, a more systematic approach to textile colorant research, principally led by the German dye industry, resulted in a better understanding of both chemistry and color–structure relationships. At the beginning of the twentieth century, Germany manufactured 85 % of the world’s synthetic dyes with 10 % being produced in Britain, France, and Switzerland. A century later, manufacture is concentrated in Asia, particularly China and India, because of a lower cost base, although some of the biggest suppliers remain headquartered in Europe. The global market for textile colorants is worth several billion dollars . The textile industry is the largest consumer of dyes and organic pigments. Some of the biggest-selling textile dyes are truly commodities, being produced in volumes of over 1,000 tonnes each year for supply at just a few dollars per kilogram or even less.
Nomenclature and Composition of Commercial Textile Colorants
Traditionally, the trade names which manufacturers give to colorants are typically comprised of three parts . The first element is a brand name, often incorporating elements of the producer’s name, the type of colorant, and/or its intended use. The next part indicates general color, occasionally with modifiers to highlight shade or application characteristics to which the manufacturer wants to draw attention. The last part of the name is made up of a code of letters and numbers for further differentiation of the colorant from others – these may be related to color, application properties, as well as strength. For example, the now-defunct manufacturer, Holliday Dyes and Chemicals, marketed the dye: Polycron Yellow C-5G 200%. “Polycron” was the name shared by the company’s disperse dyes developed solely for the coloration of polyester; “C” denoted the disperse dye application category into which this colorant fell; “5G” indicated that it had a relatively greenish hue; and the suffix “200%” signified that its tinctorial strength was double that of the market norm (“100%”), i.e., twice the amount of active dye was present per unit mass of colorant. Many other companies employ this system of nomenclature for their colorants. However, reliance on commercial names alone will not necessarily enable informed colorant selection (i.e., which colorant to use for a particular substrate and application technique) nor show which textile colorants in the market are equivalent. Fortunately, many manufacturers link their products to the Color Index (CI) generic naming system which assists users in making sense of the vast array of textile colorants that are commercially available. For instance, Polycron Yellow C-5G 200% has a CI Generic Name assigned to it, CI Disperse Yellow 119, thereby allowing users to identify equivalent products. While colorants with the same CI Generic Name ought to contain the same main colored compound, they may not be exactly equivalent. Variations in impurity content, whether inadvertent or through deliberate inclusion of shading components, may affect color or other application properties. Differences in physical form can affect performance. Members of several types of dye class typically include substantial quantities of additives, present as processing aids, such as dispersing agents or buffers to stabilize pH. Another source of variation is the presence of a diluent, for example salt or dextrin, which is added to standardize certain dye types to a desired tinctorial strength. A further potential complication is that some textile colorants do not have a CI Generic Name, either because they are mixtures of colored components as is often the case with navy and black dyes, or simply because one has not been disclosed or assigned.
Textile Colorant Structure
The constitution of commercial dyes and pigments used for textile coloration can be described as lean: each part of a colorant molecule has one or more purposes as will be illustrated in some of the following sections. These functions may be related to adjustment of color (e.g. hue, intensity, brightness), physical character (e.g. solubility, crystal structure, volatility), dyeing behavior (e.g. substantivity, leveling), fastness (to, e.g. light, heat, moisture), and so on, although there will be occasions when substituents are present merely for convenience or cost . Often, dye design involves an element of compromise as certain properties will have a degree of mutual exclusivity, e.g. rapid dyeing and good leveling behavior at the expense of good wash fastness. The most general structural classification centers on the key molecular features of a colorant that gives rise to its color. Azo derivatives are the largest such class of textile colorants, although there are many others of significance.
Azo Textile Colorants
Carbonyl Textile Colorants
Other Textile Colorant Structural Types
There are numerous other structural classes of textile dyes [1, 3, 5, 7] and pigments [4, 5], each occupying its own application niches. A few examples are presented in Fig. 3. The triarylmethane class dates all the way back to Mauveine, although the importance of such purple, blue, and green dyes has diminished with the development of alternative chemical types of superior fastness . The same is true of yellow to red fluorescent xanthene dyes as textile colorants. However, the phthalocyanine class remains as much prized as ever since its commercialization in the first half of the twentieth century . This colorant type furnishes robust and tinctorially strong bright blue to green dyes and pigments with diverse uses in the textile arena, especially when they are in the form of copper complexes [3, 4]. Other classes occupy more specific sectors in terms of color and/or application. For example, triphenodioxazines are exploited as intense bright blue dyes for natural fibers, while nitrodiphenylamines are cost-effective yellow colorants of good photostability for synthetics . Polymethine dyes are a broad class of chromophore of which certain subclasses are employed in textile coloration: two such types, hemicyanines and diazahemicyanines, are shown in Fig. 3. Both families are useful for intense bright reds, while the latter also supplies blue colorants .
Textile Colorant Application Classes
Major textile colorant application classes
Principal textile substrate(s)
Wool, silk, nylon, modified polyacrylonitrile
Cotton and other cellulosics, acetates
Polyacrylonitrile, modified nylon, and polyester
Cotton and other cellulosics, polyamide
Polyester, acetates, nylon, polyacrylonitrile
Cotton, wool, silk, nylon
Cotton and other cellulosics
Cotton and other cellulosics, polyamide
These colorants are employed predominantly on cellulosic fibers [7, 8]. They are insoluble azo compounds that are synthesized within the textile substrate during the dyeing process from soluble precursors. The colorants generated are physically trapped as solid particles within fiber pores, so dyeings display good wash fastness and photostability. There are numerous variations in technique and materials, but the usual method is to apply a naphthol coupling component followed by a diazo component, which reacts with the coupler to produce a non-ionic colorant. The diazo component is an aromatic amine, which is diazotized as part of the application process or supplied as stabilized pre-diazotized materials, for example, aryldiazonium tetrafluoroborate salts (i.e., ArN2+BF4-). Azoic textile colorants have largely been sidelined industrially owing to the greater convenience and economy of other systems but are still of commercial interest in the red shade area.
These non-ionic colorants are hydrophobic like the synthetic textile substrates to which they are applied [8, 9]. Originally developed for cellulose acetate fibers, their principal use is the coloration of polyester: the importance of this textile material means that disperse dyes have become one of the two most important types of textile colorant [3, 7]. They have only sparing water solubility and so are applied as fine dispersions in water (apart from when they are used in transfer-printing ink films). Dye particle sizes are typically distributed in the range of around 0.1–1μm diameter for supply either in solid or liquid dispersion form. These forms are achieved by milling in the presence of dispersing agent, usually anionic polymeric materials such as lignosulfonates or arylsulfonic acid condensates, to inhibit reaggregation and maintain dispersion stability. During exhaustion dyeing, a small proportion of colorant is in aqueous solution: it is from this phase that dye is adsorbed onto the textile and diffuses within it. Colorant lost from the aqueous phase is replenished by dissolution of dye remaining in suspension. As disperse dyes must have relatively compact structures to enable them to diffuse satisfactorily within hydrophobic textiles, commercial ranges therefore consist mainly of monoazo and anthraquinone derivatives: the latter are important in the bright red and blue sectors, but the former dominate the rest of the spectrum. Several other chemical dye classes are employed, but these tend to occupy niches, such as yellow nitrodiphenylamines. Brown and black disperse dyes are usually formulated using mixtures of azo dyes because of the technical and economic difficulties in creating single-component colorants of small molecular size with the desired hue.
While used primarily for the coloration of media other than textiles, pigments can be printed onto fabrics. Examples of organic pigments used as textile colorants in this way include yellow acetanilide and red naphthol monoazo derivatives as well as blue copper phthalocyanines . The insoluble colorant is incorporated into a paste, which is applied to the textile fabric by a printing process (e.g., screen, roller, etc.). After curing of the paste, normally achieved by drying, the pigment is physically bound to the fabric. Pigments are also employed in the mass coloration of many types of textile fibers in which the colorant is dispersed in a solution or melt of the polymer prior to fiber formation so that the pigment particles become trapped within the fibers as they are produced. The pigment types mentioned above as well as numerous other chemical classes are employed in this capacity.
These colorants are prepared by heating aromatic compounds with sulfur or a sulfur compound to produce compounds of large molecular size that contain disulfide (–S–S–) linkages between the aromatic components [1, 3, 5]. Dyes of this kind are chemically complex and, in the majority of cases, their structure is unknown. Their application resembles that of vat dyes (see below) . The initially water-insoluble dyes are reduced in the presence of alkali to a water-soluble “leuco” form containing water-solubilizing thiolate (–S−) groups which diffuse into the fiber in the presence of electrolyte. At the end of dyeing, these groups are then oxidized in situ to regenerate the insoluble disulfide which remains trapped in the fiber. Sulfur dyes are widely used for the production of dull deep shades of reasonable wet fastness because of their low cost .
The enormous variation in both the chemical and physical characteristics of textiles has forced the emergence of a diverse array of coloration technologies. As a consequence, many different types of colorant have been commercialized for application to textiles. Pigments are used in the printing of textiles or in the mass coloration of polymer intended for fiber manufacture, while dyes are applied industrially in different ways ranging from exhaustion dyeing to transfer printing. The wide spectrum of techniques available to a dyer or a printer has only been made possible through the development of colorants that possess chemistries and physical properties tailored for application to specific types of textiles. There is no such thing as a “universal” colorant that can be applied satisfactorily to all textiles for all intended outlets nor is one likely to materialize in the near future. It is widely accepted that the chances of new application classes or structural types being introduced are remote. Nevertheless research along these lines continues alongside incremental work to improve on the technical properties, economy, and environmental impact of existing textile colorants.
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