Application ranges of dyes and pigments

A vat dye is a water-insoluble colorant containing two or more keto groups. It can thus be brought into aqueous solution by a reduction process (vatting), which converts the vat dye into its alkali-soluble enolic (leuco) form. As the soluble sodium enolate the leuco vat dye has substantivity for cellulose. The application of vat dyes to cellulosic fibres (virtually the only fibre type on which their outstanding fastness properties can be exploited) thus proceeds in four stages  

Vat dyes are used to colour both components in pale depths on polyester/cellulosic fibre blends [44] but coloration of the polyester component in this case is more closely analogous to disperse dyeing (section 1.6.5). Anthraquinone disperse dyes resemble those vat dyes that are substituted anthraquinone derivatives and in both instances it is exclusively the virtually water-insoluble keto form that is absorbed by the polyester fibre. 

The vat dyes section of the Colour Index incorporates a subgroup called solubilised vat dyes. These are sodium salts of sulphuric acid esters of the parent leuco vat dyes, such as Cl Solubilised Vat Blue 6 (1.50). In contrast to the leuco compounds, the vat leuco esters dissolve readily in water at neutral pH. They have relatively low substantivity for cellulose and thus have been used mainly in continuous dyeing and printing. In the presence of an oxidant in mineral acid solution (sodium nitrite and sulphuric acid, for example) the leuco ester is rapidly decomposed and the insoluble vat dye regenerated. Thus application of a vat leuco ester represents a simpler (but more costly and less versatile) alternative to conventional dyeing methods via the alkaline leuco compound. 

In the Colour Index the sulphur dyes are classified into four subgroups. Conventional insoluble disulphide brands, taking the generic Cl Sulphur dye designation, are converted to the leuco compound by adding sodium sulphide to a boiling aqueous dispersion of the parent 

These dyes are capable of reacting chemically with a substrate under suitable application conditions to form a covalent dye-substrate bond. 

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Before moving on to a description of the application ranges of dyes and pigments, it is appropriate to trace briefly the developments in understanding of the relationship between colour and chemical constitution. This subject has been reviewed most thoroughly elsewhere [30-33] and the intention here is only to outline the basic principles so that the reader can appreciate the need for such a variety of structural types of colorant. The requirements of colour and application are often in conflict and this forms a major part of the subject matter in succeeding chapters. 

In Chapters 3-6, the commercially important chemical classes of dyes and pigments are discussed in terms of their essential structural features and the principles of their synthesis. The reader will encounter further examples of these individual chemical classes of colorants throughout Chapters 7 10 which, as a complement to the content of the earlier chapters, deal with the chemistry of their application. Chapters 7, 8 and 10 are concerned essentially with the application of dyes, whereas Chapter 9 is devoted to pigments. The distinction between these two types of colorants has been made previously in Chapter 2. Dyes are used in the coloration of a wide range of substrates, including paper, leather and plastics, but by far their most important outlet is on textiles. Textile materials are used in a wide variety of products, including clothing of all types, curtains, upholstery and carpets. This chapter deals with the chemical principles of the main application classes of dyes that may be applied to textile fibres, except for reactive dyes, which are dealt with exclusively in Chapter 8. 

Organic modifiers have been frequently employed in CE to increase the solubility of hydrophobic solutes in the aqueous buffer system. Unfortunately, many organic modifiers are UV absorbent and cannot be used without considerable loss of sensitivity of detection. A contactless conductivity detection system has been developed which extends the application range of UV-absorbing solvents [ 119]. As both natural pigments and synthetic dyes absorb in the visible part of the spectra, the application of UV-absorbing organic modifiers in their CE analysis does not cause detection problems. 

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