Лако-красочные материалы — производство

Colorants may be classified usefully in two separate ways, either accord­ing to their chemical structure or according to the method of application. The most important reference work dealing with the classification of dyes and pigments is the Colour Index, a publication produced by the Society of Dyers and Colourists, Bradford, England. This series of volumes provides a comprehensive listing of the known commercial dyes and pigments and is updated on a regular basis. Each colorant is given a C. I. Generic Name, which incorporates its application class, the hue and a number, which simply reflects the chronological order in which the colorants were introduced commercially. It is extremely useful that this system of nomenclature for dyes and pigments is more or less universally accepted by all those involved in their manufacture and application, and so it is frequently used throughout this book. The Colour Index provides useful information for each dye and pigment on the methods of applica­tion and on the range of fastness properties. The volumes also list the companies that manufacture each of the products, together with trade names, and give the appropriate chemical constitutions where these have been disclosed by the manufacturer.

In the chemical classification method, colorants are grouped according to certain common chemical structural features. The most important organic dyes and pigments, in roughly decreasing order of importance, belong to the azo (-N=N-), carbonyl (C=O) (including anthraquinones), phthalocyanine, arylcarbonium ion (including triphenylmethines), sulfur, polymethine and nitro chemical classes. A discussion of the principal structural characteristics of each of these chemical classes, together with a discussion of the major synthetic strategies used in the manufacture of these chemical groups of organic colorants, may be found in Chapters

3- 6. Added to this, Chapter 9 contains a discussion of the most important inorganic pigments, a group of colorants that has no counterpart in dye chemistry.

To the textile dyer whose role it is to apply colour to a particular textile fibre, the classification of dyes according to the method of application is arguably of greater interest than the chemical classification. Dye and pigment molecules are carefully designed to ensure that they have a set of properties that are appropriate to their particular application. Obvious requirements for both types of colorant are that they must possess the desired colours, in terms of hue, strength and brightness, and an appro­priate range of fastness properties. Fastness properties refer to the ability of a dye or pigment to resist colour change when exposed to certain conditions, such as to light, weathering, heat, washing, solvents or to chemical agencies such as acids and alkalis. For textile applications, dye molecules are designed so that they are attracted strongly to the mol­ecules of the fibre to which they are applied. The chemical and physical nature of the different types of textile fibres, both natural and synthetic, require that the dyes used, in each case, have an appropriate set of chemical features to promote affinity for the particular fibre concerned. The chemical principles of the most important application classes of textile dyes are described in Chapters 7 and 8. The application classes discussed include acid dyes, mordant dyes and premetallised dyes for protein fibres, direct dyes, reactive dyes and vat dyes for cellulosic fibres, disperse dyes for polyester and basic (cationic) dyes for acrylic fibres. In contrast to textile dyes, pigments tend to be versatile colouring materials requiring less tailoring to individual applications. With pigments there­fore, classification according to application is relatively unimportant. Pigments are simply designed to resist dissolving in solvents with which they may come into contact in paint, printing ink or plastics applications.

There is a third method proposed for classifying colorants which is in terms of the mechanism of the electronic excitation process. According to this method, organic colorants may be classified as donor-acceptor, polyene, cyanine or n-n* chromogens. While this method of classification is undoubtedly of importance theoretically, it is arguably of lesser practi­cal importance, since the vast majority of commercial organic dyes and pigments for traditional applications belong to the donor-acceptor cat­egory. The mechanism of the electronic excitation process, an under­standing of which is fundamental in establishing relationships between the colour and the molecular constitution of dyes, is discussed in the following sections of this chapter.