Лако-красочные материалы — производство

Colour may be introduced into manufactured articles, for example tex­tiles and plastics, or into a range of other application media, for example paints and printing inks, for a variety of reasons, but most commonly the purpose is to enhance the appearance and attractiveness of a product and improve its market appeal. Indeed it is often the colour which first attracts us to a particular article. The desired colour is generally achieved by the incorporation into the product of coloured compounds referred to as dyes and pigments. The term colorant is frequently used to encompass both types of colouring materials. For an appreciation of the chemistry of colour application it is of fundamental importance that the distinction between dyes and pigments as quite different types of colouring materials is made. Dyes and pigments are both commonly supplied by the manu­facturers as coloured powders. Indeed, as the discussion of their mole­cular structures contained in subsequent chapters of this book will illustrate, the two groups of colouring materials may often be quite similar chemically. However, they are distinctly different in their proper­ties and especially in the way they are used. Dyes and pigments are distinguished on the basis of their solubility characteristics: essentially, dyes are soluble, pigments are insoluble.

The traditional use of dyes is in the coloration of textiles, a topic covered in considerable depth in Chapters 7 and 8. Dyes are almost invariably applied to the textile materials from an aqueous medium, so that they are generally required to dissolve in water. Frequently, as is the case for example with acid dyes, direct dyes, cationic dyes and reactive dyes, they dissolve completely and very readily in water. This is not true, however, of every application class of textile dye. Disperse dyes for polyester fibres, for example, are only sparingly soluble in water and are applied as a fine aqueous dispersion. Vat dyes, an important application class of dyes for cellulosic fibres, are completely insoluble materials but they are converted by a chemical reduction process into a water-soluble form that may then be applied to the fibre. There is also a wide range of non-textile applications of dyes, many of which have emerged in recent years as a result of developments in the electronic and reprographic industries (see Chapter 10). For many of these applications, solubility in specific organic solvents rather than in water is of importance.

In contrast, pigments are colouring materials that are required to be completely insoluble in the medium into which they are incorporated. The principal traditional applications of pigments are in paints, printing inks and plastics, although they are also used more widely, for example, in the coloration of building materials, such as concrete and cement, and in ceramics and glass. The chemistry of pigments and their application is discussed in more detail in Chapter 9. Pigments are applied into a medium by a dispersion process, which reduces the clusters of solid particles into a more finely-divided form, but they do not dissolve in the medium. They remain as solid particles held in place mechanically, usually in a matrix of a solid polymer. A further distinction between dyes and pigments is that while dye molecules are designed to be attracted strongly to the polymer molecules which make up the textile fibre, pigment molecules are not required to exhibit such affinity for their medium. Pigment molecules are, however, attracted strongly to one another in their solid crystal lattice structure in order to resist dissolving in solvents.