Лако-красочные материалы — производство

The synthetic organic pigment industry developed towards the end of the l9th century out of the established synthetic textile dye industry. Many of the earliest organic pigments were prepared from water-soluble dyes rendered insoluble by precipitation onto colourless inorganic substrates such as alumina and barium sulfate. These products were referred to as ‘lakes’. A further significant early development was the discovery and commercial introduction of a range of azo pigments, which provided the basis for the most important yellow, orange and red organic pigments currently in use. These so-called classical azo pigments offer bright in­tense colours although generally only moderate performance in terms of fastness properties. A critical event in the development of the organic pigment industry was the discovery, in 1928, of copper phthalocyanine. This blue pigment was the first product to offer the outstanding intensity and brightness of colour typical of organic pigments, combined with an excellent set of fastness properties, comparable with many inorganic pigments. The discovery stimulated the quest for other chemical types of organic pigment which could emulate the properties of copper phthalocyanine in the yellow, orange, red and violet shade areas. This research activity gained further impetus from the emergence of the auto­motive paint market and the growth of the plastics and synthetic fibres industries, applications which demanded high levels of technical perform­ance. The range of high-performance organic pigments which has emerged includes the quinacridones, isoindolines, dioxazines, perylenes, perinones and diketopyrrolopyrroles, together with a number of im­proved performance azo pigments.

Organic pigments generally provide higher intensity and brightness of colour than inorganic pigments. These colours are due to the n-n* electronic transitions associated with extensively conjugated aromatic systems (Chapter 2). Organic pigments are unable to provide the degree of opacity which is typical of inorganic pigments, because of the lower refractive index associated with organic crystals. However, the combina­tion of high colour strength and brightness with high transparency means that organic pigments are especially well suited to printing ink applica­tions. The range of commercial organic pigments exhibit variable fastness properties which are dependent both on the molecular structure and on the nature of the intermolecular association in the solid state. Since organic molecules will commonly exhibit some tendency to dissolve in organic solvents, organic pigment molecules incorporate structural fea­tures which are designed to enhance the solvent resistance. For example, an increase in the molecular size of the pigment generally improves solvent resistance. In addition, the amide (-NHCO-) group features prominently in the chemical structures of organic pigments, because its presence enhances fastness not only to solvents, but also to light and heat, as a result of its ability to participate in strong dipolar interactions and in hydrogen bonding, both intramolecular and intermolecular. The incor­poration, where appropriate, of halogen substituents and of metal ions, particularly of the alkaline earths and transition elements, can also have a beneficial effect on fastness properties. The following sections provide an overview of the more important chemical types of commercial organic pigments, together with some discussion of the structural features which determine their suitability for particular applications.