Organic pigments: general principles

Abstract

This introductory chapter presents an overview of the general principles underlying the structural chemistry, manufacturing processes, and application technology of organic pigments. The coverage provides a fundamental theoretical and practical basis for the chapters that follow in this series that are devoted to specific chemical classes of industrially significant organic pigments of the azo, phthalocyanine, carbonyl, dioxazine, and metal complex classes. The initial sections cover the fundamental differences which mean that dyes and pigments are considered universally as two separate types of colorant, based on their solubility characteristics. They also provide discussions of the contrasting chemical, technological, and performance features of organic and inorganic pigments. An outline of the most important historical features in the development of the synthetic organic pigment industry is then presented, from its origins in the 19th century that followed soon after the development of the industrial synthetic dye industry, through its expansion in the 20th century, to its current position as a mature global industry. A section then follows that describes the functions that organic pigments are required to perform in their application, mainly their optical functions that include not only color properties, including hue, strength, brightness, but also the contrasting requirements for transparency or opacity as demanded by specific applications. The pigments are also required to resist the conditions and agencies that they might encounter in applications, assessed as fastness properties, such as fastness to light, heat, solvents and chemicals, amongst many others, to an extent that specific applications demand. The principles, in broad terms, of the ways in which chemical structures determine colour and performance of organic pigments are discussed, with focus not only on the influence of molecular structure, but also on the effect of the crystal structural arrangement and the particulate structure, including particle size and shape and its distribution, on application performance. This is important as these pigments are applied as a dispersion of finely divided crystalline solid particles that are insoluble and are ultimately trapped mechanically in their application medium, often a polymer. The manufacture of organic pigments is discussed in broad terms. The overall process may be considered in stages, initiated by the chemical synthetic sequence in which the pigment is formed, followed by a conditioning stage where the crude product thus obtained is modified to optimise its performance properties, and finally finishing where the product is processed into a form, or preparation, that is suitable for its intended applications. Finally, the technological principles underlying a broad range of the most important application areas for organic pigments, which are mainly in paints, inks, and plastics, are discussed.