Лако-красочные материалы — производство

Natural inorganic pigments, derived mainly from mineral sources, have been used as colorants since prehistoric times and a few, notably iron oxides, remain of some significance today. The origin of the synthetic inorganic pigment industry may be traced to the rudimentary products produced by the ancient Egyptians, pre-dating the synthetic organic colorant industry by several centuries (Chapter 1). The range of modern inorganic pigments was developed for the most part during the 20th century and encompasses white pigments, by far the most important of which is titanium dioxide, black pigments, notably carbon black, and coloured pigments of a variety of chemical types, including oxides (e. g. of iron and chromium), cadmium sulfides, lead chromates and the struc­turally more complex ultramarine and Prussian blue.

The structural chemistry and properties of the important chemical types of inorganic pigments are dealt with in the sections which follow, together with an outline of the most important synthetic methods. The colour of inorganic pigments arises from electronic transitions which are quite diverse in nature and different from those responsible for the colour of organic colorants. For example, they may involve charge transfer transitions, either ligand-metal (e. g. in lead chromates) or between two metals in different oxidation states (in Prussian blue). In ultramarines the colour is due to radical anions trapped in the crystal lattice. Inorganic pigments generally exhibit high inherent opacity, a property which may be attributed to the high refractive index which results from the compact atomic arrangement in their crystal structure. Various synthetic methods

are employed in the manufacture of inorganic pigments. Frequently, the chemistry is carried out in aqueous solution from which the pigments can precipitate directly in a suitable physical form. In some cases, high temperature solid state reactions are used (e. g. mixed phase oxides, ultramarines), while gas-phase processes because of their suitability for continuous large-scale manufacture are of importance for the manufac­ture of the two largest tonnage pigments, viz. titanium dioxide and carbon black.