Classification of inorganic pigments by chemical composition:
- salts (complex salts, aluminosilicates, carbonates, phosphates, etc.);
- elements (zinc dust, carbon black, aluminum powder, etc.);
- oxides (oxides of zinc, lead, titanium, chromium, etc.).
By color, inorganic pigments are divided into two large groups: chromatic (all colored inorganic pigments) and achromatic (gray, white and black pigments). In turn, chromatic pigments are also divided into two groups: green, brown, blue, purple and red, orange and yellow.
Key Properties of Inorganic Pigments
Chemical properties of inorganic pigments
The chemical properties of inorganic pigments are completely determined by their composition. For example, lead carbonate (or lead white) is fairly easily destroyed even by weak acid. Titanium dioxide (white) has exceptional resistance in many aggressive environments. It only breaks down in heated concentrated sulfuric acid. Zinc oxide (also a white pigment) is amphoteric. Blue ultramarine pigment contains sodium sulfides, so it is unstable in acids. The blue pigment iron blue is also quite easily destroyed when exposed to weak alkaline solutions. It contains an iron ferricyanide complex.
Most pigments contain several elements, which means they are not chemically pure. Research shows that almost all the properties of pigments are determined by their structural features, not by their chemical composition. The chemical composition only determines the ability to form a particular structure.
Often technical products with a certain macro- and microstructure, variable composition, rather than pure chemical compounds, are used in the production of pigments. The properties of pigments are greatly influenced by impurities, which are often specially introduced into the composition. The additives are needed to give some specific properties to the pigments. Such additives can be pigment surface modifiers or surface-active substances etc. Inorganic compounds, various polymers and surfactants can be used as modifiers. For example, oxides of zinc, calcium, magnesium, silicon, silicon and aluminum phosphates, aluminum hydroxide, titanium phthalate, etc.
For pigments to be easily dispersed in film-forming substances, and for the resulting dispersion (enamel, paint, etc.) to be stable, surface-active substances (surfactants) are added to the pigments.
Special additives can be added to the pigment during synthesis, for example, to ensure that the pigment crystallizes in a particular crystalline system.
Crystalline structure of inorganic pigments
Crystallinity is the most important property of a pigment, because each small particle is a crystal of a certain geometric shape. The shape and characteristics of the crystal determine the properties of the pigment. Anisotropy is the most characteristic feature of the crystal state. It is the difference in properties in different directions. Anisotropic are the optical, thermal, magnetic, and electrical properties of the crystal, as well as the strength and growth rate of the crystal.
Isomorphism and polymorphism are also important features of the crystal state. Polymorphism is when the same substance can exist in several crystal forms. Polymorphic modifications of the same pigment differ in physical properties (density, hardness, color, etc.). Crystalline modifications are designated by letters of the Greek alphabet (?-ZnS, ?-ZnS). Pigments are more commonly referred to by historically established names, such as sphalerite and wurtzite.