25 ноября, 2015 
Pokraskin The distinction between pigments and dyes, which is based on the differences in their solubility characteristics, has been discussed in detail in Chapter 2. A pigment is a finely divided solid colouring material, which is essentially insoluble in its application medium. Pigments are used mostly in the coloration of paints, printing inks, and plastics although they are applied to a certain extent in a much wider range of substrates, including paper, textiles, rubber, glass, ceramics, cosmetics, crayons, and building materials such as cement and concrete. In most cases, the application of pigments involves their incorporation into a liquid medium, for example a wet paint or ink or a molten thermoplastic material, by a dispersion process in which clusters or agglomerates of pigment particles are broken down into primary particles and small aggregates. The pigmented medium is then allowed to solidify, either by solvent evaporation, physical solidification or by polymerisation, and the individual pigment particles become fixed mechanically in the solid polymeric matrix. In contrast to textile dyes where the individual dye molecules are strongly attracted to the individual polymer molecules of the fibres to which they are applied, pigments are considered to have only a weak affinity for their application medium, and only at the surface where the pigment particle is in contact with the medium.
Pigments are incorporated to modify the optical properties of a substrate, the most obvious effect being the provision of colour. However, this is not the only optical function of a pigment. The pigment may also be required to provide opacity, most critically in paints, which are generally designed to obscure the surface to which they are applied. Alternatively, and in complete contrast, high transparency may be important, for example in multicolour printing, which uses inks of four colours, the three subtractive primaries, yellow, magenta and cyan, together with black. In this process, transparency is essential to ensure that subsequently printed
colours do not obscure the optical effect of the first colour printed. As with dyes, pigments are required to exhibit an appropriate range of fastness characteristics. They are required to be fast to light, weathering, heat and chemicals such as acids and alkalis to a degree dependent on the demands of the particular application. In addition, they are required to show solvent resistance, which refers to their ability to resist dissolving in solvents with which they may into contact in their application, to minimise problems such as ‘bleeding’ and migration.
In chemical terms, pigments are conveniently classified as either inorganic or organic. These two broad groups of pigments are of roughly comparable importance industrially. In general, inorganic pigments are capable of providing excellent resistance to heat, light, weathering, solvents and chemicals, and in those respects they can offer technical advantage over most organic pigments. In addition, inorganic pigments are generally of significantly lower cost than organics. On the other hand, they commonly lack the intensity and brightness of colour of typical organic pigments. Organic pigments are characterised by high colour strength and brightness although the fastness properties which they offer are somewhat variable. There is, however, a range of high-performance organic pigments which offer excellent durability while retaining their superior colour properties but these tend to be rather more expensive. The ability either to provide opacity or to ensure transparency provides a further contrast between inorganic and organic pigments. Inorganic pigments are, in general, high refractive index materials which are capable of giving high opacity while organic pigments are of low refractive index and consequently are transparent.
This chapter provides an overview of the characteristic structural features of the most important commercial pigments. If it seems that the chapter places greater emphasis on inorganic pigments, this is because the various chemical classes of organic pigments are dealt with, to a certain extent, in Chapters 3-6. In individual cases there is some discussion of structure-property relationships. Such relationships are rather more complex with pigments than with dyes, because of the dependence of the colouristic and technical performance of pigments not only on the molecular structure but also on the crystal structure arrangement and on the nature of the pigment particles, particularly their size and shape distribution. The section on inorganic pigments presents an outline of the synthetic procedures used for their manufacture. Discussion of the synthesis of organic pigments is omitted as this is dealt with in relevant earlier chapters concerned with the specific chemical classes. The manufacture of a pigment may be considered as involving two distinct phases. The first of these is the sequence of chemical reactions in which the pigment is formed. The second phase, which may run concurrently with the synthetic sequence or which may involve specific aftertreatments, ensures that the pigment is obtained in the optimum physical form. This can involve, for example, development of the appropriate crystal form, control of the particle size distribution and modification of the surfaces of the particles. Organic pigments are commonly prepared in as fine a particle size as is technically feasible to give maximum colour strength and transparency. In contrast, the particle size of many inorganic pigments is controlled carefully (often in the range 0.2-0.3 ^m) to provide maximum opacity. Surface treatments are commonly used to improve the performance of pigments. For example, treatment with organic surface — active agents may lead to an improvement in the ease of dispersion into organic application media, while coating the particles with inorganic oxides, such as silica, may be used to improve the lightfastness and chemical stability of certain inorganic pigments.
Опубликовано в рубрике 