Pigments particle size/shape

Hardness and Abrasiveness. The abrasiveness of a pigment is not identical to its intrinsic hardness, i.e., the hardness of its primary particles. In practice the Mohs hardness is not therefore a useful indication of the abrasiveness of a pigment. Abrasion rather depends on pigment particle size and shape and is usually caused by the sharp edges of the particles. A standard test procedure for determining abrasiveness does not exist. A method based on the abrasion of steel balls by the pigment is described in [1.12],... 

Particle size and shape. The particle size, shape, and distribution of a pigment influence the rheological properties, shade, gloss, weathering characteristics, and ease of dispersion. 

Binder Demand n That amount of binder needed to completely wet a pigment by displacing the air voids. This is determined primarily by the particle size, shape, chemical composition, and density of the pigment and the particle size, degree of polymerization and wetting properties of the binder. (Whereas the binder demand refers to a particular pigment-vehicle system, oil... 

B, M and n are related to particle size, shape and surface. A is independent of particle size but may depend on particle shape and surface. A may be related to particle-particle interaction in the dispersion. Equations (4.47) and (4.48) predict a linear relationship between log qpi or log Op and c(). This is illustrated in Figs. 4.22 and 4.23 which also show the effect of average particle diameter (volume to surface ratio dj2) of the pigment [30]. The smaller the size, the higher the slope. This is illustrated in Fig. 4.24 which shows the variation of the exponents B and n with particle diameter. As is clear, both B and n increase with decreasing dj2-... 

Pigments and Extenders. Pigments are selected for use in house paints based on thek appearance and performance quaUties. Appearance includes color and opacifying abiUty. Performance quaUties include ultraviolet light resistance, fade resistance, exterior weatherabiUty, chemical resistance, as well as particle size and shape. Toxicity profiles and safety and health related properties are also important criteria in pigment selection. 

The value of pigments results from their physical—optical properties. These ate primarily deterrniaed by the pigments physical characteristics (crystal stmcture, particle size and distribution, particle shape, agglomeration, etc) and chemical properties (chemical composition, purity, stabiUty, etc). The two most important physical—optical assets of pigments are the abiUty to color the environment in which they ate dispersed and to make it opaque. 

The most commonly measured pigment properties ate elemental analysis, impurity content, crystal stmcture, particle size and shape, particle size distribution, density, and surface area. These parameters are measured so that pigments producers can better control production, and set up meaningful physical and chemical pigments specifications. Measurements of these properties ate not specific only to pigments. The techniques appHed are commonly used to characterize powders and soHd materials and the measutiag methods have been standardized ia various iadustries. 

Pigment-related aspects, which involve the chemical constitution, crystalline modification, particle size distribution, particle shape, surface structure, preparation, and processing of the pigment powder, especially in terms of drying and milling. 

It has often been observed that the coloristic properties of an organic pigment are a function not only of the size of particles but also of their shape. This is due to the anisotropy of the optical properties in different crystallographic directions within the crystal forms of a pigment. In 1974 [5, 6], it was demonstrated that of the equally sized but differently shaped particles of beta copper phthalocyanine blue, the almost completely cubic, i.e., more or less isometric form produces greenish blue shades, while acicular forms are responsible for reddish blue hues. The optical behavior of ordered pigment particles in systems has been reported in the literature [7, 8]. 

Like the natural iron oxide pigments, the synthetics are used for colouring concrete, bitumen, asphalt, tiles, bricks, ceramics and glass. They are also used extensively in house and marine paints. Because the shapes of the particles can be accurately controlled and the particle size distribution is narrow, synthetic iron oxides have a greater tinting strength than the natural ones and so, are chosen where paint colour is important, i. e., for top coats. Red iron oxides are used in primers for automobiles and steel structures. 

Particle Size. The important physical data for inorganic pigments comprise not only optical constants, but also geometric data mean particle size, particle size distribution, and particle shape [1.8]. The standards used for the terms that are used in this section are listed in Table 1 ( Particle size analysis ). 

---