Pigment particle size 477 - structure

A highly concentrated dispersion of carbon black is first prepared with a portion of the binder and solvent. The viscosity of this concentrate is a function of the particle size, structure, and surface chemistry of the black, the type of binder and its interaction with the pigment black, and the proportions of black, binder, and solvent. The final paint is made from the concentrate by adding more binder and solvent, its carbon black concentration is 3-8% referred to the solids content. Wetting agents are sometimes added to improve dispersibility and prevent flocculation. A number of concentrates for paint manufacture e.g., carbon black-nitrocellulose chips or carbon black -alkyd resin pastes, can be obtained from paint producers. 

Carbon grades differ in particle size, structure, surface chemistry and purity. Unless the reason for using carbon is high quality pigmentation, a coarse grade is usually used in plastics. However, a fine particle size of 20 nm is preferred for UV protection. 

The third factor, and certainly one of the most important, is related to particle size distribution, presence of surface treatments, the chemical structure of the dye and pigment, and the chemical bonding that may be involved between the colourant and the polymer. " All four will dramatically influence the stabilizing or destabilizing effects of the dye or pigment on the polymer. An increase in pigment particle size and the presence of a surface treatment may, for example, reduce photocatalytic activity. Chemical structure is probably the most important, a well-known example being the marked difference in photochemical activity between anatase and rutile. 

Vaterite is thermodynamically most unstable in the three crystal structures. Vaterite, however, is expected to be used in various purposes, because it has some features such as high specific surface area, high solubility, high dispersion, and small specific gravity compared with the other two crystal systems. Spherical vaterite crystals have already been reported in the presence of divalent cations [33], a surfactant [bis(2-ethylhexyl)sodium sulfate (AOT)] [32], poly(styrene-sulfonate) [34], poly(vinylalcohol) [13], and double-hydrophilic block copolymers [31]. The control of the particle size of spherical vaterite should be important for application as pigments, fillers and dentifrice. 

The methods used to convert these vat dyes into a suitable physical form (and in some cases, crystal structure) for use as pigments have been carefully guarded industrial secrets, revealed only in patents. The general principles are clear, however. One method is to reduce the vat dye in the usual manner to bring it into solution and then to re-precipitate it under very carefully controlled conditions. The other is to subject the dye to a fine grinding operation. Whichever approach is used, the aim is to reduce the mean particle size to below 1 J,m (1000 nm). 

In the manufacture of quinacridone pigments only the first and last of the four routes outlined have been operated commercially. Synthesis is followed by the milling processes necessary to give products with the crystal structure and particle size required for their use as pigments. 

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. 

Normal-phase HPLC has also found application in the analysis of pigments in marine sediments and water-column particulate matter. Sediments were extracted twice with methanol and twice with dichloromethane. The combined extracts were washed with water, concentrated under vacuum and redissolved in acetone. Nomal-phase separation was performed with gradient elution solvents A and B being hexane-N,N-disopropylethylamine (99.5 0.5, v/v) and hexane-2-propanol (60 40, v/v), respectively. Gradient conditions were 100 per cent A, in 0 min 50 per cent A, in 10 min 0 per cent A in 15 min isocratic, 20 min. Preparative RP-HPLC was carried out in an ODS column (100 X 4.6 mm i.d. particle size 3 jum). Solvent A was methanol-aqueous 0.5 N ammonium acetate (75 25, v/v), solvent B methanol-acetone (20 80, v/v). The gradient was as follows 0 min, 60 per cent A 40 per cent A over 2 min 0 per cent A over 28 min isocratic, 30 min. The same column and mobile phase components were applied for the analytical separation of solutes. The chemical structure and retention time of the major pigments are compiled in Table 2.96. 

The chemical structures of betanin and indicaxanthin found in the prickly pear are depicted in Fig. 2.150. Pigments were extracted by homogenizing fresh fruit flesh with methanol (1 5, w/v). The suspension was fdtered and the liquid phase was applied for spectrophotometry and RP-HPLC. Liquid chromatographic separation was performed in an ODS column (250 X 4.6 mm i.d. particle size 5 pan) at ambient temperature. Gradient elution started with 1 per cent aqueous acetic acid and changed to 12 per cent solvent B in... 

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. 

X-ray investigation of inorganic pigments yields information on the structure, fine structure, state of stress, and lattice defects of the smallest coherent regions that are capable of existence (i.e., crystallites) and on their size. This information cannot be obtained in any other way. Crystallite size need not be identical with particle size as measured by the electron microscope, and can, for example, be closely related to the magnetic properties of the pigment. 

Chromium(III) oxide crystallizes in the rhombohedral structure of the corundum type space group D3d-R3c, Q 5.2 g/cm3. Because of its high hardness (ca. 9 on the Mohs scale) the abrasive properties of the pigment must be taken into account in certain applications [3.44], It melts at 2435 °C but starts to evaporate at 2000 °C. Depending on the manufacturing conditions, the particle sizes of chromium oxide pigments are in the range 0.1-3 pm with mean values of 0.3-0.6 pm. Most of the particles are isometric. Coarser chromium oxides are produced for special applications, e.g., for applications in the refractory area. 

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

The most commonly measured pigment properties are elemental analysis, impurity content, crystal structure, 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 are not specific only to pigments. The techniques applied are commonly used to characterize powders and solid materials and the measuring methods have been standardized in various industries. 

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