Pigments platelet-shaped

It seems more important to focus on problems that may arise in the course of a quantitative evaluation of electron photomicrographs of an organic pigment. Automatic image analyzers cannot extract information from images which indicate more or less agglomerated, nonisometric, or even platelet-shaped particles. 

The pigments are formed in the fish scales as platelet-shaped crystals (0.05 pm x 1-10 pm x 20-50 pm). A commercial synthetic process for producing purines with this crystal shape has not been found. An aqueous suspension of fish scales is, therefore, extracted with organic solvents to dissolve and remove the proteins. The remaining dispersion contains purine crystals and scale which are separated from one another by a complicated washing and phase-transfer process [5.216]. 

Bismuth-containing special effect pigments based on platelet-shaped crystals of bismuth oxide chloride (bismuth oxychloride, BiOCl) have been known for a long time (see Section 5.3.1.3). 

Natural pearl essence is isolated as a silky lustrous suspension from fish scales. The organic pigment particles in the suspension are platelet-shaped with a very high aspect ratio (0.05 pm x 1-10 pm x 20-50 pm), and consist of 75-97% guanine and 3-25% hypoxanthine (natural fish silver) [5.122, 5.123, 5.128, 5.130]. 

Special care has to be taken to ensure parallel alignment of the platelet-shaped pigment particles during their application. In most cases the pigments can be dispersed homogeneously by simply and carefully stirring them into a suitable liquid system. 

The newest form of pearlescent pigment is a coated mica. The mica provides a transparent platelet shape with its two parallel faces. These faces are coated with a high-refractive-index layer of titanium dioxide or iron oxide. An absorption color layer can be overlaid on the oxide layer, giving additional reflective surfaces. These coated micas have high operating temperatures and are nontoxic. They are supplied in a range of particle sizes, usually from 10 to 50 xm. Loadings of coated mica pearlescents usually run about 1.5-3 percent. 

Pigments with shape factors (such as platelets or needle-like shapes) that orient in the film to give desirable improve resistance of electrical connection properties. 

Synthetic pearlescent pigments are either transparent or light-absorbing platelet shaped crystals. They can be monocrystalline as in Pb(OH)2 x 2 PbC03 and BiOCl or possess a multi-layered structure in which the layers have differing refractive indices and light absorption properties. 

Fine particles of malleable metals in elemental form are also used as pigments in coatings for aesthetic as well as protective purposes. They are generally platelet-shaped particles, but some are spherical. 

Aluminum pigments are the most often used metallic pigment in surface coatings. They are prepared from high purity aluminum metal by ball milling fine aluminum powder in the presence of a hydrocarbon solvent and grinding aids such as stearic acid and then formed into platelet-shaped particles. They are normally supplied as pastes in hydrocarbon solvents and occasionally in powder form. 

The first interferential pigments were created on the basis of natural muscovite mica as a substrate and coated with titanium or iron oxide. Other substrates have subsequently been used in this type of pigment, notably the silica flake and the alumina flake. The substrate provides the necessary platelet shape, while the interference colour is determined by the thickness of the metal oxide layer and its refractive index. 

The presence of mica in pearlescent pigments only partly accounts for the appearance of the pigment. A very thin layer of the inorganic oxide titanium dioxide (TiC>2) or iron oxide (Fe2C>3) or both is coated on the mica platelets. The various colors and pearlescent effects are created as light is both refracted and reflected from the titanium dioxide layers. The very thin platelets are highly reflective and transparent. With their plate-like shape, the platelets are easily oriented into parallel layers as the paint medium is applied. Some of the incident light is reflected... 

Flake aluminum pigments with varying platelet thicknesses and shapes are used for corrosion protection [5.175]. For standard specifications, see Table 1 ( Aluminum pastes and pigments ). They are coated with a water-repellent, fatty film and are therefore particularly suitable for conventional solventbome coating systems. They have outstandingly good weather resistance [5.175]—[5.177]. 

The barrier effect can be achieved, for example, by using pigments with a platelet-like or lamellar particle shape. This allows them to form a wall of flat particles within a paint film and therefore water and electrolytes have to take an extended/less direct route through the paint film to the substrate. 

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