Transparent Iron Oxide Pigments

Pigment transparent iron oxide red opaque iron oxide red iron mica 

Shade yellowish red yellowish red to violet metallic brown to black 

Transparent yellow iron oxide is manufactured by precipitation of iron(ll) hydroxide or carbonate from a very dilute iron(ll) salt solution and subsequent oxidation, e.g. with air, to FeOOH. The quality is determined by parameters such as the concentration of the iron(ll) salt during precipitation, precipitation temperature, oxidation time. 

Transparent red iron oxide is manufactured by thermal decomposition of yellow iron oxide and elimination of water at temperatures of300 to 500 °C. It is best to start from the dried, crushed filter cake. The pigment is subsequently ground. Pigments with particularly low conductivity values are obtained by additional treatment [5.186]. 

It is also possible to obtain transparent red iron oxide with a hematite content of 85% directly by precipitation of iron(l 1) hydroxide or carbonate from iron(II) salt solution and oxidation with air at ca. 30 C in the presence of one of the chlorides of magnesium, calcium or aluminum [5.189]. 

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Transparent iron oxide pigments have exceUent weatherabiHty, Hghtfastness, and chemical resistance, comparable to opaque iron oxides. 

Mixed metal oxide pigments containing iron oxide are also used (see Section 3.1.3). Magnetic iron oxide pigments are discussed in Sections 5.1.1 and 5.1.2. Transparent iron oxide pigments are described in Section 5.4.1. Methods of analysis and specifications of iron oxide pigments are listed in the standards given in Table 1. 

A comparison of transparent iron oxide pigments formed by the precipitation and the combustion process is shown in Figure 5.41. 

Transparent iron oxides have the same fastness properties as opaque iron oxide pigments, but much higher color strength and significantly higher color purity. Table 5.19 summarizes the properties of selected transparent iron oxide pigments [5.192]. 

Transparent iron oxide pigments are characterized by high UV absorption making them particularly suitable for the coloration of plastic bottles and films for food packaging and the manufacture of wood protection coatings. The precipitated, needle-... 

Use Levei 10-20% (transparent iron oxide pigment) 5-15% (inorg. pigments) 10-30% (oig. pigments) borage Redose containers immediately after use Disperbyk -107 [BYK-Chemie GmbH BYK-Chemie USA]... 

Use Levei 10-20% (transparent iron oxide pigment) 5-15% (inorg. pigments) 10-... 

Iron oxide-coated sand (IOCS), for arsenic removal, 3 279, 284-285 Iron oxide control, in industrial water treatment, 26 133 Iron oxide pastes, 19 402 Iron oxide pigments, 19 397-402 production of, 19 385 transparent, 19 412 economic aspects of, 14 557-559... 

Figure 82. Electron micrograph of a transparent red iron oxide pigment (Sicotrans Red 2815)...

Worldwide consumption of transparent iron oxides is 2000 t/a. They are mainly used in the production of metallic paint in combination with flaky aluminum pigments and in the coloring of plastics for bottles and fibers. 

Another very important application is the dispersion of pigments that are difficult to disperse (e.g., carbon black, transparent iron oxides, phthalocyanine blue and green, and perylene red). The use of CAB and two-roll milling is the most efficient method of dispersion. 

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... 

P.R.171 is used in plastics and in paints. Its lightfastness in PVC equals step 7 to step 8 on the Blue Scale, depending on the exact composition of the tested system, the pigment concentration, and the Ti02 content. Incorporated in plasticized PVC, P.O.171 is migration resistant and heat stable up to 180°C. It is used in conjunction with organic yellow pigments, frequently also with iron oxides, to produce shades of brown. Shades of bordeaux are accessible in deep transparent colorations. 

As a pigment, each iron oxide has an optimum particle size which is that with the maximum scattering cross section. This optimum particle size is lower, the higher the refractive index of the mineral. For hematite, the size corresponding to the maximum in scattering/absorption cross section is ca. 1 pm. As the particle size decreases, the relative scattering cross section drops to zero and the relative absorption cross section levels out. As a result, very small particles of hematite are transparent. 

Thermal decomposition of iron pentacarbonyl. Very finely divided red iron oxide is obtained by atomizing iron pentacarbonyl, Fe(CO)5, and burning it in excess of air. The size of the particles depends on the temperature (580-800 °C) and the residence time in the reactor. The smallest particles are transparent and consist of 2-line ferri-hydrite, whereas the larger, semi-transparent particles consist of hematite (see Chap. 19). The only byproduct of the reaction is carbon dioxide, hence, the process has no undesirable environmental side effects. Magnetite can be produced by the same process if it is carried out at 100-400 °C. Thermal decomposition of iron pentacarbonyl is also used to coat aluminium powder (in a fluidized bed) and also mica platelets with iron oxides to produce interference or nacreous pigments. 

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