Mixed oxides, pigments

Fe, Co, Ni. This provides a range of colours in which the excellent durability characteristics of the host crystal structures are retained. An important commercial example of a mixed oxide pigment based on the spinel lattice is cobalt aluminate blue (Cl Pigment Blue 28), which is usually represented as C0AI2O4,... 

At one time, pigments based on iron oxide WCTe known for great stability but poor tinting strength. The trend towards elimination of the use of compounds of lead, cadmium, and mercury has driven the development of improved iron oxide pigments and of new mixed oxide pigments that cover a wide range of colors. 

About 100,000 t of titanium dioxide aimuaHy are used as formulation components in the production of glass (qv), ceramics, electroceramics, catalysts, and in the production of mixed-metal oxide pigments. 

Mixed-Metal Oxide Pig ments. Mixed-metal oxide pigments can be considered a subcategory of complex inorganic color pigments. The name, mixed-metal oxides, does not, however, represent the reaUty as these pigments are not mixtures but rather soHd solutions or compounds consisting... 

Mixed-phase oxide pigments are manufactured by high temperature (800-1000 °C) solid state reactions of the individual oxide components in the appropriate quantities. The preparation of nickel antimony titanium yellow, for example, involves reaction of Ti02, NiO and Sb203 carried out in the presence of oxygen or other suitable oxidising agent to effect the necessary oxidation of Sb(m) to Sb(v) in the lattice. 

Currently produced iron oxide pigments are listed in Table 2.7. Similar pigments occur naturally in many parts of the world, but are of course mixed with other mineral substances. 

Molybdates/lead chromates Luster pigments Mixed metal oxide pigments Iron blue pigments Ultramarine... 

Oxides and oxide -hydroxides Iron oxide pigments Chromium oxide pigments Mixed metal oxide pigments chromium oxide cob chromium oxide hydrate green alt green and b lue iron oxide red iron oxide orange chromium rutile orange iron oxide yellow nickel rutile yellow, chromium rutile yellow iron oxide brown zinc iron spinell, Mn-Fe-brown... 

Electrically-conducting Ti02 pigments have been produced by an aftertreatment to give a coating of mixed oxides of indium and tin, or antimony and tin [2.62]. These pigments are applied to fibers used in photosensitive papers for electrophotography, and for the production of antistatic plastics. 

Mixed Metal Oxide Pigments. The starting material is a Ti02 hydrolysate, which is calcined with oxides of transition metals to form chromium rutile or nickel rutile pigments (see Section 3.1.3.1). 

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. 

The solutions of the iron salts are first mixed with alkali in open reaction vessels (Fig. 23, Route A) and oxidized, usually with air. The quantity of alkali used is such that the pH remains acidic. The reaction time (ca. 10-100 h) depends on the temperature (10—90 °C) and on the desired particle size of the pigment. This method yields yellow pigments (a-FeOOH) [3.16], [3.17]. If yellow nuclei are produced in a separate reaction (Fig. 23, Route A, tank c), highly consistent yellow iron oxide pigments with a pure color can be obtained [3.18],... 

The term mixed metal oxide pigment denotes a pigment that crystallizes in a stable oxide lattice, and in which the color is due to the incorporation of colored cations in this lattice. Such compounds are regarded as solid solutions. The American Dry Color Manufacturers Association has abandoned the term mixed metal oxide inorganic colored pigments , it denotes these colorants as complex inorganic color pigments [3.73],... 

The only stable oxide lattices that have so far been of value as mixed metal oxide pigments are those with spinel, rutile, and hematite structures. These lattices possess not only good thermal and chemical stability, but also have a high refractive index which is important for good optical pigment properties. 

Spinel Mixed Metal Oxide Pigments. The crystal structure of spinel, MgAl204, is made up of an approximately cubic closely packed array of oxygen ions (Fig. 26). The unit cell contains 32 oxygen ions. Of the 64 possible tetrahedral sites, 8 are occupied by magnesium ions of the 32 possible octahedral sites, 16 are occupied by aluminum ions. 

The mixed metal oxide pigments are usually produced by reaction of the components in the solid state at 800-1400 °C. The reactions proceed more readily if the components are reactive, finely divided, and intimately mixed. The reactants are... 

Wet mixes are usually dried before calcination. Calcination is performed continuously in rotary or tunnel kilns, or batchwise in directly fired drum or box furnaces. The temperature at which the mixed metal oxide pigments are formed can be reduced by adding mineralizing agents [3.75]. In the case of chromium rutile pigments, addition of magnesium compounds [3.81] or lithium compounds [3.80] before calcination improves thermal stability in plastics. 

The formation of rutile mixed phases in the system Ti-Ni-Sb-0 is described in detail in [3.92], When nickel ions are to be included in the pigment and antimony trioxide is used to provide the antimony ions, oxygen must be present to ensure that nickel and antimony occupy the titanium sites in the lattice. If oxygen is excluded, only small amounts of nickel and antimony are taken up in solid solution, and no useful products are formed. The presence of oxidizing agents (e.g., nitrates) is necessary for the production of antimony-containing mixed metal oxide pigments. 

There are as yet no national or international (ISO) standards for mixed metal oxide pigments, and none are expected in the foreseeable future. However, the general tests of chemical, physical, and optical properties described in ASTM, DIN, or ISO standards for pigments may be utilized, such as DIN 6174, DIN 55 986, and ISO 787 Part 16 for the optical properties, and ISO 787 Parts 1-20 for the physical properties (see also Table 1). 

There are no standard methods for the chemical analysis of mixed metal oxide pigments. They are first decomposed (e.g., with peroxide in the case of rutile mixed phases, or soda/borax for the spinels) and the appropriate methods are then used to determine the elements. 

Mixed metal oxide pigments are thermally stable, water-insoluble materials. They are not classified as hazardous substances, and are therefore not subject to international transport regulations. When stored under dry conditions their pigment properties do not deteriorate. 

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