Pigment properties

Scattering Power. The refractive indices of rutile and anatase are very high (2.70 and 2.55, respectively). Even after incorporation in a wide range of binders, they lie 

Pigment Class Ti02 (min.), wt% Water-soluble salts, wt% Volatiles (max.), wt% 

Country Company Location Chloride Capacity, 103 t/a Sulfate Total 

France Thann et Mulhouse (Rhone-Poulenc) Le Havre 95 95 

Poland Yugoslavia Czechoslavakia Total, Eastern Europe Agrokhim Zachem Cinkarna Precheza 

Taiwan China Metal Chemicals Chin Shin - 15 15 

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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. 

Schoefs, B., Plant pigments properties, analysis, and degradation, Adv. Food Nutr. Res., 49, 41, 2005. 

In practice, diarylide yellow pigments are frequently selected for their excellent fastness to a variety of organic solvents, in which they perform much better than monoazo yellow pigments. Properties such as migration or recrystallization... 

A number of resinated grades are produced in order to provide higher transparency and to optimize other aspects of pigment properties in application. For reasons connected with process engineering, the resin is typically incorporated as a metal (calcium) resinate. In the past, types of P.R.57 1 additionally contained certain amounts of barium sulfate. 

Although there is one route [3] which describes the direct synthesis of the tetrachlorothioindigo pigment by oxidation of 3-hydroxy-4,7-dichlorothionaphthenone with oxygen in an aqueous alkaline medium, this is somewhat of an exception. In most cases, it is necessary to modify the crude thioindigo derivative by appropriate aftertreatment in order to develop the desired pigment properties. 

With the exception of the anthraquinone-azo series and the salts of hydrox-yanthraquinone sulfonic acids, all of the listed compounds have known a long history as vat dyes before their pigment properties gained commercial recognition (Sec. 3.3). 

Kneading or milling the crude indanthrone in the presence of finishing agents, such as polyols, or milling it with salt also affords a product which provides useful pigment properties. 

Flavanthrone, like indanthrone, must be extremely pure in order to develop useful pigment properties. Subsequent finishing converts the thus prepared material into an appropriate product for use in paints or plastics. 

In addition to 6,14-dibromopyranthrone, there are some mixed halogenated pyranthrone derivatives which exhibit equally useful pigment properties. 

These methods produce yellow to red compounds which exhibit satisfactory pigment properties. 

At the completion of the reaction, the aniline is separated from the iron oxides by steam distillation and the umeacted iron removed. The pigment is washed, filtered and dried, or calcined in rotary kilns to hematite (Plate 20.1, see p. XXXIX). Considerable control over pigment properties can be achieved in this process by varying the nature and concentration of the additives and the reaction rate the latter depends on pH, the rate of addition of iron and nitrobenzene and the type and particle size of the iron particles. Two advantages of this method are that a saleable byproduct, aniline, is produced and that there are no environmentally, harmful waste products. 

Important pigment properties and the methods for determining them are described later. 

Hydrolysis is carried out in brick-lined, stirred tanks (n) into which steam is passed. The hydrolysate does not have any pigment properties, but these are strongly influenced by the particle size and degree of flocculation of the hydrolysate (mean particle size of hydrolysate is ca. 5 nm, and of Ti02 pigments 200-300 nm). 

The zinc salt and BaS solutions are mixed thoroughly under controlled conditions (vessel geometry, temperature, pH, salt concentration, and stirring speed, see (a) in Fig. 20). The precipitated raw lithopone does not possess pigment properties. It is filtered off (b2) and dried (c) ca. 2 cm lumps of the material are calcined in a rotary kiln (d) directly heated with natural gas at 650-700 °C. Crystal growth is controlled by adding 1-2 wt% NaCl, 2 wt % Na2S04 and traces of Mg2 + (ca. 2000 ppm), and K+ (ca. 100-200 ppm). The temperature profile and residence time in the kiln are controlled to obtain ZnS with an optimum particle size of ca. 300 nm. 

Aftertreatment. Thermal treatment at temperatures up to 1000°C improves the pigment properties of the ZnO and is mainly applied to oxide produced by the direct method. Controlled atmospheric calcination also improves the photoconducting properties of the high-purity oxide used in photocopying. 

The pigment properties of chromium oxides can be modified by precipitation of hydroxides (e.g., of titanium or aluminum), and subsequent calcining. This treatment changes the color to yellow-green, and decreases the flocculation tendency [3.54], Aftertreatment with organic compounds (e.g., alkoxylated alkylsulfon-amides) is also used [3.55]. 

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. 

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. 

Production. The raw material for the production of cadmium yellow pigments is high-purity cadmium metal (99.99 %), cadmium oxide, or cadmium carbonate. If the metal is used it is first dissolved in mineral acid. A zinc salt is then added to the solution the amount added depends on the desired shade. The zinc salt is followed by addition of sodium sulfide solution. An extremely finely divided cadmium sulfide or cadmium zinc sulfide precipitate is formed, which does not possess any pigment properties. This intermediate product can also be obtained by mixing the cadmium or cadmium-zinc salt solution with sodium carbonate solution. An alkaline cadmium carbonate or cadmium zinc carbonate precipitate is formed which reacts in suspension with added sodium sulfide solution. 

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