Pigments crystal form

A high purity titanium dioxide of poorly defined crystal form (ca 80% anatase, 20% mtile) is made commercially by flame hydrolysis of titanium tetrachloride. This product is used extensively for academic photocatalytic studies (70). The gas-phase oxidation of titanium tetrachloride, the basis of the chloride process for the production of titanium dioxide pigments, can be used for the production of high purity titanium dioxide, but, as with flame hydrolysis, the product is of poorly defined crystalline form unless special dopants are added to the principal reactants (71). 

A white pigment for rubbers and plastics characterised by high tinctorial power, fastness to light, and chemical stability. Titanium dioxide pigments are made in two crystal forms, mtile and anatase, which differ in crystal structure and crystal size. 

The starting material for making these pigments is cadmium sulphate, which must be free from iron, nickel and copper impurities. Cadmium sulphide is precipitated from the sulphate solution by adding an alkaline solution of pure sodium sulphide under controlled conditions of pH, temperature and addition rate. The yellow product is in the cubic crystal form, which is converted into the required hexagonal form by calcination at 500-600 °C in the absence of air. 

This chapter has been concerned mainly with the chemistry of making pigments but has also stressed the importance of preparing them in the correct physical form. In the case of those pigments which can be made in more than one crystal form, such as the anatase and rutile forms of titanium dioxide, this may mean that all the pigment should be in one form only. In others, a mixture of polymorphic forms may be required, and it is then necessary to ensure that the desired ratio of these forms is present. 

Fig. 2 Examples of crystal forms of organic pigment particles.

It has often been observed that the coloristic properties of an organic pigment are a function not only of the size of particles but also of their shape. This is due to the anisotropy of the optical properties in different crystallographic directions within the crystal forms of a pigment. In 1974 [5, 6], it was demonstrated that of the equally sized but differently shaped particles of beta copper phthalocyanine blue, the almost completely cubic, i.e., more or less isometric form produces greenish blue shades, while acicular forms are responsible for reddish blue hues. The optical behavior of ordered pigment particles in systems has been reported in the literature [7, 8]. 

Quinacridone is the trivial name given to the five-ring heterocyclic system exemplified by the linear irani-quinacridone (2.69). The yellow-red to reddish-violet shade pigments based on this ring system show outstanding durability and are used in plastics, in industrial and automotive finishes and in exterior finishes. Like copper phthalocyanines the unsubstituted linear quinacridone exhibits polymorphism and two crystal forms, the reddish violet P-form and the red y-form, are commercially... 

The crude precipitated cadmium yellow is washed and then calcined at approx. 600 °C, at which temperature the cubic crystal form changes to the hexagonal form. This process determines the particle size distribution, which is essential for the pigment properties. 

Molybdate red and molybdate orange [12656-85-8], C.I. Pigment Red 104 77605, are mixed-phase pigments with the general formula Pb(Cr,Mo,S)04 [3.131]. Most commercial products have a Mo03 content of 4-6 % (refractive index 2.3-2.65, density 5.4-6.3 g/cm3). Their hue depends on the proportion of molybdate, crystal form, and particle size. 

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]. 

The crystal defects of the host lattice structure aid in the incorporation of chromophores. By increasing those defects, reactants can diffuse more easily through the product layers and the pigment is formed faster. The presence of mineralizers can also positively affect the solid-state reaction (24). A mineralizer is a compound that facilitates crystal growth during solid-state reactions by providing a local environment that makes the movement of reactants through the solids mixture easier. The incorporation of the chromophore into the host lattice usually results in the formation of a substitution, or less often an addition compound. 

The rate of vatting depends not only on the concentration of dye and reducing agent but also on the crystal form, surface, and dispersion of the pigment (i.e., on its finish quality [51]). Leuco compounds are soluble in alkali. In the case of anthraquinoid vat dyes, the pH of the vat is about 13. At lower values the risk of vat acid sediments exists. Reduction is usually performed at 50-60°C. At higher temperature, over-reduction of certain dyes can occur (i.e., reductive destruction of the dye molecule). 

Titanium dioxide is found in nature in three crystal forms anastase, brooldte, and rutile. Its extreme whiteness and brightness and its high index of refraction are responsible for its widespread use as a white pigment in paints, lacquers, paper, floor covering, plastics, rubbers, textiles, ceramics, and cosmetics, see also Aluminum Catalysis and Catalysts Copper Fertilizer Haber, Fritz Iron Steel. 

Gerstner W. 1966. Crystal form and particle size of organic pigments in printing inks and paints. J. Oil Col. Chem. Assoc. 49 954-973. 

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