Pigment crystal shape

The (i-naphthol pigment Pigment Red 1 was the first red azo pigment to be submitted to three dimensional X-ray diffraction analysis [1], Today, three different crystal modifications are known, exhibiting divergent hues and crystal shapes [2] ... 

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]. 

An elegant and economical one-step synthesis of P.Y. 139 is described in l As initially obtained, the pigment is in an unsatisfactory physical, or cmde form. It is crystallized therefore in aqueous suspension in a finishing process to develop the required physical properties such as crystal shape and size, crystal phase and parti-de size distribution. This determines important application properties like hue, color strength, rheological behavior, hiding power, transparency and light and weather fastness. 

The pigments used are listed in Table 1, together with some details of composition and crystal shape. The average minor dimension of the crystals are all within the range from 10 to 40 nm. 

Figure 36 Illustrative model of a growing particle of a pigment crystal. Hot colored faces are fast growing, cool colored faces are slow growing. Modeling can be used to understand crystallization mechanisms, and thus as a tool in controlling crystal shape. (Image courtesy of Molecular Simulations Inc., San Diego, CA)...

The value of pigments results from their physical—optical properties. These ate primarily deterrniaed by the pigments physical characteristics (crystal stmcture, particle size and distribution, particle shape, agglomeration, etc) and chemical properties (chemical composition, purity, stabiUty, etc). The two most important physical—optical assets of pigments are the abiUty to color the environment in which they ate dispersed and to make it opaque. 

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. 

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]. 

In an alternative process, the starting material consists of needle-shaped particles of a-Fe203 instead of FeOOH pigments [5.9], [5.10]. The synthesis is carried out in a hydrothermal reactor, starting from a suspension of Fe(OH)3, and crystal growth is controlled by means of organic modifiers. 

Powder diffraction patterns have three main features that can be measured spacings, peak intensities, and peak shapes. Because these patterns are a characteristic fingerprint for each crystalline phase, a computer can quickly compare the measured pattern with a standard pattern from its database and recommend the best match. Whereas the measurement of spacings is quite straightforward, the determination of peak intensities can be influenced by sample preparation. Any preferred orientation, or presence of several larger crystals in the sample, makes the interpretation of the intensity data difficult. The most common structures of inorganic pigments are rutile, anatase, and spinel. 

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