Pigments, crystal size

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. 

A certain amount of shearing forces have to be applied in order to overcome the surface forces that maintain the adhesion between agglomerated pigment crystals. In practice, the shearing forces that are necessary to reduce the particles in a given pigment sample to smaller or even optimal particle size, i.e., the dispersibility of a pigment powder, depends on a number of factors ... 

For several reasons, it is very difficult to quantitatively determine the solubility of an organic pigment as related to the particle size. F. Glaser [25] found that a particle diameter of 0.3 pm represents somewhat of a threshold value. The solubility of pigment crystals above this size essentially equals that of very large particles (cx) and is therefore largely constant in moving towards finer particle sizes, however, the solubility increases appreciably (Table 7). 

BiOCI pigments are available in three grades with different pearlescent effects depending on the aspect ratio and the crystal size ... 

Pigments Density (g cm-3) Refractive index a(BET) (m2 g"1) Crystal size (pm)... 

The particle sizes relevant for inorganic pigments stretch between several tens of nanometers for transparent pigment types to approximately two micrometers. For practical applications it is very desirable to determine not only the mean particle size but also the whole distribution. These parameters must not be confused with the crystal size determined by X-ray diffraction, as pigment particles usually are not monocrystals. 

There are two major processes for the manufacture of titanium dioxide pigments, namely (1) sulfate route and (2) chloride route. In the sulfate process, the ore limonite, Fe0Ti02, is dissolved in sulfuric acid and the resultant solution is hydrolyzed by boiling to produce a hydrated oxide, while the iron remains in solution. The precipitated titanium hydrate is washed and leached free of soluble impurities. Controlled calcinations at about 1000°C produce pigmentary titanium dioxide of the correct crystal size distribution this material is then subjected to a finishing coating treatment and milling. The process flow sheet is shown in Fig. 7.1 [4],... 

P.Y. 185 can also be produced in a one-step process in a water / solvent mixture starting with the formation of 10 in an organic solvent, followed by the addition of N-methylcyanoacetamide, and carrying out the final condensation with barbituric acid. The very specific crystal size required for ink application of the pigment can be formed directly by appropriate choice of temperature and dispersant without isolation of the cmde pigment. 

When we compare the masstone reflectance curves of iron chrome brown and iron chrome black pigments, we see the result of this effect where the black pigment has an expanded absorption region that covers the optical region. As discussed, the primary cause is the larger particle size. Iron chrome pigments with crystal sizes less than 0.5 are typically brown, and greater than 1.5 appear black. 

Ease and completeness of dispersion and intensity of colour are the primary application requirements of organic pigment powder (OPP). In order to meet these requirements, pigment manufacturers can vary the crystal size and surface treatment. Such variation can influence the flowability of powders and thereby affect handling processes at both manufacturers and consumers plants. 

Pigment series Composition General crystal size and shape... 

The chemistry of the aqueous sol-gel process is quite complex due to metal and the double role of water as ligand and solvent as well as due to the large number of reaction parameters that have to be stricdy controlled (hydrolysis and condensation rate of the metal oxide precursors, pH, temperature, method of mixing, rate of oxidation, the nature and concentration of anions, etc.) in order to provide good reproducibility of the synthesis protocol. Another fundamental problem of aqueous sol-gel chemistry is that the as-synthesized precipitates are generally amorphous. The required postsynthetic annealing step to induce the crystallization process prevents any subtle control over crystal size and shape. For the preparation of bulk metal oxides, like the ceramic pigment case, these limitations play only a minor role however, in the case of nanoparticle synthesis, they constitute a major concern. 

Products for which crystal size is important include catalysts, pigments,... 

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