Pigments size distribution

Improved properties of the azo pigment Yellow 12 were also achieved in a micromixer-based azo coupling process (Scheme 4.20), providing a smaller pigment size distribution [42]. Compared with the corresponding commercially available standard, the glossiness of Yellow 12 was increased by 73% and the transparency by 66% while maintaining the tinctorial power. 

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. 

The choice of parameter used in the determination of size distribution should include consideration of the information needed in the interpretation of the data. For example, in the case of a manufacturer of paint pigment, the size parameter that best describes the hiding power (performance of the pigment) is the projected area of particles. A powdered catalyst manufacturer is primarily concerned with surface-area equivalence. 

Testing. Various test methods are provided by ASTM (16). These iaclude pigment tests of importance such as chemical analysis, presence of oversize particles, oil absorption, particle size distribution, degree of dispersion, presence of soluble components, etc. Numerous tests are also given by ASTM for the properties of filled and unfilled polymers. These iaclude, for example, such properties as impact resistance, stiffness, viscosity, tear resistance, hardness, color, and electrical resistivity. 

Theie aie several additional factors that must be considered in selecting pigments for a specific coating application (40). These factors include processing stabihty requirements, pigment uniformity and reproducibiUty, particle size distribution, dispersibiUty, and compatibiUty of all materials to be used. 

By the use of microstructured mixers, pigment and other particulate syntheses can be improved. In this way, finer particles with more uniform size distribution were yielded for the commercial azo pigment Yellow 12 (see Fig. 2) [11]. The particles formed in the microstructured mixer have better optical properties such as the glossiness or transparency at similar tinctorial power. Since the micro-mixer made pigments have more intense colour, lower contents of the costly raw material in the commercial dye products can now be employed which increases the profitability of the pigment manufacture. 

Fig. 2 Particle size distributions, (by volume) of the impeller-batch (top) and the micro-mixer-continuous-flow (bottom) processes when manufacturing the pigment Yellow 12. The cumulative distributions are given as well [11]...

The effects of the changes in the preparation methods were assessed by measuring the colour of the pigments in draw-downs and the particle size distributions of the powders. It was found that changes in all the listed variables could produce changes of several NBS... 

Two processes have been used for making copper phthalocyanine, but neither yields a form suitable for immediate use as a pigment since the physical form and size distribution of the resulting particles are far from the optimum. The product is generally referred to as crude blue and requires purification and a sometimes complex series of finishing processes before it is in a form ready for use as a pigment. 

With all pigments, the particle size distribution of the pigment as synthesised is the most important single factor in ensuring its optimum potential performance in use, but this potential can only be achieved if the particles are properly dispersed. To understand why particle size is so important it is necessary to look in some detail at the optical behaviour of tiny pigment particles dispersed in a transparent organic medium. The exact nature of this medium depends on whether the end-use is a paint, a printing ink or a plastic. 

Such simplified considerations can only serve as approximations to a more complex problem even the shade of a polycyclic pigment basically depends on crystal characteristics, such as particle size, particle size distribution, and crystal modification. 

G. Eulitz [13] studied the correlation between chemical constitution and tinctorial strength directly on dissolved pigment samples. He was able to exclude physical crystal parameters, such as particle size distribution and degree of dispersion, by achieving molecular dispersion. His analysis demonstrated the following ... 

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