Organic pigments, determination

Table 21.7 Decomposition temperatures of organic pigments determined byTGA.

Hydrogen bonds are important structural elements of organic pigments in-tramolecularly, they enforce planarity in a molecule intermolecularly, they may even play a role in determining the basic color of a pigment. 

The particle size distribution of an organic pigment powder is usually different from that found in the pigment-vehicle system and since both have practical importance, methods were developed for their determination. 

A number of phenols have proven to be valuable adsorbates for surface area determinations in organic pigments [10]. Phthalocyanines in particular have been evaluated successfully by phenol adsorption [11],... 

It seems reasonable to conclude that experimentally determined specific surface areas can only qualitatively relate to the physical characteristics of organic pigments. Instead, their value emerges in combination with other physical or physico-chemical parameters or in the context of application properties such as oil absorption [16] or wettability (Sec. 1.6.5). 

There are various methods for the determination of the size distribution of organic pigment particles, the most common are sedimentation techniques in ultracentrifuges and specialized disk centrifuges as well as electron microscopy. These methods require considerable experimental skill, since the results depend largely on sample preparation and especially on the quality of the dispersion. 

Particular difficulties similar to those associated with specific surface area determinations (Sec. 1.5.1) also accompany particle size distribution studies on surface coated organic pigments. The identity of the additive, be it an amine, a hard resin, or some other material, is less of a concern than the question of its concentration There is no information on the concentration limits above which an additive may distort the measurements but one can expect this value to be defined largely by the specific surface area and the average particle size of each individual pigment. For pretreated surfaces, sizing the pigment particles by electron mi-... 

The crystallinity of organic pigment powders makes X-ray diffraction analysis the single most important technique to determine crystal modifications. The reflexions that are recorded at various angles from the direction of the incident beam are a function of the unit cell dimensions and are expected to reflect the symmetry and the geometry of the crystal lattice. The intensity of the reflected beam, on the other hand, is largely controlled by the content of the unit cell in other words, since it is indicative of the structural amplitudes and parameters and the electron density distribution, it provides the basis for true structural determination [32],... 

There are certain accepted tests used to determine the extent to which a given organic pigment tolerates solvents. Experimental procedures commonly involve... 

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). 

Fig. 87 Systems containing organic pigments (left) and Chrome Yellow (right). Determination of resistance to acids with 1 N sulfuric add.

P. Maijanovic, M. McGarvey, R.B. McKay, Development of laboratory methodology to determine flow properties of organic pigment... 

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