8 ноября, 2015 
Pokraskin Solubility is the most undesirable characteristic of a pigment. Although pigments by definition are insoluble, in practice, many pigments exhibit a minute level of solubility, leading to noticeable problems in the systems in which they are incorporated. Generally, organic pigments exhibit such a level of solubility in organic solvents, while inorganic pigments are completely insoluble in solvents. Molecular size, substituent groups, metal complex formation and intermolecular bonding are some of the important factors that affect solubility of organic pigments. Some of the predominant problems observed in coatings as a result of solubility of pigments in the solvent or vehicle are bleeding, blooming and recrystallization.
Bleeding: In coatings, a pigment is said to bleed if some portion of it, by virtue of its solubility, migrates from the film in which it is incorporated to the medium with which it is in contact, causing discoloration of the latter. For example, if a white coating is applied on a red coating, and if the red pigment is even slightly soluble, the white coating may be discolored to pink rather than white. Such pigments are said to bleed. Resistance of a pigment to this phenomenon is known as bleed resistance or overcoating fastness.
Blooming: In coatings, blooming refers to the migration of dissolved pigment particles from the inside of a paint film to the surface, where they are deposited as a layer of pigment crystals. As solubility increases with temperature, the phenomenon is more significant at elevated temperatures, such as in baking enamels. Even after wiping the deposit from the surface, a bloom may form again as a consequence of a pigment’s solubility in the binder. The phenomenon continues until most of the pigment has crystallized either at the surface or in the film. Apart from temperature, the tendency of pigment to bloom is also the function of its concentration in the film.
Recrystallization: While milling the pigment in a vehicle, a portion of the pigment may get dissolved due to generation of the heat in the milling process. The resultant paint may meet the optical criteria of the final paint, appearing strong and bright, because the pigment is acting as a supersaturated solution of dye. However, over a period of time, the dissolved portion of the pigment starts to precipitate, causing loss in strength and brilliance. The problem is more evident as a noticeable shift in shade when organic pigments with significantly different colors and differential solubility are combined in a single composition. Recrystallization can be avoided by appropriate selection of pigments and temperature control during the milling process.
Along with solubility in solvents, the pigment should also be checked for water-soluble content. Water-soluble components are restricted to a few parts per million. Pigments prepared by precipitation or by calcination of metal salts tend to adsorb or retain traces of salts that are not always completely removed in the subsequent washing process. The presence of water-soluble matter in the paint film may impair performance of the coating. Water-soluble sulfates or chlorides in pigments used in corrosion inhibiting primers for metals can act as corrosion accelerators.
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