USO DE LENGUAJE INAPROPIADO

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. Gene- rally, organic pigments exhibit such a level of solubility in organic solvents, while inorganic pigments are completely insoluble in sol- vents. 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 pro- blems 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 pig- ments 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 dissol-

ved 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 phenome- non 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 por-

tion 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 preci- pitate, 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 com- bined 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 chlori- des in pigments used in corrosion inhibiting primers for metals can act as corrosion accelerators.

Evaluation

Tolerance to an individual solvent is tested by enclosing a certain amount of pigment powder in a piece of filter paper, which is then immersed in the organic solvent for a given amount of time. The extent of coloration of the test solvent indicates the solvent fastness of the pigment.

In another method for evaluating overcoating fastness, a uniform coating containing test pigment (generally a full shade) is over- coated with a uniform film of white paint to yield complete hiding. After complete drying, pigment migration from the base coat into the white topcoat and hence discoloration of the latter is determined visually or colorimetrically. The binder system for base coat and

white top coat are generally selected based on the actual coating system in which the pigment is to be used. Further information on bleeding tests is in ISO 787-16 and ASTM D279.

The water-soluble matter can be determined by extraction of a known amount of pigment sample with water followed by filtration and evaporation of the filtrate to dryness in order to gravimetrically determine water-soluble.

3.2.2.2 Density

Density can be a useful pointer to such properties of a pigment when dispersed in paint as suspension and density of the paint itself. According to Stokes’ law, a pigment particle will settle in paint at a rate proportional to the difference in the densities of the pigment and the medium. Therefore, paints with dense pigments exhibit quick settling during storage. The settling rate can be reduced by compensating for the high density by using a viscous medium and a pigment with low particle size.

A further use of density is in calculating important volume rela- tionships while formulating the paint, such as PVC, which affects many important properties of coatings and volume solids in paint and is used to correlate dry film thickness and wet film thickness. The density of the pigment is primarily dependent on its chemical composition and to some extent on its crystal structure. Generally, inorganic pigments are denser than organic ones, with a few excep- tions such as carbon black and iron blue pigment that have lower density than other inorganic pigments.

Evaluation

The density of a pigment may be determined by the pycnometer method at a standard temperature of 25 °C using kerosene as immersion liquid. The detailed procedure may be found in ISO 787- 10 and ASTM D153.

Many pigments are tested disregarding air entrapment on the par- ticle surfaces or in the voids between the particles. The mass (in g) of 1 cm3 _{of the pigment after tamping in a tamping volumeter} under prescribed conditions is referred to as its apparent density.

It depends on the true density, shape and size of the particle. Infor- mation on tamped volume determination is in ISO 787-11.