Лако-красочные материалы — производство

Pigments vary not only in size but also in particle shape. Although the inherent shape of pigment particles is determined from its crystal

form, it also depends to some extent on the method of synthesizing the pigment. In general, wet-ground pigments tend to have smoother and rounder edges than those produced by dry grinding. Micronized pig­ments also have smooth contours. The primary particles of pigments may be spherical, cubical, nodular (irregularly shaped), rod-like, aci — cular (needle-like) or lamellar (plate-like), as illustrated in Figure 3.4.

Particle shape of pigments also has an influence on such properties of paints as consistency, flow, brushability, settling and the mecha­nical properties of their films. Acicular pigments are several times longer than their smallest diameter. Needle or rod shapes have been found to exert a reinforcing action in paint films, similar to steel rods embedded in concrete. Acicular particles tend to settle in paint in a brush-heap structure, which is more readily re-dispersed than the compact layer formed by nodular, spherical or cubical particles. These relatively long acicular pigments may protrude through the surface of the film and thereby produce sufficient irregularity at the surface to reduce gloss. The surface roughness may be used to advantage in undercoats and intermediate coats by providing tooth for better adhesion of subsequent coatings. Lamellar pigments are extremely thin plates that reinforce the film. They tend to overlay one another in paint films, thereby making the film impermeable to the passage of moisture or gases. Nodular, spherical and cubical particles tend to have low oil absorption values, thus providing good flow properties to paint and providing the latitude to formulate paint with high PVC, while acicular, lamellar and rod-like particles have a tendency to increase the consistency of the paint due to their high oil absorption value. The abovementioned effects are not absolute, and should also be correlated with a pigment’s other properties such as particle size and tendency to agglomerate. For example, even though primary particles of carbon black are spherical, they have a high oil absorption value because of their extremely low particle size, and frequently these spherical parti­cles are transformed into chainlike structures.

Evaluation

The particle shape of a pigment can be observed through a light microscope. Nowadays, expensive but precise and more reliable techniques like electron microscopy are widely used to determine the particle shape of a pigment.