Лако-красочные материалы — производство

Particle size and particle size distribution are very important cha­racteristics of pigments because they influence various optical pro­perties of the paint such as color (Section 3.2.1.1), hiding power/ opacity (Section 3.2.1.2), tinting strength (Section 3.2.1.3) and gloss, as well as physical properties such as suspension of particles in the paint and rheology of the paint system. For high gloss in coatings, it is necessary for the surface to be as smooth as possible. Large pigment particles or aggregates may project to the surface, thereby diffusing light and reducing gloss. According to Stokes’ law, settling velocity is directly proportional to the square of the radius of the particle. Thus, smaller particles can be suspended more efficiently in a paint system than larger particles.

Pigments are produced in form of finely divided powder particles that consist of primary particles, aggregates and agglomerates (Figure 3.3).

Primary particles are individual pigment particles that can be recognized by a suitable physical method such as optical or electron microscopy.

Aggregates are an association of primary particles that have grown together and are aligned side by side, where the total surface area is smaller than the sum of the surface areas of the primary particles.

Agglomerates are an association of primary particles aligned along the corners and edges, where the total surface area does not differ appreciably from the sum of the individual surface areas of the particles.

Agglomerates present in a suspension (for example, in pigment — binder systems) that can be disintegrated by weak shear forces are known as flocculates.

The ultimate size of the pigment particle (primary particle) is depen­dent on pigment manufacturing conditions. Paint manufacturers actually have no control over the ultimate particle size because the

shear forces involved in the paint milling process are not sufficient to break primary particles; only breaking of agglomerates takes place during the milling and dispersion process.