Лако-красочные материалы — производство

In many applications, satin or low-gloss finishes are preferred over high-gloss finishes for reasons such as having an elegant and luxu­rious appearance, hiding imperfections of a large surface area or even safety reasons, such as on the dashboards of cars. Reduction in gloss can be achieved by increasing PVC or by introducing incom­patibility into the binder. These approaches are not suitable in some formulations where certain mechanical properties or film clarity

are important. Matting agents are additives that allow better control of gloss without impairing other properties of the coating. Some of the commonly used such additives are silica, polymethyl ureas and waxes.

Synthetic amorphous silica (precipitated and fumed) is the most widely used matting agent in the coating industry. These types of silica are of very low particle size and are often surface-treated to improve compatibility with all major resin systems. They generate micro-roughness into the surface of the coating film, which results in diffused scattering of the incident light and hence matting. However, they tend to increase viscosity of the systems. The refrac­tive index of silica is very close to that of common binders; therefore, they can be used as a matting agent in clear finishes without any haziness. They are very effective and are used at levels of up to 3 % by weight in paint. Natural silica such as diatomaceous silica is also used occasionally together with extenders to produce a matte finish.

Organic matting agents such as micronized polymethylurea resins and polyethylene and polytetrafluoroethylene modified polyethylene waxes are used in coatings. They are incorporated in coatings by hot precipitation or as a dispersion. Like silica, they produce matting by migrating to the surface and producing micro-roughness. This affects how light is reflected from the surface (Figure 5.10).