The industrial silicate minerals, wollastonite, talc, mica, kaolin, feldspar, and nepheline syenite are widely used in coatings as "extender pigments." Current formulation practice more appropriately uses these minerals as functional fillers. They are chosen to optimize coating stability, rheology and application properties, and to enhance dry film properties. Very fine particle size silicates, particularly certain kaolins and ultrafine talcs, are used as pigment extenders in a more literal sense. The pigment that is extended is TiO2. These extenders effectively separate the individual TiO2 particles and provide the optimum pigment spacing for maximum opacifying effect. Pigment extenders are used in this way to reduce the amount of relatively expensive TiO2 required for a given level of opacification and whiteness.
The effects of the silicates on coatings are a consequence of the following.
# The mineral's structure, which influences its particle shape.
# The volume fraction of the mineral in the coating film, which is determined by particle shape and size.
# The mineral's compatibility with the binder matrix, which depends on particle physical and surface chemical properties.