Opacity in paint is generated by differences in refractive index between the formulation ingredients. Titanium dioxide has a refractive index of 2.7 (Rutile) and 2.55 (Anatase), typical binders have a refractive index of 1.49.
Opacifying extenders are used to improve the effectiveness of the titanium dioxide in several ways, depending on the type of paint, the properties required and importantly the pigment volume concentration (PVC). Increasing the effectiveness of titanium dioxide allows the formulator to formulate a paint to the required covering power and use less titanium dioxide – reducing overall costs.
Below CPVC (critical pigment volume concentration), fine hydrous kaolin such as Supreme is used to space the TiO2 particles and avoid crowding. The particle size and shape of Supreme both make their spacing effect superior to fine calcium carbonates. Also, the lamellar particle shape of Supreme means that the effect on gloss is less than materials with a lower aspect ratio but of similar average spherical diameter.
Paints formulated above CPVC are opaque because of differences in refractive index between the ingredients AND air voids that occur in the paint film. Calcined kaolin such as PoleStar 200P has an irregular particle shape which promotes the creation of air voids. Gloss levels are affected by calcined kaolin and this must be considered when formulating the paint.
Opacilite contains numerous internal voids which generate opacity in paints above and below CPVC. When used in paints above CPVC external voids also contribute to dry film opacity, as with PoleStar 200P. Unlike most extenders which give low wet film opacity, Opacilite contributes significantly to wet film opacity, a feature that can help the painter apply the paint more effectively.
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