Kaolin is natural clay formed over many millions of year by the hydrothermal decomposition of feldspath rocks like granite or porphyry. Kaolin is mined in many locations throughout the world but the quality is often insufficient. Kaolin is composed of primary and secondary kaolinites. The primary kaolinites have in its disc structure a ratio from length to thickness of 10:1 up. The individual discs have a hexagonal crystalline form. Kaolins are used in a lot of applications in rubber products. The particle shape and the charge distribution impact on the rheological behaviour, the attainable thixotropy avoids settling of pigments and the lamellar particle shape provides positive mechanical values like a high reflecting and covering power. At the same time the surface hardness of the end product is improved.
Calcined Clay are produced by heating natural kaolin (China Clay) to high temperatures (ca. 1000°C). Calcined kaolin can be coated with silane in order to obtain a surface that chemically bounds with the polymer. Calcined kaolins are dielectric and for the most part have a high degree of whiteness. Their chemical resistance is very high, even against acids. Calcined kaolins can be used in cable insulation compounds
Calcium carbonates are composed of the crystalline mineral, calcite, which occurs naturally in the form of chalk, limestone or marble. Calcium carbonate is the most significant filler nowadays and one of the most widespread binders on earth. Calcium carbonates can be processed by crushing and fractionation to suit almost all applications. Some product forms undergo a surface treatment with a stearate ameliorating the processing and the dispersion. A good dispersibility and a high degree of whiteness combined with positive mineral properties make the use of calcium carbonate as ecological filler inoffensive. Calcium carbonates are often added as a filler for rubber applications.