Kaolin is one of the most indispensable fillers in the paint industry. It is a natural clay raw material, containing Ca and Na and is also known as china clay or kaolin.
Kaolinite is one of the most common minerals on earth. It is mined, as kaolin, in Brazil, France, United Kingdom, Germany, India, Australia, Korea, China and the USA.
Kaolinite is a soft, earthy, usually white mineral, produced by the chemical weathering of aluminum silicate minerals like feldspar. Depending on where in the world it is sourced, it can have different hues, for example, it can be coloured pink-orange-red by iron oxide, giving it a distinct rust hue. Lighter concentrations of iron oxide yield white, yellow or light orange colours.
Formation of Calcine Kaolin
During the removal of kaolin from nature, there are often undesirable impurities in it. These impurities and some of the water contained in the kaolin are removed by the calcination process.
Calcined kaolin is produced by heating ultrafine natural kaolinite to high temperatures in a kiln. The calcination process increases whiteness and hardness and alters the size and shape of the kaolin particles to provide superior optical and physical properties.
Uses and Advantages
Calcined kaolin offers better opacity and whiteness making it an excellent extender for titanium dioxide. The hard calcined kaolin particles help to improve the structural strength of a coating, and also the scrub-resistance of both water and oil-based paints. It adds corrosion and fire-resistant properties to the paint.
The disadvantage is the cost. Calcination and the subsequent milling of the hard calcined particles both require a lot of energy. This makes calcined kaolin around three times more expensive than hydrous kaolin. However, it is still less than half the price of titanium dioxide. It is, therefore, still a good option as an extender, particularly in paints above CPVC (critical pigment volume concentration).
Kaolin clay is also used in grounds, such as for painting and for gilding. The kaolin clay can be used in recipes calling for "pipe clay" and mixed with starch or flour to make a thick cream and applied to wood panel or canvas supports for painting. The ground dries quickly and several very thin layers applied one after another to form a ground typical of 19th century paintings.
Calcine Kaolin is also used in ceramics, rubber and plastics and pesticides.
For more information contact Chemgrit Coatings.