Calcined Kaolin For Coatings and Glazes

Calcined Kaolin For Coatings and Glazes

Kaolin is a naturally occurring clay mineral that is heated at high temperatures, to alter its chemical and physical properties. The heat increases the kaolin's whiteness and hardness. It also changes the kaolin's particle size and shape. It is used in coatings, glazes and other ceramic applications.

Calcined kaolin is a natural clay that has been heated at high temperatures to improve its brightness, colour and abrasion resistance. It is also used as a paint extender (specifically titanium white).

The use of calcined kaolin has increased significantly in recent years, due to the increasing demand for pigments with specific features such as high brightness and opacity. This type of kaolin is more abrasion resistant than standard raw kaolin and can be processed into different particle sizes, including hydrous kaolin.

A wide range of calcined kaolin products are available in the market. These calcined kaolins are available in the form of powders, flakes or granules. The agglomerates of these calcined kaolins can be formulated into pigmented formulations or incorporated into coatings.

There are a number of different grades and particle sizes of calcined kaolins available from Imerys, with a variety of properties to suit your application needs. Some of these kaolins are engineered to provide excellent burnish and scrub resistance, while others offer higher Mohs hardness and opacity.

Mattex(r) - This is a medium grade kaolin that offers excellent brightness, good opacity and scuff and mar resistance. It is suitable for both exterior and interior coatings and can be formulated without the need for additional matting agents, offering a cost-effective alternative to titanium white in many formulations.

Alphatex - This is a high brightness calcined clay that has been specially treated to provide a high light scatter kaolin for the coatings industry. It has a controlled pore structure that maximises light scattering, which is particularly useful for reducing sheen knockdown.

In the calcining process, kaolinite [Al2Si2O5(OH)4] is transformed into metakaolinite and then mullite. The calcination process is essential for the industrial manufacture of various ceramic and building materials as it enhances a wide range of properties and reduces the costs of production.

Brightness & Valley Abrasion

The abrasion of calcined kaolin depends on the heating time, the calcination temperature and the amount of carbon added to the clay during calcination. Traditionally, the brightness of a kaolin is achieved by heating the clay in an oxidizing atmosphere for a long period of time before calcining it. However, the abrasion of a calcined kaolin can be less than that of a conventional kaolin if the abrasion is reduced by adding carbon to the kaolin during calcination.

According to the present invention, this abrasion reduction can be achieved by sequentially heating a finely divided hydrated crystalline kaolin clay containing a small amount of a ferruginous impurity and less than 0.1 percent by weight of carbon as an impurity in a calciner to a temperature in the range of 1400degF. to 2100degF. and then heating it again in the presence of sufficient air to oxidize carbon, in this case in the calciner, to a temperature in the range of 1600degF.