Let's discuss calcined kaolin. What is it and why should we use it in our paints, paper, plastics, rubber and ceramics?
Naturally-occurring kaolin is known as "hydrous kaolin" because it contains water within its crystal lattice. This water cannot be removed by simply drying the kaolin.
Calcining a mineral is to heat it to a temperature where a phase transition occurs, usually the loss of a volatile component. In the case of kaolin it loses its waters of crystallisation.
As you heat kaolin to 800ºC, it loses water in the form of hydroxyl (-OH) groups to form Metakaolin:
2 Al2Si2O5(OH)4 -› 2 Al2Si2O7 + 4H2O
Metakaolin is an interesting material. When you add it to concrete, it acts as a pozzolan. This means it enhances the strength of the concrete, a useful property in civil engineering and oil-well cementing.
To form calcined kaolin, you need to continue to heat it all the way to 1050ºC. Two different reactions occur on the way there. Firstly the formation of spinel at 950C:
2 Al2Si2O7 -› Si3Al4O12 + SiO2
Then the spinel phase transforms to the mineral mullite plus the highly crystalline cristobalite, SiO2:
3 Si3Al4O12 -› 2 Si2Al6O13 + 5 SiO2
But enough of the chemical reactions! What we need to know is that calcining causes the kaolin structure to collapse and become denser, which improves its opacity and its whiteness. The particles become sharp and hard, but also more porous because of the voids formed where the –OH groups were. Flash calcining is a method where the kaolin is heated very quickly. This increases the number of voids compared to conventional methods of calcining.
These changes give calcined kaolin some useful properties in various applications:
Better opacity and whiteness make calcined kaolin a great 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. They also add corrosion-resistance 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. So it is still a good option as an extender, particularly in paints above CPVC (critical pigment volume concentration).
Good whiteness is useful in ceramics too. However the most useful property to the ceramist is the fact that calcined kaolin it is already calcined. Hence it will have no plasticity and there will be no expansion or contraction of this material. It imparts mechanical strength and a fine white surface texture.
It is easy to tell calcined kaolin from hydrous kaolin by looking at their data sheets. Just look at the LOI (Loss on Ignition) value. For water-washed kaolin it is around 13%, but for calcined kaolin it will be zero. The LOI test basically calcines the kaolin. Hence the LOI is the percentage mass lost due to the loss of the waters of crystallisation.
Rubber and plastics
Using calcined kaolin as a filler in rubber and plastics helps impart tensile strength. Hence it is a reinforcing filler instead of being non-reinforcing like hydrous kaolin. The fire-resistant property of calcined kaolin plus the fact that it is an electrical insulator is especially useful in the plastic covering of electric cables. It also imparts uv-resistance which is why it is often added to greenhouse film and garden furniture.
An unexpected use of calcined kaolin is as a pesticide. When sprayed onto fruit, the fine sharp calcined kaolin particles deter pests by getting into their joints and irritating them. At the same time it reflects the sun and acts as a sunscreen.
Calcined kaolin has many other important applications such as its use in paper coating and in petrochemical catalysts. You cannot deny that it is an interesting material!