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Flash Calcined Clays
If the heating of kaolinite clays is carried out very rapidly, then the outside of the clay particles can fuse, forming a glass, before dehydroxylation occurs. The water formed cannot escape and forms closed pores, giving a lighter weight aluminosilicate, which still has a defect spinel structure (see Fig. 4).
Fig. 4 ilcined kaolin
(OpaciliteTM) (reproduced with kind permission from Imerys)
Uses of Calcined Clays in Polymers
The low dielectric constant of metakaolin and calcined clays makes them particularly useful for medium- and high-voltage cable insulation use. Their low water content and absorption further help in this regard. While there is some overlap in the performance and applications of metakaolinite and calcined clays, there are significant differences between them which lead to separate use patterns.
Metakaolin is mainly used in PVC cable insulation where, in addition to the low dielectric constant, it also improves the electrical resistivity of plasticized PVC. This effect is unique to metakaolin and is thought to be due to its active surface deactivating ionic impurities. This special effect of metakaolin is absent in most other polymers, where calcined clays are then often preferred.
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