The industrial utility of kaolin covers a diverse range of applications which can be classified into six categories according to primary functions: kaolin in film formation, kaolin in fiber extension, kaolin in polymer extension and reinforcement, kaolin use by virtue of chemical composition, kaolin as a carrier, adsorbent, diluent, etc., and kaolin as a polishing agent. By far the most important use of kaolin is in film formation, especially paper coating, which commands the most stringent kaolin specifications. These include brightness, particle size, particle size distribution, particle shape, and rheology. Along with fine particle size, the most important property of kaolin for film formation is the thin platelet shape which for Georgia kaolins generally do not exceed about 0.15 μm in average thickness. Calcined kaolins for paper applications must be bright, fine in particle size, and have aggregate structures with high internal void volume for enhanced light scatter. Excluding rheology, the same specifications are necessary for kaolin in fiber extension such as paper filling.
For polymer extension and reinforcement, such as in plastics and rubber, surface chemistry and particle size are of primary importance. The finer the particle size the better the reinforcement, but dispersion is essential to realize fully this benefit. For best dispersion, the polarity of the kaolin surface should approximate that of the polymer and can be achieved by surface treatment. In some systems silanes act as coupling agents giving a strong covalent bond between the kaolin surface and the polymer, thereby offering maximum reinforcement to the composite.
Where kaolin use involves reconstitution, as in catalyst supports, cement, fiber glass, and for the production of aluminum compounds, chemical composition is most critical. As a carrier for pesticides and pharmaceuticals, catalytic activity promoted by Lewis acid and Brõnsted acid sites on kaolin surfaces can affect transformation of some organic substances. Fine particle calcined clays, subangular to subrounded, are efficient polishing agents for teeth, automobiles, and soft metals such as gold and silver. The wide utility of kaolin is a function of a broad composite of characteristics but the most important are low cost, high brightness, fine particle size, platelet shape, and hydrophilic surface chemistry.