Calcium carbonate

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Calcium carbonates may be considered as materials of two families. One is natural calcium
carbonate, a ground limestone, used as diluents to reduce compound cost. The particle
diameter is relatively large, ranging from 1 to 5µm, and bulk density is low. Ground
calcium carbonates generally mix easily into rubber. Good dispersion is obtained even at
very high loadings, up to 200 phr, without significant increases in compound viscosity.
Although ground calcium carbonates reduce cost, the magnitude of the practically
important volume cost reduction is limited because of the high specific gravity. 

The other family is precipitated calcium carbonate. Limestone is burned in a kiln driving off carbon
dioxide gas and leaving calcium oxide. The carbon dioxide gas is transferred to a calcium
oxide suspension to form calcium carbonate again. The resulting carbonates are filtered,
dried, ground and separated by size. The particle size of precipitated calcium carbonateis             below  0.1 µm, thus the small particles are reinforcing in rubber compounds. 

Coating agents such as fatty acids or lignin are used to prevent secondary flocculation.
 Without coating, they tend to stick to the walls of the internal mixer and become difficult to
disperse in rubber. Silane modification of precipitated calcium carbonate can further
increase tensile strength of the compound but not to the level achieved with a high surface

area carbon black.

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