Kaolin Clay: Pigment Extenders


Basic Chemistry of Kaolin Clay

Kaolin clay or most commonly known as ‘China clay’ is a hydrated aluminosilicate Al4Si4O10(OH)8 formed over million years by weathered granite rocks.

Its physical characteristic can be crystalline, layered or platy in shape. Its aspect ratio is 4:1 to 10:1. The Mohs hardness is between 2 and 3. It is a chemically inert material, hydrophilic and can easily be dispersed in water.

Kaolin clay is a natural mineral wherein its depository can mainly be found in China, USA and Brazil.

Different Types of Kaolin

There are three basic types of Kaolin: hydrous, calcined and metakaolin.

  • Hydrous kaolin is characterized by its fine particle size. It undergoes extra processing so it can be spray dried/surface treated or pulverized.
  • Calcined kaolin is produced by heating the natural kaolin in high temperature giving is popcorn like structure and its increase whiteness and hardness.
  • Metakaolin is produced by calcination of natural kaolin the outcome is an amorphous pozzolanic mineral mainly used as an additive in cement-based products.

Application of Kaolin and its Function as Extenders

Most china clay or kaolin clay are used in ceramics, paper, rubber, wire and cables, paints, printing inks, and even personal care and cosmetic products.

In the paint industry, kaolin clay is mainly as extenders of titanium dioxide in white paints. Either hydrous or calcined kaolin can be used depending on the properties of the paint needed. It can also be used as pigment extenders since it provides spacing for pigment particles.

Fine particle size kaolin (0.2-0.4 microns) are used in high gloss and semi-gloss paints while calcined grades are likely to be used in satin and flat paints. The particle size range allows flexibility in formulation. The recommended dosage of most kaolin is 2-15% of the total pigment in the formulation. Kaolin clay is usually used as pigment extenders mainly due to cost consideration. Using too much kaolin can prove detrimental to paint properties, opacity may suffer as well as other properties such as gloss etc., so it is better to formulate based on the recommended dosage.

The most common Imerys kaolin are Polestar 400 (calcined kaolin) and Hydrite PXN-LC (hydrous kaolin). Both grades are being promoted in the Philippine market in paint and ink applications.

Kaolin Clay Morphology

Effective Spacing by Delaminated Kaolin

Source: Pigment Handbook edited by T.C.Patton1973

Effect of Particle Size on Gloss

Sign in to leave a comment
Basic Chemistry of Nipar S-10