To optimize a paint formulation for the best cost-performance, the choice of the kaolin-based extender is crucial, and the choice is dependent on the type of coating to be produced.
Paints with PVC below critical, such as semi- and high-gloss paints, have no entrapped air in the dry film. The entire solid surface is wetted by the binder and opacity is obtained solely by TiO2 particles. At any given TiO2 concentration, opacity is improved or optimized by ensuring maximum surface exposure of the TiO2 particles to light. Using fine hydrous kaolins with a particle size similar to TiO2 (0.2 to 0.4um) will give optimum spacing of TiO2 particles, prevent further agglomeration and ensure maximum exposure to light.