Kaolin or clay is a natural fine-grained, well-crystallized hydrous aluminum silicate mineral. Depending upon impurities, it is available in very white to grayish-white grades. The primary particles of clay are of lamellar shape, which contributes to good brushability and reinforcement of films. Due to its high oil absorption value and particle shape, it is also used as a cheap flatting agent. Clay imparts thixotropy to paints and improves suspension of other pigments. It is frequently useful as an extender along with white pigments. It is hydrophilic and widely used in water-based latex paints, but excessive usage can decrease the scrub resistance of films. Poor weathering resistance and poor flow properties are some of the important limitations of its use as an extender. Some of the other uses of clay as an extender include in primers and fillers. Finer grade clay is used in electrophoretic primers.
Calcination of clay at high temperature (1000 °C) causes an irreversible loss of chemically combined hydroxyl groups from the clay crystal structure, causing an interparticle sintering of kaolin platelets and giving rise to irregular-shaped particles with a microporous structure. The major use of calcined clay is as an opacifying extender in interior matte latex paints. High hiding power is obtained by the inability of paint binders to completely fill the pores of the microporous particles, leaving air in the pores, which contributes to light scattering.
Mica (aluminum potassium silicate — K2O-3Al2O3-6SiO22H2O)
Table 3.5: Important properties of extenders
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Mica is an orthosilicate of aluminum and potassium obtained from the natural mineral muscovite. The lamellar-shaped particles of mica have a grayish pearlescent appearance. Mica is a chemically inert extender with good weathering resistance and electrical insulating properties. In coating films, the platelet-shaped particles tend to orient parallel to the substrate and increase the barrier properties of films. They also provide mechanical reinforcement to polymeric matrices, leading to tougher films. Incorporation of mica also increases washability while helping in reducing chalking, checking and cracking of coatings. It is used in anticorrosive coatings, primers, fillers and exterior latex paints. Some of the limitations of mica include poor color, poor dispersibility and high price.