The major ingredients present in undercoats are extender/filler pigments, which are essential for properties like filling, sanding, flow and durability; in general, less binder is used in undercoats. Extenders are natural or synthetic materials finely distributed into paint. The different types of extenders used in the coatings industry are China clay, talc, chalk, barytes and mica. Each of these has different sizes and shapes, such as platy, nodular, lamellar and acicular. Overall, the total extender combination is around 30-40%, so extenders have significant influence on the physical properties of the paint, such as dry adhesion and elastic properties. Different undercoats were prepared using different extender combinations and tested for dry adhesion and elastic properties. This article discusses the results of the study.
Table 1 shows the properties of the different types of extenders used in this study. China clay/kaolin is a clay material with a platy structure; its chemical composition is Al2Si2O5(OH)4. Talc (Mg3Si4O10(OH)2 is hydrated magnesium silicate and exhibits useful flatting and anti-settling properties. Platy talcs improve the toughness and durability of paint films. Calcium carbonate, sometimes called chalk (CaCO3), exhibits good flatting properties.