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Understanding calcium carbonate and dolomite as extenders
Author: Harshil Kansara, technical director, Victoria International.
If the image of coating extenders were on par with the price to performance properties as well as durability – extender manufacturers and suppliers would be highly pleased. Unfortunately, this is not always the case. Some paint chemists have reservations on the selection of extenders for paints.
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xtenders may vary in characteristics such as reactivity, sensitivity and solubility in water, as well as ph of their aqueous mediums. Reactivity may be due in part to origin and processing methods, but some is inherent in the particular extender itself an example, calcium carbonate is more readily affected by acidic conditions. The reason may lie in the knowledge of their chemical and physical characteristics. Further extenders may be used in significantly high quantities in paints relative to pigment, binder and additive levels. Paint chemists must recognise the necessity of evaluating these extenders carefully or work closely with the manufacturers and suppliers in open and effective discussions. In this article the authors examine the use of calcium carbonate vs dolomite in a typical universal matt acrylic paint and textured coating.
Three grades were evaluated: 5microns, 10microns and 45microns (textured). The main purpose is to highlight the individual physical and chemical characteristics of calcium carbonates and dolomite (naturally occurring grades), in terms of their mineralogy, crystal form, presence of impurities, and degree of refinement and processing, mainly for the paint chemist to make specific selections to achieve certain end results in their paints and coatings. EXTENDERS The range of naturally occurring white minerals and synthetic white inorganic pigments that are relatively colourless and transparent in organic binder films are referred to as extenders. Extenders are paint additives that are insoluble in the binder and solvents of formula and have little or no opacity or colour effect on the film. They are added to modify the flow and mechanical properties of
Calcium carbonate
the paint as well as the permeability, gloss and levelling characteristics of the paint film. Extenders are generally used in paints to give or enhance certain properties. They have extensive application, the major areas of use being as fillers and sanding aids in putties. It is used as filling compounds, primers and undercoats as a flatting agent to give inter-coat adhesion. It has been used as viscosity modifier in matt and semi-gloss paint, as well as a surface modifier in texture paint. Higher micron size such as calcium carbonate 45 or dolomite 45 are used to give the texture effect. There are several types of extenders available, main extenders used in the coatings industry for various properties are: calcium carbonates, dolomites, barytes, kaolin clay, talc, mica, silica, wollastonite (calcium metasilicate), perlite and vermiculite.
Dolomite
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