Different types and grades of paint provide different application and resistance properties, depending upon the kinds and levels of ingredients used to create the paint. In turn, the properties of a paint determine the general quality of the coating.
Pigments, which are finely ground particles or powders that are dispersed in paints, provide color and hiding; some are used to impart bulk at relatively low cost. There are two primary categories of pigments: prime and extenders. Prime pigments provide whiteness and color, and are the main source of hiding capability. Titanium dioxide (TiO2), the predominant white pigment, provides exceptional whiteness by scattering light; provides whiteness and hiding in flat or glossy paint, whether wet, dry, or rewetted; is relatively expensive; requires the use of an appropriate extender to ensure proper spacing of particles to avoid crowding and loss of hiding; and has more chalking tendency in exterior paints than most color and extender pigments. In contrast, color pigments - either organic or inorganic - provide color by selective absorption of light. Color pigments are compounded into liquid dispersions called colorants, which are added at the point of sale to tint bases, and to white paints designed for tinting. In the factory, color pigments are used as dry powders and in liquid colorant form to make pre-packaged color paints. Extender pigments (also known as “extenders”) provide bulk at relatively low cost. They add much less hiding than TiO2, and impact many properties, including sheen, scrub resistance, exterior color retention, and others. Commonly used extenders include clay, silica and silicates, diatomaceous silica, calcium carbonate, talc, and zinc oxide.
The binder provides adhesion, integrity, and toughness to the dry paint film by binding the pigment together. The binder affects application properties like flow, leveling and film build, and gloss development. Oil-based binders generally refer to both oil and alkyd coatings. Some coatings, particularly exterior primers, are made with combinations of oils and alkyds to achieve appropriate flexibility. Latex-based binders are generally found in most water-based (or latex) paints. Two types of latex binders are most commonly used in North America: 100-percent acrylic and vinyl acrylic (also called PVA, for polyvinyl acetate).
The liquid portion of the paint (also referred to as the “carrier”) provides desired consistency and makes it possible to apply the pigment and binder to the surface being painted. For most oil-based and alkyd paints, the liquid component is paint thinner; the liquid in shellac-based primers/varnishes is denatured alcohol; lacquer thinner is used for clear and pigmented lacquers; and water is primarily used in latex paints.
Pigments and the binder are what are left on a surface when the paint dries and the liquid portion evaporates. Together they are called the solids portion of the paint (pigments + binder = solids). The coating (i.e. paint, stain, primer) consists of the solids and the liquid (solids + liquid = coating). A paint with higher solids content will provide a thicker dry paint film for a given square-footage per gallon, which results in better hiding and durability, compared to a paint with lower solids content.
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Material was excerpted, with permission, from the website of The Rohm and Haas Paint Quality Institute (www.paintquality.com).