Binders and Pigments
All paints contain two basic components, a pigment and a binder. The pigment is the substance that gives the paint its color, and the binder is the substance that produces the paint's physical finish. After the paint is applied to a surface, the binder dries and cures, forming a thin, durable film that adheres to the surface and holds the pigment in place.
In modern paints, binders are usually either a synthetic plastic, such as acrylic or latex; a natural oil; or a synthetic resin called an alkyd. The binders are carried in a fluid solvent; in the case of latex paints, the solvent is water, while alkyd paints usually use a petroleum-based solvent.
The earliest oil-based paints were developed by artists centuries ago. These early paints used natural oils such as linseed, walnut or poppy-seed oil as a binder. For artists, the advantage of oil-based paints is that their slow drying time allows the paint to be mixed and worked for a longer period than is possible with water-based paints, and this same characteristic works to the advantage of modern house painters as well. Slow drying and curing times can produce better finishes, since the paint has more time to settle and level out brush marks before it dries.
Oil-based paints also perform better than acrylic or latex paints when applied in cold temperatures. Latex paints generally won't cure properly in temperatures below 50 degrees Fahrenheit, so they're not a good choice for exterior applications in cool weather. Oil-based paints, in contrast, will cure in temperatures as low as 20 degrees. Frost and condensation can be damaging to the paint's finish, though, so painting in humid or below-freezing conditions can be problematic even with oil-based paints.
Oil-based paints are also generally more difficult to thin and clean up than latex paints. Latex paints can be thinned and cleaned up with water, but oil-based paints must be thinned and cleaned with a solvent such as mineral spirits or turpentine.
Modern oil-based house paints more usually utilize an alkyd resin, rather than a natural oil, as a binder, and the resins are carried in a petroleum-based fluid. Alkyd resins consist of a polymer and fatty acids that together form a flexible film when they cure. Alkyd paints tend to dry more quickly than natural-oil-based paints, and they are less likely to yellow with age than paints based on oils such as linseed oil.
Like the binders in natural-oil-based paints, most alkyd binders are not water-soluble, so alkyd paints must be thinned and cleaned up with mineral spirits, turpentine or other similar solvent. Alkyd paints are also considered to be hazardous substances, so old paint should only be disposed of through a household hazardous waste program or an authorized recycling center.
Some recently developed paints employ alkyd resins dissolved in a water-based carrier. These paints combine the finish and curing advantages of traditional alkyd paints with the ease of clean up and relative environmental friendliness of water-based latex paints.