Article Outline
Paint and Varnish, liquids that solidify when exposed to air, and are used to cover surfaces for decorative and protective purposes. Paints are formed by mixing a pigment (the substance that provides color) and a binder, a fluid vehicle, such as linseed oil, that solidifies when exposed to air. A varnish is a transparent solution that solidifies into a protective coating. Opaque and colored varnishes are called lacquers.
The first uses of paint were entirely decorative. Thus, paint without a binder, consisting of iron oxide, was used for cave paintings about the 15th millennium bc. In Asia, several pigments made from ores, prepared mixtures, and organic compounds were known about 6000 bc. Indigo, a pigment extracted from the indigo plant, was known to the ancient Egyptians, Greeks, Romans, and Inca. Gum arabic, egg white, gelatin, and beeswax were the first vehicles used for these pigments. Lacquers were used to paint buildings in China about the 2nd century bc; in Europe, protective painting began about the 12th century ad. Linseed oil, although known as a paint vehicle by the Romans, appeared in artistic painting only in the 15th century. White lead, a white pigment, became widely used in the 17th century, and paint consisting of prepared mixtures of pigments and vehicles first became commercially available in the 19th century.
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Chemical Composition of Paints
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A modern paint formulation consists of several different categories of chemical compounds. The vehicle forms the adherent, skinlike coating; the pigment is dispersed in the vehicle and gives the final film its color and hiding power; and the solvent, or thinner, evaporates shortly after the coating has been laid. The vehicle can be an unsaturated, or drying, oil, which is an ester formed from the reaction of a long-chain carboxylic acid, such as linoleic acid, with a viscous alcohol, such as glycerine; or it can be a polymer. A filler, containing powdered components such as kaolin (see Clay) or barium sulfate, enhances the strength of the dried film of paint.
The molecular structure of a typical drying oil, linseed oil, is as follows:
If this substance is exposed to the oxygen in the air, the unsaturated ends on the hydrocarbon chain, shown above at the locations of the double bond, 8CH9CH8, are attacked, and an oxide, or ether, is formed, thereby cross-linking one molecule to another, to yield a tough, insoluble macromolecule with high molecular weight, as follows:
The drying oil, therefore, is a monomer when it is in the can and becomes a polymer after being applied to an exposed surface.
If the vehicle is a synthetic polymer, it is dispersed in a suitable solvent, so that as the solvent evaporates the individual macromolecules come into contact and become enmeshed with one another. The solidification is improved by the presence in the solvent of a polymerization catalyst, called a drier. The types of synthetic polymers most widely employed as paint vehicles are alkyd resins, which are polyesters of a polyhydric alcohol, such as glycerol, with a polybasic acid, such as phthalic acid, C6H4(COOH)2; nitrocellulose, in which cellulose is depolymerized, the small molecules are nitrated, and the substance is then repolymerized (see Rayon); phenolic resins; acrylic resins; epoxy resins; polyvinyl acetate resins; and polyurethanes.
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