Wednesday, January 6, 2010

Varnishes

A varnish is an unpigmented colloidal dispersion or solution of synthetic and/or natural resins to oils and/or thinners used as a protective and/or decorative coating for various surface and which dries by evaportion, oxidation, and polymerization of portions of its constituents. Not being pigmented varnishes are less resistant to damage by light than paints, enamels, and pigmented lacquers. They furnish, however, a transparant film, which accentuates the texture of the surface coated. Varnishes are frequently oleoreinous; there are two minor classes, spirit varnishes and japans. Oleoresinous varnishes are solutions of one more natural or synthetic resins in a drying oil and a volatile solvent.

The oil reduces the natural brittleness of the pure resin film. Spirit varnishes are solutions of resins, but the solvent is completely volatile and nonfilm forming. Oleoresinous varnishes were formaly of major importance, but alkyd and urethane varnishes have largely replaced them because of greater durability, less yellowing, ease of application, and beauty. Pressure to reduce the amount of air-polutting solvent in varnishes and paints, coupled with the desire for water cleanup of tools and spills, has led to the development of water thinned varnishes.

Spirit resin varnishes are solution of resins in volatile solvents only, such as methanol, alcohol, hydrocarbons, ketones, and the like. Spirit varnishes dry most rapidly but are likely to be brittle and eventually crack and peel off unless suitable plasticizers are added. The preparation of these products involves active stirring and sometimes heating, to bring about the desired solution. An important example of a spirit varnish is shellac or a solution of the resin shellac in methanol or alcohol. Japans are rarely used now. They are opaque varnishes to which asphalt or some similar material has been added for color and luster. They may be subdivided into baking, semibaking, and air-drying japans, according to their method of application.

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