Varnishes

(also lacquers), solutions of film-forming materials in organic solvents that, upon application of a thin layer to a metal, wood, or other surface, form hard, lustrous, transparent films that are firmly held on the surface by forces of adhesion. Such coatings are intended to protect objects from the destructive effects of atmospheric agents and other aggressive mediums, and also for decorative finishing of surfaces. Some varnishes are used as electrical insulation materials. The largest quantities of varnishes are used as a basis for the production of pigmented paint and varnish materials, such as enamel paints, primer coats, and sealers.

Varnishes have been known since ancient times. As early as the second millennium B.C. they were prepared in China from the sap of the lacquer tree. Several centuries before the Common Era, varnishes were prepared in Egypt from natural resins, such as amber and gum mastic. In the eighth century A.D., varnishes based on the natural drying oils, such as linseed, hemp, pop-pyseed, and nut oil, were well known. Until the 1930’s, natural resins and vegetable oils remained the basic film-forming materials for the preparation of varnishes. A gradual transition to production of varnishes based on cellulose esters began in the 1920’s, and the use of synthetic products (alkyd resins and phenol-aldehyde resins) in the manufacture of varnishes began in the 1930’s.

Varnishes are produced by dissolving film-forming materials in organic solvents, such as alcohol, white spirit, ethyl acetate, and xylene. A distinction is made among oil varnishes, alkyd varnishes, polyurethane varnishes, and epoxy varnishes, depending on the type of film-forming material (see Table 1, p. 785). Most varnishes are colorless. Soluble organic dyes, which make possible retention of the transparency of film, are used when the production of colored varnishes are necessary. Black varnishes may be produced from bitumens. Plasticizers, such as dibutyl phtalate or nondrying vegetable oils, are added to varnishes to increase the elasticity (bending strength) of the films.

Varnishes are applied to surfaces by spraying, dipping, pouring, and other methods. In the simplest case, the drying of varnishes consists in the volatilization of the solvent and the formation of reversible (fusible and soluble) films. The advantage of such films is their rapid drying at room temperature. In many cases the volatilization of the solvent is accompanied by profound chemical transformations of the film-forming material (polymerization or polycondensation), leading to the formation of irreversible (insoluble) films, which surpass reversible films in terms of resistance to high temperatures and the action of aggressive mediums. In some cases, hardeners are added to produce irreversible films. The drying of varnishes based on oil-containing film-forming materials may be accelerated by the introduction of driers, as well as by drying at high temperatures.

Varnishes containing reactive solvents are becoming increasingly widely used. Examples of such varnishes are polyester varnishes, which are solutions of unsaturated polyesters in styrene or some other solvent that is capable of copolymerization with the film-forming material during drying, to give varnish films up to 300 microns (μ) thick in a single application (varnishes without reactive solvents yield films only 10–20 u. thick in a single application).

The properties of varnishes are characterized by the viscosity index, solid residue content, spreading on the surface, and drying rate. The most important characteristics of varnish coatings are the adhesion to the substrate (see Table 1), hardness, impact and bending strength, resistance to air and light (light stabilizers are added to varnishes to increase resistance to light), electrical insulation properties, and resistance to moisture, gasoline, and mineral oils.

Varnishes are used in all areas of the national economy and everyday life. Alkyd varnishes are produced in the greatest quantity both in the USSR and abroad; this results from the availability and relatively low cost of the initial materials, as well as from the good technical properties of the varnishes. Varnishes based on epoxy resins, polyurethanes, polyacrylates, and silicones are also becoming increasingly important.

Use in art Various types of varnishes are used in decorative and applied art; in engraving, to coat metal plates prior to etching; and in painting, to form a protective layer on the painted surface, to impart a uniform gloss to oil paints (coating varnishes), and to eliminate dulling in oil painting (retouching varnishes). Varnishes for painting are prepared by the solution of pure, preferably colorless natural resins (dammar, mastic, and sandarac) in vegetable drying oils, alcohol, turpentine, or other solvents.