VARNISHES

The Uses of VARNISHES

Varnishes are nonpigmented paints, which dry to a hard-gloss, semigloss, or flat transparent film by a process comprising evaporation of solvent, followed by oxidation and polymerization of the drying oils and resins.
The varnish is manufactured by cooking the drying oil (usually linseed oil, tung oil, or mixture of the two) and resin together to a high temperature to obtain a homogeneous solution of the proper viscosity. The varnish is then thinned with hydrocarbon solvents to application viscosity. Varnishes were widely used in the 19th and early 20th centuries as spar varnishes for use on the wooden spars of ships, furniture, and floors. The original spar varnish was a phenolic-tung oil varnish; the tung oil provides high cross-linking functionality, and the phenolic resin imparts hardness, increased moisture resistance, and exterior durability. The types of oils and resins and the ratio of oil to resins are the principal factors, which determine the properties of a varnish. The bulk of the market for these traditional types of varnishes have been almost completely replaced by a variety of other products, especially to uralkyds that provide greater abrasion and water resistance. Uralkyds are also called urethane alkyds or urethane oils. They are alkyd resins in which a diisocyanate, usually toluene diisocyanate, has fully or partly replaced the phthalic anhydride usually used in the preparation of alkyds. Uralkyds are superior in performance over alkyds or epoxy esters. These days the term varnish refers generally to the transparent coatings, even though few of them today are varnishes in the original meaning of the word.