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.
