66 PAINTING MATERIALS
Surface Film (see also Resins and Synthetic Resins) is sometimes used as a
slightly more general definition than 'varnish' of the layer or layers of film
material applied over a completed painting (see also Protective Varnish).
Synthetic Resins. These are complex, amorphous, organic, semi-solids or
solids that are made by chemical reactions from a variety of raw materials.
They approximate the natural resins in many physical properties: lustre, fracture,
comparative brittleness at ordinary temperatures, insolubility in water, and
fusibility and plasticity when heated or exposed to heat and pressure. They com-
monly differ from the natural resins in chemical constitution and in their behavior
to chemical reagents (see Ellis, Synthetic Resins and Their Plastics, p. 13). Most of
the synthetic resins that have been developed for commercial purposes are pre-
pared from unsaturated, organic compounds by the chemical process known as
' polymerization' (see Polymerized Resins) or from oxygen-containing compounds
(particularly hydroxy) by condensation (Ellis, op. cit., p. ay). Some resins are
produced by the combined effect of both polymerization and condensation. In both
these processes, simple and usually liquid substances are changed into solid but
generally plastic substances that are useful for molding or for the preparation of
paints, varnishes, and lacquers. The poly vinyl resins (see Vinyl Resins), the acrylic
resins (see Acrylic Resins), the styrene resins (see Styrene Resins) and some of
the alkyd resins (see Alkyd Resins) are good examples of linear or additive
polymer resins that dissolve in organic solvents and, hence, have become com-
mercially practicable for the preparation of colorless lacquers and varnishes.
Many of the most important synthetic resins, historically and commercially,
are prepared by a chemical process known as condensation. This is similar to
polymerization, except that the primary reaction of simple molecules to form
macromolecules is accompanied by dehydration, and the condensed product is
not a multiple of the molecular weight of the simple starting substance (or
monomer). Condensations may also take place between unlike molecules. In
the formation of some resinous substances, both polymerization and condensa-
tion are supposed to take place and this process is known as * polymerization
condensation * or c multicondensation/ Although in some cases linear molecules
are supposed to be formed in the early stages of condensation, the products
usually end as three-dimensional molecules and, hence, are usually infusible
and insoluble in organic solvents* The condensation resins are of minor im-
portance in the preparation of paints, varnishes, and lacquers, but are of major
importance in the preparation of molding compounds. The most important are
made from formaldehyde and phenolic compounds. Bakelite is a typical example-
Certain of the phenolic resins are soluble in oil and can be used to make oil
varnishes directly (Ellis, op. cit,, p. 1141), but most of these products are dark in
color. Light-colored resins from the condensation of urea and formaldehyde are
of commercial importance in the preparation of molding compounds and coating
compositions*