74 PAINTING MATERIALS
that of pines, the larch balsam is secreted in the heart of the tree. Therefore, for
tapping, a hole must be bored deep into the tree near the base. There are many
old recipes naming Venice turpentine as a varnish ingredient. De Mayerne, XVII
century (Berger, IV, 185, 259, and 269), gives several which call for Venice tur-
pentine. In one, admixture with sandarac resin is mentioned. Doerner (p. 125)
recommends it as a *. . . non-yellowing painting medium where an enamel-like
effect of the colors is desirable/ It must be sparingly used, however, especially with
dark, slow-drying colors, or it will dry poorly and give an unpleasant gloss. It is
not recommended as a finishing varnish.
Vinyl Acetate (see Vinyl Resins).
Vinyl Ester (see Vinyl Resins).
Vinyl Resins (see also Synthetic Resins). The so-called vinyl resins may be
considered as the polymerized derivatives of vinyl alcohol, CH2 = CHOH. Al-
though polymerized vinyl alcohol has been produced commercially and is a
water-soluble, film-forming substance, it is not so important as the polyvinyl
chlorides or acetates, particularly the latter. Although the vinyl halides have
been known for about one hundred years, it is said that the acetate was first dis-
covered by Klatte in 1912 (German patent, no. 281,687). Since about 1930 the
vinyl resins have acquired a considerable industrial importance because certain
ones can produce protective coatings which are colorless when applied and which
are free from after-yellowing. It is claimed (Ellis, Chemistry of Synthetic Resins,
p. 1025) that coatings of the acetates and chloracetates are transparent to ultra-
violet light. Their acid number is usually rated as zero. They are used in the
preparation of thermoplastic molding compounds as well as in coatings. Vinyl
acetate is synthesized from acetylene and acetic acid; some mercury salt (as
mercurous sulphate) is employed as a catalyst. The monomer is a mobile, color-
less, ethereal-smelling liquid, which boils at 73° C. Polymers of varying viscosity
can be made, depending upon the concentration of the catalyst. Polyvinyl acetate
now comes on the market in coarse, granular form (shavings). Since it has a high
cold-flow it is difficult, in warm weather, to keep it from caking; hence, it should
be packed in small containers, and stored at as low a temperature as possible.
The vinyl acetate resin is a colorless solid, horny, and a little rubbery, but not
brittle. It is soluble in alcohols, ketones, esters, and chlorinated and aromatic
hydrocarbons, but is insoluble in aliphatic hydrocarbons (petroleum naphthas)
and water. It softens at 30° to 40° C.; it has outstanding light and heat stability.
According to Curme and Douglas (p. 1124), it has a relatively high water absorp-
tion—3 to 5 per cent in 16 hours at 60° C. Coatings may be made by dissolving
the solid vinyl acetate resin in various combinations of organic solvents mentioned
above^Since the vinyl polymer resins have a high solubility in some of the sol-
vents, it is possible to prepare lacquers that have a high solids content. Although
polyvinyl acetate resin is insoluble in water, it does swell when immersed in it
and becomes leathery and pliable when a large amount of water has been ab-