MEDIUMS AND ADHESIVES 75
sorbed. The surface, however, is unaffected after drying. The dried film is quite
permeable to water vapor. The water absorption (A.S.T.M.) is given as 2 per
cent in 144 hours. A noticeable property of polymerized vinyl acetate resin is
its low refractive index, 1.4665. This property makes it valuable as a fixative for
granular surfaces, since it does not cause so much darkening as do films with
higher refractive index. Polymerized vinyl acetate can now be obtained in
different viscosities. Where penetration is desired, a low-viscosity resin should
be used. Since films of vinyl acetate are naturally quite flexible, it is not necessary
to incorporate plasticizers except for special purposes. If necessary, any of the
common plasticizers, like the phthalates, abietates, or glycollates can be used;
in general, much less plasticizer is required to impart a definite degree of flexi-
bility to these films than is required for cellulose nitrate. Vinyl acetate polymers
have conspicuous adhesive properties when used with a wide variety of materials
—cloth, paper, porcelain, metal, stone, leather, and wood.
Polyvinyl chloride is an odorless, chemically inert, and difficultly thermo-
plastic, synthetic resin. It has a more limited solubility than do the polyvinyl
acetates. It is partially soluble in acetone, but is insoluble in most ketones, is
resistant to acids, alcohols, and water, and is soluble in dioxan, ethylene dichloride,
and chlorobenzene. The refractive index at 20° C. is 1.544. The heat and light
stability of this resin is poor (see Curme and Douglas, p. 1124). It is brittle;
hence, when used industrially, it is always plasticized.
A synthetic resin that is in many respects superior to straight polyvinyl ace-
tate or polyvinyl chloride is one that is produced by the co-polymerization of the
two. This is not just a physical mixture but is a product formed by simultaneous
polymerization of the monomers. Those now in commercial production contain
65 to 80 per cent vinyl chloride. The polyvinyl chloride in the co-polymer may be
regarded as being internally plasticized by polyvinyl acetate. Curme and Douglas
(!oc. cit.) have described many of the properties and uses of these vinyl co-
polymers. They are colorless, tasteless, odorless, non-toxic, and permanently
thermoplastic compounds. Their chief advantage over the polyvinyl acetates lies
in the fact that they are not swelled by water. Moreover, they are stable to
alcohol and to petroleum naphthas. They are soluble only in ketones and related
compounds, esters, and chlorinated compounds, but they only swell and dissolve
slightly in aromatic hydrocarbons. Heat and light stability are not quite so good
as in straight polyvinyl acetate, but are much better than in the chloride. The
vinyl acetate-chloride co-polymer darkens on prolonged exposure to heat or
direct sunlight, but may be improved by the use of small amounts, usually i or
2 per cent, of stabilizers such as the commonly used lead pigments or lead stearate,
lead oleate, or slaked lime. The co-polymers are marketed as white, fluffy powders
which may be dissolved in mixtures of the solvents mentioned above, and the
solutions can be diluted with cheap aromatic hydrocarbons like toluene, The
refractive index of the vinyl co-polymer is 1.53.