MEDIUMS AND ADHESIVES 67
The manufacture of resins by the synthesis of macromolecules through the
processes of polymerization and condensation is not far removed from the pro-
cesses that nature uses in building up resins, drying oils, cellulose, rubber, and
proteins. Carothers (* Polymers and Polyfunctionality') has touched upon the
role of polymerization in the growth of living organisms. He says (p. 42): * Reac-
tions of polymerization appear to be uniquely adapted to the chemistry of vital
growth because they are the only reactions that are capable of indefinite structural
propagation in space.'
The synthetic resins formed by polymerization and condensation appear to
have very good chemical stability. In the process of their formation, all unsatu-
rated groups are stabilized by the formation of cross linkages. Many of them are
stable, even to strong acids, bases, and oxidizing agents. Among the cellulose
derivatives only cellulose nitrate is known to be unstable and to have poor aging
qualities (see Cellulose Nitrate). Cellulose acetate, on the other hand, appears
to have very good chemical stability (see Cellulose Acetate). The permanence
of artificial coating materials is usually considered from two points of view: first,
the tendency to turn yellow, and, secondly, the tendency to become brittle with
age. The first of these seems to be associated with ultra-violet light absorption
and the second is attended by complete loss of solvent and plasticizer. The syn-
thetic resins proper—the vinyl, alkyd, and the methacrylate resins—appear to be
inherently more plastic than the cellulose derivatives. In the technical literature
there has been found no mention of continued polymerization as a factor that
might cause the embrittlement of polymer resins.
Synthetic Resins, physical properties. The usefulness of the synthetic resins
depends upon various physical properties like hardness, clarity, transparency to
ultra-violet light, adhesiveness, refractive index, moisture permeability, and
thermal behavior. Although hardness is not a primary requisite for protective
coatings on works of art, in special cases it may be a desirable property. Kline
and Axilrod have recently made a study of the hardness of clear, synthetic
plastics in connection with their use in aircraft windshields. In their tabulated
results on scratch resistance, an acrylate resin led the list. The vinyl and styrene
resins and cellulose acetate and nitrate gave intermediate results, and were much
alike. Vicker's indentation-hardness tests placed these materials in different order.
Here a styrene resin led the list and the other synthetic resins were less easily
indented than cellulose derivatives. Synthetic resins and natural resins, like many
other substances, owe their exceptional physical properties of strength, elasticity,
hardness, and plasticity to their peculiar molecular structure.
Cellulose plastic coatings and synthetic resins, in this order, have high moisture
permeability as compared with coatings made from natural resins and waxes.
A study made by Gettens and Bigelow on the moisture permeability of an ex-
tensive list of materials that have been suggested or used for protective coatings
allows the comparison of the moisture permeability of synthetic resins and plastics