MEDIUMS AND ADHESIVES 49
certain cases (as in tung oil and safflower oil) the oil is converted into a gelatinous
or rubber-like material. The effect of heat is thus to diminish the iodine number
(unsaturation) and to increase the viscosity, but in the absence of air there is no
oxidation. Typical figures for various commercial polymerized oils are given by
Leeds as follows (see Hilditch, p. 400):
PER CENT SPECIFIC SAPONIFICA-
LOSS ON GRAVITY IODINE TION
THICKENING 15° C. NUMBER NUMBER
Raw oil — 0.9321 169 194.8
Thin oil 3 0.9661 100 196,9
Middle oil 6 0.9721 91 197.5
Strong oil 12 0.9741 86 190.9
Such polymerized oils are often known as * lithographic varnishes.' Not much is
known of the nature of the constituents of the polymerized oils. There is a little
loss of volatile products of decomposition but the chemical structure remains
essentially the same. During the heating polymerization takes place—molecules
of linseed oil unite to form larger molecules, and these larger molecules are less
liable to the chemical changes that produce yellowing, cracking, and disintegra-
tion. Such oils dry more slowly but the resulting film is much more durable than
the film of raw or boiled or blown oil, and white lead ground in stand oil yellows
very slightly. From the physical standpoint, mainly in view of the observed
viscosity relationships, it is now thought that, like the oxidation product (linoxyn),
the fully polymerized glyceride must be a solid, colloidal structure and that the
thickened or stand oils are systems in which the colloidal polymerides are dis-
persed in the unchanged portion of the fatty oil.
Polymerized Resins (see Synthetic Resins and Polymerized Oils). Polymerized
resins are synthetic resins that are formed from simple, monomeric compounds
by the chemical process of polymerization. Bender, Wakefield, and Hoffman say
(p. 125): * Polymerization is the term we use to denote change of a substance
without loss or gain of material, but generally with a transfer of energy to a less
fusible, less chemically active form of higher average molecular weight.* The
vinyl, acrylic, and styrene artificial resins are formed by this process. An example
will serve to explain it. The unsaturated, organic compound, styrene, has the
simple formula, CH = CH^. Styrene is a clear, sweet-smelling liquid that boils at
CeH5
146° C. and has a molecular weight of 104. In this form it is known as thee mono-
mer/ Under suitable conditions, however, as when warmed or when a catalyst is
added, or even when allowed to stand for a long time, the monomeric units com-
bine to form solid polystyrene which may consist of thousands of monomeric
units and have a molecular weight estimated to be in the hundreds of thousands
(Ellis, * Tailoring the Long Molecule,' p. 1135). These polymeric molecules are