MEDIUMS AND ADHESIVES 41
is better explained on the basis of a three-dimensional polymerization than by
the simple splitting off of fatty acids.
The oxidized product from linseed oil is known as linoxyn, and this refers more
often than not to the final material obtained. That must be looked upon as
a conglomerate of unsaturated glycerides which have partially or completely
undergone addition of a molecule of oxygen at each double bond, the un-
saturated glycerides which have not been attacked, any saturated glycerides
originally present in the oil, and perhaps a small amount of other compounds
formed by slight decomposition and further oxidation of the first oxygen-
addition product. Oxidized linseed oil, or linoxyn, has certain physico-chemical
resemblances to the typical colloid gelatin. For example, the viscosity relationships
of linoxyn and of gelatin are qualitatively similar and both materials swell when
treated with suitable liquids, water in the case of gelatin and hydrocarbon solvents
in the case of linoxyn. It is supposed that linoxyn consists physically of a solid
lattice-work of oxidized glyceride molecules enclosing the liquid (unchanged)
glycerides, the whole forming a perfectly homogeneous, solid jelly. Most drying
oils other than linseed oil do not give so satisfactory a film on exposure to air;
linseed oil itself, if intensively oxidized, becomes thick and finally crumbles into
soft fragments. In these cases, separation of the colloidal solid phase from the
homogeneous medium has taken place, and, instead of a clear, solid film or jelly,
a more or less coagulated and heterogeneous system of colloidal solid interspersed
with clear jelly is produced.
The utility of the oxidized products from various fatty oils differs widely and
bears little relationship to the original state of unsaturation of the oil. Linseed oil
is the most suitable vehicle for paints; perilla oil is more unsaturated but gives an
irregular film; tung oil is equally unsaturated but has more tendency to separate in
the heterogeneous phase, so that the films produced are frequently dull or mat.
Soya bean oil, safflower oil, sunflower-seed oil, and other oils of fairly high iodine
numbers and pronounced drying properties do not yield such satisfactory films as
linseed oil; the products tend to be softer and more gummy. Fish oils oxidize very
readily but the products also tend to be gummy. These variations are doubtless
conditioned to a large extent by the general type of unsaturation present. Thus,
in oils of the poppy-seed class, there is little linolenic acid present, but there is an
abundance of linoleic acid; tung oil contains a linoleic acid (eleostearic acid)
structurally different from the ordinary variety; the fish oils contain only traces
of linolenic acid, as a rule, but do contain fair quantities of oleic and linoleic acids
and marked amounts of unsaturated acids with twenty and twenty-two carbon
atoms and the equivalent of four or five ethylenic linkages. On the other hand,
perilla oil contains more linolenic acid than linseed oil and yet does not give so
satisfactory a product. The most abundant constituent of linseed oil is believed
to be dilinoleo-linolenin (Hilditch, pp. 388-396):