jg PAINTING MATERIALS

Saponified Oils) one gram of the fatty material; and the acid number is the number
of milligrams of caustic alkali required exactly to neutralize the free acidity of
i gram of the material. The meaning of the iodine number is explained above.

Oils, drying process. Drying oils (see Oils and Fats) are suitable vehicles for
pigments and for mixture with resins because of two distinct processes which
occur in them when they are exposed to heat or to the action of atmospheric
oxygen: (i) A thickening occurs, and, in certain cases, the oil becomes a jelly
(gelation). This is undoubtedly from some kind of association or polymerization
of the molecules, and takes place when the oil is heated for a time at a temperature
of about 250° C. (See Polymerized Oil.) (2) When a drying oil is exposed to air
it becomes a solid, rubbery mass or, if exposed in thin layers, it becomes a clear,
hard solid. There are two principal processes recognized in this * drying' of an
oil, namely, oxidation and subsequent polymerization of the oxidation product
to form aggregates of high molecular weight. The oxidation is effected by the
addition of oxygen to the unsaturated glycerides without any great amount of
molecular disruption. The ' drying * of oils is imperfectly understood. For linseed
oil, there exists the largest amount of experimental data, but the process for all
* drying * oils is essentially the same.

When exposed to air, linseed oil absorbs oxygen, first slowly, then, for a time,
more rapidly. After that, the rate progressively diminishes as the process nears
completion. At ordinary temperatures the period of induction is from one to three
days, and the process is complete in about twenty to thirty days; at 100° C. the
whole process occupies only six or seven hours and the induction period is less
than half an hour. If a small percentage (about o.i to 0.3 per cent as metal) of
certain salts such as lead, manganese, or cobalt linoleates or resinates (abietates)
are present in the linseed oil, the oxidation period is greatly shortened; this is
mainly because the induction period is eliminated and oxygen absorption sets in
immediately at its maximum rate. The weight increase of the oil film on * drying *
is from oxygen absorption less the diminution resulting from the escape of volatile
products of decomposition. The true oxygen absorption is about 28.7 per cent
of the original weight of the oil and the primary process is one of addition of a
molecule of oxygen at each ethylenic linkage or double bond. In the case of trilin-
olein, the first product of the action is apparently a substance of the formula,

,0-0, ,0-0,

Trilinolenin yields a similar triperoxidic compound:
,0-0, ,0-0, ,0-0,
(CHa-CH8-CH-CH-CHrCH.CH.CHi!-CH-CH-(CH2)7COO)JCa8.
These compounds may be further oxidized and break down into carbon dioxide,
acetic acid, acrolein, and non-volatile oxidation products of lower molecular