46

PAINTING MATERIALS

great extent the fineness of the pigment. The oil absorption factor being relative
to the surface conditions of the pigment is independent of its chemical composi-
tion or specific gravity/ Williamson observes: ' The fact that no simple relation-
ship exists between oil absorption and consistency is explained as follows: Two
factors which act in opposition to each other—i.e., degree of wetting and soap
formation—control the values obtained from oil-absorption measurements,
whereas the consistency of the ground pastes is controlled by soap formation
alone.' By empirical methods Wolff and his collaborators have derived a mathe-
matical expression for the consistency of a paint as a function of pigment con-
centration, and from this they calculate the critical oil content of paints. They
have found that this critical point indicates the pigment-fixed binder ratio which
yields the optimum paint properties from the point of view of drying, water
resistance, etc. Elm ('Fundamental Studies of Paints') has studied experiment-
ally the relationship which exists between the critical point of a paint as deter-
mined according to Wolffs method and the durability of the same system on
exterior exposure. Results showed that the critical point falls within the range
of pigment concentrations yielding good durability, and the conclusion that paint
durability is a function of paint consistency seems justified. There are other
factors besides pigment-vehicle ratio which affect the durability of a paint.
J. Schmidt says that for each pigment there is an optimum acid value for the
oil used in grinding and that this affects working quality and permanence.
Oils, yellowing. All fatty oils (see Oils and Fats) have a tendency to yellow
with time; darkness and dampness increase this tendency and it is also acceler-
ated by certain pigments. It varies with different oils and somewhat, also, with
the particular sample and. the treatment it has received. Poppy and walnut oil
have, in general, less tendency to yellow than has linseed oil. Cold-pressed linseed
oil yellows more than oil that has been thickened in the sun, but stand oil (see
Polymerized Oil) is superior to both. Impurities may tend to increase yellowing,
but complete removal of them does not remove the tendency, for pure, synthetic
trilinolenic glyceride turns color badly. Furthermore, the yellowing of an oil is
independent of the free acid content. Various vegetable oils turn yellow, to some
extent, according to degree of unsaturation. Other things being equal, the least
unsaturated oil, that is, the oil with the lowest iodine number, has the best color
retention. Polymerization of oils reduces their iodine numbers and makes them
less susceptible to yellowing. This is especially true if they are heated in an inert
atmosphere, such as nitrogen or carbon dioxide, so as to prevent the formation
of oxidation products. The nature of the chemical change that causes yellowing
is not known. It is well known, however, that drying oils yellow more readily
in the dark than when exposed to light. In fact, films that have become badly
yellowed after storage in the dark may be partially bleached when exposed to
the light. Werthan, Elm, and Wien (p. 775) found that white linseed oil paints
yellowed more in red light than in blue light and, hence, wave-length of light
seems to be a factor.