68 PAINTING MATERIALS
with natural resin and wax coatings. Recent studies of moisture permeability of
coatings usually have been limited to coatings for special purposes, or to the
effect of adding various resins or plasticizers to certain types of coating bases.
The methods of measuring permeability of industrial products have not been
standardized, and so it is impossible to compare the values obtained. Natural
and synthetic resins, when added to cellulosic coatings, usually reduce moisture
permeability, but the addition of liquid plasticizers increases it. There are many
other influences on permeability. Within limits it is held to be inversely propor-
tional to film thickness, and conditions under which films are applied and dried
no doubt affect it. Solvents retained in the film, as well as liquid plasticizers
that are hygroscopic, appreciably increase moisture permeability* The age of
a lacquer film, up to the point where it begins to break down as a result of natural
causes, would be expected to have an effect on moisture permeability. Wray
and Van Vorst have found that the moisture permeability of cellulose nitrate
and vinyl resin lacquers decreases with age. Their findings were based on measure-
ments carried out over a period of one year. Kline, who has recently made a study
(p. 236) of synthetic resin finishes for aircraft, believes that water vapor is absorbed
and transmitted through the films by a process of chemical diffusion. He found
that an aircraft finish made with a .cellulose nitrate base is very permeable to water
vapor. He also found (p. 244): * The additional protection afforded by the thin film
of wax (carnauba) against passage of moisture was found to be considerable, being
in some cases equivalent to doubling the thickness of the original film/ It seems
that the use of wax coatings may help to overcome one of the inherent disadvantages
of synthetic resin coatings, namely, a high moisture permeability. Where lacquer
films with high moisture impedance are required, the use of chlorinated rubber
coatings (see Rubber, chlorinated) may be considered.
Other than that furnished by the study of Gettens and Bigelow, it has been
impossible to find data by which the moisture permeability of a large number of
synthetic resins may be directly compared. The Hercules Powder Company, in a
booklet entitled Ethyl Cellulose^ furnishes the following comparative figures for
moisture permeability of some plastic materials. These data are in terms of a K
value which represents the grams of water permeated per hour/sq. cm./cm. thick-
ness of film by a modified Gardner method.
Cellulose acetate........................ 5.0-7.0 X io~6
Ethyl cellulose......................... 2.72 X io~*
Cellulose nitrate........................ 1.68 X io"6
Benzyl cellulose........................ 1,12 X icr6
Tornesit (chlorinated rubber).'........... 0.80 X Io1""6
In the selection of protective coatings for special purposes, it is sometimes
necessary to take into account the refractive index of the coating base. Coatings
with high index of refraction sometimes cause changes in value of colors and a