I2 PAINTING MATERIALS
with the esters and ketones. In recent years the glycol ethers have become impor-
tant cellulose nitrate solvents. Solutions of pyroxylin in simple solvent mixtures
do not make very good surface coatings. Well compounded cellulose nitrate
lacquers are complex in composition. Pure cellulose nitrate solution, like pharma-
ceutical collodion, dries out to a brittle film which shrinks as it hardens. For this
reason it is necessary to incorporate with the solution liquid or plastic materials
which are retained in the film and keep it flexible. Camphor and castor oil have
long been used with cellulose nitrate. The former, however, is readily lost from
the film since its vapor pressure (for this purpose) is high. Castor oil has a ten-
dency to develop rancidity and an unpleasant odor on standing, and it makes the
film too soft if used in slight excess. In recent years synthetic plasticizers, like the
triphenyl or tricresyl phosphates or dibutyl phthalate, have come into favor.
In addition to plasticizers, nearly all pyroxylin lacquers contain certain
amounts of resin, either natural or synthetic. Resins increase the body of the
film, enhance the gloss (where this is desirable), and improve the adhesion, par-
ticularly to metal and to glass. Dewaxed dammar is used where a pale lacquer
is required. Shellac, copal, elemi, mastic, sandarac, the phenolic and the vinyl
resins, and others are compatible with cellulose nitrate. Besides the solvents used
for taking the cellulose nitrate into solution, it is usually necessary to add small
quantities of solvents which have a higher boiling point. Such solvents are known
as ' blush resistants.' If the main solvent or solvents evaporate too rapidly, they
may chill the surface to which a lacquer is applied and cause water to condense
in the film; this, in turn, causes the film to turn white (blush or bloom). Small
amounts of such solvents as diacetone alcohol, the glycol ethers, and the lactates
are commonly used for this purpose. These high-boiling solvents also improve the
brushing and spraying qualities.
Cellulose nitrate has two main shortcomings. In the first place, it is not stable
to light, particularly strong sunlight. Devore, Pfund, and Cofman say (p. 1836):
The action of sunlight or ultra-violet light on an unpigmented nitrocellulose film is
accompanied by a variety of phenomena in addition to the gaseous decomposition.
The film becomes acid, its brittleness increases, its tensile strength decreases, and
after prolonged exposure the film becomes yellow. The viscosity of a solution pre-
pared by redissolving an irradiated film is lower than that of the solution from which
the film was cast.
In their experimental work they found that there is a sharp peak in the curve
indicating a strong maximum of decomposition per unit energy in the region
represented by lines near 3130 A. Gloor found that sunlight not only subjects a film
of nitrocellulose lacquer to stresses incidental to normal temperature change, but
that it also promotes photochemical changes in the film itself. His data indicate
that the principal effect of ultra-violet light is a pronounced local denotation and
degradation, while the effect of heat is the same but more general. The second