MEDIUMS AND ADHESIVES n
considered as a complex alcohol, with three hydroxyl groups for each unit mote"-
cule. These alcoholic radicals can be esterified by acids and the acetic and nitric
acid esters have tremendous importance in industry.' These cellulosic coating
materials can not properly be called * synthetic-resins/ although they may be
used for lacquers and molding compounds in a similar way. The raw, modified
cellulose materials are, for the most part, light-colored or white, powdery or flaky
materials that do not have a resinous lustre or fracture. Moreover, they are
natural products prepared by dissimilar chemical processes. Cellulose acetate and
cellulose nitrate are frequently classified as * plastics/
In recent years there have been developed some new cellulose materials which
are similar to cellulose acetate and which are said to be superior in many respects.
Among these is cellulose acetobutyrate, which is more highly miscible with resins
and plasticizers than is cellulose acetate. Lacquers can be made from it which
are tough, flexible, and resistant even to out-of-door weathering. Cellulose aceto-
butyrate is a white, flaky material; it gives a colorless film which transmits all
visible and ultra-violet light in the solar spectrum and does not yellow or discolor.
The refractive index of the pure film, is 1.47 at 25° C. Another derivative is ethyl
cellulose, a cellulose ether, which is softer and more extensible than the cellulose
esters and, hence, requires little or no plasticizer. Benzyl cellulose is another
cellulose ester, suitable for lacquer formulation.
Cellulose Nitrate (see also Cellulose Coatings and Cellulose Acetate). Cellu-
lose nitrate, also known as gun cotton or pyroxylin, has been known for nearly
one hundred years. (Wilson [p. 11] says that the cellulose nitrates are broadly
and incorrectly termed ' nitrocelluloses.') It was not until after 1920, however,
that its manufacture became important through the demands of the wood- and
metal-finishing industries. It is made by treating cotton linters or high-grade
tissue paper with a mixture of concentrated sulphuric and nitric acids, which is
partially diluted with water. Dry cellulose nitrate is a voluminous, white or
faintly yellow solid which is readily flammable and deflagrates if brought near
a naked flame. For shipping purposes it is usually moistened with alcohol or
some other organic liquid. It is sold on the basis of its viscosity in standard
. solution; for example, a specification that the cellulose nitrate is * R. S. one half
second cotton * indicates that when it is made up in a standard solution (regular
solvents), one half second is the time required for a standard steel ball to fall
through ten inches of the solution contained in a one-inch-diameter, vertical
column at 25° C. (A. S. T. M. method). One half second cotton is used extensively
for preparing lacquers, but cellulose nitrate is prepared commercially with a vis-
cosity as high as 200 seconds.
The best solvents for cellulose nitrate are the organic esters, ethyl acetate,
butyl acetate, amyl acetate, and ketones, like acetone and diacetone alcohol.
Paraffin hydrocarbons, coal-tar hydrocarbons, and even the lower alcohols have
little or no solvent effect, although these solvents may be used as diluents along