10

PAINTING MATERIALS

lose acetate lacquers to blush in humid weather because of the rapid evaporation
of such solvents as acetone and methyl acetate, but it is now possible, by proper
choice of high-boiling solvents, to prepare lacquers with a high degree of blush
resistance.
Most cellulose nitrate plasticizers are incompatible with cellulose acetate and
it is hard to prepare a satisfactory plasticizer. Moreover, few natural resins are
compatible with it and that has prevented the development of a cellulose acetate
lacquer with good adhesion. Recently, however, it has been found that some of
the aikyd synthetic resins (glycol phthalate) may be used with cellulose acetate
in the combined role of resin and plasticizer. Cellulose acetate is superior to
cellulose nitrate in that it does not yellow or become so much degraded in sun-
light. It is chemically more stable and, also, the solid cellulose acetate is nearly
non-inflammable, in contrast with cellulose nitrate. Hill and Weber have recently
made a study of the comparative stability of cellulose nitrate and cellulose acetate
motion picture films. From their oven-aging tests they found that cellulose ace-
tate retains its flexibility and weight much better than cellulose nitrate. On
artificial aging, cellulose acetate remains neutral but cellulose nitrate increases
greatly in acidity. They conclude that a cellulose acetate film appears to be a
stable substance.
As an impregnating material, cellulose acetate has little value because solu-
tions are too viscous and have too low solids content. Advantage may be taken,
however, of this high viscosity in impregnating and stiffening old fabrics, because
the cellulose acetate does not appreciably penetrate and darken. This is important
in the conservation of old textiles. Plenderleith (p. 12) suggests that a I per cent
solution of cellulose acetate in acetone, applied in several coats, ba used for
strengthening brittle fabrics. He also recommends it (p. 19) as a cement for re-
pairing old ivories. A film of cellulose acetate may be used in place of glue for the
sizing of artists' canvas. It has long been used as a 'dope' for aeroplane wing
fabrics. Care must be taken in the application of cellulose acetate—and, for that
matter, of almost any cellulose ester coating—not to have solutions that are too
thick or viscous, especially on smooth surfaces. Such coatings have poor adher-
ence and are liable to peel. It has been observed that films of cellulose acetate
applied as a thick lacquer to a smooth paper base can be stripped as intact films
from the paper without any difficulty. The incorporation of synthetic resins with
the film helps to alleviate this shortcoming.
CelMose Coatings (see also Cellulose Acetate and Cellulose Nitrate). Several
plastic and coating materials are derived from cellulose which is the principal
carbohydrate constituent of many woody plants and vegetable fibres. Cotton
fibre and delignified wood are the most important raw materials for the produc-
tion of these derivatives, many of which are esters. The cellulose coating materials
are colloidal in nature. They may be dispersed in organic solvents and in this way
used as lacquers. Wilson says (p. 11); * For practical purposes cellulose may be