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Oct., 1944 PROPERTIES AND BEHAVIOR OF FILM 251
tation of the cellulose nitrate or acetate molecules in the base in the machine direction. When the molecules are aligned it is evidently easier to increase the distance between them (by thermal agitation or by the introduction of moisture) in a direction perpendicular to the alignment. In practice this means that changes in temperature and humidity will have a slightly greater effect on transverse pitch than on longitudinal pitch.
Permanent Dimensional Changes. — Photographic film undergoes a gradual but continuous permanent shrinkage throughout its life, at a rate which depends on the type of film and the conditions under which it is used or stored. The permanent shrinkage of a photographic film is due principally to the following causes :
(1) Loss of residual solvent or volatile material other than moisture. Shrinkage due to this cause is increased by heat and moisture and reduced by preventing free access to the air. The effect of moisture is two-fold — it accelerates diffusion of solvents from the interior of the base and also renders the emulsion more permeable. (Dry gelatin is an effective barrier to many solvents.) Shrinkage resulting from the loss of volatile material from the base is greatest in the widthwise direction because of the orientation of the cellulose nitrate or acetate molecules.
(2) Plastic flow of the base. The compressive force of the emulsion upon the base results in a certain amount of plastic flow or permanent shrinkage. Dimensional changes of this type are increased by heat because the base is softer and more plastic at higher temperatures. Moisture also increases the plasticity of the base, but it reduces the contraction of the emulsion, and the latter has the greater effect. Consequently, an increase in relative humidity at constant temperature generally decreases this type of shrinkage. Plastic flow of the base may also be produced by stretching in -handling or processing which, of course, will result in an extension rather than shrinkage. Dimensional changes of this type are increased by heat, moisture, the tension applied, and the time during which it acts. Shrinkage or stretch caused by plastic flow of the base is slightly greater in the widthwise direction (cf. Table 3).
(3) Release of strain or recovery from deformation. If film base is stretched during manufacture under conditions which do not permit reorientation of the cellulose nitrate or acetate molecules, deformation or creep occurs resulting in a lengthwise extension and a widthwise contraction. Rapid cooling retards recovery of the deformation (primary creep) due to "freezing-in of strain." This strain may be released at some time during the life of the film with a consequent lengthwise shrinkage and widthwise expansion. Where such a strain exists, the rate of recovery is increased by both heat and moisture.
It will be seen from the above that the shrinkage of photographic film is extremely complex. Several different processes are going on simultaneously, and each is affected in a different manner by heat, moisture, and other factors. It is not always easy to predict how a