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Oct., 1944 PROPERTIES AND BEHAVIOR OF FILM 263
cated effect on the subsequent widthwise curl of the film when unwound, which we will attempt to analyze. It is common practice to wind raw stock emulsionin and processed film emulsion-out. When film is rolled emulsion-in, the emulsion is compressed and the base stretched in the lengthwise direction. Plastic flow takes place to relieve the stress and, under suitable conditions, the film takes a certain degree of permanent set. Then, when the film is unrolled the emulsion is stretched lengthwise and the base compressed. This compresses the emulsion and stretches the base in the widthwise direction which tends to make the widthwise curl more positive.
When film is rolled emulsion-out, the effect is the opposite to that described above. Unwinding film having a permanent set stretches the base and compresses the emulsion in the lengthwise direction. This compresses the base and stretches the emulsion widthwise, which tends to make the widthwise curl more negative.*
The smaller the diameter of the roll the greater is the deformation of the film and, therefore, the greater will be the change in curl from this cause. (We have found somewhat larger curl changes at the core end than at the outside end of 1000-ft rolls.) An increase in either relative humidity or temperature increases curl changes of this type because of the increased plastic flow and permanent set.
The above discussion illustrates a few of the complexities in the behavior of motion picture film. In addition to the factors described, there are undoubtedly others, even more obscure, which influence curl. The curl of the film at any given time will be determined by all these effects, some of which act in opposite directions, and each of which may be influenced in a different way by heat, moisture, and other factors. For these reasons it is not always possible to predict what the curl of motion picture film will be at any given point in its history.
In the data in Table 6, the effects of shrinkage on curl are difficult to distinguish from the effects of plastic flow. However, in the case of film wound emulsion-in, plastic flow produced by transverse flattening would tend to give a zero curl, while plastic flow produced by longitudinal winding would tend to give a more positive curl. The fact that the curl actually becomes increasingly negative at low relative humidities as the temperature increases, must be attributed to shrinkage of the base. At higher relative humidities the effects of shrinkage are masked by other factors.
* It is assumed that at least several feet of film are involved, so that the weight of the film itself when unrolled and stretched out will prevent lengthwise curl. If a strip of film only a few inches in length is taken from a roll, it will frequently curl lengthwise rather than widthwise, particularly if previously wound to a small diameter. This behavior is sometimes referred to as "clockspring" from the obvious analogy.