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262 J. M. CALHOUN Vol 43, No. 4
tics of film tend to change on storage even where the moisture content of the film remains the same. Table 6 illustrates the effect of several storage variables on the curl level (at 50 per cent RH) of developed Eastman Nitrate Motion Picture Fine-Grain Positive film over a period of 2 months. It will be observed that the curl of the film becomes more positive under some conditions and more negative under others. The film wound emulsion-in behaves differently from the film wound emulsion-out. The changes in curl are greater at high storage temperatures and low storage relative humidities.
It may help to clarify some of the peculiarities in film behavior if we attempt an explanation of the curl changes which occur during storage. The following are believed to be the more important factors responsible for the effects observed in Table 6 :
(1) Shrinkage of the base. Shrinkage of the base in the widthwise direction tends to counteract the pull of the emulsion and, therefore, makes the curl less positive or more negative. The influence of storage conditions on the shrinkage of film has already been described in detail. An increase in either storage relative humidity or storage temperature increases shrinkage of the base and, we believe, accelerates curl changes due to this cause.
(2} Plastic flow of the base and emulsion during storage in roll form — (a) Transverse flattening of the film. The effects of plastic flow on curl are rather complex. If a strip of film having moderate curl is held flat under a weight, one of the 2 elements (emulsion and base) will be stretched and the other compressed. Since neither the base nor emulsion is perfectly elastic, a plastic flow takes place which reduces the strain. This tends to make the film flatter regardless of whether the curl was previously negative or positive in direction.
Motion picture film wound in tight rolls is somewhat analogous to the strip of film under a weight, since the tension in the roll prevents the film from curling in the widthwise direction. The magnitude of this effect is directly related to the degree of curl at the time of winding which determines the widthwise stress applied in the roll and, therefore, the amount of permanent set. In other words, the more curly the film at the time of winding, the greater will be the change in curl.
Curl changes caused by transverse flattening of the film are increased by heat during storage because plastic flow is greater at higher temperatures. Relative humidity (or rather the equilibrium moisture content of the film at the time of winding) has the opposite effect to that expected. Although film undergoes greater cold flow at high relative humidities under a given load (cf. Fig. 12), the curl at the time of winding is usually highest at low relative humidities. Since this latter factor determines the magnitude of the stress applied in the roll, curl changes due to this cause are greatest when the film is wound when in equilibrium with air at . low relative humidities.
(6) Longitudinal winding of the film. The film in roll form is flat only in cross section and is wound or curled in the lengthwise direction. This has a compli