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244
J. M. CALHOUN
Vol 43, No. 4
effects are apparently due to a plasticizing action of the moisture between the molecules of the base, and demonstrate that a moist film is weaker and is more easily stretched or distorted than a dry film. This should be borne in mind in the handling of film in processing machines.
The effect of temperature at constant relative humidity on the mechanical properties of film is, in general, in the same direction as the effect of an increase in relative humidity at constant temperature. Heat, like moisture, makes the film base softer and more
pliant. This reduces the tensile strength and modulus of elasticity, and increases the tendency to plastic flow. Therefore, film is particularly susceptible to various distortions at elevated temperatures. However, it should be remembered that an increase in temperature is generally accompanied by a decrease in relative humidity and vice versa, so that the effect of reduced moisture may partially compensate for the effect of heat.
The Folding Endurance and
FIG. 12. The effect of relative humidity on the Young's modulus and cold flow of Eastman Nitrate Motion Picture Positive film in the lengthwise direction at 70 F. Load for cold flow test, 1900 Ib per in.2; time loaded, 7 days; recovery time, 24 hr.
Tearing Resistance of Film. — The folding endurance and tearing resistance of nitrate and safety motion picture film base are also recorded in Table 3. The folding endurance (number of folds) was determined with the Schopper fold testing machine, which folds the sample sharply back and forth until it breaks. The tearing resistance was measured with the Elmendorf tear tester. A tear is first started by hand, then a weighted disk revolves which continues the tear, and an indicator measures the tearing resistance. It may be seen that the nitrate film base has a higher folding endurance and a higher tearing resistance than the safety base. The tearing resistance of film base is lowest in the lengthwise direction owing to its partially oriented structure (that is, the film tears more easily along the "grain"). The increased