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Oct., 1944
PROPERTIES AND BEHAVIOR OF FILM
247
the second case it is under compression. When the film is bent with the emulsion side out, the emulsion adhering tightly to the base tends to prevent the outer surface of the latter from stretching. Once the emulsion cracks the stress is localized and the base is much more likely to rupture.
If a film is bent slowly there is less chance of a break than if it is bent rapidly, because there is more time for elastic or plastic deformations to occur, which help to dissipate the stress. For this reason, film breaks due to brittleness under adverse conditions are more likely to occur in high-speed equipment. The design of motion
020
0 20 40 60 80
RELATIVE HUMIDITY -96
_40 0 +40 +80
TEMPERATURE -°F
FIG. 14. The effect of relative humidity and temperature on the vise brittleness of undeveloped Eastman Motion Picture film in the lengthwise direction. (Dotted line indicates double film thickness corresponding to minimum brittle point.)
picture equipment should exclude sharp bends in the film travel, particularly with the emulsion side out, whenever possible.
The film brittleness test we have used consists of breaking a loop of film, emulsion side out, between the jaws of a vise closed at a uniform speed. The distance between the jaws of the vise at the instant the film cracks is a measure of the brittleness of the film. Fig. 14 illustrates the effect of relative humidity and temperature on the "vise brittleness" of nitrate and safety motion picture film. The measurements at 70 F were made in air-conditioned rooms. For the subzero temperatures, it was necessary to precondition the film samples to the desired relative humidity at 70 F, seal them in small individual metal containers, and then cool to the desired temperature. The tests were then made quickly (in less than a minute) in a cold room where the humidity was not controlled.