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246
J. M. CALHOUN
Vol 43, No. 4
folding endurance and tearing resistance of the film at high relative humidities are illustrated in Fig. 13. This is apparently due to the fact that the film base is softer and more limp when moist, and it tends to "give" rather than rupture.
The mechanical properties described in the above sections affect the performance of film on continued projection, and indicate some of the reasons for the supremacy of nitrate base in the motion picture field. Nevertheless, safety film has made rapid strides in recent years, and its physical properties now approach more closely to those
of nitrate than was formerly the case. This is illustrated in Table 3 with the comparison between Eastman Safety Motion Picture film base manufactured in 1937 and that manufactured in 1943. The improvement in the safety base is particularly marked in the case of folding endurance. Mechanical tests of this nature give an indication of what may be expected from a given film in use, although they are not a direct measure of practical performance. These laboratory tests also help to give a clearer
60 80
HUMIDITY 96
FIG. 13. The effect of relative humidity on the folding endurance and tearing resistance of Eastman Nitrate Motion Picture Positive film in the lengthwise direction at 70 F.
understanding of the behavior of the film, and are useful in manufacturing control to make sure that the highest possible quality is maintained.
Brittleness. — Brittleness is another important physical property of a photographic film. Difficulty from brittleness is generally encountered only at low relative humidities or at very low temperatures. When a piece of film is sharply flexed or bent, the surface on the inside of the bend is compressed, while that on the outside is stretched. Therefore, a crack generally starts on the outside of the bend. Emulsion coated film is more brittle than the base alone, partly because the emulsion itself is more brittle which increases the chance of a break starting. Film is more likely to break if bent with the emulsion side out than if bent with the emulsion side in because in the first case the emulsion is under tension, whereas in