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260 J. M. CALHOUN Vol 43, No. 4
In normal film the emulsion is firmly attached to the base so that it cannot contract as much as it would otherwise. Therefore, when the moisture content of film is reduced the film bends or curls toward the emulsion side to reduce the strain. In addition, the base is compressed slightly (cf. Table 5) and the emulsion stretched (compared with the dimensions it would assume if unattached). An analogy is sometimes drawn between this behavior and that of a bimetallic strip which consists of 2 elements, each having a different thermal coefficient of linear expansion.
The curl measurements in Table 5 give the average altitude of the arc (that is, the maximum distance between the arc and the chord) formed by 35-mm film in the widthwise direction. If the film base itself were perfectly flat, the emulsion coated film would be nearly flat
TABLE 5
The Effect of Relative Humidity on the Curl of Fresh Undeveloped Eastman Nitrate Motion Picture Fine-Grain Positive Film
Widthwise Humidity Contraction from 70 per cent RH, per cent
Relative Humidity, per cent
Stripped Emulsion
Uncoated Base
Normal Film
Curl of 35-mm Film, in.
70
0
0
0
-0.03
50
0.8
0.10
0.15
+0.05
20
1.8
0.25
0.35
+0.20
at high relative humidities and become more positive in curl as the moisture content is reduced. However, in most motion picture films the base is treated so that it will have a small negative curl to counteract the pull of the emulsion. The film, therefore, has a slight negative curl at 70 per cent RH, a small positive curl at 50 per cent RH, and a curl at 20 per cent RH which is not as large as it would be if the base itself were flat. It is desirable that the curl amplitude, that is, the difference between the curl at low and at high relative humidities, be as small as possible.
Curl in Processing. — When film is immersed in the processing solutions, both the base and emulsion swell. The curl generally disappears and the film is essentially flat as it enters the drying cabinet. Here the back of the film base begins to dry while the face is kept moist by contact with the wet emulsion. This causes the film to curl in a negative direction. As the emulsion dries, it contracts more than the base and finally pulls the latter until the curl