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248 J. M. CALHOUN Vol 43, No. 4
It may be seen in Fig. 14A that the brittleness of motion picture film at 70 F is virtually negligible above 20 per cent RH, but increases rapidly as the relative humidity is reduced below 20 per cent. There is very little difference in brittleness between the present Eastman nitrate and safety motion picture film although the safety film made a number of years ago was considerably more brittle than the nitrate.
Fig. 14B shows that the brittleness of motion picture film increases as the temperature is lowered. At about — 25 F and 60 per cent RH the ,film has the same degree of brittleness as at 70 F and 14 per cent RH. The effect of subzero temperatures on the brittleness of film is of interest where motion pictures must be taken in arctic regions or at high altitudes. Attention is drawn to the fact that relative humidity has an important bearing on the brittleness of film even at temperatures below the freezing point of water. Apparently the moisture in the film does not crystallize at these temperatures, but remains in an amorphous or supercooled liquid state, and continues to plasticize the emulsion and film base. This is probably the reason that more trouble from brittleness is not encountered in the arctic, where the outside relative humidity is generally high. Nevertheless, precautions should be taken to prevent film from drying out prior to use at low temperatures. Additional care should be given to the handling of film and the proper operation of equipment at either low relative humidities or low temperatures.
Tackiness. — Although tackiness is not a mechanical property of film it is discussed in this section for convenience. The tendency of film to stick to itself, or to metal parts of cameras or projectors, seldom occurs except at very high humidities, particularly when warm. Under these conditions the emulsion may become somewhat soft and spongy and in extreme cases plasticizer may diffuse out of the base and cause tackiness. The most difficulty from tackiness in practice occurs with green prints which have been insufficiently dried. The emulsion tends to stick in the projector gate, especially if the latter is warm. This builds up tension on the film and may cause a tear or break. Particles of emulsion may also pile up in the gate as a hard, horny deposit and cause trouble. Adequate drying after processing tends to reduce this difficulty, but in any case new prints should be waxed' or lubricated in some way before the first projection.