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252 J. M. CALHOUN Vol 43, No. 4
given film will shrink when subjected to unknown conditions of storage and handling.
The largest portion of the permanent shrinkage of most motion picture films results from the first of the above causes, that is, loss of volatile material from the base. The exact magnitude of the dimensional changes produced by compression by the emulsion, and release of strain in the base are difficult to determine because they are generally masked by the larger effects of solvent loss. With some low-shrink films, the shrinkage of the base alone may be compared with the shrinkage of the emulsion coated film to obtain an estimate of the plastic compression in the latter? This is not possible with ordinary motion picture films because the emulsion influences the rate of solvent loss which masks the effect of plastic flow of the base. Compression by the emulsion and release of strain in the base are generally of secondary importance in the shrinkage of motion picture film, except in special cases where a high degree of dimensional stability is required, or where extremely severe storage conditions are encountered.
Although temporary dimensional changes in a motion picture film are always greatest in the widthwise direction, permanent shrinkage may be greatest in either the lengthwise or widthwise direction, depending on which of the 3 causes described above predominates. In cases where the release of strain is large, the permanent shrinkage is greatest in the lengthwise direction. However, since loss of volatile material predominates in most motion picture films, permanent shrinkage is generally greatest in the widthwise direction (cf. Table 4).
The Shrinkage of Raw Stock. — The shrinkage of motion picture film prior to exposure is generally small, where the film is kept in tight rolls in closed metal containers which retard the loss of both solvents and moisture. In the case of unexposed nitrate motion picture film in 1000-ft rolls in taped cans which have not been opened, the actual shrinkage (temporary and permanent) seldom reaches 0.1 per cent in 6 months at 70 F. However, where cardboard boxes were temporarily substituted for cans during the war a somewhat higher shrinkage was encountered. The necessity of using rubber substitutes in adhesive tape during the war has also resulted in slightly greater shrinkage.
Film in 16-mm X 100-ft rolls wound on cores shrinks more rapidly than the same film in 35-mm X 1000-ft rolls, because there is a