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how these values will change with age of the film. If progress is to be reasonably rapid, the manufacturer cannot afford to wait years to see how his product behaves on keeping. Therefore, the film base is subjected to " accelerated aging."
In this form of test, the film is kept at an elevated temperature for a short period of time, on the assumption that this will simulate storage for a much longer time at normal temperatures. It cannot be said with certainty that this follows on all occasions, but accelerated aging provides a useful basis from which to predict behaviour of the film.
Comparative Values on Safety and Nitrate Base
Having discussed the various characteristics which will be considered in comparing safety and nitrate film bases, we will now examine the results of such tests, summarised in Table I. The films to be compared will be nitrate, and those safety bases shown in Fig. 1.
Tensile Strength and Modulus of Elasticity. — The tensile strength of the newsafety base is considerably improved over the older types, and approaches that of nitrate. Young's modulus is appreciably higher for the new film, but in this respect the material is still somewhat inferior to nitrate.
Folding Endurance and Tear Strength. — Folding endurance is in the same range for all products with the exception of the earliest type of safety base, which was much inferior in this respect, a fact which probably had something to do with the
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f ,NMIGM ACETYL SAFETY ^ ACETATE PROPIONATE SAFETY
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Fig. 4. Viscosity retention of film base at 100° C.
Fig. 5. Rates of swell in water at 68c F. (heavy
curves), and rates of shrinkage upon drying at
70° F. 50% R.H. (light curves) for motion picture
positive films.
reputation for brittleness which is still regarded as a characteristic of safety motion picture film. Tear values for the new film are somewhat below those of nitrate or the previous safety materials, and might be the cause of some concern if this property should prove important.
Retention of Tear Strength at 100c C— As seen in Fig. 2. the safety films show practically no change in values for tear strength on accelerated aging tests up to 30 days. Nitrate film, on the other hand, deteriorates rapidly, and afaer 48 hours shatters when subjected to the test.
Retention of Flexibility at 100° C— As with tear strength, very little deterioration is apparent with safety films. "Nitrate film, however, became completely brittle after ten days (Fig. 3).
Viscosity Degradation at 100c C— Under accelerated aging, nitrate film base, as indicated in Fig. 4, shows serious degradation, which is anticipated from previous experience with this material. Up to 30 days, change in the safety base is negligible.
Rate of Swell in Water and Rate of Shrinkage on Drying.— Compared with nitrate, the degree of swell of safety film is high, although the new safety is superior in this respect to the old. The factor of importance, however, is the rate of swell and shrink, rather than the ultimate values, since sudden changes in dimension may