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688 J. R. HILL AND C. G. WEBER [j. s. M. p. E.
of 95 per cent was about 3 days, compared with 30 days for ovenaging in dry air. The decrease in viscosity of acetate film for 3 days of aging was 6.5 per cent. This is a greater change than for the same time of aging in a dry atmosphere (Fig. 6), but is small compared with the change found for nitrate film, and indicates excellent stability even under these extreme conditions.
(5) Effects of Aging upon Gelatin Emulsion. — Ordinary ovenaging in dry air had no effect upon the gelatin emulsion of acetate films so far as could be observed. The aging at 100°C, 95 per cent relative humidity, resulted in some softening of the emulsion and a tendency of the emulsion to separate from the base. The first visible evidence of deterioration of nitrate film with natural aging is a discoloration and softening of the emulsion. Tests of the emulsions from such films showed that oxidation, as indicated by relatively high ammonia nitrogen content, and hydrolysis as indicated by relatively high amino nitrogen content, had both taken place. Results of tests on the emulsion of oven-aged film showed that the oxidation was increased from 0 to 0.89 per cent and hydrolysis from 1.89 to 3.90 per cent by oven-aging. This apparently results from contact with the products of decomposition of cellulose nitrate, as there was correlation between the acidity of the film and condition of the emulsion. Hence, the softening of the emulsion of nitrate films often observed after natural aging may be said to be an indirect result of the decomposition of the cellulose nitrate film base.
VI. SUMMARY
On the basis of the test data, the cellulose acetate type of safety film appears to be a very stable substance. It is a comparatively new material on which no natural-aging data are available. However, data on the relative resistance to accelerated aging of the acetate film, nitrate film, and permanent-record papers are very favorable for the acetate film. A specification, proposed by Scribner,7 for permanent-record paper of the highest quality requires a retention of folding endurance of 85 per cent after heating for 72 hours at 100°C. Acetate film retained 50 per cent after 150 days at the same temperature. Nitrate film, known to be relatively impermanent, had no folding endurance after 10 days of heating.
The relatively high chemical stability of acetate film is shown by the fact that aged samples showed no increase in acidity, whereas nitrate film became distinctly acid. For permanent-record paper, the copper