International projectionist (Jan-Dec 1950)

VOLUME XXV FEBRUARY 1950 NUMBER 2 The 35-mm Projection Positive Film By ROBERT A. MITCHELL IV. The Film Base: Nitrate and Triacetate Stock WHILE various types of non-inflammable film stock have been tried from time to time ever since the earliest days of motion pictures, nitrate stock has been used almost exclusively for professional films because of its superior wearing qualities. Nevertheless, dissatisfaction with nitrate film has been voiced frequently by exhibitors, projectionists, and exchange workers because of its dangerous inflammability, and by custodians of film libraries because of its impermanence. The search for non-inflammable filmbase plastics failed to produce a commercially acceptable material, however, until 1946 when the Eastman Kodak Company introduced high-acetyl safety film and immediately instituted an intensive manufacturing program aimed at the complete replacement of nitrate stock by 35-mm safety film. Curiously, there is nothing new about cellulose triacetate, the chief ingredient of high-acetyl safety base. Much of the literature on cellulose plastics published during the period 1905-1910 contains numerous references to the triacetate of cellulose. In fact, as early as 1910 the nitrates, acetates, formates, propionates, butyrates, palmitates, stearates, benzoates, acetonitrates, aceto propionates, acetobutyrates, propionobutyrates, etc., of cellulose had been the subjects of intensive research. The present belated flurry of activity on the part of film manufacturers in the cellulose triacetate field may be attributed, in part, to improvements in methods of manufacture. Requisites for Projection Any film stock intended for use in theater projection must measure up to high standards on the score of (1) tenacity (2) pliancy (3) permanence toward water (4) permanence toward heat (5) flammability characteristics (6) permanence toward time, and (7) transparence. Table A shows that new nitrate film of good quality is somewhat superior to high-acetyl safety film in tenacity and pliancy; is inferior in transparence and permanence toward heat; and is vastly inferior to high-acetyl film in flammability characteristics and permanence toward time. The two types of film are comparable in the matter of permanence toward water, a matter of considerable import NOTE: Tables A and B appear on the pages immediately following. ance to processers, who fear slack loops caused by excessive film swelling in the developing machine and excessive tension and distortion of the film caused by a too rapid shrinking during the drying stages. But Table A does not tell the whole story. After prints have been subjected to considerable use — say, to 50 or more projections and to numerous transits to and from theaters — the superiority of nitrate over high-acetyl film in the matter of tenacity and pliancy almost vanishes. Why is this? Relative Deterioration Characteristics The data on permanence toward heat and time show that whereas nitrate film deteriorates rapidly by repeated exposure to heat and with age (weakening the .film and increasing its brittleness) high-acetyl safety film is relatively immune to these causes of film deterioration, and therefore retains its original physical properties almost indefinitely. The greater degree of cold flow of high-acetyl film, however, indicates that this type of film stock is more susceptible to "curl deformation" than is nitrate, thus giving rise to somewhat more pronounced focus-drift effects. This is unfortunate ; but it is entirely possible that this defect will soon be overcome by incorporating special materials with the plasticizers used in the manufacture of the film. Specific physical data anent the two types of film base are given in Table B. Nitrocellulose Fire Hazard Nitrate motion picture film is dangerously inflammable. The emulsion-supporting base of this type of film is celluloid, a plastic material consisting of the INTERNATIONAL PROJECTIONIST • February 1950