International projectionist (Jan-Dec 1956)

INTERNATIONAL PROJECTIONIST VOLUME 31 MAY 1956 Evaluating Film Base Materials By ROBERT A. MITCHELL The ability of film base materials to withstand the many stresses of repeated projection is discussed in this article. Nitrate and triacetate bases are considered, and an estimate is made of the new polyester base which is expected to be in use before long. THE PROJECTION process imposes severe conditions upon any plastic used as the base, or emulsion support, of theatre-release positive film. If the film fails to meet the requirements of the projection process, if it fails to hold up under conditions of intense heat and mechanical stress, the pictures on the screen will be very poor. Moderately nitrated cellulose combined with suitable plasticizing agents (camphor, castor oil, tricresyl phosphate, etc.) was universally used for the base of professional 35-mm motionpicture film during the fifty years between the late 1890's and the late 1940's. Nitrate film was a remarkably satisfactory material for projection prints despite its dangerous inflammability and marked tendency to deteriorate even in storage. In fact, certain of the excellent operating characteristics of nitrate film have never been equalled by film of any other type. Nitrate Was Standard So good is fresh nitrate film of the best quality that many of its physical characteristics are accepted, even today, as standards which must be met by new and experimental materials intended for use as theatre-release positive base. The earlier diacetate and acetopropionate safety film -base materials failed to come up to the high performance standards of nitrate base, and were accordingly rejected by theatre projectionists even though such films eliminated the fire hazard. Only in comparatively recent times has an acetate of cellulose been produced having physical properties similar to those of fresh nitrate stock. The cellulose triacetate safety film in use at the present time is almost as good as fresh nitrate film and very much better than worn and shrunken nitrate film. As all but the youngest members of our craft will recall, nitrate prints gave us beautifully steady and sharply focused pictures on our screens. Nitrate film was tough, flexible, and rigid in the projector gate. It was resistant to tearing and could stand a great deal of abuse with no significant deterioration of picture quality. The explosive inflammability of nitrate base was a constant worry, and occasionally the cause of disaster. Many of us projectionists, in fact, viewed the passing of nitrate film in 1947 with a certain amount of regret because of understandable prejudice against the unsatisfactory safety films we had occasionally encountered previous to that year. Anyone who has ever projected diacetate safety film in a theatre cannot easily forget how film of that type (no longer manufactured) buckled, shrank, and became brittle. Diacetate film gave pictures that jumped and fluttered in and out of focus. And when triacetate safety film entered the field, projectionists were justifiably irked by splicing difficulties. The film exchanges had failed to notify the craft that regular nitrate cement would not produce solid joins in the new safety film! And that was not all. Perfection of Acetate Being accustomed to the characteristics of nitrate film, projectionists were quick to notice such deficiencies of triacetate film as slighdy lower resistance to the tearing action of sprocket teeth on the perforations, and a tendency of the perforation margins to warp under the influence of heat and buckle the film. Also, triacetate film was more sensitive than nitrate to slight irregularities in the surfaces of INTERNATIONAL PROJECTIONIST MAY 1956