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178 Transactions of S.M.P.E., May 1924
The work which follows is, for the most part, necessarily tabular and graphic, an attempt, however, has been made to offer as simple an interpretation as possible in discussing those problems which may be of direct and practical interest.
A strip of motion picture film is composed of a thin cellulose support about .005 of an inch thick coated on one side with a layer of gelatin less than .001 thick, the latter serving the purpose of holding the crystals of sensitive silver salts in position.
While the coating of gelatin reduces the flexibility of the cellulose support, it fortunately adds the necessary ''body" to the film, for it will be shown that for equal thicknesses the coated film has far greater projection strength than the uncoated but more flexible base. This observation leads us at once to the conclusion that the edges of film should be carefully guarded against any practice which will injure or remove the gelatin coating even though such injury ma}^ not reach beyond the perforations. This condition is strikingly shown in an experiment to be described and illustrated later.
The first part of this paper will deal with the tensile strength and plasticity of nitrocellulose film base, both uncoated and coated; the second part with conditions affecting the projection strength of motion picture film.
Nitrocellulose film base is a flexible, plastic material which, however, undergoes gradual decomposition. Its plasticity is promoted by the presence of residual high boiling solvents or mincible nonvolatile bodies. When warmed, the plasticity of the material is greatly increased. It has already been mentioned that gelatin coated film base has greater projection strength than uncoated film base of equal thickness and, since both plasticity and flexibility are reduced by the gelatin coating, it follows that beyond a certain limit, plasticity and flexibility decreases rather than increases the projection life of motion picture film. This is not difficult to understand when it is remembered that the cross section presented to the sprocket tooth must take the blow without appreciable yield. It is the constant excessive yield of the plain film base in projection which eventually tears the corners of the sprocket holes. The support or body which the less plastic gelatin coating gives to the film base, accounts for the enormous increase of the projection life of coated film.
We may anticipate a later treatment of the factor, that the condition of the physical contact of film base and gelatin determines the value of the combination with respect to its projection life. The degree of adhesion of gelatin to film base will be shown to have a marked influence on the useful life of the finished product.
The Physical Tests
The machine used in making the tensile strength and plasticity tests shown on the charts, is a Scott Serigraph. Any other standard tensile strength tester is, of course, equally adapted to such work; asfor example, the Schopper machine.