International projectionist (Jan-Dec 1949)

desirable characteristics — toughness, flexibility, resistance to heat and moisture, etc.. but unfortunately there are no suitable solvents in which to dissolve it for the film-casting process. A reduction in the number of acetate groups of atoms attached to the cellulose molecule is the purpose j)f the hydrolysis reaction. The older safety-film bases were usually made from cellulose diacetate, a substance very soluble in a number of organic solvents. But in comparison with nitrate film, the diacetate film had poor wearing quality and an excessive tendency to swell when wet and to become brittle when dry. The new and superior cellulose triacetate used for high-acetyl safety film is prepared by carrying the hydrolysis only a fraction of the amount employed in the manufacture of the diacetate. Cellulose triacetate shares to some extent the desirable qualities of the higher acetates, yet it is sufficiently soluble in certain organic solvents to be made into film. Preparation of Film Base The purified and dried nitrocellulose or cellulose acetate is dissolved in volatile solvents to make a syrupy "dope" suitable for the film-casting process. Solvents used to dissolve nitrocellulose include various mixtures of diethyl ether, methyl alcohol, ethyl alcohol, isoamyl alcohol, acetone, methylethyl ketone, isoamyl acetate, ethyl acetate, butyl acetate, and various "Cellosolves," of which 1-ethoxyethan-2-ol is representative. The solvent mixtures for cellulose triacetate usually contain considerable proportions of dioxane. Various materials which improve the flexibility, toughness, and wearing quality of the finished film are incorporated One of many gigantic machines at Eastman's Kodak Park which convert the honeylike substance known as "dope" into endless sheets of the familiar transparent film base. The material is so clear as to be nearly invisible as it passes through the machine. INTERNATIONAL PROJECTIONIST txtmbz tn all xU frtettfo % g>m$mx a (Smttttga into the dope before casting. These filmimproving materials, called plasticizers, include such substances as camphor, castor oil, ethyl phthalate, butyl phthalate, and tricresyl phosphate. Plasticizers are used in both nitrate and high-acetyl acetate film. By film "casting" is meant the operation of spreading out the honeylike dope on a polished surface in order to expel the volatile solvents and obtain a thin pelicle (film) of cellulose plastic-cellulose ester mixed with plasticizers. The film-casting machine is a large and complex apparatus. All air admitted to the film-casting room is washed free of dust and conditioned as to temperature and humidity; and the machine, itself, is kept scrupulously clean. The heart of the film-casting machine is a metal drum which, in some installations, is 20 or more feet in diameter. (An endless metal belt is sometimes used in place of the drum.) The outer rim of the drum is several feet in width and very highly polished. Application of 'Dope' Dope is applied to the rim of the slowly revolving drum by means of a special spreading arrangement which allows the thickness of the film to be controlled. The speed at which the drum revolves is such that the greater part of the volatile solvents evaporates from the film of dope before one complete revolution has been made. An aspirating ventilator placed over the rim of the drum hastens solvent evaporation. The coagulated film of nitrate or acetate base is continuously detached from the drum and drawn away by a separate roller. The film, several feet in width, is so transparent as to be nearly invisible as it passes through the remainder of the film-casting machine. Various other rollers complete the drying operation, and the film is finally wound up to await the emulsion-coating process. The pale yellow light-sensitive coating applied to one side of the transparent film base (to both sides in the case of duplitized stock) is called the emulsion. The term "emulsion" signifies a suspension of minute particles of one substance in another, the two substances being immiscible, or insoluble, in one another. Preparation of Emulsion An emulsion is not a true solution. Kerosene, for instance, is immiscible with water. But if a little kerosene is added to water and the mixture shaken very vigorously, a milky fluid is obtained. The whitish mixture is an emulsion of kerosene droplets suspended in water. An emulsion of kerosene and water breaks down very quickly, the two liquids separating, unless an emulsifier is added. Soap acts as an emulsifying agent for kerosene in water; so if a small piece of soap is shaken up with the two immiscible liquids, a milky-white suspension is obtained which lasts a long time. The same phenomena are observed in the case of liquids and solids which are insoluble in them. The chief light-sensitive ingredient of photographic emulsions 35-mm processing machine at Kodak Park INTERNATIONAL PROJECTIONIST DECEMBER 1949