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Oct., 1944 PROPERTIES AND BEHAVIOR OF FILM 229
has sufficient strength to be stripped from the metal surface. It is then passed through a complex series of heated chambers to drive out the remaining solvent.
Unfortunately, in practice it is not possible to remove every trace of solvent from the film base. Fig. 1 illustrates the shape of a typical curing curve for green film base and shows that the rate of solvent removal decreases rapidly as .curing proceeds. The last traces of solvent are held very tenaciously by the base. In commercial production it is obviously impossible to increase the curing time beyond a certain limit. Therefore, in spite of the improvements made in recent years, there is always some residual solvent in the film base which, with small amounts of plasticizer, gradually diffuses out of the film throughout its life, resulting in a certain amount of shrinkage and related troubles.
A word concerning the structure of film base may be helpful at this point. Cellulose nitrate and acetate molecules are generally considered to be long chains which may or may not be grouped together in clusters or bundles called micelles.2 The exact dimensions of the molecules or micelles probably vary even within the same sample, but it is their approximate shape which is of interest here. McNally and Sheppard,3 following a study of the optical properties of cellulose nitrate and acetate films, concluded that whatever the nature of the particles involved " — it appears that they must be unsymmetrical in shape, rod or lath-like, having one axis considerably longer than the others.'' All of the available evidence obtained from studies of the physical properties and dimensional behavior of film base confirms this view.
Since the film base is pulled rather than pushed through the curing chambers, it is unavoidably stretched and strained to some extent in the machine direction (lengthwise). If the base is sufficiently warm and soft when stretched, the molecules (or micelles) tend to become partially oriented in the direction of stretch. This produces a slight "grain" in the film base which affects the strength, tear resistance, shrinkage, and other physical properties of the film which will be described later. If the base is not sufficiently warm and soft when stretched, the deformation produces a mechanical strain in the film.4 When the base is cooled rapidly, the deformation remains — a phenomenon referred to as the "freezing-in of strain."5 This deformation or strain may be released sometime during the life of the film with a consequent lengthwise shrinkage.