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CELLULOSE
CELLULOSE TRIACETATE
ACETYL
Fig. I. Chemical composition of cellulose esters used in safety film base. A, acetone soluble cellulose acetate ; 8, high acetyl cellulose acetate ; C, cellulose acetate propionate.
CELLULOSE
TRIPROPICNATE
PROPIONYL
away as a thin film. This then passes through further heated " curing " chambers where more solvent evaporates, and the film attains its final form. In passing through the curing chambers the film has to be pulled along to some extent. As a consequence, a certain amount of orientation of the cellulose ester molecules takes place. The practical effect of this later is to cause differential shrinkage across the film and along its length, so that we usually find when measuring a processed film that the percentage of shrinkage in width rather exceeds the lengthwise shrinkage.
II. MECHANICAL AND PHYSICAL CHARACTERISTICS
The final evaluation on the quality of a film base is, of course, its behaviour under practical working conditions. However, a very good estimate of its properties can be Tnade from a study of certain mechanical and physical characteristics which are capable of measurement with considerable accuracy.
Mechanical Characteristics
From this aspect there are four mechanical characteristics we will consider : tensile strength, which is a guide to the ability of the film to withstand hard wear ; the mod id its of elasticity, indicating its stiffness and rigidity ; the folding endurance, which determines flexibility ; and the tear value. showing the ability of the film to withstand stresses of that character.
Tensile Strength. — If a strip of film is secured at one end and a tension applied to the other, it will stretch. As the tension is increased it will stretch further, and at first the amount of stretch will be constant for a given increase in tension. However, after a certain limit has been reached the degree of stretch will become very much greater, and the point at which this occurs is known as the yield point. If the tension continues to increase the film will finally snap. The tension at which this occurs is a measure of the tensile strength.
Modulus of Elasticity is the ratio of tension to stretch within the first part of the curve, and is indicative of the stiffness and rigidity of the support. Thiimportance in resisting curl and buckle in the gate during projection.
Folding Endurance is measured by the number of times the material can be rapidly flexed back and forth without breaking. Good flexibility is obviously of importance for film which must travel devious paths in camera, printer and projector.
Tear Value is measured by the energy required to continue a tear which has been started by hand.
All these measurements will vary according to the temperature and the