Journal of the Society of Motion Picture Engineers (1930-1949)

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Oct., 1944 PROPERTIES AND BEHAVIOR OF FILM 243 The Strength, Elongation, Modulus of Elasticity, and Cold Flow of Film. — In Table 3 are recorded some of the mechanical properties of nitrate and safety motion picture positive film base compared with metals and rubber. The tensile strength and elongation measurements on the film base were made with a Schopper dynamometer operating at a speed of 100 mm per min. The values obtained are not absolute values, but depend somewhat on the rate of testing because of the tendency of the material to exhibit plastic flow. Young's modulus for the film base was determined by quickly measuring the elongation produced by the application of a small tension below the yield value. The cold flow or permanent set was determined by measuring the residual extension remaining after the load and recovery cycle specified. The data show that both nitrate and safety film base lie between steel and rubber in tensile strength, elongation at break, and Young's modulus. The nitrate base has a higher tensile strength, lower elongation at break, higher Young's modulus, and lower cold flow than the safety base. The differences between the lengthwise and widthwise properties are due to the part ally oriented structure of the film base produced by slight stretching during manufacture. The film is slightly stronger in the direction of orientation (lengthwise) and is more easily stretched widthwise. For this reason, Young's modulus is highest lengthwise and cold flow is highest widthwise. The effect of relative humidity and temperature on the mechanical properties of motion picture film is very important in practice. Fig. 11 demonstrates that an increase in the moisture content of film reduces the tensile strength and increases the elongation at break. Fig. 12 shows that an increase in moisture content reduces Young's modulus and increases the degree of cold flow. These 20 40 60 80 RELATIVE HUMIDITY 9b 15 FIG. 11. The effect of relative humidity on the ultimate tensile strength and elongation at break of Eastman Nitrate Motion Picture Positive film in the lengthwise direction at 70 F.