INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vol. 18, No. 1
The Time Factor and Yield Value of Cellulose Estersls2 By S. E. Sheppard, E. K. Carver, and S. S. Sweet EASTMAN KODAK Co.,ROCHESTER, N. Y.
I
T WAS pointed out by the authors a t the recent Plasticity Symposium at Lafayette College3 that films of cellulose esters are not perfectly elastic and flow slightly even under small loads, and that the rate of flow is proportional to some
L o a d in Kg Figure 1
high power of the load, instead of being proportional to the difference between the load and yield value as is the case with many plastics. Typical curves, obtained by measuring the rate of stretch of films under different loads, are shown in Figures 1 and 2. Figure 1 is a plot of rate of flow against force applied-i. e.,
a curved line, such as Bingham4found to occur with cellulose ester sols, and that when the logarithm of rate of flow is plotted against force we get a straight line, as Ostwald6 obtains with various sols and gels. Since the material flows even under the lightest of loads there can be no definite yield point. What, then, is the significance of the very definite yield points and b r e a k i n g loads obtained by the various strength testers? To comprehend t h i s question better it will be necessary to consider the photograph ( F i g u r e 3 ) of t h e Schopper paper tester as it is used in this laboratory for film strength determinations. A motor, M , serves to pull down Figure 3 the threaded bar. B. which holds the film; F, by means of the clamp, C. As the film is pulled down, it lifts the weight, W , on the end of the lever, thus applying an ever-increasing load to the film. The magnetic tapper, T , connected to a time line records the downward movement of the clamp, while the rotation of the drum, D,caused by the movement of the weight, W , records the load. Thus in a few moments a complete stress-strain diagram can be obtained. Such a diagram is shown in Figure 4. The exact shape of the curve depends very much on the nature of the film. Often FILM STRiTCH
TEST
LOAD Ill WE
Figure 4
5
6 Load in KQ
7
8
Figure 2
it is an ordinary plastic flow curve such as one obtains with many colloidal substances. It will be observed that it gives 1 Presented before the Division of Cellulose Chemistry at the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 to 10,1925. Communication No. 247 from the Research Laboratory of the Eastman Rodak Company. Sheppard and Carver, J . Phrs. Chrm., 89, 1244 (1925).
* *
the secondary period of elasticity is absent. Usually the curve is nearly perpendicular a t the yield point, but in certain cases the yield value is indicated merely by a change in the slope of the curve. By keeping in mind the fact that the rate of flow increases rapidly with the load, we can readily comprehend the signifiSecond Colloid Symposium, Evanston, Ill., June, 1924. Proposed earlier by de Waele, Oil and Color Chcmisls Assoc., 6 , 33 (1923). and for starch pastes by Farrow and Lowe, J. Tcrrilc Insf., 14, T414 (1923). T h e latter proposed and used a formula identical with that suggested by Ostwald, Kolloid-Z., 86, 99 (1925). 4
6
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INDUSTRIAL A N D ENGINEERING C H E - I S T R Y
January, 1926
Figure 5
stretched a certain amount the rate of flow decreases. It may decrease regularly throughout the period of stretch, thereby giving an indefinite yield point, as illustrated by Figure 7, or it may decrease suddenly, as if the film suddenly came to the end of its tether, thereby giving a curve as illustrated in Figure 8. Intermediate types of curve are also obtained, and while it appears probable that the shape of the curve is related to the structure, no definite statement can be made at this time. The fact that the region of apparently perfect elasticity is shorter according as the rate of stressing is slower has perhaps considerable bearing upon the application of stress-strain tests. Tests run a t a rate which is suitable for material to be exposed to rapid changes of stress-e. g., motion picture films-are not suitable for materials to be exposed to relatively slow changes of stress-e. g., paint films.
cance of the stress-strain diagrams. When the pull is first applied the film stretches slightly and beCELLULOSE NITRATE gins to flow very slowly; as the load is increased 0 L = 15 0 kg. the flow increases until finally the flow is just as TotalE=S5cm rapid as the pull. At this point there will be no PerrnE = 4 5 c m increase in the load, and the curve will be vertical-i. e., we shall be a t the yield point. Since the yield point is merely the point at which the flow is equal to the rate a t which the pull is applied, we can readily see that the yield point must vary with this rate. That this is true 1 can be seen from Figure 5, which is made up of stress-strain diagrams obtained with identical material but with different rates of pull. The yield Fieure 7 point and breaking load, therefore, are merely accidental values depending on the rate a t which the pull is applied. In Figure 6 is plotted the logarithm of rate of pull against breaking load, showing a rough proportionality between the two. The question of the “second elastic portion” which sometimes occurs on the stress-strain curves deserves some attenCEUULOSE N I T ~ A T E tion. The cause of it appears to be that when the film has Sample 114-6
--
&La= 11.0 kg.
Total E = 7.2 c m . Perm,E=54cm
Figure 8
Summary
It is shown that films of cellulose esters are imperfectly elastic, and flow slightly even under very small loads. The yield points, therefore, as obtained by the ordinary strength testing machines depend largely upon the speed a t which the machine is run.
Calendar of Meetings
Figure 6
American Petroleum Institute4th Annual Meeting, Los Angeles, Calif., January 19 to 21, 1926. American Ceramic Society-Atlanta, Ga., February 8 to 13, 1926. American Chemical Society-7lst Meeting, Tulsa, Okla., April 5 t o 9, 1926. American Electrochemical Society-Chicago Beach Hotel, Chicago, Ill., April 22 to 24, 1926. Association of Chemical Equipment Manufacturers-2nd Chemical Equipment Exposition, Cleveland, Ohio, May 10 to 15, 1926. American Institute of Chemical Engineers-Berlin, N. H., June 21 to 23, 1926.
