STRESS-STRAIS CURVES FOR PLASTIC SULFUR AXD RAW RUBBER AT VARIOUS TEMPERATCRES BY JOHN D. S T R O S G
Introduction A great deal of information regarding the elastic properties of a substance is revealed by its stress-strain curve. Materials which have very similar elastic properties should have stress-strain curves which have the same general shape and which change in the same manner with a change of temperature. Using the stress-strain curves as criteria to classify elastic substances, one would expect to find all types of substances from quartz on the one extreme to rubber on the other. There are no substances which would fall very near rubber in such a classification except, perhaps, the so-called plastic-sulfur.' It is the object of this research t o establish the similarity of the elastic properties of raw rubber and pla& sulfur by the determination of stressstrain curves for each substance at various temperatures. These substances might be called "plastic-elastic" substances because they are neither purely elastic substances like quartz nor pure plastics like clay, but they exhibit some properties in common with both of these types of substances. A stress applied to either of these substances causes a reversible or elastic deformation and an irreversible or plastic deformation as well. This plastic deformation or plastic flow comes into most experiments, designed to show the elastic properties of these substances, to such an extent that their elastic properties cannot be brought clearly into focus. I n order to determine the true stress-strain diagrams of these substances, the plastic flow must be either measured and subtracted from the total extension in order to give the elastic deformation, or it must be eliminated. The ideal conditions for getting stress-strain data would be realized if the load could be applied instantly and if the elongation could be measured just after its application. Then there would be little time for plastic flow to occur. Practically, the closest approach to these conditions is to apply the load as rapidly as possible and determine the extension simultaneously with the application of the load. This means, that the testing machine must be equipped with an automatic recording device. The curves given in this paper were determined on machines that were equipped in this manner. The rate a t which the load was applied was sufficiently rapid so that the errors introduced by plastic flow were judged to be of the same order of magnitude as the experimental errors due to other uncontrollable factors. Experimental Suljur. Plastic sulfur filaments about 0.04" in diameter were prepared by pouring re-distilled sulfur which had been heated for some time a t 3 0 0 T yon Weimarn: Kolloid-Z., 6 , 2 j0 (1910). Elastischer kautschukartiger Schwefel
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into a cooling brine whose temperature was slightly below zero. By exercising care the plastic sulfur could be obtained in clear yellow filaments of surprising uniformity. Plastic sulfur prepared in this way was remarkably stable. Some filaments, immersed in water, retained their elastic properties for more than a day. It is unfortunate that the sulfur cannot be made to retain these useful properties permanently. If a means of “vulcanizing” and stabilizing plastic sulfur could be devised it might prove a worthy substitute for rubber especially since it costs but a few cents a pound. Tho stress-strain data for sulfur were taken on a modified Jolly Balance (See Fig. I ) . This machine was equipped to record automatically the applied load and the resulting extension.
Operatzon of Jolly Balance. Several turns of the filament about the sample hooks were made when the latter were in this zero position. The ends of the filament were then fixed. This gave a total length of 1.00 cm of filament per turn. The load and extension were automatically recorded by a curve traced with a pen. The record sheet and pen both move as the sample is tested. The sheet being attached to an aluminum plate, is raised vertically by turning the crank and thus the load is applied to the sample and a movement of the pen to the right occurs as the sample stretches. From the position of the pen at any instant both the load and corresponding extension may be determined. However, since the vertical motion of the sheet is shared between the extension of the sample and the extension of the load spring the extension of the sample must be taken into account FIG.I Jolly Balance modified for Study of before the load can be determined. I n Plastic Sulfur order to do this two empirical curves were dotted: first. the horizontal Dosition of the pen (x) was plotted against the separation of the sample hooks under zero load, and second, the vertical position of the pen was plotted against the separation of the sample hooks under no load. As the sample was stretched, by turning the crank, the curve from which stress-strain data were determined was traced by the pen on the sheet. Three curves were traced a t each temperature with similar sized samples. X curve was then constructed which was a mean of these curves. From this constructed curve the data
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given in Table I were taken. Y-Yo is the total vertical displacement of the sheet minus the part of this displacement necessary to stretch the sample. The two pulleys b and c are part of an arrangement for reducing the horizontal displacement of the pen caused by a given extension of the sample. The stress-strain curves plotted from these data appear in Fig. 2 . Rubber. The raw rubber was milled from smoked sheet stock into sheets approximately 50 mils in thickness. From these sheets narrow strips about 3.5 cm. long were cut and their ends were lapped _ _ and pressed together so that they formed rings about I cm in diameter, (the splice being placed over one of the hooks which held the sample, when it was put in the machine to be tested). Stress-strain data for rubber was taken on a modified Schopper Testing Machine (See Fig.3). The load and extension were automatically recorded by a curve traced with a capillary pen on a sheet of paper. The angle, about the axis of the load arm, through which the pen is moved is a measure of the load applied to the sample. The difference between the angular deflection of the extension arm and the load arm is a measure of the extension of the sample. The pen is moved in toward the load arm axis by a silk thread runFIG.2 Stress-strain Curves for Plastic Sulfur ning from the extension arm (a) over a pulfey (b) to the pen (c). The movement of the pen toward the axis of the load arm is equal to the cord of this angular difference, but the cord is approximately equal to the arc for small angles, so that the movement of the pen toward the axis of the load arm was accordingly taken as proportional to the extension of the sample. The sample may be immersed in a cooling bath and tested while it is in this bath. Samples of raw rubber were tested at -45'C, -55OC, -6o"C, and -65OC. From the dimensions of the sample and the curve traced by the pen, the stress-strain curves were constructed. (See Fig. 4). The absolute accuracy of this work is not claimed to be great since only small samples could be used. Some error was introduced due to the nonuniform stretching of the sample, since the portion of the sample behind the hook was no doubt restrained in its extension by friction against the hook. This particular error has, however, a similar effect on the stress-strain curves of both the rubber and the sulfur. The maximum deviations of the stress as read from the constructed curve and as read from the most divergent of the curves traced by the Schopper
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Machine are: for A, 5 ’ 3 ; for B, 4+70; for C, 7%. The average deviation of the diameter of the thread was about 2 . 5 % . It does not seem unreasonable to estimate the probable error as 470.
FIG.3 Modified Schopper Machine
FIG.4 Stress-strain Curves for Rubber
Discussion It will be observed there is some little similarity between the stress-strain curves for the raw rubber and for plastic sulfur both in regard to their shape and the change of shape with temperature. The inflection point in the curve which is so characteristic for rubber is also distinctly present in the curve for sulfur taken a t -3.6’c. S o doubt the reason that this inflection point is not more marked is that the load per unit of “original” cross area is plotted as stress instead of the actual stress. If the actual stress were plotted the ordinates of the curve would be greater than shown by an amount directly proportional to the extension of the sample. This may also be given as an explanation of the retroflex portion of the sulfur curve taken a t - 10.3’C and of the flat portion a t the inflection point of the raw rubber curve (the - 6o0C curve). This curye for rubber is very similar to a stress-strain curve for g u t t a
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percha or balata a t ordinary temperatures.‘ The curves for rubber given here do not agree quantitatively with the data given by Kroger and LeBlanc2 in a paper “Vulcanization by Cooling”. This discrepancy is no doubt due to the different rates of elongation.
summary The stress-strain curves of raw rubber and plastic sulfur are determined at various temperatures. The stress-strain data were recorded automatically and this made possible a greater speed of testing. For this reason it is believed that the data are fairly independent of plastic flow. Attention is drawn to the marked similarity between the stress-strain curves of raw rubber and plastic sulfur. If it had been possible to run either material on both testing machines this would have been done. Schenectady, New York August 26, 1927. 1
Park: Ind. Eng. Chern., 17, 152 (1925)
* Kolloid-Z., 37, 206 (1925).
