INDUSTRIAL A N D ENGINEERING CHEMISTRY
September, 1924
927
Consistency of Rubber-Benzene Solutions'" By Winslow H. Herschel B U R ~ AOF U
STANDARDS,
HE rubber experimental stations in the Dutch East Indies have found that, although the behavior of rubber during and after vulcanization cannot be predicted from determinations of the so-called relative viscosity, this test, combined with vulcanization tests, gives valuable indications, and in certain investigations viscosity determinations alone may be of use. I n the method adopted by the rubber experimental stations, using the Ostwald viscometer, ratio of time of flow of the rubber solution to time of flow of benzene a t the same temperature of 30" C. (86" F.) is called the "relative viscosity," although, strictly speaking, it is only a t very low velocities of flow that the ratio of time of flow is equal to the relative viscosity. Since it is practically impossible to have the solution, as run, of exactly the desired concentration, a diagram is furnished to correct results to the standard concentration of 1 per cent. An examination of this method shows the following possible objections:
T
1-Inasmuch as the time of flow is greatly reduced if the crepe rubber is milled before being dissolved, the question naturally arises whether the rolling received by the crepe a t the plantation would not have the same effect, so that tests on benzene solutions should take account of the method of preparing the crepe. %-There seems to be no adequate reason for comparing the time of flow of the solution with that for benzene. Water has the advantage of being a definite chemical compound of'known viscosity, easily procurable, and of high purity. C. P. benzene is less readily procurable. The difference in time of flow of commercial and C. P. thiophene-free benzene ww found to be hardly greater than the experimental error with an Ostwald viscometer at 30' C., being 17.4 seconds with the former and 17.3 seconds with the latter. With water at 30' C. and a time of flow of 21.0 seconds, the percentage error in timing would not be so great. 3-Since the equation connecting absolute viscosity with time of flow contains two instrumental constants, it is necessary to determine two times of flow with water, a t two temperatures, to calibrate the viscometer so that its readings may be readily convertible into viscosity in c. g. s. 'units, provided that the benzene solutions of rubber are not plastic but viscous. The distinction between viscosity and plasticity is very important and will be considered later. 4-As Ostwald viscometers are of glass, it is impracticable to make them all of exactly the same dimensions. A correction must therefore be applied to reduce all results to those of a standard instrument. This correction is made most easily by calibrating each instrument with water and expressing all results as viscosity in poises, the poise being the c. g. s. unit of viscosity. If, however, the solution is not viscous but plastic, the magnitude of the correction depends upon the nature of the solution, as well as upon the dimensions of the instrument, and a single time of flow is inadequate to determine the consistency of the solution.
DISTIXCTION BETWEEN VISCOUS
AND PLASTIC MATERIA4LS
Viscosity is the constant ratio of shearing stress to rate of shear. A material is considered plastic if the apparent viscosity (ratio of shearing stress to rate of shear) varies with the rate of shear. Viscosity can be expressed by a single numerical value, but the consistency of a plastic material must be expressed by an equation or other, means which completely .defines the variable relation between rate of flow and the force that produces its4 1 Presented under the title "Method of Determining Consistency of Benzene Solutions of Rubber" before the Division of Rubber Chemistry a t t h e 67t.h Meeting of the American Chemical Society, Washington, D. C., April 2 1 to 26, 1924. * Published b y permission of the Director, U.S. Bureau of Standards. . 8 Sheeley, THISJOURNAL. 15, 1109 (1923). 4 Hall, Bur. Standards, Tech. Paper 234, 351 (1923).
WASHINGTON. D. C.
It is to be expec€ed that true solutions and mixtures of miscible liquids will be viscous, but that mixtures of a liquid with a powder or finely divided material will be plastic, if of sufficiently high concentration. I n cases of doubt tests are necessary, the essential requirement being that successive trials be made a t different rates of shear. The standardization of the rate of shear, as has been proposed,6 may enable differentlaboratories to obtain the same numerical results but the complete relation between the rate of flow and the force which produces it cannot be obtained by a single test. Two different plastic materials which show the same apparent viscosity on a single test would not necessarily have the same consistency. The Ostwald viscometer is not suitable for the measurement of plasticity as ordinarily used, because the pressure head, and consequently the rate of flow for a given material, is fixed. By tests with the Bingham viscometer,6 in which the rate of flow can be varied, the ratio of rate of flow to pressure head was found to be a variable, thus showing that, a t least in some concentrations, benzene solutions of rubber are plastic. The 1 and 2 per cent solutions of crepe rubber in commercial benzene, which were used in these tests, were prepared in accordance with the method of the Delft I n ~ t i t u t e . ~It was found that if the crepe was milled before being dissolved, a 2 per cent solution would be viscous, and further milling resulted in a solution of lower viscosity. This is in agreement with the previous observation that the apparent viscosity decreases with the length of milling of the rubber before it is dissolved. As the concentration is decreased a point must be reached where the solution is viscous. The exact concentration where this change takes place has not been determined, but solutions of between 0.03 and 0.47 per cent concentration have been reported as viscous.8 COXCLUSIONS 1-Tests of the consistency of benzene solutions of rubber should be made by a method which permits the distinction between a viscous and a plastic material to be observed and reported. 2-Results should be expressed in poises when the solution is found to be viscous. 3-At least two numerical values are required to express the consistency of plastic materials such as (at least in some cases) benzene solutions of unmilled rubber. 6 Shulenberger, Paznt, Ozl Chem. R e v , 72, August 17, p 10, a n d f4, 1164 (1922), 16, 310 October 5 , p 10 (1921), see also THISJOURNAL,
(1924) 6
7 8
BUY Standards, Scr Papev 298, 64 (1917). Kolloidchem. Berhefle, 10, 83 (1918-19) C A , 18, 1065 (1924)
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