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ANALYTICAL CHEMISTRY
indicates to steel men that coal must be low in total sulfur to produce low-sulfur coke by usual methods. Only one coal mixture was tested under one set of conditions and thus one cannot absolutely rule out the possibility that with another coal mixture under the same oven conditions some preferential evolution of one form of sulfur might occur. The data of Lowry et al. ( b ) show that this effect does not occur, however, with a wide variety of coals. The main hope for lower sulfur coke from coal entering the oven with a given sulfur content would seem t,o be in adjustment of coking conditions. ACKNOWLEDGMERT
The authois are indebted to the late J. H. Slater and Earl C. Smith of the Republic Steel Corporation for their part in sponsoring the study, and to Robley D. Evans and John IT. Irvine, Jr., of the Massachusetts Institute of Technology for their valuable guidance. They are further indebted to the following Republic Steel personnel for their assistance in the experiment: C. H. Flickinger, chief chemist, Cleveland District, and R. L. Horn, and to the Bureau of Mines for checking the analytical work. Finally, they acknowledge the participation of the follon4ng personnel at ilrthur D. Little, Inc., who assisted in preparation
of the pyrites and in obtaining and analyzing the samples: E. A. Rietzel, J. L. Utter, A. R. Almeida, and E. L. Pepper. LITERATURE CITED (1) Allen, E. T., Crenshaw, J. L., Johnston, John, and Larsen, E. S.,Am. J. Sci. (4), 33, 169-236 (1912). (2) Eaton, S.E., Hyde, R. W.,and Old, B. S., Am. Inst. Mining Met. Engrs., Iron and Steel Diu., Metals Technol., 15, No. 7, Tech. Pub. 2453 (1948). (3) Lowry, H. H., ed., “Chemistry of Coal Utilization,” Vol. I, pp. 427, 487, New York, John Wiles & Sons, 1946. (4) Ibid., p. 947. (5) Lowry, H. H., Landau, H. G., and Naugle, L. L., Trans. Am. Inst. Mining M e t . Engrs., 149, 297-330 (1942). (6) Powell, A. R., and Parr, S. W., U. S.Bur. Mines, Tech. Paper 254 (1921). (7) “Scott’s Standard Methods of Chemical Analysis,” Furman, N. H.. ed., 5th ed., Vol. I, pp. 906, 913, New York, D. Van Nostrand Co., 1939. (8) Thiessen, G., I n d . Eng. Chem., 27, 473-8 (1935). (9) Treadwell and Hall, “Analytical Chemistry,” 8th ed., Vol. 11, p. 336, New York, John Filey & Sons, 1935. (10) U. S.Steel Corp., “Methods of the Chemists of the U.S.Steel Corporation for the Sampling and Analysis of Coal, Coke, and By-Pioducts,” 3rd ed., pp. 77-80, Cainegie Steel Corp., 1929. RECEIVED October 27. 1948.
Mechanical Stability Test for Hevea latex 11. G . D4WSON The Firestone Tire & Rubber Company, Akron, Ohio
The mechanical stability test is a rapid, simple method of estimating the colloidal stability or quality of Hevea latex by high speed stirring. Latex particles start to agglomerate as soon as the peripheral speed of the agitator reaches a certain minimum value. Progressive flocculation continues until mechanical coagulation occurs. The end point
H
EVEA latex is a complex, colloidal suspension that is con-
tinually changing with age, temperature, agitation due to centrifuging or transportation, preservatives, or addition of compounding ingredients. Ever since latex became an article of commerce, the need has been recognized for a quick, reliable test of the colloidal stability of a given sample. Three general approaches have been made to this problem of measuring the stability of latex: mechanical, chemical, or heat. The first method is based on the application to the latex particles of a mechanical shear by rapid stirring, slow stirring, shaking, or rubbing. Chemical stability involves the addition of a “sensitizing” agent such as zinc oxide, ammonium sulfate, calcium sulfate, or sodium silicofluoride to the latex, and measurement of its effect by the resultant increase in viscosity or decrease in mechanical stability; while heat involves thermal agitation of the latex particles as well as possible changes in the coating on the particles. Although many tests have been devised based on one or more of these approaches, the simplest and most efficient in time and sensitivity is the mechanical stability test. This test, as generally run using a high speed stirrer, has been more or less generally accepted as a standard test, but there is some question as to whether it gives a completely accurate index of the stability under conditions that prevail in ordinary handling operations. It has been Tidely used in various forms for a number of years,
is defined as the time in seconds required to coagulate 0.5 to 1.0% of the total solids. If the shear is constant, the time is proportional to the colloidal stability, which depends upon the interfacial film between the latex particles and the serum. The mechanical stability time depends critically upon the size, the total solids, and the temperature of sample.
and no doubt a number of studies have been made on the effect of different variables, but no publication attempting to summarize such studies has appeared in the literature. In 1930, Morris ( 7 ) used a Hamilton Beach drink mixer “to compare the stabilities of the various latices when subjected to vigorous agitation.’] Several investigators had made exploratory experiments with the method before 1930, but the first published account of a high speed stirring test for the estimation of latex stability was given by Soble (9)in 1936. He specified a Hamilton Beach mixer, a 118-ml. (&ounce) square bottle, and a 50-gram sample at 30% total solids. Soble also stated that the addition of 770 zinc oxide decreases the (mechanical stability) time by one half. This is the first mention of a combined mechanical and chemical stability test where some chemical such as zinc oxide is added to “sensitize” the latex. In 1937, Jordan (6) included a mechanical stability test, using a Hamilton Beach drink mixer, in a list of proposed physical testing methods for the examination of rubber latex and rubber latex compounds. Murphy (8) devised a test apparatus which was designed to reproduce under controlled conditions the hand rubbing test in which coagulation is brought about mainly by the combined influence of friction and evaporation, It consisted of a molded rubber nose, as a rubbing element, which rotated with a sun and planet motion over a glass plate on which the latex sample was spread. The end point was defined as the number of seconds for the film to commence to break u p into small particles of coagulum. Davey and Coker ( 3 )objected to the high speed stirring test on the basis that it tends to break up the coagulum as it is formed; hence, they proposed a tester having a cylindrical ( 5 X 2 cm., 2 X 1 3 / l s inch diameter) impeller, with three vertical fins, rotating a t
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V O L U M E 21, NO. 9, S E P T E M B E R 1 9 4 9
Figure 3. Appearance of Latex Coagulum on Cheesecloth a t End Point
of the procedure. All this emphasizes the need for further study of the many variables and rigid standardization of the test, Figure 1. Mechanical Stability Tester
APPARATUS
The test apparatus (Figure 1) is essentially a vertical shaft, high speed stirrer capable of maintaining B constant speed of 14.000 r.p.m. for the duration of the test. I"
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t h e ~ d rrubb&coAenti i Martin (6)~substitutedsodium silicofluoride for zinc oxide in the test. ~
The mechanical components consist of FI Dumore grinder No. 5 (The Dumore Co., Rscine, Wis.) equipped with IL special quill designed for vertical rather than horizontal oneration.
Divisioi of Rubbe; Chemistry. AMERICANC H E M I C A SOCIETY. ~
widely used fa; come io years, 6ut its irregular shape makes it diffioult to reproduce; hence, a plain polished stainless steel disk 0.844 (2'/aB) inoh in diameter and 0.0625 (l/ls) inch thick is
the old standard. The bottle employed in these tests is a flst-
tkbtnne havine an inside diameter of 26/x, inches (55 mm.). The bottle holder is so constructed that the bottle may be conveniently lowered or raised until the agitator is any desired distance from the bottom of the bottle and exaotly in the center. Constant temperature of the sample is maintained by a water tank with a window m shown in Figure 1. I
I
PROCEDURE
Figure 2. Propellers and Bottles Used i n Mechanical Stability Determinations
Dilute the latex to exactly 51.5% total solids (50% dry rubber oontent) with distilled water.
ANALYTICAL CHEMlSTRY
1068
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Figure 5. Figure 4.
8
10 12 14 SPEED (THOUSAND R P M )
Effect of Speed of Stirrins on Mechanioal Stability Time
Menisow Fall D u e to Foam Collapse ae Test Appmadws End Point
1% of the total solids i s measured
6y filterinp%e latex through
Dart oi the Drocedure. a t least &til the operator becomes skilled at &mating %heend paint by the visual Gethad described below. The approach to the end point can usually be noted visually by a drop of the meniscus of the latex due to ioam collapse (Figure 4) and loss of turbulence. Dip a glass rod into the latex and draw it across the palm of the hand (clean and moist); the presence of small floos of coaqulum is evident a t the end point, as can bP further shown by filtering the latex through cheesecloth. One can also introduce a dron oi latex onto z lame surface oi distilled water ih a Petri dish or witch glass. The lat& immediately spreads ou1 into a large film which permits as," inspection for rninnt,e Rook? present st the end point. FACTORS AFFECTING THE T E S T
Speed of Stirring. The greater the speed of stirring the less the time required to reach a given degree of coagulation or the end paint (Figure 5). A Benedict ring-type propeller (16/,8-inch diameter) was used in all these determinations. All latices used were commercid ammoniated concentrates (61 to 62% total solids) designated by letters 4 . to T in Figures 5 to 13. Abnormally high mecbanical stability time values obtained at speeds below 8000 r.p.m. can be explained by assuming that at low speeds the shear has been reduced below the minimum required to overcome the repulsive forces between the latex particles. Diameter and Thickness of Propeller. A series of stainless steel and plastic disks varying in diameter from 0.75 to 1.47 inches (Figure 2) was used a t 14,000 r.p.m. in testing several latices. Meahmical stability time i s inverse!y proportional to the di-
Figure 6. Relation of Propeller Size to Mechanioal Stability Time
V O L U M E 21, NO. 9, S E P T E M B E R 1 9 4 9
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Figure 10. Effect of Latex Sample Size on Mechanical Stability Time
stability time obtained by changing the diameter of the propeller (Figure 6), i t is concluded that most of the shear or rub takes place a t the edge of the propeller rather than a t the bottle wall (Figure 8). Shape and Inside Surface of Test Bottle. The rub or shear between the latex and the bottle is a t a minimum when the bottle is perfectly round and smooth. When the standard test bottle was lined with 0.25-inch mesh hardware cloth the mechanical stability time for a given latex decreased from 1050 to 600 seconds, indicating that the total shear had increased almost 50%. Roughening the inside surface or changing the shape of the bottle increases the shear at the wall and decreases the mechanical stability time for the latex. Distance of Propeller from Bottom of Test Bottle. The mechanical stability time should decrease as the distance between the propeller and the bottom of the test bottle is decreased if the shear or rub takes place between the bottom of the propeller and the bottom of the test bottle. However, Figure 9 shows that there was little change in mechanical stability time when the propeller was lowered progressively from 1 inch to 0.5 inch, to 0.25 inch from the bottom. Decreasing the distance to 0.125 inch from the bottom of the test bottle gave marked increases in mechanical stability time values, which indicates that the shear had been decreased rather than increased. Consequently, the test should not be run with the propeller less than 0.25 inch from the bottom of the test bottle and the magnitude of any possible errors would be decreased if the distance was increased to 0.5 inch. Size of Sample. With all other conditions constant, the larger the sample of a given latex the greater the mechanical stability time value (Figure 10). When the sample size T%-asreduced to 40 ml. abnormal results were obtained, which indicated that the rate of shear as well as the sample size had been decreased. The rate of change of mechanical stability time with an increase in sample size is greater for a high stability latex than one of low
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PER SOLIDS Figure 12. Effect of Dilution of Sample on Mechanical Stability Time
stability. This phenomenon has been noted for several of the other factors studied, and indicates a measure of the probability that two particles will stick together upon collision. Assuming constant shear, the probability depends upon the nature, extent, and degree of hydration of the coating or interfacial film on the individual latex particles. Ammonia Concentration. Ammonia concentrations greeter than 0.4% have little effect on the mechanical stability of latex
1071
V O L U M E 21, NO. 9, S E P T E M B E R 1 9 4 9
is really a combination of two stability tests-heat and mechanical. Therefore, temperature is very critical and close control should be maintained while the test is being run. MECHANISM OF MECHANICAL STABILITY TEST 28031
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Mechanical Stability Time in Relation to Temperature of Latex Sample
(Figure l l ) , but concentrations below this figure represent a greater drop in p H and a corresponding more rapid drop in mechanical stability time was observed. Here again, the rate of change of stability time for a high stability latex is much greater than the rate for a low stability latex. Replacement of Air by Oxygen or Nitrogen. S o change in mechanical stability time values could be obtained by running the test in an atmosphere of pure oxygen or nitrogen instead of air. This indicates that the test involves only a physical change rather than a chemical reaction such as oxidation. Total Solids of Sample. At high total solids concentrations (61 to 62%) all natural latices tend to have low mechanical stability (Figure 12). As the total solids is decreased the stability increases very rapidly, indicating that not only are the latex particles farther apart, but there is probably a solvation effect, Dilution is critical and should be done very carefully. Two latices that had a daerence of 270 seconds a t 51.5'% total solids differed by 480 seconds a t 45% total solids. The lower the total solids content of the sample the greater the sensitivity of the test; however, this advantage is offset by longer running time. The present recommended dilution of 51.570 total solids appears to be a satisfactory compromise. Extrapolation of the curves in Figure 12 to zero stability time indicates that the total solids would be GSaJo-the approximate limit of concentrating latex. Temperature. The mechanical stability time of a given lates varies inversely with the temperature and the rate increases rapidly as the temperature of the latex is lowered below room temperature (Figure 13). The stability of a poor quality lates can be increased from 400 seconds a t room temperature to 1800 seconds by cooling t o 10" C. This increase is very significant in the storage, aging, and compounding of latex. Heating latex from 25" to 60" C. causes a decrease in mechanical stability time, but the rate of change decreases rapidly as the temperature approaches 60" C. and the mechanical stability is always low. This
High speed stirring appears to agglomerate or cause the single latex particles to gather into minute clumps or flocs. Van Dalfsen (12) called this microflocculation. If the mechanical force applied is sufficient to overcome the repulsion of like charges on the latex particles (this corresponds to the shear a t minimum peripheral speed of the agitator), then the agglomeration takes place a t a rate depending upon the probability of particles' touching where there is no coating. If the mechanical shear is constant, the time required to reach a certain degree of agglomeration or coagulation depends upon the colloidal characteristics of the latex being tested. I n turn the colloidal stability of natural latex depends upon the interfacial film ( 1 ) between the latex particles and the serum in which they are suspended. This coating is made up of lipides ( d ) , soaps, and proteins which possess an electric charge (11) and a certain degree of hydration. If one of these factors is reduced, the stability of the latex is reduced; if one or more of these factors is increased either by natural reactions in the ammoniated latex during aging ( I I ) or by the addition of proteins, soaps, lipides, or water, the stability of the latex is increased. The test described is entirely mechanical; the shear or rub takes place at the outer edge of the agitator. This shear is directly proportional to the peripheral speed of the agitator. The process of agglomeration or microflocculation of single latex particles starts as soon as the test is started and proceeds as a result of continual mechanical agitation until the flocs become large enough to be visible without magnification. If stirring is continued a single large ball of coagulum is produced which occludes liquid lates. The "end point" is merely an arbitrary stopping point in this progression from billions of single latex particles to one big ball of coagulum. This has been arbitrarily defined as the point where 0.5 to 1.0% of the total solids has been coagulated to a size large enough to be retained by cheesecloth (Figure 3 ) . Experience has shown that this end point is reproducible with an accuracy of 3% or better. ACKNOWLEDGMENT
The writer wishes to thank 0. D. Cole, J. K. Liska, L. A Wohler, and E. RI. Glymph for their interest and encouragement in this work, and the Firestone Tire 8: Rubber Company for permission to publish this report. The author is indebted to Joe Koch for the design and to F. S. Grover for the fabrication and assembly of the mechanical stability tester. LITERATURE CITED
(1) Cockbain, E.G.,Rubber Age, 62,649 (1948). (2) Crude Rubber Committee, Div. of Rubber Chemistry, Rubber Chem. Technol., 14, 290 (1941). (3) Davey, W. P.,and Coker, F. J.. Trans. Inst. Rubber Ind., 13,368 (1938). (4) Homans, L. N. S., van Dalfsen, J. W., and van Gila, G. E., ,Vature. 161 (4083).177 (1948). Jordan, H. F., Brass; P. D:, and Roe, C. P., IND.ENG.CHEW, AXAL.ED., 9,182(1937). Martin, George, Reu. g e n . caoutchouc, 18,90 (1941). Morris, V. K., Firestone Tire & Rubber Co., Drivate communication, February 1930. (8) Murphy, E. A.,Proc. Rubber Tech. Conf., London, 159 (1938). (9) Koble, R. J., "Latex in Industry," p. 181,New York, Palmerton Publishing Co., 1936. (10) Novotny, C. K., and Jordan, W. F., IND.ENG.CHEM.,A N A L E~.,13, 189 (1941). (11) Seifriz, W., Science, 102,378 (1945). (12) van Dalfsen, J. W., Rubber Chem. Technol., 14, 315 (1941). RECEIVEDKovember 17, 1948. Presented before the 53rd meeting of t h e Division of Rubber Chemistry, AMERICANCHEMICALSOCIETY,Detroit. Uich.. Xovember 10, 1948.
