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I N D UXTRIAL 24ND ENGIXEERING CHE*MISTRY
or similar materials a less complete aggregation may be accomplished. The filterability of the treated Revertex was not so great as that of the aggregated latex. It has been found that considerable difficulty may be encountered in obtaining the exact degree of aggregation and consequent filterability when repeating experiments, especially if latices are used which have been obtained from different sources or from trees tapped shortly after a rest period. The addition of pigments will in some cases cause marked increase in the degree of aggregation. In view of these factors, the development of a process which always yields a latex in a definite state of aggregation would seem to be a difficult problem. Acknowledgment
The writer wishes to acknowledge his indebtedness to N. A. Shepard for his helpful suggestions and willing counsel during the course of this work.
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Literature Cited Anode Rubber Co., British Patent 252,673 (1927). Ditmar, British Patent 214,224 (1924). Dunlop Co., Ltd., British Patent 285,938 (1926). Dunlop Co., Ltd., Canadian Patent 284,565 (1928). Hauser, “Latex,” p. 3, Steinkopff. Hauser, Zbid., p, 94. Hauser, I h i d . , p. 95. Hauser, Ibid., p. 121. Hauser, German Patent 412,060 (1923). Hauser, M. I. T.Lectures, 1928. Hazell, British Patent 295,700 (1929). Hopkinson and Gibbons, U. S. Patent 1,542,388 (1925). Hopkinson and Gibbons, U. S. Patent 1,632,759 (1927). McGavack and Rumbold, IND.ENG.CHEM.,Anal. Ed., S, 94 (1931) MacKay, I n d i a Rubber J . , No. 10 (1930). Ostwald, Fischer’s Handbook of Colloid Chemistry, p. 263, Blakiston. Smith, U. S. Patent 1,678,022 (1928). Stevens, “Latex,” p. 21, British Rubber Growers Assocn. Stevens, Ibid., p. 9 . Venosta, British Patent 233,458 (1924).
Effect of Storage on Milled Crude Rubber’ C. M . Carson THE
GOODYEAR TIRE82 RUBBER COMPANY.
AKRON, OHIO
Smoked sheets which have been milled to different months which the p r e s e n t U R k n o w l e d g e of degrees of plasticity and stored in bale form for periods paper attempts to cover. cured rubber, aged for up to 9 months show a decided increase in modulus, various periods of time Experimental Methods plasticity, and recovery values. The increase in reis fairly extensive. The fact covery value is the most noticeable, the change being Smoked sheets were milled that uncured rubber also un180 per cent of the original, if the rubber is baled at under definite procedures to dergoes certain changes, even 40-50” C. and stored at 10-20” C. for 9 months. produce four different plasover short storage p e r i o d s When the aged, milled rubber is mixed in a tube ticity grades ranging f r o m ranging from a few hours to stock and processed on a tubing machine, the stock is slightly to thoroughly plastiseveral days, is also known rougher and the speed of extrusion is slower than a cized rubber, and stored a t in a g e n e r a l way. It was similar stock containing freshly milled crude rubber. considered i n t e r e s t in g to two temperatures (10-20’ C.) The plasticity of stocks which are subjected to tubing and a t 55” C., for periods up s t u d y t h e effect of l o n g e r operations is shown best on an extrusion type plasto 9 months. For convenaging periods along this same tometer in preference to the compression type. line. ience in handling, the rubber Milled crude rubber “freezes” at temperatures below was baled, 225 pounds per TThen p l a s t i c i z e d or so0’ C. and thaws at room temperature of 15-25’ C. bale, using pressures of 60 to called broken-down rubber It may be permanently frozen by being placed under 70 p o u n d s per square inch is not used within a reasonslight pressure for several months, freezing temperato exclude air; and in order a b l e p e r i o d , it r e g a i n s a ture being unnecessary. In either sheet or milled not to overlook any effect of certain amount of “nerve” form this type of frozen rubber requires a temperature original t e m p e r a t u r e , the and b e c o m e s more difficult of about 50’ C. to thaw, whereas temporarily frozen bales were prepared with rubto handle in ordinary facrubber will thaw at room temperature. ber a t three temperaturestory processes. This p a p e r 43”, 72”. and 100” C. deals Drimarilv with the effect Plasticity values are based on the Williams plastometer of stoiage a t “different temperatures on milled crude rubber, in an attempt to translate the general term “nerve” into defi- under the following testing conditions: a 1-cc. pellet, under nite physical properties such as plasticity, modulus, and rate 10 kg. pressure for 3 minutes a t 70” C . , the compressed height of cure. This investigation was conducted on a factory scale being expressed in millimeters times 100. The regain or in order to permit comparisons between typical factory recovery value is based on a 1-minute recovery expressed operations and these physical properties, and a total amount in the same way. The four plasticity groupings used in of 40,000 pounds of rubber was used. this work were 390,350,325, and 275. The first was obtained The literature contains a number of articles dealing with by one breakdown on an 84-inch mill, the second by one breakchanges taking place in rubber a t certain definite tempera- down on a 60-inch mill set a t a somewhat tighter gage, the tures. Among these is an article by Griffiths ( 1 ) appearing third by remilling 390 rubber, and the fourth by remilling 325 in 1926, in which he evaluated “nerve” in terms of extrusion rubber. plasticity. He showed that the plasticity figure did not Results increase for periods up to 30 hours after the rubber had been cooled below 55” C. He did not continue his experiment beyond 30 hours, and it is this longer period extending into The effect of storage is most noticeable in the recovery value with but slight change in the plasticity figure, except for 1 Received March 16, 1931. Presented before the meeting of the the longest storage periods. The recovery value increases Akron Rubber Group of the American Chemical Society, February 19, 1931. consistently under all conditions inyestigated up to 9 months,
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the greatest increase being with 275 plasticity rubber baled a t 43" C., and stored a t room temperature (10-20" C.). This increase is about 180 per cent of the original recovery figure. The least increase, 80 per cent, occurred with 390 rubber baled a t 100"C. and stored a t 55" C. The modulus of the rubber showed a decided increase similar to that noted in recovery. However, modulus was increased most by storage a t 55" C., whereas the plasticity and recovery figures had been increased most by storage a t room temperature.
COOLING TIME
OF
MILL E D . BALED CRUDE RueeER
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taken at 700 per cent elongation on the Goodyear autographic machine. The most marked effect is the increase in modulus shown by storage a t 55" C. (Figure 5). Up to 6 months there was a consistent increase in modulus of samples stored either a t room temperature or a t 55" C. The last 3 months showed a still further increase in modulus of bales stored a t 55" C., but not in those stored a t 10-20" C. Table I-Increase
i n Plasticity Values Due t o Different Original and Storage Conditions 3 MONTHS 6 MONTHS 9 MONTHS
Temperature stored: 1&200 c. 55-60' C. Temperature baled: 40-45" C. 70-75' C. Over loOD C.
%
%
%
6.8 6.6
6.1 7.9
13.7 10.5
6.4 7.6 4.9
6.8 7.3 6.6
10.7 12.1 8.3
It was impossible to get a complete range of cures on every sample, but this was obtained on every tenth sample. These results, taken in connection with the stress-strain curves and with hand tests, gave a comparison of rate of cure before and after aging. It was found that a t 3 months 33 per cent of the samples had become faster curing than originally, a t 6 months this percentage had increased to 45 per cent, and a t 9 months to 50 per cent, with a corresponding decrease in samples showing a slower rate of cure. The stress-strain curve showed a decided tendency to become steeper with increased aging time, which is an added indication of the faster rate of cure noted above. Table 11-Change
Figure 1
The change in rate of cure was usually one of gradual, but slight, increase; i. e., the best cure changed from 40 minutes a t 126.5" to 35 minutes a t 126.5"C. Figure 1 is a cooling curve showing the time required by rubber, baled a t different temperatures, to come to equilibrium. Measurements were made by thermocouples inserted in the rubber a t the time the bales were prepared. The increase in physical property values is shown in Figures 2 to 5. Figure 2 shows the percentage increase in recovery due to storage temperature. The recovery value is used in preference to the plasticity increase, since it was found to be a more definite and uniform figure. Storage a t room temperature of 10-20' C. was found to give a gradually increasing recovery value, ending a t 130 per cent in 9 months. Storage a t 55" C. caused a smaller increase of 110 per cent in 9 months. The effect of baling temperature is shown in Figure 3. Rubber baled a t 43" C. increased in recovery to a figure 180 per cent higher than the original, in 9 months, while rubber baled a t 100"C. showed a much smaller increase. Figure 4 shows the effect of the original plasticity groupings in which the rubber had been divided. I n this case the increase in plasticity value was plotted. As would be expected, the softest rubber showed the greatest increase, while the rubber which had been worked the least showed the smallest change. I n general, the average plasticity increase was sufficient to place the rubber in the next higher plasticity group. Table I shows the percentage increase in plasticity. The modulus changes were of a somewhat different nature. All samples were cured in the following formula: rubber 100, zinc oxide 4, sulfur 6, and captax 0.5. The cure was 40 minutes a t 126.5" C., the modulus being
Flatter Steeper N o change
in Shape of Stress-Strain Curves after Aging 3 MONTHS 6 MONTHS 9 MONTHS
%
%
%
14 37 49
2 56 42
6.5 67.5 26
Action of Aged Rubber in Factory Stocks
When compounded in regular factory stocks, the aged rubber was found to act practically the same as stocks containing freshly milled rubber, on mills and calenders. However, in comparing regular factory runs of tube stock, it was found that t'he former was much rougher, more porous, and slower
Figure 2
tubing than the latter. The physical tests showed the Williams plasticity figure to be equal to or lower than in the freshly milled stock, indicating a smoother, faster tubing compound. The modulus figure was higher in the stock containing the aged rubber, it was also slower curing even though the rubber itself seemed to be faster curing. This is explained by the fact that the rubber used in the tube stock
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had been aged only 3 months or less and the trend toward faster rate of cure was not yet so noticeable. Since rough tubing could not be explained by a slower curing stock and since higher modulus is not necessarily a cause of rough tubing, the question of the low Williams plasticity figure is interesting. It appears that the Williams machine, being a compression type plastometer, is entirely satisfactory as a gage of milling and calendering operations, but in dealing with a tubing machine it is necessary to use a different type, such as the extrusion plastometer, which approximates more nearly actual tubing principles. When samples of tube
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cases the modulus and tensile were much higher and the cure faster in the stored rubber than in the fresh rubber. I n three samples the stored rubber was completely ruined by the age test, whereas the same rubber when fresh had withstood this test. (3) The 12-day oxygen test was not so severe on the stored rubber as the nitrogen bomb had been. Nevertheless, the samples were all inferior to the same rubber when fresh. (4) The logical conclusion to be drawn from this is that the natural antioxidant has been destroyed by storage. Change i n Chemical Properties of Aged Rubber
A change in chemical properties of rubber is sometimes an indication of the nature of physical changes taking place. Table I11 shows the chemical changes occurring in several bales. C h a n g e s in R u b b e r on Storage f o r 9 Months N2 I N ACETOXE ALCOHOLIC ACID N2 IN ACETONE EXTRACTED EXTRACT KOH EXTRACT NUMBER EXTRACT RUBBER BALE Before After Before After Before After Before After Before After Table 111-Chemical
ON
R E C O V E RTIGURE Y
%
%
%
%
%
%
%
%
0.021 0.025 0.016 0.018
0.53 0.53 0.56 0.46
0.44 0.42 0.38 0.39
0.011 0.015 0.52 0.011 0.015 0.52 0,010 0,014 0.59
0.40 0.33 0.43
Stored at - 5 . 6 O C. _....___
165 167 168 173 174 175 176
Figure 3
stock containing aged rubber were compared with samples containing freshly milled rubber on an extrusion type plastometer, they mere found to have a higher plasticity figure in the same degree as their action on the tubing machine would lead one to expect. Age Tests on Aged Rubber
Samples of rubber from a number of bales were put up in the testing formula previously used. Modulus tests were run on the fresh stock and on the same stock after 12-day oxygen and nitrogen aging. After 9 months' aging at, -5.6" and a t 55" C. the bales were re-
9
1.10 370 297 1.00 360 273 0.95 362 204 0.90 230 204 Stored a t 55' C. 3.65 3.20 0.35 1.07 266 236 0.85 275 205 3 . 6 0 3.25 0.42 3.75 3 . 4 5 0.48 1 . 2 5 233 217 3.50 3.37 0.35 3.40 3 . 3 2 0 . 3 0 3.50 3.10 0.40 3.55 2 . 9 8 0.45
0.015 0.014 0.010 0.014
The effect of storage is exactly the opposite of the effect of heat on rubber (4) except in nitrogen distribution. In the latter case the acetone extract and acid number increased. The decreased acid number and acetone extract may be due to a polymerization of part of the fatty acids, making them acetoneinsoluble. Weight is given to this theory by the increased alcoholic potassium hydroxide extract. Alcoholic potassium hydroxide would reverse the polymerization process and extract the fatty acids from the rubber. The fact that the alcoholic potassium hydroxide extract increased more than the acetone extract decreased may be explained by supposing that some other non-rubber constituents have been rendered soluble by aging. The change in nitrogen distribution is in line with what we would expect. "Frozen" Rubber
Among the many peculiar properties of rubber, that of freezing and thawing has been little investigated, in spite of its presence, during a t least a part of the year, in this climate.
y
I I I I I I
I
LoLTLkTdaA
I I I I I I I I
Figure 4 Figure 5
sampled and retested both before and after 12 days' oxygen and nitrogen aging. The results were as follows: (1) The aged rubber conformed to the general average of all stored rubber; i. e., modulus and tensile were higher after storage than before. The rate of cure was somewhat faster. (2) Aged rubber after 12-day nitrogen bomb test in the testing formula was much poorer aging than the fresh rubber. In most
It is occasionally encountered in midsummer and in rubber shipped direct from the plantations. Van Rossem and Dekker (s), as a result of a study of frozen smoked sheets, advanced the theory that the cause of the frozen conditions was crystal growth. The density, hardness, and light adsorption of frozen rubber a t various low temperatures were suddenly
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Vol. 23, No. 6
decreased a t about 36-38" C., indicating a melting of crys- Two adhering sheets, 3 by 3 inches thaw in 3 minutes; three tals. They evidently did not encounter frozen rubber of sheets of the same size thaw in 4l/2 minutes. This can be the type which has not been exposed to freezing temperature. carried on up to the dimensions of a bale, which thaws comLeblanc and Kroger (3) and Kroger (6) used sufficient pres- pletely in 24 hours if not surrounded by other bales. sure on rubber to cause a change of state (aggregation) which The thawing of temporarily frozen rubber is a different type they regarded as analogous to the effect produced by cold of change. Simple exposure to a moderate room temperature and, in a way, resembling vulcanization. will thaw sheets in a few minutes. The thawing of a bale is, Apparently, milled rubber undergoes the same change as of course, a longer process, but extreme temperature is not smoked sheets. Several bales of milled rubber were placed necessary except as a means of hastening heat transfer. in a local ice plant a t -5.6" C. and examined a t various interNone of the baled, milled rubber was found to be pervals during 38 weeks. The bales showed normal cooling manently frozen, even after 38 weeks at -5.6" C. However, curves and came to equilibrium in 4 to 6 days, depending upon a number of 1-pound samples which were deformed by a their original temperature. When removed from the ice rubber-cutting machine and later stored under slight pressure plant, the bales thawed in 5 to 7 days at room temperature for nearly a year a t room temperature (5-35" C.) assumed varying from 15" to 30" C. the opaque, horny condition of permanently frozen smoked Frozen rubber, whether in sheet or milled form, assumes sheets and did not thaw out a t room temperature. It would an opaque, creamy color and becomes very difficult to work. seemi therefore, that pressure is one cause of permanent This condition may be either permanent or temporary. By freeiing. the former is meant rubber which remains unworkable, Acknowledgment boardy, and opaque at room temperature; and by the latter, rubber which will thaw a t room temperature. Most frozen The writer wishes to express his thanks t o G . K. Hinshaw rubber is in the second class. Both usually have the same J. P. Maider for helpful criticism in the preparation of and appearance, although bales of smoked sheets may assume this article, and to R. P. Dinsmore for permission to publish their natural brown color and still remain boardy. By cutting a sheet the opaque, creamy color will be seen on the it. freshly cut edge. Literature Cited The thawing of frozen rubber is a very definite temperature effect. A single sheet of permanently frozen crude rubber (1) Grifiths, Trans. Inst. Rubber I n d . , 1, 308 (1926). (2) Kroger, Gummi-Ztg., 40,782-4 (1926). will remain boardy for months a t room temperature. If (3) Leblanc and Kroger, Kolloid-Zlg., 37, 205-14 (1926). placed in an oven at 50" C., it will thaw in a few minutes. A (4) Park, Carson, and Sebrell, IND.ENG.CHEM.,20, 478 (1928) piece of smoked sheet 3 inches square thaws in 2 minutes. (5) Rossem, van, and Dekker, Kaalschuk, 6, No.1, 2-5 (1929).
The Reduction of Carbon Disulfide'" J. A. M i t ~ h e l l Emil ,~ Ott,a and E. Emmet Reid' CHEMISTRY LABORATORY, JOHNS HOPKINSUKIVBRSITY, BALTIMORE, MD.
T
H E reduction of carbon disulfide has not received m u c h a t t e n tion. I n 1856, Girard (4) obtained thioformaldehyde by using zinc and d i l u t e d hydrochloric acid as r e d u c i n g agents, according to the reaction :
C&'+ 4H+ H&:S H2S The gas formed during the reaction had a strong odor of leeks but only hydrogen sulbeen fide and carbon disulfide were identified. In particular no volatile mercaptan was observed. It is probable that the thioformaldehyde always polymerizes as formed. Gawalowsky (3) reduced carbon disulfide with zinc and either sulfuric acid or a strong solution of potassium hydroxide and obtained in both cases gases having the odor of rotting cabbage. This description of the odor is in agreement with the present writers' observations. The gas obtained by the reduction with sulfuric acid immedi-
ately gives a pure black precipitate from lead nitrate solution, whereasthat from the reaction with alkali yields a gas giving a fiery orange-red p r e c i p i t a t e . Mittasch ( 6 ) found that when hydrogen a n d c a r b o n disulfide were conducted over nickel powder, which was heated t o a m o d e r a t e temperature, the disagreeable odor of certain organic sulfur compounds was apparent. Although no accurate analyses could be obtained, the presence of mercaptans and alkyl sulfides was assumed. The vapors were condensed together with unchanged carbon disulfide in an ice-salt mixture. Similar investigations were conducted by Sabatier and Espil (7). The nickel was heated to 180" C. and a product of very disagreeable odor was obtained, which they thought might be methylene dithiol, H2C (SH),. Its precipitated salts were yellow for mercury, white for cadmium, yellow-
Presented before the Division of Petro1 Received March 3, 1931. leum Chemistry at the 81st Meeting of the American Chemical Society, Indianapolis, Ind., March 30 t o April 3, 1931. 2 This paper contains results obtained in an investigation on a study of the Reactions of a Number of Selected Sulfur Compounds listed as Project 28 of the American Petroleum Institute Research. Financial asssitance in
this work has been received from a research fund of the American Petroleum Institute donated by John D. Rockefeller. This fund is being administered by the Institute with the co6peration of the Central Petroleum Committee of the National Research Council. 8 American Petroleum Institute Research Fellow for summer of 1930. 4 Director, Project 28.
+
A study of the reduction of carbon disulfide has shown that under favorable conditions methylene dithiol, HzC(SH)Z,is formed. Owing to the presence of excess of carbon disulfide it is impossible to isolate the methylene mercaptides since they react at once with carbon disulfide to form trithiocarbonates. Several such salts have been prepared. It is proved thatthey are salts of methylene ditrithiocarbonic acid, H2C(S.CS.SH)2,which is obtained free as a heavy brown oil. It is also possible that the lead salt of an interS.CS.SH mediate monotrithiocarbonic acid, H&' 7 has 'SH
