INDUSTRIAL AND ENGINEERING CHEMISTRY
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.
This may be one reason why the former produced poor blues and the latter strong ones. It was believed that the entrance of the higher polysulfides within the crystal is of paramount importance. Therefore in view of the geometry it is better to start with no structure or with compounds which rearrange themselves during the reaction so as to permit the lattice to be built around the polysulfide#. I n this case, the framework being already formed, the polysulfide entered with difficulty and in fact its sulfur content was only about 7%.
Vol. 45, No. 3,
LITERATURE CITED (1) (2)
Gessler, A. E., and Kumins, C. A, U. S. Patent 2,535,057 (1951). Insley, H., and Ewell, R. H., J . Research iVutZ. Bur. Staizdards, 14%
792 (1935). (3) Kumins, C. A., U. S. Patents 2,544,693-2,544,695 (1952). (4) Pauling, L., “Nature of the Chemical Bond,” p. 364, Utica, pu’. Y., Cornell University Press, 1939. (5) Ibid., p. 367. (6) Rice. O., “Electronic Structure and Chemical Binding,” p. 311: New York, McGraw-Hill Book Co., 1940. (7) Singer, J., 2. anorg. Chem., 204, 232, 237 (1932).
ACKNOWLEDGMENT
(8) Zerr, G., Rubencamp, R., and hiayer, C., “Treatise on Color Manufacture,” pp. 177-200, London, Charles Griffen &- Co.,
The authors wish to express their thanks to the Interchemical Corp. for their encouragement in this project and for permission t o publish the data.
RECEIVED for re\,iew June 7, 1952. ACCEPTEDNovember 20, 1952. Presented before the Division of Physical and Inorganic Chemistry at the 121st Meeting of the ANERICANCHEMICAL S O C I ~ TBuffalo, Y, N. Y.
1908.
High-Quality Deresination of Guayule d
F. E. CLARK, T. F. BANIGAN, JR., J. W. JIEEKS, AZTD I. C. FEUSTEL U . S . Natural Rubber Research Station, Salinas, Calif.
L
ABORATORY and pilot plant investigations are being
carried on a t this station in order to develop guayule so that it might serve as a domestic source of natural rubber, especially for critical uses where Hevea is considered essential. Removal of the deleterious resin fraction, which normally comprises about 20 to 2574 of the weight of the resinous crude, constitutes one means for effecting quality improvement. The work reported in this paper, which presents results obtained in the experimental deresination of guayule rubber, is a continuation of studies reported earlier from this laboratory ( 5 ) . I n the previous report, Chubb, Taylor, and Feustel reviewed the work of earlier investigators in the field and described a method for producing a high-quality deresinatedrubberbya batch countercurrent acetone extraction of comminuted freshly harvested guayule shrub, followed by the usual pebble milling operation for recovery of the rubber. The present paper surveys progress made toward development of a n alternative and possibly more economical process involving acetone deresination of resinous rubber after recovery from pebble milling. Resinous guayule rubber recovered from shrub by the normal milling process is in the form of small spongy particles known as “worms.” The term “worm deresination” is therefore used to designate deresination methods where the milling operation precedes the acetone extraction step, in contrast t o “shrub deresination” where the reverse order is followed. While the current work was in progress, Wood and Fanning ( I S ) reported a method for deresinating wild rubber of high resin content by mastication in an internal mixer simultaneously with extraction by a continuously renewed solvent (acetone). Tests conducted with Mexican resinous guayule and chilte appeared to offer some promise and may point to a possible application of this method for the deresination of certain of the wild rubbers which are obtained in a massive solid form. Guayule rubber worms, which comprise the milling agglomerates of a number of rubber-bearing cells, are by virtue of their small size (ca. 0.5-mm. diameter by 2.0-mm. length) and spongelike structure, so well adapted to solvent deresination that i t is unnecessary to resort to the more rigorous and cumbersome technique of masticative extraction. Acetone deresination of the
worms has also been found to be very rapid, However, these important advantages are lost if the resinous worms are dried or otherwise allowed to become compacted before deresination. MATERIALS AND METHODS
The resin-containing rubber worms were prepared from freshly harvested 5- to 7-year-old guayule shrub, variety 593, by the pebble milling process (IO). Rubber and cork were separated from bagasse in a flotation tank, and then the cork was removed from the rubber by baica treatment (6). A final scrub milling completed the clean-up of the resinous m’orms. Whenever it was not practical t o deresinate the worms soon after scrub milling, they were placed in covered stock pots containing about an equal volume of water and were occasionally stirred with a paddle t o minimize clumping. In certain instances, especially where storage of small quantities was contemplated for longer periods of time, a small amount of formaldehyde war, introduced and/or the worms were refrigerated. Laboratory scale experiments conducted here and elsewhere ( 5 , l a ) have repeatedly shown acetone t o be a practical choice of solvent for the deresination of guayule worms. Hence our recent investigations have largely been limited to this solvent. The acetone used throughout this work was of technical grade. Since this work was undertaken with the dual purpose of providing a background of information to aid in large scale extractor design and to supply deresinated rubber for tire testing, it became desirable to adopt several approaches to the subject. Thus one phase of the program dealt with countercurrent extractions without agitation where acetone was percolated through static beds of worms packed loosely in various containers such as glass tubes, glass boats, etc. Factors such as flow rates a t equilibrium, composition of miscella (resin-solvent mixture), and solvent retention volumes were determined. Another phase of the study included countercurrent extractions with agitation where higher acetone-to-rubber ratios made stirring possible, thereby making it practical to investigate diffusion rates, temperature effect, and miscella composition. The third phase of the current work comprised the preparation of approximately 4000 pounds of deresinated rubber by means of a series of runs
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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573
individual components. Further reuse of this miscella in additional runs resulted in a shift of the two high points toward each other, with eventual attainment of a single higher peak. I n such cases much of the resin actually precipitates out in a viscous phase owing to its relatively poor solubility in the aqueous acetone of the advancing solvent front. Fortunately, this precipitation phenomenon has not been very noticeable in miscellae containing less than about 12% resin. The use of this extraction tube provided a n essentially enclosed apparatus in which it was practical to determine the per cent miscella retention of the extracted rubber a t the end of each run under free drainage conditions. In one series of extractions, the following miscella retention ratios (weight of miscella to weight of resin-free rubber) were found for separate batches of rubber deresinated to the following levels of residual resin content: 1.04 to 1 a t 1.75% resin content; 1.086 to 1 a t 3.11y0 resin content; and 1.207 to 1 at 6.49% resin content. A reasonably linear relationship is indicated. Consequently, the expression
l O 0 r
I
I
1
4
I NUMBER Of
I
10 I5 STAGES OF E X T R A C T I O N
20
Figure 1. Progressive Composition of the Miscella in Countercurrent Batch Deresination of Guayule Worms and Shrub
made in a single-cell extractor, operated batch countercurrently with respect to solvent. This was also an extraction with agitation as miscella-rubber ratios were sufficiently high to permit stirring of the extractant rubber. The amount of resin in the crude rubber was determined by a 16-hour extraction with hot ethyl alcohol in a modified American Society for Testing Materials apparatus (1). Resin in the miscella was determined as total solids after evaporating the acetone and water. Acetone and water in the miscella were determined by specific gravity measurements with a correction factor applied for the influence of the dissolved resins.
+
2/ = 0.0352 0.977 where g = miscella retention ratio and 2 = residual resin contect may be used for approximations throughout this range. EXTRACTIONS WITH EXCESS SOLVENT
This term has been chosen to apply generally to those cases where acetone ratios are sufficiently high ( i 2 . 5 to 1) to permit vigorous stirring within the system. A series of laboratory scale extractions was carried out in a 10-liter stainless steel stock pot
COUNTERCURRENT EXTRACTIONS WITHOUT AGITATION
These experiments were designed primarily to determine the distribution of water and resin in the miscella of a countercurrent extraction. In one series of runs, the apparatus consisted of a glass tube (122 by 5.3 em. in diameter) fitted with a solvent inlet tube a t the top and a regulated withdrawal tube a t the bottom. The apparatus was mounted a t about 7' from horizontal. The tube was loosely packed with moist resinous worms. Fresh acetone at room temperature was introduced continuously at the top while miscella was withdrawn at the bottom at a rate of 4 liters an hour, until a total of 15 1-liter fractions were collected. The tube was kept filled with the solvent throughout the extraction. After the first run was completed, the drained deresinated rubber was removed and replaced with another batch of resinous worms. Extraction was then resumed with the miscellae of the preceding run reused in the order in which they had been collected. One liter of fresh acetone was added a t the end. The drained deresinated rubber was removed, again replaced with resinous worms, and the miscellae of the previous run were reused as before; again 1 liter of fresh acetone was added at the end of the run. This was repeated until four runs had been made and a total of 22 1-liter fractions had been collected. The rubber samples were analyzed for resin, and the miscella fraction for acetone, water, and resin. The resulting distribution curves for the miscella are plotted in Figure 1, together with data reported earlier for guayule shrub deresination ( 5 ) plotted on a comparable basis. A similarity may be noted in the shape of theseresindistribution curves. The double peaks evidently denote a partial fractionation of the resins based on solubility differences among certain
o A t 20.0. x 39-40'0. 0
84-58*0.
* I 0
Figure 2.
10
20
30
I * 60
40 60 T I M E I N MINUTES
I
I
I
70
80
90
)O
Effect of Temperature on Extraction Rate of Resin by Acetone
which was fitted with a n impeller stirrer and surrounded with hot water heating coils. I n each run of the series, 110 grams of moist resinous worms (ea. 56y0 moisture) were added to 7 liters of rapidly stirred solvent. Small samples were screened out a t prescribed intervals and analyzed for resin content. The solvent-to-rubber ratio of 115 t o 1 was deliberately set very high so that the solvent would be essentially uninfluenced by the trace (less than lye) of water and resin introduced in each case. This technique made feasible a n evaluation of temperature influence on extraction rate. It also made possible determinations of the effect of systematic addition of water and resin on the rate and extent of deresination. Three series of experimental extractions were conducted in the above manner. The curves for these are plotted in Figures 2, 3, and 4. Figure 2 shows that extraction with acetone a t 20' C. proceeds more slowly than at 40' and 55" C. There appears t o be very little difference a t the two higher tempera-
INDUSTRIAL AND ENGINEERING CHEMISTRY
514
tures where deresination isvirtually complete afterabout 8minutes. The presence of water in the acetone (Figure 3) limits the extent of extraction very decidedly, but has little effect on the time required to reach an apparent equilibrium. The data of Figure 4 illustrate a similar phenomenon for extractions with acetone containing various amounts of added resin. The seemingly discordantly high resin content of these samples results partly from m i s c e l l a retention, The data of Figure 2 a t 20' and 55' C. have been replotted in Figure 5 to t e s t c o n f o r mance with Fick's diffusion law ( 4 ) , a general e q u a t i o n which applies to I S % WATER nonstationary mass transfer by diffusion. T h e derivation of the law and a dis0 IO 20 30 40 10 cussion of condiT I M E IN MINUTES t i o n s requisite Figure 3. Effect of Water on Extracfor its application Rate of Resin by Acetone at tion to this 30" C. type of extraction are presented elsewhere ( 2 ) . The law applies in its simplest form only t o constant rate processes-that is, t o the straight-line portion of a plot of log,, q/qo versus t where q/qo is the fraction of extractant (resin) remaining a t time, t. Although the diffusion c6efficients cannot be calculated from the current data, it is valid to show by comparison of the respective slopes that the diffusion coefficients a t the two temperatures bcar the following relationship: D6b'
= 1.85 0
2 ~ '
Diffusion is moderately accelerated by increased tempci a t u i c! in this instance, However, it should be emphasized that thc straight-line portion of these plots-that is, the part whrl P deresination is limited by a low order, constant rate diffusion-occurs well toward the end of the extraction. I n a practical sense, this means that a t 20' C. about 91%, and a t 55" C. about 947, of the resin initially present in the worins is removed very rapidly to yield rubber with resin contents of about 2.25 and 1.5%, respectively. Further deresination proceeds a t a greatly decreased rate and fortunately does not appear necessary for attainment of the high physical quality desired in the final product. COUNTERCURRENT EXTRACTIONS WITH AGITATION
A series of countercurrent batch extractions was made on :I laboratory scale to determine the number of stages necessary a t 30" C. for good deresination a t a low acetone-to-rubber ratio. These extractions were designed t o develop a set of conditions adaptable t o larger scale operat'ion (see section on Production of Rubber for Tire Tests). The experiments were conducted in a 10-liter stainless steel stock pot fitted with a screen-covered drain and a n efficient air stirrer. h 2-kg. charge of moist (ea. 50% water) resinous worms was stirred with 10 changes of acetone, comprising 1625 ml. per stage. Each stirring period was 5 minutes; each draining period was 2 minutes, except for the first and second which were 5 and 3 minutes, respect'ively. The niiscella fractions that had been collected a t each stage, with the
Vol. 45, No. 3
exception of those removed, were reused consecutively with another 2-kg. charge oi moist resinous worms. Additional fresh acetone was added a t the end to replace the misccllae of the two stages removed and to provide one extra stage to help attain equilibrium. The two stages removed were 1, which contained mostly water, and 7 , which had the most resin. After several additional runs were made in this manner, it was found desirable to remove some of the miscella from stage 2 in order to maintain equilibrium. I n this case, equilibrium may be defined as that point at which the amount of water and resin carried in with the rubber is just balanced by the amount removed from the system. The acetone which is contained on the rubber and in the miscella removed at equilibrium can be recovered readily and reused to keep the system balanced. I n these experiments 20 stages proved sufficient to maintain equilibrium at a resin level of 2'36 in the rubber and 10% in the miscella. I n a tight system evaporation losses would be negligible but here they amounted to 110 ml. per stage. This value is only approximate because more acetone is lost during the later stages than during the first stages where the acetone is diluted by water.
I < . 0
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T I M E 11.1 M I N U T E S
H g u r e 4. Effect of Added Resin on Extraction Rate of Resin by Acetone at 30' C.
A4fterequilibrium was rcached, 5 additional runs were made and the results were averaged and plotted in Figures 6 to 9. Table I illustrates the inaterials balance as obtained in these cxperiment8. Under these conditions 4.8 grams of acetone were needed to produce 1 gram of deresinated rubber, not counting evaporation loss. This ratio is based on the current findings and is not necessarily the optimum figure. Of this amount, 3,5 grams appear in the miscella and may be recovered by distillation, and 1.3grams are ret.ained on t'he rubber from which it may be recovered a t the time of drying. Removal of the stage 7 miscella accounts for 85% of the resin, leaving only 7,8y0of the original resin in the deresinated rubber. About 90% of the entering water is removed in the miscellae of stages 1 and 2, and the remainder is removed in stage 7 miscella. The resin content of the miscella is plotted in Figure 6. A peak of about 1Oyoresin content is shown a t stages 7 t o 8 a t which point miscella is removed from the system. Only one peak was obtained with this agitated extraction in contrast to the two peaks obtained with two examples of unagitated extractions, shrub deresination ( 6 ) , and one case of worm deresination (see Figure 1). Since unagitated deresinations more closely fit' the conditions for ideal differential extraction than do the stirred or agitated systems, the two peaks probably reflect solubility differences among various resin components, as postulated above. I n the present case a deflection may be noted in the curve a t
March 1953
575
INDUSTRIAL AND ENGINEERING CHEMISTRY
stage 3, which could suggest a TABLE I. WATER,RESIN, AND ACETONE B A L A N CFOR ~ S LABORATORY BATCHDERBSINATION secondary peak. Operating with a one-peak system is sim(Grams per gram of resin-free rubber) pler than operating with a twoIn out Standard Standard peak system, because it requires deviadeviathat miscella be removed a t Grama tiona Grams tion only one point in the system Water removed in Stage 1 miscella 0.748 0.076 Water removed in Stage 7 miscella 0.108 0.026 in addition to that removed a t Water removed in '/a of Stage 2 miscella 0 .294 0.027 the point where the wet rubber Water in wet worms 1.140 0.028 1.150 enters. Figure 7 shows that Resin removed in Stage 1 miscella 0.0086 0.0012 Resin removed in Stage 7 miscella 0.2590 0.0147 the miscella does not become Resin removed in 1/a of Stage 2 miscella 0.0215 0.0032 Resin left in deresinated rubber 0.0244 0.0020 water-free until the tenth stage Resin in wet worms 0.314 0.010 0.3135 under these operating condiAcetone removed in Stage 1 miscella 0.608 0.038 tions. Up t o that point the Acetone removed in Stage 7 miscella 2.430 0.131 m i s c e l l a c o n s i s t s of water, Aoetone removed in '/a of Stage 2 miscella 0.471 0.015 acetone, and resin. Acetone removed on deresinated rubber 1,299 0,010 The resin retained by the 4.808 Acetone lost by evaporation and samrubber a t any given stage may pling 2.387 be seen in Figure 8. Of particAcetone added to system 7.195 0.839 7.195 0 Standard deviations were determined from data of five runs made a t equilibrium. ular interest is the fact t h a t no deresination takes place until the eighth stage, by which time the dehydration of the rubber vented most but not all of the fines from coming through the drain. is virtually complete. The apparent increase in the worm resin The acetone and miscella were preheated to 30" C. Stirring was content during this early period results partly from the retention accomplished with a n air stirrer. About 30 pounds of deresinof high-resin-content miscella on the samples. There is also a ated rubber were produced in each run. The ratio of miscella t o deposition of resin caused by water entering the system with rubber was kept the same a t each stage as in the laboratory scale the rubber and diluting the miscella beyond its resin solubility experiments. However, a few changes were made for convenpoint. The portion of the curve in Figure 6 from stage 0 to the ience. The number of stages was reduced from 20 to 16. The point of maximum removal is, in fact, a resin solubility curve in miscellae of stages 1, 2, and 5 were removed for acetone recovery water-acetone mixture of progressively changing composition after each run. The only column still available for this work produced acetone containing about 5% water. This acetone 1.00 I seemed unsuitable for €he last stages but was used for stage 1 in place of miscella. D r y acetone was added at the end of each extraction to maintain the desired total of 16 stages. An acetoneto-rubber ratio, somewhat higher than that used in the laboratory extractions (4.8 t o l), was employed here to assure the thorough deresination required for this test rubber. More efficient extraction equipment would undoubtedly lower this ratio. 12
0.05
.
-
-
IO
TEMP. 55.C.
3 2
0
I
0.03k
2 8
i
z
$ 6
0.01 0
Figure 5.
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P
Application of Fick's Diffusion Law to Resin Extraction 0
The retention of acetone, water, and miscella by the rubber is shown in Figure 9. The sum of the water and acetone curves is not quite equal to the miscella curve, the difference being the resin content of the miscella. The water retention of the rubber at stage 1 is greater than that originally present. This is due to the fact that the wet worms were squeezed a t stage 0, whereas at the succeeding stages free drainage was employed. PRODUCTION OF RUBBER FOR TIRE TESTS
A series of larger scale deresinations was made to obtain sufficient rubber for truck tire road testing. For this production, a 35-gallon stock pot fitted with a drain was used. An 80-mesh stainless steel wire-screen basket, placed inside the pot, pre-
0
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B
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IS
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NUMBER OF STAGES
Figure 6.
Progressive Resin Content of the Miscella in Countercurrent Batch Deresination
Approximately 1yo of phenyl-2-naphthylamine was added to the rubber by washing the deresinated worms in an acetone solution of the antioxidant. The rubber was dried t o constant weight w e r a 2-hour period in a circulating air oven a t 40" C. Meeks and Feustel (8) have shown that molecular weight loss based on solution viscosity is negligible if the rubber is handled in this fashion. Over 4000 pounds of deresinated rubber was produced by this procedure. I n general, the resin content of this rubber ranged
INDUSTRIAL AND ENGINEERING CHEMISTRY
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CONCLL-SIOn-S
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p 2
V O l . 45, No. 3
‘0
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0
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8 NUMBER
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20
Figure 7 . Progressive Acetone and Water Content of the Miscella in Countercurrent Batch Deresination
In laboratory and pilot plant experiment,s conducted here recently, it has been shown that rubber of uniform high quality can be prepared routinely from guayule worms. Deresination of worms is feasible because of the relative ease with which resins are removed from the rubber in this favorable physical forin. The accompanying x-ater helps to preserve the worms as a loose aggregate. Moreover, the process is unique in that the presence of water need not, impair the effectiveness of the deresination since the Tmter is efficiently displaced in the early stages of the extraction. 1-ery little acetone is required for the water removal once equilibrium has been established in a continuous countercurrent system. Equilibrium was maintained in a laboratory scale batch countercurrent, extractor at a resin level of 2% in the issuing rubber and 104:, in the viithdrawn miscella for a n acet,one to resin-free-rubber ratio of 4.8 to 1. The rate a t which equilibrium is established a t any given stage is very rapid and cannot be readily described hy Fick’s diffusion law. The acetone required to maintain the equilibrium, defined above, can be efficiently recovered in a tight syst’em together with the resin which contains potentially valuable organic by-products (3,7 , 9). Countercurrent extract,ions conduct,ed without agitation have revealed an interesting t,endency for certain resin components t o partition int,o tn-0 groups, based apparently on solubility differences in the miscellae. The dubious advantage of such a rough fractionation probably fails to offset various op~rational difficulties encountered in these systems. This work has been carried out only with bat,ch systems. However, the process seems inherently adaptable to cont.inuous operation. hccordingly work is currently- in progress toward the development of a continuous countercurrent extractor.
NUMBER OF STAGES *
4CKNOWLEDGhIEVT
Progressive Resin Content of Rubber in Countercurrent Batch Deresination
Figure 8.
The authors wish to thank Kenneth W. Taylor, Ralph 1, Chubb, and the other members of the pilot plant staff for the preparation of the rubber used in this work. They wish to thank Rolla H. Taylor and n’illiam P. Ball for the BIooney viscosity deteiminations. They also wish to thank Ruth \-. Crook and Clay E. Pardo for the analytical data and William J. Gonans for technical help in certain phases of the work. LITERATURE CITED
0 O
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4
6
8
IO
I
1
I
I
12
14
16
I0
LO
N U M B E R OF S T A G E S
Figure 9. Progressive Retention of Water, Acetone, or Miscella by Rubber i n Countercurrent Batch Deresination
from 1.5 t o 2.5’%, the Mooney viscosity from 90 t o 96, and the tensile strength of the vulcanizate (ACS KO. 2 formula) from 3600 to 3800 pounds per square inch, This rubber was prepared for road test evaluation, as carcass stock in large truck tires. One of the leading rubber companies fabricated the tires, which were tested by the test fleet of the Synthetic Rubber Division (formerly Office of Rubber Reserve, Reconstruction Finance Corp.). It was reported recently (11) that the guayule tires proved at least equal t o the Hevea controls in all important respects on the basis of one severe road test (50,900 miles a t 45% overload).
Am. Soc. Testing Materials Standards, Designation D 297--23T, Philadelphia, 1948. Banigan, T. F., J r . , ISD. ENG.CHEM.,45, 577 (1953). Banigan, T. F., Jr., \leeks, J. TI’., and Planck, R. K., L-. S. P a t e n t 2,549,763 (April 24, 1981). Barrer, R. hl., “Diffusion in and Through Solids,” Cambridge University Press, 1941. Chubb, R. L., Taylor, E. C., and Feustel, I. C., I n d i a R u b b e r W o r l d , 123,557 (1951). Cumming, J. M., and Chubb, R. L., Chem. R- M e t . Eng., 53, (9), 125 (September 1946). Haagen-Smit, A. J., and Siu, R., J . Am. C h e m . Soc., 6 6 , 2068 (1944). l l e e k s , J. TI-,, and Feustel, I. C., India R u b b e r W o r l d , 125, 187 (1951). Murray, C. W., and Walter, E. D., J. Am. Chem. Soc., 67, 1422 (1945). and Chubb, R. L., IKD.ENG.CHEM.,44, 879 Taylor, K. W,, (1952). U. 8 . 82nd Congress. Congressional Record 98(24) :A992. “Uses of Guayule Rubber in Heavy-Duty Truck Tires.” (Extension of remarks of Hon. Jack Z. Anderson of Calif.) Fell. 19, 1952. Williams, I., C.S. P a t e n t 2,390,860 (Dec. 11, 1945). K o o d , J. W., and Fanning, R. J . , Rubber A g e , 68, 195 (1950). RECEITED for review June 30, 1952.
ACCEPTED October 8 , 1952.
