CRYPTOSTEGIA LEAF RUBBER SAM R. HOOVER, THOMAS J. DIETZ, JOSEPH NAGHSKI, AND JONATHAN W. WHITE, JR. Eastern Regional Research Laboratory, U. S. Department of Agriculture, Philadelphia 18, Pa.
c
out on fermentation as a step RYPTOSTEGIA, a The leaves of Cryptostegia grandiflora have been investiin the recovery of the leaf rubleafy tropical vine n& gated as a possible emergency source of rubber, suppletive to Madagascar, produces menting the latex rubber from the stems. The leaf rubber. An anaerobic fermentation by Clostridium roseurn ber consists of two distinct portions; 85 to 90% is a relaa latex which contains rubber. This rubber has been tively low-polymer rubber which occurs in the chlorenwas worked out in which more than 60% (dry weight) of the chyma cells, and 10 to 15% is a latex rubber of better marketed from time to time; its properties are generally quality which occurs in the latex ducts. Isolation of the leaves is digested. After two leaf rubber by a combination of fermentation and chemdays’ incubation the leaves similar to those of hevea rubber (1). The leaves of the ical extraction, and the characterization of vulcanizates are well digested and disinte plant contain benzene-soluble are described. grated. This slurry is paased over a vibrating screen (80 X material considered to be rub80 meshes to the inch). The liquid and protoplasts pass through, ber (4). Two species, Cryptostegiu grandiflora and C. macEagascuriensis, were introduced into this hemisphere, and the former has and the bagasse (cuticle, veins, and small stems) remain on top. become widely dispersed throughout the West Indies, Central The latex ducts in cryptostegia leaves are closely associated with America, Mexico, and the extreme southern portions of the United the veins and, owing to their length, are trapped in the bagasse. The bagasse is suspended in one half the original volume with States. For a number of years the United States Department of Agriculture has conducted botanical investigations upon Cryptowater and again screened to recover an additional quantity of protoplasb that are trapped mechanically. The bagasse is freed stegia as a possible source of rubber (18). This work conof excess water by pressing and then dried. The protoplasts firmed the existence of rubber in the leaves. Jenkins reviewed (specific gravity 1.17-1.27) are recovered from the liquor in the information available in January, 1943,on Cryptostegia as an which they are suspended by gravity settling and decantation; emergency source of rubber (6). they yield a slurry containing about 4-7% solids. The slurry is Properties of the leaf rubber became of interest when plantations were established with the primary aim of producing latex further freed of soluble materials by rediluting with water, settling, and decanting. Figure 1 shows the fermentation equiprubber from cryptostegia stems,,since large quantities of leaves ment. The bacteriologicalaqpects of this fermentation have been would thus be made available from normal pruning operations. Moreover, it was possible that the plant would be grown for the described elsewhere (1.2)). production of leaf rubber alone, provided its quality was high and This paper presents two procedures for the recovery of the cell rubber from the fermentation products. The first consists in satisfactory means of recovery could be worked out. In fact,, isolation of the protoplasts followed the labor required to tap the vines by a “caustic cook” to liberate the for latex practically precluded the employment of this process in the rubber. The photomicrograph of Figure 2 illustrate the successive United States and Mexico. stages in this process. By the second The characterization and isolation procedure all the leaf rubber is exof the leaf rubber have been of primary interest. Whittenberger, tracted by bcneene. Chemical and physical data on the raw rubber obBrice, and Copley (19)demonstrated tained by these processes are given, that only 10 to 15% of the rubber and certain properties of the vulin the mature leaf occurs in the canizates are evaluated. latex ducts; 85 to 90% is embedded in the individual chlorenchyma cells CAUSTIC-CREAMING PROCESS as discrete globules. This “cell rubber” is, of course, not directly Previous work demonstrated that available by tapping the latex systhe protoplasts obtained by fermentem of the leaf and has not been tation could be digested by dilute alkali and thus release the rubber satisfactorily recovered by pebblemilling procedures of the type used globules (1.2). Boiling a 10% susin the production of guayule rubber. pension of the protoplasts for 20 The latex duct rubber of the leaves minutes in water containing 2% sodium hydroxide was the standard is essentially the same as the rubber procedure in the work described obtained by tapping the latex systern in other portions of the plant. here. One per cent powdered JZF In the dried leaves it is coagulated (4,4’-diphenylphenylenediamine), and therefore contains the nonbased on the amount of rubber, was rubber constituents of the latex, STEAMadded as antioxidant. The suspenwhich are largely removed during sion containing the liberated rubber VALVE coagulation of latex obtained by globules formed a “cream” when it tapping procedures. DRAIN stood without stirring. The cream ExtenRive studies were carried Figure 1. Anaerobic Fermentation Tank was siphoned off the surface, dis803
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 37, No. 9
ofmethodsfor their recovery. About 4 liters of the liquor produced hy boiling the protoplasts inalkslineaolution was allowed to eream overnight in II depth of npproaimntely 25 cm. A small sample (0.1 ml.) of the cwas taken oE and diluted for microS00pk observation (sample A) afte wluch the dispersion WBS itirred thoroughly aith B mechanical stirrer, sampled (R), and subjected to entrifugation a t Zoo0 r.p.m. (925 X gravity) far ditlerent eriods. After centrifugation the rugbe, wa8 completely removed from the surface and the liquor underneath analyzed for rubber (Table I). Micmseopie observations were carried out upon the liquor of the 2@minute esmple (C). The snalytieal results reported io this paper were obtained by Willits snd ea-workem ($9). The samples wore extracted with benzene, benzene insolublos were determined by fdtrntion of the benzenesolution, and rub ber hydrocarban was then determined gravimetrieeUyby precipitation asthe tetrahromide. Kesins were calculsted by diffemnoe. Rubber could not be quantitatively extracted from the raw leaves unless s preliminary treatment with oxalic acid was 61)plied. Resinsend rubber were determined by n modified Spence-CaldweU procedure. The samples were prepared for micrascopio ewuninsticn by midng 0.01 ml. of esoh with 0.01 ml. glyoeml on a slide, carefullyevspornting the water, and covering with B cover slip. Brownian motion of the particles wan
tsrw number of fields selected at nmdo&. B calibrated eyepiece mierometer'was used. (The count of the
d i e s t globules in whole li uor B is an estimate obtained by es%ulation from the count of the centrifuged sample C. This wns necessitated by the densenessof the slide ofthe whole li "or and the number of bacterial e& present which caused difficulty in counting thesmnllerpsttacles. bn qpreciable m i in this e+irpsts would not sEect the weight dratmbution markedly, for the total weight of this fraction was only 4% of the whole.) The data are presented in Table I1 and Figure 3. Data for hevea btex taken from L u w (8) m included in Figure 3 for camparisan. Fig2. Photomicmgraphe of Material Prepared fmm an F, Hybrid of To calculate the resulk, the globC. madagaacariensia and C. g r n d f i m ules were mumed to be homogeneous and identical in composition, independeqtly of ske. This, sFumption IS ~ustlfiedwithin the llrmts of BCcuracy desired here. Analysia of the datashowsthst 91% of the total number of globules in the whole liquor were present in the two mnallest~&a elsssiJicatiom(1andZrdiameter), yet p e r 4 in water, and reorenmed. After it had been washed in this they constituted only 16% of the weight of the rubber present. Thaee smaller particles were almost entirely absent in cream A rn-r-4 times, thepH wm adjusted to 4.6 with wetic acid, which rose during 16 hours'standing. Moreover, they constituted the glabuies ~,clumpodz,,the separated in leas all the globules in liquor C sfter a centrifugation equivalent to than half an hour, and elem serum remained. If the nlknline 300 hours* The specificgravity of the globules waa determined by the equiliquor was adjusted to pH 4.6 without prior creaming, B heavy librium position they ssawned when oentrifuged in water-aleohol precipitateof pmteinsoeow msterial trapped the rubber globules mixturea. The a v e w e specific gravity of the gtohules recovered and carried them down with it, by crenming was 0.92.although for 8ome it was 88 high as 0.96. The 8pecific gravity of the a W n e oook liquor was 1.02. S u b CR?AAKCNQ STUDEES.Tho distribution of particle size and density of the globules were determined to facilitatedevelopment atitution of these vnlues snd the microsrOpiurUy determined
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1945
805
nate the effect of long exposure to the air during creaming. Tho cell rubber was recovered in a total time of less than 3 hours by centrifuging the alkaline cook li uor adjusting the pH to 4.6, (AT925 X GRAVITY)OF ALKALINE DISPERSION and washing the cream twice. Tab?e If1 gives analytical data OD Centrifu ation Rubber Content of Proportion of typical preparations. The crude rubbers obtained both by Time, &in. Liquor, Mg./100 MI. Total Rubber, % gravity creaming and by centrifuging were soft and tacky, and 0 198 (100) contained a large proportion of acetone-soluble material (resins). 6 49 26 10 50 Rubber deresinated by precipitation from benzene solutions 26 20 46 23 with acetone was prepared for further studies of its properties. 34 40 17 It was firmer than the resiniferousmaterial but still relative1 soft ttnd tacky. Inasmuch as the physical properties indicateithat TABLE 11. NUMBER AND WEIQRTDISTRIBUTION OF GLOBULES it was a relatively low olymer rubber, i t was extracted with methyl ethyl ketone (MI&), which is considered to be a solvent OF CRYPTOSTEGIA LEAFCELL RUBBER for the lower-molecular-weight fraction of hevea and for depol % in Gravity % in Av. Diam: merized rubber (3 The leaf cell rubber was almost completeg Cream A Liquor C of Globules, soluble in MEK Table IV). ComDarative data were obtained AI Wt. No. Wt. No. Wt. -Na. .. upon a sample of smoked sheet of cryptostegia latex rubber which 1 4.2 0 0 had been exposed to the air for several months. The MEK 2 12 24 10 0.3 2.6 8.6 9 23 4 0 0 solubilit indicates that the latex rubber is comparable to hevea 6.5 5 21.4 .2 26 17 13 latex rugbet in this res ect. Cheyney found that 5 to 8% of 25 26 crepe and vulcanized rugber and 25% of reclaim were extracted 8 0.8 27 24 12 28 1.1 4.2 9.6 by MEK (9). 11 2.4 18 Further evidence concerning the molecular weight of the cell 12.6 1.1 8.9 rubber was obtained by measuring the viscosity of dilute benzene solutions of the rubber. These experiments were based on the methods of KemD and Peters (6). Several samples gave molecular weight val6es in the range k0,OOO to 16,006. These mdii in Stokes equation gave satisfactory agreement with the obresults are consistent with the physical properties and solubility served rate of creaming. relatione, all of which indicate that the leaf cell rubber is of relaMicroscopic and chemical data indicate that a maximum of tively low molecular weight. about 8OoJ, of the rubber in this sample could be recovered by diThe possibility that there had been oxidative breakdown of the rect creaming of the suspension. Attempts to find-creaming rubber during drying, storage, and shi ment of the leaves from agents or conditions which would facihtate the separahon of the Cuba was investigated. Several sampres of mature leaves were smaller particles were not successful. prepared a t Yuma by (a)air drying and (b) sterilizing by canning These data were obtained on globules prepared from a typical the fresh leaves in water. It was assumed that oxidative breaksample of Cuban leaves having a rubber content of 8.2%. A down would be minimized in the sterilized sam lea. These sample of mature selected leaves later obtained from the U. S. leaves were shipped to Philadel hia and globule rubter was preRubber Company plantation at Yuma Aria., had a rubber conpared from each by the anaerokc fermentation process. There tent of 6.9%, and a large proportion oi the rubber globules were was no significant merence in the acetone-soluble contents 30 to 40p in diameter. It is obvious that the rate of creaming (resins) of these two pre arations, nor did they m e r from de nds upon the size of the globules. It has been well estabresults previously obtainezupon Cuban !eaves (Table IV). It LsEd that the rubber content (IS) and the size of the globules was therefpre concluded that no appreciable oxldative breakvary with the age of the leaves (9, I@, and therefore the rate of down was caused by drying and stonng the leaves. creaming varies with the tnaturity of the leaves. COMPOUNDING AND PHYSICAL PROPIGRTIES. Samples of crude cell rubber prepared by the caustic-creaming process were compounded b a modified A.S.T.M. evaluation procedure. The recipes (Take V) were similar to those employed by McKemon and Lindquist compounding rubber obtained from goldenrod by extraction methods (IO). Considerable difficulty was encountered in securing satisfactory dispersion of the compoundmg ingredients in the soft crude stock. Because of the limited quantities of rubbers available, a test slab 0.030 inch thick was substituted for the usual 0.075- or 0.lqO-inch s$bs. Also, sandwich-type, stainless steel mol& havlng exceptionally broad bands were used became of the exceasive plasticity of the com unded stocks. A curing temperature gum stock recipes, exce t for certain of 260' F. was used for slow-curing Sam les which were cured a t 274". %e thin test specFens probatly had. slightly greate? tensile strengths than specmens of standard thckness, but the increase would probably
TABLE I. REMOVAL OF CELLRUBBERBY CENTRIFUGATION
qQuzp
2.
2';
Fb
tql t4
tg
L-
a
v
/
CENTRIFUGED
I TABLE 111. COMPOSITION OF CRYPTOS~QIA LEAF CELL GLOBULESOBTAINEDBY CAUSTICCREAMING PROCESS
;ff 50 75 100 OU MULATIVE W f . OF PARTICLES ( X I 0
25
Figure 3. Cumulative Weight Distribution of Cryptostegia Cell Rubber Globules COMPOSITION OF LEAF CELL RUBBER. Samples of globule rubber were prepared by ravity creaming and centrifuging. The first sample was allowe8 to cream for 14 days. The cream was then drawn off, diluted with water, and recreamed three times at &hour intervals to remove the alkali and other contaminants. The fins1 cream, containing about 50% solids, was dried in vacuo at 60' C. Other samples were creamed for 2 to 4 days, and an antioxidant (JZF) was introduced into the alkaline liquor. The recovery of rubber hydrocarbon by thi~ method ranged from 70 to 7570, which agrees weil with the data obtained in the creaming etudies. Samples were also prepared by centrifugation to elimi-
Sample Gravity-creamed Gravit -creamed CentriLged Centrifuged
TABLE IV.
% Rubber Hydrooarbons 08.8 67.5 62.7 62.8
Benienb
%
Pnsoluble
30.9
0.8 4.8 1.8 1.6
Material
Rains 88.2 46.6 86.2
COMPOSITION OF CE'YPTOSTEQIA LEAFCELL RUBBER^ AND SOLUBILITY IN MEK Rubber, Reains, Benrene Sol. in
Souroe of Rubber % % Insol..% MEK,% Typical Cuban leaves 39 4 93 Dned Yuma leaves 64 b7 33 3 91 94 60 37 Canned Yuma leavw Typioal Cuban leaves, p td. from beniene soh. by 2 v o f aoetone 91 9 0 100 Latex, smoked sheet (Haiti) 9 17 * Rubber waa determined aa tetrabromide. insolublea by direct extraction and resina by difference (SO). Solubility in MEK WM determined by direat extraction.
..
..
INDUSTRIAL AND ENGINEERING CHEMISTRY
806
Vol. 37, No. 9
Table VI1 gives typical data on the rubber content and bulk of the raw leaves and fermentation products. The increase in RUBBERSTOCKS rubber content of the extractor charge averaged 2.9 fold on a -Recipe, Partsweight basis and 3.3 fold on a volume basis. Recovery of rubber A B 0 was essentially quantitative in all four experiments. These Crude cryptostegia rubber 100 100 100 Zinc oxide 6 6 6 fermentations were run for two days, as in the previous work. Mercaptobenzothiazole (Captax) 1 1 1 Stearic acid 4 4 4 Unpublished work has shown that greater decomposition can be Di henylguanidine (DPG) 0.5 0.5 0.5 obtained in additional time; the gain, however, is not great, and SuPfur 3.5 3.5 2.5 Reinforcing blaok (Standard Micronex) . . . 30 30 the products become progressively more slimy and difficult t o filter. RATEOF EXTRACTION. Ease of extraction of both resins and TABLE VI. COMPOSITION AND PHYSICAL PROPERTIXS OF CRYPTOSTEGIA LEAFCELLRUBBER RECOVERED BY CAUSTIC CREAMING rubber from the fermented leaves was markedly increased. Data to illustrate this effect were obtained by periodically weighing ----Recipe A---Recipe Bthe extract of small samples extracted by hot percolation of solvent ResinifResiniferous Deresierow Deresiin the regular analytical equipment (Figure 4). The resin extracCentri-* nated, centr(nated, Creamed fuged creamed fuged creamed tions were run for 24 hours; the residues were dried in a current of Composit,ion % air until free of acetone, and then extracted with benzene, Both Rubber hidrocarbon 68.3 52.7 86.0 52.7 85.0 extractions proceeded more rapidly in the fermented materials. Resins" 30.9 45.5 14.3 45.5 14.3 Benzene-insol. mateThe difference in the rate of extraction of rubber was especially rial 0.8 1.8 0.7 1.8 0.7 great. In fact, the rubber could not be completely recovered P h sics1 properties dptimum cure. min. 25 20 20 35 20 from untreated leaves even with prolonged extractionl, whereas Cure temp., e F. 260 274 260 274 274 Optimum tensile, lb./ extraction of rubber from the fermented leaves was essentially aq. in. 400 1000 1500 650 1950 complete in 2 hours under the conditions employed. Lack of Ultimate elongation, % 720 630 680 360 550 pilot-plant equipment made it impossible to obtain data under Modulus a t 500%. conditions comparable to those used in large-scale solvent exlb s q in '-' ... .... 600 1750 Hardness' * (Duromtractors. eter) 25 28 37 57 60 METHODS OF EXTRACTION. Solvent extraction can be applied * Includes 1% of J Z F antioxidant. to both protoplasts and bagasse when the two fractions are separated by screening. The bagasse contains the latex duct rubber of the leaves, which presumably is of better quality than not be greater than 200 pounds per square inch. Table VI shows the cell rubber. Any protoplast rubber not released by fermentensile properties of the cryptostegia leaf cell rubber as recovered tation would also be in this fraction. by caustic creaming. Although in practice the fermentation products would probTo determine the effect of resins on the properties of cell rubber, ably be recovered and dried without fractionation, for purposes the resiniferous stock was dissolved in benzene and precipitated by acetone. Table VI also shows the composition and physical of evaluation the two fractions were prepared by screening the properties of the deresinated rubber. It is evident that the defermented slurry (through SO mesh) and drying in air at 70" C. iesihnated cryptostegia leaf cell rubber recovered by the causticThe protoplast fraction was ground in a disk mill to pass a 20creaming process was superior in tensile properties to the resinifmesh screen. The bagasse was cut through a l/a-inch screen in a erous creamed product. Improvement in tensile strength resulting from deresination is probably due in part to improved rotary knife cutter. The extractions were carried out in an imdispersion of the ingredients as well as to reduction of nonrubber provised steam-jacketed Soxhlet-type extractor of approximately constituents in the specimen. 0.8 cubic foot capacity. Two types were used: (1) direct extraction with benzene and precipitation of the rubber with 2.5 SOLVENT-EXTRACTION PROCESS volumes of acetone, and (2) acetone extraction (deresination) The possibility of direct solvent extraction of the fermentaLion followed by benzene extraction. The rubber obtained by both products appeared worthy of investigation. This method would procedures was freed of solvent and dried in vacuo, after the have the advantage of recovering the latex duct rubber in the addition of 1% powdered JZF antioxidant (estimated on the leavw a3. well as the cell rubber not recovered by creaming. amount of rcbber). Fermentation as an aid to solvent extraction of rubber was LEAFRUBBER FRACTIONS. Table VI11 shows the composipatented in 1873 ('7). That rubber can be more easily extracted tion and tensile properties of the cell and bagasse rubber fracfrom fermented plant materials has been repeatedly demontions m prepared by process 1. The treatment did not comstrated (16,1'7). Moreover, the removal of nonrubber constitpletely deresinate either sample, and the bagasse fraction contained more than 20% nonrubber material. I n this experiment uents and the consequent increase in rubber content of the extractor charge decreases the cost of extraction greatly. Im35% of the total rubber was in the bagasse portion, an indication provement in aging properties caused by leaching of metal salts, that separation of the protoplasts was incomplete and that latex etc., during fermentation might be an additional advantage. rubber was contaminated with cell rubber. Nevertheless, the This factor was not investigated, however, as i t was impossible crude bagasse rubber was considerably firmer and less tacky to prepare the leaf rubber satisfactorily by any method which than the cell rubber. The tensile properties of the bagasse fracdid not involve fermentation or leaching. tion were definitely better and probably reflected the better quality of the latex duct rubber it contained. The protoplasts were then extracted by procedure 2, first with acetone and then TABLE VII. ENRICHMENT I N CRYPToSTEGIA LEAFRUBBER BY FERMENTATION BY with benzene. The benzene solution was Clostridium roseum AT 40" C. FOR Two DAYS divided into two portions; one was precipEnriohment Factor Original Leaves Fermentation Producta itated with acetone, and the second was Ex t. D r wt., Vol., Rubber Content Dry wt., Vol., Rubber Content for Rubber k. Cu.ft. T o - L b 7 lb. cu.ft. % LbT evaporated dry in vacuo. Both preparaCRYPTOSTEGIA LEAF TABLE V. RECIPESUSEDIN EVALUATING
-
...
826N30 824N3 824N4 824N5
16.44 52.9 51.8 52.2
0.85 2.7 2.7 2.7
4.2 2.8 2.5 2.2
0.684 1.455 1.257 1.170
6.02 18.06 17.82 17.74
0.27 0.82 0.81 0.84
11.0 8.0 6.7 6.6
0.682 1.434 l.i90 1.170
2.6 3.1 2.7 3.0
3.2 3.3 3.3
3.2
1 Willits and co-workera (10) had previously found it neceesary t o w e special chemical treatments prior to extraction t o obtain complete extraction of rubber from untreated leaves.
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1945
'
tions of the cell rubber were almost free of resins. Table IX presents the composition and tensile properties of the two preparations compounded by the gum recipe A. The difference between the two samples is slight, and the results agree well with those obtained with the deresinated cell rubber prepared by the creaming method (Table VI). The tensile properties of both samples were superior to those shown by the lesa completely deresinated sample of cell rubber in Table VIII. WHOLE LEAF RUBBER. Properties of the mixture of duct and cell rubber which would be obtained if the fermented residues were extracted without fractionation were determined. The small amount of higher-grade duct rubber would certainly not justify extraction for this fraction alone, but there would be no point in discarding it if solvent extraction were to be utilized for recovery of the leaf rubber. For this experiment and the following one on blending with GR-S, several fermentations -re carried out (Table VII). The leaf fractions were separated, dried, ground, and extracted with acetone and benzene. [The acetone extract, "resin", investigated by White and Senti (18)aa a byproduct of possible value, contained ursolic acid and higher paraffins typical of leaf and fruit cuticle waxes.] The benzene extracts were freed of solvent in vacuo. Yields ranged from 95 to 105%, on the basis of the rubber content of the leaves fermented. The rubber from the two fractions was combined on a cold mill in the s a n e proportions as the fractions bore to the original amount in the leaves (27% bagasse rubber to 73% cell rubber). The mixture was so tacky that it was unsuitable for processing according to standard rubber mill techniques. It should be noted here that the solution method of compounding developed by McKennon and Lindquist (IO) for solvent-extracted goldenrod rubber would be directly applicable to cryptostegia leaf rubber and would make it suitable for processing on conventional rubber mill equipment. However, this type of compounding and precuring was not investigated in this experiment. When this stock was compounded on a roll mill by recipe A and cured in a standard 6 x 6 inch A.S.T.M. test slab mold, it was too soft to be retained in the mold, and the resulting vulcanizate ww porous. Presumably i t could have been compounded and cured in the smaller quantity and special mold used in the previous work.
TABLE VIII.
RE-
TABLE X. COMPOSITION AND PROPERTIES OF WHOLECRYPT+ STEQIA LEAF RUBBER (CONTAINING 27% BAGASSERUBBERAND 73% CELLRUBBER)
Cell Rubber BagWe Rubber
.
86.4
10.7 2.9
20 1060 780 160 81
77.1 22.0 0.9 20 2050 720 1000 40
OF RECOVERY BY PRECIPITATION OR TABLE Ix. EFFLCT EVAPOUTIONON COMPOSITION AND PROPERTIES OF SOLVENT-EXTRACTED CELLRUBBER
Pptn. from Bensene Evapn. of by Aoetone Bensens Compoaition, yo Rubber hydrocarbon Resins Benmne-insol. material
95.8 3.8 0.9 25
Compounded by gum recipe A.
20
1700
1609
