KUBBER FROM GUAYULE \ I . S. Y I S H I \ I t R i ] . F K i \ I C
\.
HlHOSi\\ i, A L D ROBERT E\IERSO\l
Ctdiforniu I n s t i t u t e of Technolog?, Prcmdena, Calif.
Jordan-type mill may be substituted for the conientional pebble mill for the extraction of rubber from guayule shrub. The Jordan is more rapid in action than the pebble mill, and may be used on young shrub fresh from the field. The crude rubber extracted by the Jordan mill contains less acetone-benzene ineolubles than pebble mill rubber. Additional adiantage can be gained bj subjecting the chopped shrub to a caustic cook before Jordaning. This treatment further shortens the milling time required to achiele a giten percentage recoiery and further reduces the acetone-benzene insolubles. The acetone solubles are slightly reduced by the caustic cooh,
and the stability and appearance of the crude rubber are improied. T ulcanizates from Jordan-extracted rubber, from either raw or caustic-cooked shrub, show phjsical properties somew hat superior to those of commercial guaj ule rubber extracted in pebble mills. Deresination of the Jordan-extracted rubber with acetic acid gites a produrt which is superior to guayule rubber deresinated with the solients preiiouslj used for this purpose. Rubber deresinated with acetic acid is stable in the crude state, elen without the protection of antioxidants. Vulcanizates subjected to simple laboratory tests show physical properties equiialent to those of No. 1 smoked sheet.
E
indispensable for plantation rubber production, because of the hand labor involved in tree tapping (4). The world-wide advance of industrialization is likely to bring about economic changes which will require the payment of higher wages in thts areas where plantation rubber is produred. Even a moderate increase in wages would be a severe handicap t o plantation rubber, and would place guayule in a strong competitive position, provided its quality would permit it t o be substituted for smoked sheet. Rubber production from guayule is consistent Kith a high wage-level economy, because of the small number of manhours of labor required per ton of rubber. Plantation rubbei production, on the other hand, requires a relatively large amount of labor per ton of rubber. The purpose of this communication is t o describe a process for the production of good quality guayule rubber which might be acceptable as a substitute for smoked sheet. This rubber is highly resistant to deterioration in the crude state, and when vulcanized its physical properties are equivalent to those of vulcanized smoked sheet. The cost should be competitive with the traditional methods of producing guayule rubber. This process also promises to be adaptable t o decentralization, which would make guayule cultivation profitable in areas too remote to be exploited with centralized factory methods of rubber extraction; thus large arras of lo^ cost land might be utilized for rubber production.
.4RLT in 1942 the United States Department of Agriculture
launched a comprehensive program to develop the guayule shrub (Parthenium argentatun) as a domestic source of natural rubber. I n productivity it conipares favorably with the plantation rubber tree (Hevea brusiliensis). Van Iterson (10) quotes figures which show that average plantation trees produce only about 300 pounds of rubber pel acre per year. This figure includes only those trees in actual production, and omits the trees which are too young to be tapped. A rubber tree must be eight to tn-elve years old before it becomes productive. Cultivated guayule, on the other hand, is reported (9) to produce an average of -100 pounds per acre per year. Guayule lays down a new increnient of rubber each year of its groxth, and the sum total of all f h e annual increments is harvested in a single operation. The growth period from planting to harvest may be from two to seven yrars, or possibly longer. Planting, cultivation, and harvesting are adapted to mechanization and require a minimum of hand labor. They fit readily into the -4merican pattern of large scale agricultural techniqur. Guayule has enjoyed only a limited development because of the inferiority of the rubber produced from it. Guayule rubber tends to soften and sometimes liquefy in the crude state; Tyhen vulcanized, its phj-sical properties are markedly inferior to those of vulcanized smoked sheet (plantation rubber). Khile small nniounts of guayulc can be advantageously blended with other types of rubber, it has riot been found practical commercially to ube it unhlended. Doering ( 3 ) reported on service tests of tires and tubes made entirely of guayule rubber, and concluded it would be a poor substitute for plantation rubber. In spite of the substantial sunis expendrd by the Department of -4griculture t o improve guayule rubber by modifying the treatment of the s!irub and the extraction process (1 7 , page 477), no practicable process ha$ yet been developed for the production of guayule rubber of a quality which would make it an acceptable substitute for smoked sheet. Reference is made to a process of lates estraction, ‘which produces rubher of greatly improved quality. It is also known (1, 18) that the resin content of guayule can be rcdueed by microbial action (so-called retting), and that this rcwlts in improvement of the rubber. These processes demonstrate the possibility of substantial improvement in the quality of guayule rubber. Ccrtain economic factors made it advisable t o investigate the possibilitics of developing guayule rubber as an acceptable substitute for smoked sheet. An abundance of cheap labor is 1
Present addrees, University of Illinois, Urbana, Ill.
EXTRACTION
Commercial guai-ule rubber is ordinarily estracted by chopping and crushing the shrub, and then grinding with water in a pebble mill for 90 minutes or longer. The milled slurry is discharged into tanks of water in n-hich the wood fiber or bagasse sinks and the rubber, together with particles of cork, floats to the surface nhcre it ih skimmed off. The cork is water-logged by a subscquent treatment (either boiling or pressure), and after a second skimming the cork-free rubber is ready for drJ-ing. The pebble milling breaks up the tissue cells of the shrub which contain t,he rubber particles, initially of microscopic size, and causes them t o agglomerate into larger particles. The agglomerated particles of rubber (called “worms”) can be recovered as soon as they are large enough to float free from the sinking bagasse. Some small particles are always lost in the bagasse, and some bagasse fiber is always entrapped in the floating rubber. The factory process has been described in detail ( 1 7 ) . Vie have introduced two changes in this general method of processing the shrub. K e cook the chopped shrub in caustic soda
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Vol. 39, No. 11
mesh screen. This was generally done by passage through a power meat grinder, but equally good results were obtained Tvith other methods of chopping. No crushing rolls were used. A coarse cut without much fine material makes circulation of the Grams Crude Rubber per 376 G . Chopped cook liquor and rinse water easier. Crushing gives too much fine Shrub (Dry Wt.) Run material. No. 5 min. 10 min. 15 min. 25 min. The chopped shrub was mixed, quartered, and mived again. 1 31.05 34.20 35.90 38.00 I t n a s then divided into equal samples, of about 1 kg. each, by 2 31.75 34.30 35.90 38.20 3 31.20 34.15 35.85 38.55 passing repeatedly through a Jones divider. Table I shows that 31.60 '34.40 36.05 37.60 ? duplicate samples prepared in this way give rubber yields in close 31.70 33.95 35.85 36.50 agreement. Storage under refrigeration for several weeks docs 35.30 6 30.40 34,40 36.25 31.70 33.80 35.80 36.75 not alter the results obtained with duplicate samples stored in 32,OO 33.80' 35.75 37.45 sealed cans. Av. 31.4 34.1 35.8 37.3 One can from each batch of chopped shrub was taken for moisture determination and for double solvent analysis of resins and rubber hydrocarbon. Xoisture was determined by drying a t 60" C. in an air oven. This temperature is arbitrary. Higher temsolution before milling, and use a steel mill resembling a paper peratures diivc off volatile materials other than nater, some of pulp Jordan, instead of the pebble mill, for the comminution of n hich are lost a t 60' also. The material for double solvent analysis as dried thoroughly the shrub and agglomeration of the rubber worms. The caustic and ground to a fine powder, duplicate samples of 2.5 or 5 grams cook softens the fiber of the shrub, and shortens the milling time were evtracted n i t h acetone and then with benzene, following the required for a given degree of rubber recovery. It also elimiSpence-Caldn-ell procedure (26). The acetone and benzene exnates the cork and improves the quality of the extracted rubtracts w r e dried a t 70" C. and weighed. Agerite Resin D (polymerized trimethyldihydro uinoline) n-as added to the benzene exber, particularly by reducing the quantity of insoluble impurities. tract to retaid oxidation %ring the drying operation. The perT h e Jordan has certain advantages over the pebble mill, even for centage of acrtone solubles and benzene solubles in the dry shrub the milling of raw shrub. I t is more rapid in action than the v-as computed and used as a basis for establishing the rubber conpebble mill, and is adaptable to both small and large scale operatent of the other cans in the batch, on the assumption that the benzene >olubles represent rubber hydrocarbon. t,ions. K e can work n-ith batches of 100 grams, Lvhile thv smallest To extract crude rubber, the procedure was as follows: The pebble mills that give satisfactory action require charges of shrub was first cooked in caustic Lvith 5 parts rvater to 1 part dry about 5 kg. m-eight of shrub, allowance being made for the moisture present Most of the work reported here \vas carried out, iii t x o miniain the qhrub. The required amount of water and caustic was brought t o a boil, and the shrub added to the boiling solution. ture Jordan units2. 17-e also made a few extractions with a comThe niais was boiled slowly for 30 minutes, time being taken from mercial Jordan loaned by the Wood Conversion Company. This the moment when boiling started again after the chopped shrub unit (knon-n as a Claflin) was powered with a 20-hor.sepo~~er was added. motor, and was installed in the espcrimentnl plant of t,he EmerThe cooked mass n a s dumped into a wire basket and rinsed in slon lv running water for about 30 minutes. The temperature of gency Rubber Project, U. S. Forest Service, a t Salinas, Calif. the rinse water was usually about 60 O C. After rinsing, the liquor Several large Jordans n-ere later installed a t the factory operated draining from the cooked chips had a p H of about 8. by the Forest Service in Salinas, and these units m r e found t o be effective in rubber recovery. XIILL OPERATIOS -1smight be predicted from an examination of various patcrits on the recovery of rubber from guayule (11),almost any mechallilling was usually carried out with a ratio of 5 parts Fvater to 1 nism which comminutes the shrub ivill produce a slurry from part initial dry weight of shrub, no allowance being made for the which rubber can be separated after dilution and flotation. loss in dry nwght due to cooking. I n charging the mill, it was assumed that the cooked chips were about 50% water. The reThere appears to he no record of Jordaning as a method of rcquired amount of water was measured out, and put into the mill, covery, although Bradshaw (2) patented the use of a Jordan or the mill m s set in motion so that the water was circulating, and similar machine to reduce t,he chopped shrub to fine particles then the shrub chips were added as rapidly as possible. This propreliminary to pebble milling. Our conclusion from three years' ccdure was followed to avoid clogging the mill, which occurred readily if the entire charge was put in before circulation was experience with a variety of Jordan-type mills is that' quantity started. One half gram of Agerit,e White (sym-di-p-naphthyl-pand quality of rubber recovered are less dependent upon design phenylenediamine) was also added to each 500 grams dry \wight and construction of the mill than on technique of preparation of of a mill charge to retard deterioration of the crude rubber. Agerthe shrub and conditions of operation of the mill. Every unit ite White was found t o be the most effective antioxidant for retarding the decrease of benzene solubles and the increase in acen-hich n-e built \vas capable of extracting rubber and, if run long tone solubles during storage of the crude rubber. enough, n-as capable of high percentage recovery. The units Figure 1 is a diagram of the setup. The slurry passes from which gave more rapid comminution also gave good recovery in hopper H into the small end of Jordan mill L. It is ground beshorter t,ime. It would therefore be misleading to emphasize t m e n the stationary and rotating blades as it passes t o the large end of the cone, and enters ret,urn hose R, through which it is fed the structural details of any particular unit, especially since back into the hopper. Xilling is usually continued for 15 or 30 Jordans are already well known and fully described. Instead, minutes (time being t,aken from the moment when all the charge attention will be given to the techniques of shrub preparation and has been added) at about 60" C. At the end of the desired period operation of the mill. valve V i s turned to discharge the slurry into flot,ation tank T , and rinse water is poured into the hopper. A charge of 300 grams of shrub ground with 2. 5 liters of water would be discharged into a PREPARATION OF SHRUB flotation tank containing about 10 gallons of water a t 50" C. The flotation tank is stirred violently with a stick for & moment Shrub was used from the local field and from various plantings after the slurry is run in, and then the surface is gently agitated t o in California which were maintained by the Emergency Rubber Project. Most of the work was done on shrub one t o tn-o years knock particles of bagasse off the curds of rubber as they float t o the surface. ilfter 5 minutes the rubber is skimmed and transold, although some extract,ions were made wit,h shrub three or ferred t o a kettle of water and boiled 30 minutes. I n the case of more years old. The same technique was adequate in all cases, raw shrub the boiling is necessary to remove cork. T o avoid unand it was not necessary to dry the shrub t o any special moisture content before processing. The shrub, as received, was deleafed necessary differences in treatment of rubber from raw and cooked b y immersing the tops in boiling n-ater for 15 minute's and then shrub, t,he rubber from cooked shrub is also boiled 30 minutes, although there is no cork to be water-logged. However, boiling beating on a cement floor. The deleafed shrub was washed to eliminates some of the bagasse particles which are almays trapped remove adhering lumps of soil and chopped to pass a l/r-inchin the floating rubber. The boiling breaks up the rubber into fine 2 Structural changes which improved the extraction efficiencies of these particles and permits bagasse splinters to separate out, and the mills were made from time to time. These changes are responsible for difrubber forms a uniform layer which can be skimmed off on a wire ferences in rubber recovery shown in different tables, in spite of comparascreen for final drying. ble conditions.
T.4BLE
:
'
INDUSTRIAL AND ENGINEERING CHEMISTRY I. YIELDS FROM MILLIKC: DUPLIC.4TE SAMPLES O F SHRUB FOR 5, 10, 15, ASD 25 MINUTES
November 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
Ordinarily a temperature of 60” C. !vas used for the flotation tank; a felv experiments with temperatures from 5’ to 85” C. showed no difference in the effect on rubber from raw shrub, but with rubber from cooked shrub, a l o a temperature in the flotation tank makes skimming easier because the worms are coarser. I n pebble milling it is believed that slow dilution of the slurry in the settling tank favors recovery of cleaner rubber; therefore some experiments were run to see if this were a significant factor in Jordan milling. The slurry was either diluted suddenly, by discharging into the empty settling tank and immediately dumping in a large volume of water, or diluted slowly in a very large mill hopper so that a large amount of IT-ater could tie added, and then allowing the mill charge to flow sloivly illto the previously filled settling tank. Sloir or fast, dilution made no difference in the amount of crude rubber recovered or in the acetone solubles or benzene insolubles of ‘the crude rubber. The screens of skimmed rubber were dried in a circulating air oven a t 40” C., usually for about 72 hours. The dried rubber was peeled off the screens, weighed, and sampled for determinations of moisture, acetone solubles, benzene solubles, and acetone-benzene insolubles. Bromination was used as a check on the benzene solubles, to support the assumption that this fraction represents rubber hydrocarbon. I n the case of the rubber from ralv shrub, not all the benzene solubles could be brominated, but with cooked shrub the rubber estimated as tetrabromide agreed TTell n-ith the figure for benzene solubles, provided there had been no appreciable softening of the crude rubber prior to analysis. Softening is accompanied by a decrease in both the benzene solubles and the brominatable fraction of the benzene solubles. After softening has taken place, the brominatable fraction no longer s h o w a constant relation to the total benzene solubles but varies over a wide range. The bagasse was collected by straining the effluent from the flotation tank through a cloth bag. The dried bagasse was weighed and sampled for anal of acetone and benzene solubles.
1479
preparation of shrub samples for analysis, and that this is one of several possible reasons why more rubber is found by mill estraction than by analysis. Therefore it appears more logical to base estraction efficiency on rubber hydrocarbon found in bagasse plus crude than o n shrub analysis. This method also avoids recovery figures higher than 100co hich is common when calculations are based on shrub analysis. The figures for extraction efficiency in the following tables are based on benzene solubles in bagasse plus crude, but the rubber content on the basis of shrub analysis is specified so that this basis can be used if desired.
fr.
Hopper in which charge is placed
J . Jordan v. \ alve K. Return hose D.
Discharge hose
T. Flotation tank P. Pulley
EFFECTIVENESS OF PROCESS
The efficiency of the extraction process may be determined in two Trays. The rubber hydrocarbon (benzene solubles) of the crude rubber can be compared with the rubber hydrocarbon present in the sample of chopped shrub, on the basis of doublc, solvent analvsis of the shrub, or it niay be compared with thca total rubber hydrocarbon found in the bagasse plus the crudts rubber. More rubber hydrocarbon was regularly found in the bagasse plus crude rubber than had been found by analysis in the initial shrub sample. There are several possible explanations:, Sanipling errors might make the shrub analysis fail to represent the correct figure for rubber hydrocarbon content of the shrub sample. In this case it xould be expected that the error would sometimes bc in one direction and sometimes in the other, but we have nearly aln-ays found that the difference Tvas in favor of mill extraction rather than shrub analysis. A second possibility is that the shrub analysis fails to extract all the benzene solubles present. Since n-e had no proper facilities for carrying out the sulfuric acid digestion specified in the Spence-Caldwell procedure, this possibility muet be considered. But while the acid digestion may be necessary n-ith older shrub where the rubber content is high, evidence is lacking that there is any need for it with young shrub such as we werr using, vhere the percentage of rutiber is 101%-. At the Emergency Rubber Project laboratories in Salinas, where acid digestion rras regularly used, analysis of the shrub generally Phoived less rubber hydrocarbon than did that, of bagasse plus crude. ‘4third possibility is that the drying of the shrub sample preparatory for double solvent analysis results in some loss of rubber hydrocarbon, so that the thoroughly dried sample is not comparable with the moist shrub used for estraction of crude rubber. T O test‘ this possibility, chopped shrub was xetted and treated with various water-soluble antioxidants such as hITD (mtoluenediamine) prior to desiccation and fine grinding for analysis. This resulted in the recovery of more benzene solubles in the analysis, but never enough to make up the total found in bagasse plus crude by milling. K e accept this effect of antioxidants ns evidence t h a t some rubber is ordinarily lost in the
Figure 1.
Diagram of Aliniature Jordan Setup
The effectiveness of the extraction technique depends not only on the completeness with which the rubber hvdrocarbon is recovered, but also on thc frcedom of the crude rubber from objectionable impurities. The nitrogen content is generally small. The principal impurity is resin (acetone solubles), and the second is material insoluble in both acetone and benzene (designated “benzene insoluhles” in the tables). The tabular data s h o n that the yield and quality of recovered rubber depend upon several controllable variables. But it cannot b: assumed that the same combination of cooking and milling techniques will be optimal for all types of shrub, because the age, season of harvest, extent of drying after harvest, and other characteristics of the shrub influence thc recoverability and quality of the rubber. Fuither experience may be expected to show the desirability of modifying the technique to compensate for differences in the condition of the shrub. iZlthough sodium hydroxide has been used to improve the quality of guayule rubber, a caustic cook folloired by a milling technique appears to be a n innovation. 11-e tried treating the ground shrub n i t h cold caustic solutions, cooking in a double boiler, open boiling, and pressure cooking. Cold soaking is less effective than hot processing. KO attempt was made to establish the relative merits of the different methods of cooking, nnd open boiling was generally used because it is simple and effective. Cooking with other chemicals used in the paper pulp industry was also tried such as sodium hydrosulfite and combinations of sodium sulfide and sodium hydroxide. Sodibm hydrosulfite gave rubber of poor quality. The use of sulfides had interesting effects on acetone solubles but showed no outstanding advantages over sodium hydroxide alone. Table I1 shoiTs the effect of caustic concentration. A charge of 500 grams of chopped shrub (290 grams dry weight) was cooked in a double boiler with 2.5 parts water to 1 part dry Jveight of
1460
INDUSTRIAL A N D ENGINEERING CHEMISTRY 1
L I
d
VI
W
W z I
t
TABLE11. EFFECT OF Car-STICCONCESTRATION ON Loss IF DRYWEIGHTAND o s RT-BBER RECOVERY AT .4 MILLIXO TIMEOF 20 NI~WTES Caustic Concn "c of D r y Vit of Shrub 0
,
, Loss
wt
i n Dry
, (7c
L
26.0
15 25
44 0 16.0
Crude Rubber Recovered, Grams 9 5 3 4
16 7 19 6 18 0
Acetone Sols i n Crude Rubber, as 70 of T o t a l Sols 26 9 19 5 26 0 24 6 22 3
shrub for 30 minutes, rinsed, and milled lor 20 minutes a t about 85" C. .i cook with caustic gave poorer recovery than no caustic, and concentrations of 5T0 or higher were needed to improve recovery appreciably. Acetone solubles are also higher with the 1% cook than with the plain water cook. The acetone solubles are definitely loiver with the higher caustic concentrations but it, is evidently impossible t o saponify more than a small part of the acetone soluble material. Since the yield did not increase appreciably from 5 to 2 s C c caustic, a concentration of 10% was arbitrarily selected, partly because this is approximately the concentration used in the paper pulp industry. However, the water-shrub ratio of 2.5-1 made such a thick mass that circulation was poor; later, when the u b of the double boiler was abandoned, difficulty was esperienced in getting a uniform cook, so the wat,er-shrub ratio was raised to 5-1, and the caustic was raised to 2 0 5 of the dry weight of shrub to keep the concentration in ivater the same as before. -4 raustic concentration of 10cTc of the dry shrub is sufficient ivith a water ratio of 2.5-1, and this ratio is satisfactory with indirect heat or with mechanical circulation of the mass during the cook. Table I1 shon-s the improvement in yield due to cooking. for an arbitrarily fixed milling time. Table I11 and Figure 2 show that the yield from raw shrub can be brought up to nearly the wnic level as the yield from cooked shrub by greatly prolonging the milling time. I t is therefore incorrect to conclude from Table I1 that cooking increases the yield. It merely shortens the time required to reach a given level of recovery. For cwmiplc, with raw shrub 96y0 of the total rubber hydrocarbon (benzene solubles) is recovered in the crude rubber after 240 minutcts of milling; the same recovery is attained with cooked shrub after only 20 minutes, and after 40 minutes the cooked shrub gives a recovery of 99yc. The insolubles are lower, and the acetone solubles continue to diminish with milling in sharp contrast to the r a ~ vshrub, where acetone solubles remain practically the same for all milling times.
lco
Vol. 39, No. 11
In the case of both raw and cooked shrub (Figurr, 2 ) , 70:; of the rubber is recovered aft,er less than 5 minutes of milling. The factory pebble mills, on the other hand, ordinarily recovcr only about 70Cb of the rubber from young shrub after about 1.5 hours of milling. The miniature ,Tordam can attain this level of rrcovery in even shorter time if the rlearance between rotor and stator cones is decreased; but throughout the esperimcntal runs reported here the clearance was kept, large to minimize w a r , in ordrr t o avoid confusing the effects of mill w a r with those of factors under investigation. The effect of pH has not yet been thoroughly investigated. Our erperirnce indicated that milling at ncxutral or slightly arid pII values gives small rubber mrnis'which are difficult to wcovcr, and are high in insolubltis and in acetone solubles. Slightly alkaline milling (pH 8-9) gives coarse worms which arc easy to float, and are lox in insolubles and acetone solubles. Hotvever, in the case of very fresh shrub, milled raw, alkalinit!. appt'ars to be disadvantagPous (Table VI). Table IV shows that there is better recovery of ruhber wh(xn the milling temperature is kept at 50" than at 90" C. There is no apparent difference in insolubles, but the acetone solubles are slightly lower a t the lower milling temperature. An important factor is the ratio of water to shrub during niilling. Table V presents data on an experiment with raw shrub, in which the ratio was varipd from 6-20 parts of water to 1 part of shrub. The shrub was milled a t 80" C. Analysis indicated that each charge contained 33.5 grams of benzene solubles. Recovery of benzene solubles is much higher for the 6 to 1 ratio, for both long and short milling. Acetone solubles arc also highcr for the thicker slurry, although the difference is not striking. I t is interesting to note that the acetone solubles are lower for thc, longer milling time and for the thinner slurry, but the benzcane insolubles remain fairly constant' regardless of milling time or water ratio. K i t h thinner slurry, the total benzene soluhl~~s fourid are appreciably lower.
T.ABI.E 111. DEPESDESCE O F Y I E L D O K D U R A T I O S OF FOR RAWASI) CAT-STIC-COOKED SHRI-B"
~~IL1,ISG
~z;$~~~
.%nalysia of Recovered Total Benzene Solubles Crude Rubber Found, G r a m s sols,R ~ AI i I l i iiy Benzene In covered In bagasse in Crude Tiiiie, Grains Acetone insols.. I n crude M i n . recovered sols., (3 Yo rubber bagasse crude Rubber
+
Raw Shrub
10
52.54 X 2 8
20 40
64 19
.5
08.80 73.11 73 78 76.46
80 120 210
0 1 2 5 10 20 40 80
2 0 3 0
160 320
32 3
37.1 49 1 57 4 68.3 73 7 7G 3 73.9
71 8 74.3
22 8 23 1 21 8 21.7 21.0 21.0 21,4
4 62 4 28 3.99 4.06
3.79 3.76 3 61
37 8 42 7 47 8 51 5 %54 8 5.5 2 58.0
13 4 11.7 8.0 6 5 4.7 3.0 2 0
Caustic-Cooked Shrub 24 5 2 72 22.8 26.3 26.2 25 4 24 8 2,lG 351 16.j 23..i 197 44 8 1i.G 21.7 1.74 .A3 1.09 50 0 20.2 54,9 1 6 19 8 1.54 0 43 1 59 5G 6 19 4 0 21 1.38 56 7 18.7 1 7 -1 1 41 56.5 0.14 66.7 0.13 17.1 1.38
51.8
54.1 p3.8 58 0 .59 5 58 2 60.0
!6 3 .~1,6
51.6
33 4 5j.3 3 . 5
n7.1 56.9
5G 7 06 8
74 0 78 5 8.5 7 88 8 92 1 94.H $16 7 40.0 46 0 GI 3
78..j 87 5 96.4 99 3 99 .5 90 2 99.3
l l a w Shrub Controls for Cooked Series %Ob
20e 40 240
63.7 G2.0 67.0 73.3
20 3 20 3 20 3 19 .5
6.14
47 (i 46 8 49.6
8 00
54 4
6 00
6.13
8.9 9.3
56.3
7.1 3 9
56.7 58.3
55.6
84 2
83 3 87 4 93 4
Each set of values regresents the average of two duplicate runs. AIilliiir a t 90" C., wirh 6 par.s water t o 1 p a r t dry weight of shrub. For the raw series, shrub analysis showed 54.4 granis of benzene solubles per 560 grams of d r y matter. For the cooked series a n d the raw controls of this series, 525 grains of dry matter were used for each charge. S o shrub analysis was made for this cooked series. T h e raw shrub controls of the rooked series are inrluded to show t h a t duulicate samoles yield less rubber when extracted raw t h a n when cooked before extractiin h S t a r t of cooked series. C E n d of cooked series. '1
WIO
November 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY,
1481
comparisons were made of total benzene solubles from fresh arid
TABLE I\-, EFFECT OF TEMPERATURE o s RECOVERY OF RLYBBER from dried shrub, dcleafed by hand t o avoid coagulation of the (Extns. froin raw shrub milled 20 mi?. with 10 parts water t o 1 part dry w-t, of shrub; each charge contalned 33.5 grams benzene sols.) Extn
*
~
.inalysis of Recovered Total Benzene Sols. Efficiency' Crude Rubber . Found, Grams Benzene In Recovered Grams Acetone insols., I n crude In bagasse in Crude recovered sols., 5; YG rubber bagasse crude Rubber 7.3 32.7 77.7 3.93 25,4 33 32 20.6 7 5 34.0 4.30 26.5 34 68 19.4 12 9 29.0 3.68 16.1 21.39 21.4 11.5 29.9 61.2 4 7.5 18.4 24.67 21 5
z~e"s"o;~,
;\111IIIIy
Temp.,
C. ;lO
.50 90 90
TIBIE V.
+
:;:!
EFFECTOF WATER-SHRI-R R . ~ T I O o'i R L B B N R RECOVERY Renzene
E&-;:
Annlysib of Recovered l'utal Benzene Sol,. Crude Rubber - Found, Grams in I11 Crude WaterBenzene Shrub Grams Acetone insols, I n crude In bagawe Rubber, % rubber bagasse A crude Ratio recorered sols., % 20-IIinute Milling Time ti-1 33.22 22 7 4.04 24.7 7.9 32.6 76 0 6-1 37.33 22.7 -1.80 27.6 6.9 34 5 80 0 10-1 31.53 22.5 4.00 23 2 10.8 33 8 69.0 10-1 30.16 23.2 3.44 22 2 9.6 31.8 70.0 20-1 23.78 22.3 4 60 17.7 14.4 32.1 55.0 '20-1 23.78 20.6 4 54 17.3 13.6 30.9 .56.0 80-Minute Nilline Time 29.3 8.5 21.0 4.15 6-1 39.04 37.i i8 0 4.95 39.19 20.6 29.2 5,l 8-1 34 3 85 0 38.19 20 4 4 84 28 5 5.7 10-1 34 2 83.0 4.25 7 3 32 66 19 4 24.8 32.1 77.0 10-1 3 80 18.2 12.1 20-1 23 92 18.8 30 6 59.0 9.6 4.76 19 9 29.5 68 0 20-1 26.22 19 9
latex by heat and deleafed by boiling in the usual way. These experiments require the comparison of equivalent samples of ivhole shrub, subjected to different treatments prior t o chopping. It is more difficult to sample whole than chopped shrub, and special precautions n-ere taken to obtain aliquots as nearly comparable as possible. Two hundred uniform shrubs were harvested from our field at, Xanzanar (eighteen months old) and divided into size classes as quickly as possible by weighing on a spring balance. The exceptionally small and esceptionally large shrubs were rejected and the remaining individuals were dealt into four piles, in order of increasing weight, a1tc.rnately clockwise and counterclockwise. The total fresh weights of the final samples differcd by less than 1%. One pair of samples was processed immediately, and the othrr pair was dried before processing. One member of each pair was deleafed by hand, and the alternate member by boiling. After deleafing, each sample was ground and divided into two equal parts by passing through the Jones divider. One part was milled raw, and the other was caustic-cooked and t,hen milled. The upper half of Table VI s h o w results obtained with the eight samples from a single harvest, and the lower half represents a later harvest. .is a standard of comparison for the total benzene solublw found in bagasse plus crude after the different treatments, did not use figures from shrub analysis, but, rather the benzene solubles found after a 30-minute mill extraction of caustic-cooked shrub, since more benzene solubles are alIvays found in this way than by shrub analysis. A separate caustic cook control extraction was made for each treatment of raw shrub, in case t,he amount of benzene solubles should be altered by the treatment. I n Table VI, the caustic cook run precedes the corresponding r a x run or runs. The column headed "Benzene solubles unaccounted for" shows zero for the caustic cook extractions, implying that the highest possible figure for benzene solubles is obtained in this way. Table VI shows that the total benzene solubles for the four control runs of the first harvest ranged from 36.4 to 42.5 grams, and for the two control runs for the second harvest, 35.9 and 31.1 grams. These differences might be regarded as evidence that the different treatments had actually altered the amounts of benzene solubles in the different aliquots, but such a conclusion is open t o doubt because the differences might also be due to the inherent difficulties in sampling whole shrub. The purpose of these experiments was to seek evidence of latex losses in the milling of ran ~
Thicker slurries have been used, especially in the case of cooked shrub, and good .results have been obtained M-ith ratios of 3.5 to 1. Thicker slurries require special precautions to ascertain that all parts of the mass have an equal chance for repeated passes through the mill. To avoid stilling, we used a narrow cylindrical receiver instead of the cone shown in Figure 1. LOSSES OF BENZENE SOLUBLES
(
The condition of the shrub at the time of milling is believed to be an important factor in determining the amount of rubber recoverable by pebble milling. According to Lloyd (fd) the rubber in guayule occurs in the form of latex particles confined within the parenchyma cells. It is known that the latex can be coagulated by partial drying of the shrub, and that the yield of rubber is better from dried than from fresh shrub. The improved yield from dried shrub is generally attributed t o the coarulation of the latex. and it is believed that the milling of fresh shrub results in unavoidable losses of latex O F S H R U B COXDITION A S D PREPARATION O N U N b C C O C N T 4 B L E TABLE \-I, EFFECTS particles in the effluent, because of failure. LOSSES OF BESZENE SOLUBLES of pebble milling to agglomerate these .4nalysis of Crude Rubber and Bagasse particles into worms which can be Total Preparation of Shrub Acetone Benzene Benzene Benzene benzene Benzene skimmed after flotation. Such losses are sols. sols. unAIilling sols. in insols. in sols. in sols. in believed to be especially serious with crude, crude, crude, bagasse, found, accounted CondiDeleafing Pretreatment t i v e , grams f o r b , 7c grams grams 7% tion method of chipsa min. % young shrub (one to two years old). 36.4 0 36.0 9.4 30 27.0 2.4 C.C. Fresh By band Our experience with the milling of 31.1 -14.4 26.7 4.4 30 29.0 7.1 B y hand Sone 38.6 0 38.2 0.4 30 30.0 2.5 caustic-cooked shrub did not produce C.C. Boiling 35.2 3.1 38.3 - 1.0 30 32.0 4.3 Boiling Sone a n y convincing evidence of appreciable 40.1 0 39.6 0.5 16 C.C. 30 28.0 Dried By hand losses of benzene solubles as latex par38.1 - 5.0 33.7 4.4 30 29 0 4.1 By hand Sone 40.2 0 39.8 0 4 30 3 2 . 0 2 . 5 C.C. Boiling ticles in the effluent. It is alFsys pos42.5 + 5.0 37.5 5.0 30 31.0 5.3 Sone Boiling sible to recover more benzene solubles 35.9 0 35.6 0.4 30 29.5 3.0 C.C. Fresh B y hand 30.7 -14.5 27.0 3.7 by milling cooked shrub than by shrub 6.5 30.6 None 30 By hand - 4.2 2.2 34.4 32.2 6.5 27.6 None 120 By hand analysis, even from young shrub fresh 23.0 -36.0 12.9 10.1 4.5 30 31.0 By hand TSPinrnill from the field. Probably the caustic Dried B y hand C.C. 30 28 0 2 0 30.7 0.4 31.1 0 - 4,s 3.2 29.6 26.4 28 0 4 5 Sone 30 B y hand cook is an effective coagulant, but it is + 2.0 1.8 31.9 29.1 26.5 4.0 None 120 By hand 29.6 - 4 8 possible that the Jordan milling of rax 25.6 4.0 26.0 3 5 TSPinniill 30 By hand C.C. = caustic cook: TSP = trisodium phosphate. shrub would result in appreciable losses b Extent to a h i c h each raw milling falls short of accounting for the total benzene solubles found in of latex such as are believed to occur in the corresponding caustic-cook milling. pebble milling. T o test this possibilitr, 0
0
'
6
INDUSTRIAL AND ENGINEERING CHEMISTRY
1482
TABLE VII. COMPARISOS O F GV.4YUL.E AND
Source of Crude Rubber
E . R . P , Salinas
RCBBERS ON
A.C.S. 11
B.A.I.C. FoRafrt..ks
Before .Iuing After .i,qing Curing Elonga- Modulus Tensile Tensile Loss in Time, tion a t a t GOO%, a t break, a t break, tensile, blin. break, % Ib./sq. in. lb./sq. in Ib./sq. i n . 55
factory, Texas shruba
60 75 90 120
860 830 820 800
570 750 760 730
2510 2360 2790 2623
1012 810 553 619
60 68 80 76
Continental Mexican Rubber Co., Pedros unit
60
75
835 840 810
446 505 476 485
2355 2410 1930 1775
685 436 264 1037
71 82 86 42
875 860 820 830
405 495
2200
1702 1575 1093 1435
22.6 36.5 48.0 39.7
1932 1832 1183
29.4 37.0 47 7 23.4
Miniature Jordan, raw shrub
90 120
60
75 90 120
785
580 555
2480
2115 2375
60 845 476 2735 495 76 855 2915 90 790 560 2260 120 780 540 2115 5 rorniula XI used only for this factory rubber, which Tonox Aliniature Jordan, caustic-cooked shrub
lG17 contained 0 4%
shrub. After the different aliquots of whole shrub had been pretreated they were ground, so that the cont,rol for each test run may be regarded as a correct sample, even though the controls may not be comparable among themselves because they represent different samples of whole shrub. The results for the first harvest arc consistent with the interpretation that the rubber must be coagulated t o be fully recoverable. The fresh, hand-deleafed shrub showed 14.4% benzenc solubles unaccounted for as compared t o the caustic-cook control: deleafing by boiling reduced the loss to only 1%. Drying, which is also believed t o coagulate the rubber, resulted in a loss of 5% in the case of hand deleafing and a gain of 5Tc for dele'afing by boiling. This gain is to be regarded as a sampling error, or perhaps loss of some of the control rubber in the skimming process. The second harvest was intended to show whether the losses in the case of fresh, hand-deleafed shrub were impossible to recover, or whether they could be recovered by longer milling. The harvest was again divided into four samples. With handdeleafed fresh shrub milled 30 minutes, t'he unaccounted loss was 14.5Yc, in aggreement with the corresponding figure for the first, harvest. When the milling period was lengthed t o 120 minutes, the loss was reduced t o 4 . 2 5 . I n the case of the dried shrub, lengthening the milling time from 30 to 120 minutes reduced the loss from 4.8$3 t o a gain of 2.5%. Our conclusion from these esperimcnts is that, if the unaccounted losses of benzene solubles are due to uncoagulated latex, then Jordan milling, likc boiling or drying, is capable of coagulating the latex. With fresh shrub, longer milling is required t o at'tain a given level of rubber accounted for than with dried shrub. Table TI also shows two 3O-nlinute runs with raw shrub, in which the mill slurry was made alkaline with trisodium phosphate. The latter was tried to determine ivhether it would act as an emulsifying agent, leave more rubber in the form of latex, and hence increase the benzene solubles Unaccounted for. With fresh shrub, trisodium phosphate raised the fraction unaccounted for to 36%, which is consistent with the hypothesis that trisodium phosphate causes more rubber to remain in finely divided form. I n the case of dried shrub, trisodium phosphate did not change the benzene solubles unaccounted for. It made no substantial change in the yield, but the rvornis vc-ere coarse and easy to skim. I n conclusion it may be said that the miniature Jordans can recover close to of the rubber hydrocarbon present in the shrub, either raw or caustic cooked. I n either case the process is more rapid than pebble milling. Figure 2 and Table I11 show that, to obtain good recovery, sufficient time must be allowed for the milling operation. Other factors favoring good recovery are low milling temperature and lo^ ratio of water to shrub.
Vol. 39. No. 11
Our results do not indicate the need for special aging or conditioning of the shrub before the rubber is extracted, except to the extent that these treatments shorten the duration of milling required t o attain a given percentage recovery of benzene solubles. Caustic cooking surpawcs other treatments in its effect on shortening milling time, and also makes a wbstantial reduction in bcnzcne insolubles and a small reduction in acetone soluhles DERESINATIOS KITH ACETIC ACID
The crudc rubber extracted in miniature Jordans, whethcbr from ralv or cooked shrub, is not muchmore stable t,han ordinary commercial guayule rubber ext'racted in pebble mills. The physical properties of vulcanizates of Jordan-milled rubber have been found to be somewhat superior to those of ball-milled rubber, probably because of the lower benzene insolubles. But the crude rubber, whether from caustic-cooked or raTv shruh generally tends t o soften and eventually t o liquefy, like the rubber from pehblc mills. The caustic cook delays softening and liquefaction, but does not prevent it. Tonox (p-p'-diaminodiphenyhnethane) is known to be a good preservative for crude guayule rubber, and we found that 0.5% Tonox, milled into the Jordan rubber from caustic-cooked shrub, prevents softening indefinitelp. The rubber from raw shrub, whether Jordan- or pebble-milled, eventually softens even if treated n i t h Tonox. .In any case, Tonox is not an ideal preservative because it acts as a secondary accelerator when the rubber is compounded and cured, and Tonostreated rubber is less resistant to Geer oven aging than rubber without Tonox. The softening of guapde rubber is generally attributed t o its high resin content, and deresination has often been recommended as a means of making it an acceptable substitute for smoked sheet. Various solvents have been used for deresination of guayule (8j, and it is reported that the removal of the resins, far from protecting the crude against deterioration, makes it extremely unst,able a n d subject to liquefact,ion. Hoxever, when the deresinated rubber is preserved with Tonox, it is much more stablc than t,he Tonox-treated resinous rubber and can be kept in the crude state indefinitely without deterioration. Morris et a!. ( I S ) report from physical tests with deresinated guayule rubber that it is superior to resinous, but that the elongation and tear resistance are lo~v,and that deresination does not offer sufficient advantages t o justify t,he expense. Guayule resin is a mixture of a large number of components, and it is reasonable to expect that the effect of the various components on the rubber will differ appreciably. Exploratory experiments shoived that,, if the bark and the ~voodwere separated and then extracted, the rubber from the rvood contained less resin but \vas more prone to liquefaction than the rubber from the bark. This suggests that deteriorhtion may be dependent upon charact'er as viell as quantity of resin. Haagen-Smit and Siu ( 6 ) studied the oil obtained from guayule by steam diqtillation, separated it into forty-five fractions, and kindly supplied us with samples of the various fractions, which we tried on guayule rubber. Some of these fractions were much more potent than others in bringing about, oxidation and deterioration of the rubber. Some seemed to exert a preservative effect. The deteriorative effect was associated with the presence of osygrn in the molecule. h suggestion was made that it might be possible t o extract the osygen-containing resins with cold concentrated sulfuric acid. Although it did est,r:tct niaterial from the rubber, no concentration of acid n a s found which would act as an effective solvent without damaging the rubber. Experiments were then made with hydrochloric, nitric, and phosphoric acids, and n-ith organic solvents such as acetone, dioxane, methanol, ethanol, glycerol, acetic acid, etc. Hydrochloric and nitric acids produced objectionable changes in the rubber, suggestive of breakdown. Phosphoric acid did not seem t o damage the rubber but was too viscous to give good ext,raction. The
November 1947
organic solvents all caused more or less swelling of the rubber, and the solvent-extracted rubber n-as extremely prone to deterioration as soon as it was washed and dried. The acetic acid, on the other hand, did not cause sn-elling of the rubber nor any apparent breakdown. Rubber extracted with acetic acid and washed in !-rater n-as light gray, not sticky, and tougher than ordinary guayule. IT-e still have samples of the first batch, prepared in July 1944. Although no antioxidant has been added other than the Agerite White used in milling, this rubber ha> shon-n no tendency to soften or liquefy. Exposure to a temperature of 70" C. for 48 hours in the Geer oven fails t o make this rubber flow, although ordinary guayule rubber becomes almost liquid n-ith this treatment. The rubber deresinated with acetic acid can be handled on the conipounding mill as easily as smoked sheet. I t does not stick to the rollers objectionably and is very easy to process. The properties of the cured rubber are given in the next section. To deresinate the rubber with acetic acid, it is first dried on screens in the ordinary way. The dried sheets are porous like rubber sponge. If the sheets are carefully peeled from the drying screens, the porosity is riot lost, and it seems to be a n important factor in successful deresination. The sheets are stacked in crocks, separated by layers of filter paper, and covered with glacial acetic acid. The acetic acid is changed occasionally. After extraction, the rubber is washed in running water until no odor of acid remains, and dried. This rubber still contain? 2.5 to 4 . 5 5 acetone solubles (depending on the length of extraction and the thickness of the sheets extracted), 1.5% acetonebbnzenc insolubles, and more than 90% brominatable benzene solub1c.s. The loss in m i g h t due to dercsination is 17 t o 19%. The acetic acid is used repeatedly so that it becomes quite dark. The acid that has been used longest contains, presumably, a (,oncentrate of the substances causing deterioration. If it is diluted with equal parts of water, a cloudy yellow precipitate appears. This floats to the surface after standing, and leaves a clear colorless layer of acetic acid and water. The oily precipitate can be filtered off or taken up in ether. If the precipitate is separated by filtration and exposed to air, it' deteriorates rapidly, and the stages of deterioration are closely similar to t h r course of deterioration of resinous guayule rubber. Attempts are being made to identify the deterioration of guayule rubber with some fraction or fractions contained in this extract. Khether or not acetic acid deresination proves to be practicable on a commercial scale, the results described here demonstrate the porsibility of preparing from guayule a crude rubber which is free from tendency to liqlefy. CURISG A S D PHYSIC.AL TESTIXG
*
1483
INDUSTRIAL AND ENGINEERING CHEMISTRY
For physical testing, 400-gram samples of crud(' rubbcr were compounded on a laboratory mill equipped with 6-inch-diameter rollers, follon-ing .I.S.T.lI. procedure. The temperature of the rollers was 130-1-10" F. The compounded sheets 11-ere allowed t o rest 24 hours before curing. Cures were made in a fourcavity A.S.T.11. mold; each cavity n-as 6 inches square and 0.075 inch deep. Curing temperature s as 257" F. (20 pounds steam), checked from time t o time by a mercury thermometer inserted in a pool of mercury in a steel plate between the plattenu of the press. The cured dabs rested 24 hours before samplts m r e cut for testing modulus a t 6 0 0 7 elongation, ultimate elongation, and tensile strength at break. Aging tests were made by exposing samples t o rapid air circulation in t h e Geer oven at 100" C. for 24 hours. S o one compounding formula has been accepted as studying the physical properties of guayule rubber. Boone (f6) used a pure gum formula containing diphenylguanidine (DPG) as the only accelerator. Hauser and leBeau (6) uscd formulas containing a combination of mercaptobenzothiazole (Captas) and DPG. Morris et a l . (13)tried out a number of srrrlerator combinations and reported that dibenzothiazyl di-
sulfide together with zinc dimethyl dithiocarbamate gave the best all-round characteristics for guayule. Hauser and leBeau ( 7 ) showed that the acetone solubles in guayule rubber influence the amounts of stearic acid and sulfur required to give satisfactory cure; since the quantitl- and character of acetone solubles may differ widely from one sample of guayule to another, it can hardly be expected that a single formula n-ill Le satisfactory for comparing different, types of guayule and for comparing guayule with smoked sheet. Therefore, it seemed best to modify the formula in accordance nitli the requirements of the rubber, and in this 11-ay seek to give a more significant cornpaikon of properties than could be achieved with any single formula. We had neither time nor material for a thorough study of all the variables in even a simple pure-gum formula. The work of Hauser and leBeau clearly indicated the importance of accelerator and stearic acid in relation t o guayule resin, and n-e confined our modifications of formulas almost entirely to these two factors. The sulfur requirement merits more attention than \-re have given it, especially in relation to aging, but in general we follon-ed Hauser and IeBeau in adhering to a concentration of 3 parts of sulfur per 100 rubber. The pure gum formulas recommended by the A.C.S. Division of Rubber Chemistry for t,he testing of Ilevea rubber ( 1 4 ) were used: h . C . 6 .I
A.C.S. I1
100 6 0.5 3.5 0.5
Rubber Zinc oxide Stearic acid Sulfur Captax
100
6
4.0 3.5 0.5
B.A.I.C. 100 6 4.0 3.5 1.0
A.C.S. formula I is for testing good grades of smoked sheet, and the formula I1 is for wild and slow-curing rubbers. Formula I does not give a satisfactory cure n-ith any guayule rubber. Forniula I1 has been extensively used by the U. S.Forest Service Emergency Rubber Project at Salinas for testing guayule, but most of the rubber tested cont,ained about 0.5y0 Tonox as a ative. Tonox acts as a secondary accelerator, enhances the activity of the Captax, and therefore constitutes a modification of formula I1 which should not be overlooked. Guayule rubber n-hich contains no Tonos gives a soft and unsatisfactory cure with formula 11. For rubber without Tonox the Emergency Rubber Project doubled the amount of Captax in formula I1 and called this the B.rl.1.C. formula.
TIBLLVIII. C O V P ~ R I S O O FS AIEXIC~N, Six,~.v~s, ASD XINIATVRE JORD %piRI-RBER.o s FOR\ITTLA A Before Aging Elonga- AIodulus Tensile Curing tion a t a t 60070, a t break, Time. break, Source of Ib./ lb / % sq. in. sq. in Crude Rubber N i n .
E.R.P.,Salinns 0 factory. Texasshrub (Tonox)
28 35 40 45 60 80
E . R . P . , S a l i n a s 20 factory,locnl 30 cultirated 45 shrub,Tonox 60 painted on bale Continental 28 Mexican 35 Rubber Co., 40 Pedros u n i t 45
875 830 813
770
780 770 790 790 743 695 810 820
After .Aging Elonpa- Tensile tion a t a t break Lossin break, Ib./ 'tensile, R sq. in. 5 735 2140 3 parts 35 815 650 2895 660 2XO 12 vtwy rapid curing of the sampk. stearic 40 SI5 710 3050 acid 50 820 630 2855 630 ?ii3 20' Table 1-111 s h o w that the rubber extracted in miniature 60 790 720 2633 . , 90 780 770 2690 690 2280 13' .Jortinns, both from raw and rooked shrub, is superior to t l i v Forniula B, 2 5 850 496 2790 , .. 1Icsican ruhber, and approximatrly equal t o the Salinas ruhhcr 2 parts 40 830 600 3020 700 2233 23 stearic 60 780 670 2510 780 2665 -3" i n tensile strength. T h r Jordan rubbrr is definitely superior in acid 75 790 670 28'20 760 2790 1 1 rwistance to Geer oven aging. Table IX shov-s that thc aging 90 795 640 2793 7.50 2320 17 tnncc of the rubber from caustic-cooked shrub can hc lura -1negative value for loss in tensile indicates a gain. thctr improved by using less stearic arid. The rubbcr x.ns extracttbd in the 20-horsepon.cr Claflin-type Jordan operatrd :it Table 1-11compares various sanlples of guayule rubt)c,r r)ll th,. Salinas. This rubber was tested o n formulas h and B, providing 3 part,- and 2 parts of stearir acid, respectively. Thc markod TvhicI, B..k.I.C. formula with formula 11 used for Salinas chareffect of st T h r possibility remains that the caustic cook damages tlit. Sulfur 3 :32 rubber in some Tvay, but that the damage is concealed iiy thrl Stearic acid 3 Captax 1 1 superiority of the caustic-cooked rubber in other respects. If DPG 0.5 0,a thr caustic cook could be avoided entirely, and its advantagtss I n the case of the Tonox-treatrd Salinas rubber, thv ('aptax arid avhieved by some othrr means, the rubber might possibly shon. DPG were cut to 0.5 and 0.25 part, respectively, t o c~oiii1)t~tisatc other advantages which are not obvious hecause of daniagc from the caustic cook. These tests on rubber Cxtracted in Jordaris froni raw and from raustic-cookcd shrub show that, rcihiniferous guayule, rubbrr ran he compounded t o TABLE s. COMPARISOS O F FORMCLAS FOR C O . \ l P O l - N D I S . ( ; (; I , . $ Y ~ - I . F : I{I.BRER DERESISATED WITH ACETICAr11) give pure-gum tensiles up to about. 3600 pountls (Base formula: 100 parts rubber, 5 zinr oxide, 3 -uliur) per square inch and reasonably good resistance t o Before Aging After Aging -~ aging in the Geer oven. Compared wit,h sanip1t.s Elonpa- LIodulus Tens= Elonga- Tensile of guayule rubber extracted in the conventional Curing tion at a t 600C;,, a t break, tion a t a t break, Loss in Time, break, Ib./ Ib,/ hreak. lb.! tensile, pebble mills in Mexico and a t Salinas, the JordanVariables in Formula IZIin. 5 7 sq. In. sq. in. '7r sq. i n . C; extracted rubber s h o w definite superiority. .I Stearic 2, Captax 1, 30 725 ., 5020 ... .. D P G 0.5 40 685 .. 4550 ... .. characteristic common to all the guayule rubber Stearic 1. Captax 0.5, 45 920 355 2500 790 3030 -21" tested is that Captax and DPG gave good currs, DPG 0.1 60 890 530 3120 750 2780 11 90 865 490 2770 710 5090 24 but much more accelerator 11-as required than is 120 855 560 2895 690 1740 40 ordinarily provided in formulas for smoked shetat. Stearic 1. Captav 0.8, 20 830 760 3080 67: 2750 11 D P G 0.2 30 770 1170 3500 663 3305 6 The rubber preparrd by deresination with actstir 60 750 1530 3750 620 2560 32 acid is quite different from the resinous rubber and, Stearic 2. Captax 0.8, 10 950 210 1515 .,. .. D P G 0.2 15 815 745 3190 480 745 69 in some respects, is inore similar to smoked shwt . 20 775 1205 3670 ... ,. The first sanipks xere compounded on formula 30 745 1880 3960 375 Si0 ig 50 720 2090 3950 315 420 89 €3, which \vas found to give more acceleration than , 80 685 2055 3500 355 590 83 necessary. A wriw of formulas ivith less acceleraStearic 2 , Captax 0.60, 20 880 400 2050 355 285 Si DPG 0.15 30 800 770 3025 310 275 91 tion were tried, in order to find the curing require780 1090 3260 ... .. .. 760 1275 3470 260 255 93 iiirnts, and comparisons Jvere made with smokrd 65 753 1320 3400 ... .. . 92 . sheet. Table X s h o w the results obtained with 80 765 127.5 3415 285 "0 A negative loss in tensile indicates a gain. acid-dercsinated rubber compounded in various nays, brginning with formula B. The remarkably
T.~BI.E 1s. EFFECT O F STEARIC .%CID COSCESTR.kTIOS O N h G I S G RLSISTAXTE O F R ~ B B EFROM R CAI-STIC-COOKED SHRL-B
, , ,
, ,
c.
::
(I
November 1947
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
STEARICACID COSTESTO N T ~ B LSEI . EFFECTOF REDUCED AGISG RESISTASCE Before .iging Stearic Elonga- I I o d u l u s Tensile ;Icid per Curing rion a t a t 600%,, at break, 100Perts Tir!ie, break, Ib./ Ib.! Rubber IIin, 7 sq. i n . sq. in. Guayule Rubber 1 0 40 800 1030 3780 3550 30 780 1160 3350 65 7.50 1180 -!O JO 63
810 810 800
1 .0
% 75
725 685 660
0.3
30
i3.i 710 6iO
0 ;,
-.iOI,)
TABLE
,
3040 3155 3135 Smoked Sheet 4245 2155 2510 4025 2490 3550 1290 1990 4245 2370 4080 2460 i10
780 810
After I g i ng Elonga- Tensile rion a t a r break, Loss in break, lb.! tenrile. R sq. in. Cb 625 ,550 565
2265 1650 2100
40 53.5 37.3
705 i39 715
2475 2855 2620
30.7 Q.5 16 4
236 200 215
280 220 250 370 425 410
300 265 235
X I I . RESL-LT~ O F P H Y S I C A L TESTS OS SO. 1
SHEET
95 95 93 92 90 90 SMOKED
(Base formula: 100 parts rubber, 5 parts zinc oxide, 2 parts stearic acid, 3 Darts sulfur) After Aging Before Aging Elonga- Tensile Ilodulus Tensile tion a t ar break, Loss in C 11ring Clonga- a t 600'X a t break, break, Ib.! te;$le Ti me, Ib.! lb.! rion a t rC sg. i n . C sq. in. Xin. "c sq. in. Cantax 0.60. D P G 0.15 20 760 1320 3530 250 330 111 30 7l.i '2190 4060 .. .. ... 40 680 2380 3810 ... , . . . 20 890 2853 3990 i5n 206 Bi 65 ti30 2700 3115 80 653 "40 3390 iin is5 135 Captax 0.8 20 875 440 2320 ... . . 30 815 760 2710 6G5 3ii5 -33" 50 765 1133 3150 615 3190 - 1.3 80 743 1360 3383 610 3003 11 110 720 1405 3100 400 950 66 0 . I' negative loss in tensile indicates a gain.
high tcnsile of 3000 pouiid? per squart' inch is superioi~t o any value, reported for guayule rubbcr deresinated hy othcr methods, arid is definitely above the tensiles ordinarily obtainable Jvith sniokcd sheet, even when DPG is used in addition t o Captas. Formula B gives too little elongation, and provides inore acceleration than is required by this ruhber. The nest formula tried \vas A.C.S. I (standard for smoked sheet) with only 0.5 part Captax and n o DPG. This failed t o give a full cure, evcii :ti'tc,r 120 minutes iri the press. The 120-minute cure gave an cllongation of 1 1 0 0 7 n-ithout breaking, and a modulus of only 345 pounds per square inch a t 1050'; elongation. Since it ~va' impossible to approach a satisfactory cure n-ith the A.C.S. formula, formula B IT-as used with various proportions of Captas, DP(:, arid stearic acid until a good cure was achieved. This iic'st tkw batches in Tabltl S sho\v the modifications of formula €3 arid thv results obtained. The nest to last batch shon-s excellent tensile and modulus arid satisfactory rlongation, but the aging resistance is poor. I t via.: first thought that this might be due t o too niuchacceleration, so mother batch (last in Table S ) \vas tried with0.6 part Captas and 0.15 DPG. The elongation is good, tensiles are Ion-er, but aging resistance is still very poor. Table XI ~hon-sthat thiq is b(wuse the stearic acid is too high. -1lthough 2 parts of stearic acid vere not too much for the resinous rubber, it is rrsponsible for bad aging in the acid-deresinated rubber. Table S I , with the same formula as the last batch in Table S except for stearic acid, shows that aging resistancr is better n-ith 1 part of stearic acid and still better n-ith 0.5 part, although in this case the modulus and tensile are appreciabl?- reduced. More work is needed to establish the best coinhination of stearic acid and accelerators for this rubber. Table SI1 shows some tests with smoked sheet, for comparison
1485
with those in Table S for the acid-extracted rubber. The first batch is on the same formula as the last batch of Table X. The smoked sheet gives loner elongation, but higher modulus and tensiles. Aging resistance is very poor and, in the case of tht. smoked oheet, cannot be improved by reducing the stearic acid, as shown by the lon-er half of Table XI. This formula obviously gives too much acceleration for smoked sheet. The second series of Table SIX shows that if the DPG is omitted and tht, Captas content raised t o 0.8 part, the elongation and aging resistance are great1)- improvrd, but the maximum tensile falls below that of the acid-estractcd rubbpr for corresponding tightness of cure. These coniparisons show that guayule rubber deresinated with acetic acid can he compounded and cured on formulas with Ies3 acceleration than is required for resinous guayule. -4lthough thts amount of acceleration required is much closer to the amount required for curing smoked sheet, the deresinated guayule still requires more acceleration than the smoked sheet. The fully cured guayule has physical properties and resistance to Geer over1 aging equivalent to smoked sheet, and the guayule will stand more DPG than the smoked sheet without sacrifice of aging resistance, Pspecially if the stearic acid is kept low. By increasing the DPG to the proportions used with resiniferous guayule, the tensile strength of the acid-deresinated guayule can hc raiqed to 4000 and 5000 pounds per square inch, or above the range of tensiles ordinarily reported for smoked sheet. ACKXOW'LEDGhIENT
The work reported in this communication was done at the California Institute of Technology and the Manzanar Relocation Ccnter of the \Tar Relocation Authority. Grateful acknowledgment is made to the \Tar Relocation Authority and the C. P. Army for administrative support: to the U.S. Forest Gi,rvicc,, the Earhart Foundation, the Iiirkhill Rubber Company, arid th(. U.S.Rubber Company for financial support; and to thc Lo. Angeles Standard Rubber Company and V.S.Rubber Coinpan!. for cooperation and advice in the compounding and physical testing of the rubber. Helpful cooperation was also recc,ivcd from many individuals, and special gratitude is due thr, c'vacuc~'. a t LIanxariar, people of Japanese descent, who xvorked on the, many different capacities, oftrn und(br 1 LITERATURE CITED
(1) Alleii, P.J . . Nagli>ki. J..and Hoover, S.K..J . Bart.. 47, 559 -70 (1944) f2) Bradshan.. G . B.. U. Y. Patent 843,567 (1907). (3) D o e r i n g . J. H.. ISD. Esc;. CHEM.,26,541-3 (1934). (4) Gho-h. H. H.. "Realm of Rubber: Record of Exibting C'oriditiona i l l lluhber IriduJtry." Calcutta, J. B. Dayniond, 1928. ( 5 ) Haagen-Smit. A . J., and Siu. Ii.,J . -4772.C h e m . Soc., 66, 2068-74 (1944). .4., arid IeBeau. D. S.. India R ubber lTodti, 106, 447
( 7 ) Ihid., 107, 568-70 (19131. (8) Ibid.. 108, 37-9 (1943). (9) House oi Repreientatives. hearing before Coninlittee on . i g r i culture on H.R.6299, 77th Congress, 2nd seazion. Jan. 7-S,
1942, page 23. (10) Iteraon. G . v a n J r . , India Rubber J . , 80, 869-79 (1936). (1.1) Lawrence. W. A , , U. S.Patents i41,258 (1903). 760,459 (19041. 982,373 (1911); de la Corte, -1.T., I b i d . , 814.675 (19061: D.. Ihid,, 1,169,137 (1915); Carnahan. G. H., Itiid.. 1.671.570 11928). 1,959,902 (1934): Teandle. Williani, Ihid.. 11695,676 (1928). Lloyd, F.E., Plant P h y ~ i o i . 7, , 131-8 (1932). Morris, IC. E., Jame-, R.R., and Kerkenthin. T. A , , India RI(!,lier W o r l d . 105, 565-9 (1942). Sackett, G . A , , Ihid., 110, 295-6 (1944). Ypence. D., and Boone, C. E., Bur. Staiidards, T ech. P a p e r 353 1-echini.
(12)
(13) (14)
(15)
11927). (16) Spence. D.. aiid Caldwell, 11.L., ISD. Es-c:.CHEM.,-4s.~~. 1,;n.. j~
5, 371-5 (1933). (17) U. S.Dept. of - i g r . , Ea,tern Ilegional I(caearch Lah., IndiaJiitiber. Tl-orld, 109,-475-7 (1944). (18) V-hite, J. IT,, Jr.. Saghski, Joseph, Allen. P. J., Hoover, S. I:.. aiid \\-illaman, J. J., IridiuRitbber T o d d , 111. 570-3 (19451. ~
Kt.(.tI\.ED
l f a y 20, 1946
