Recovery of Rubber Latex from

Recovery of Rubber Latex from Guayule Shrub J

J

EDWIN P. JONES Guayule Emergency Rubber Project, U . S . Department of Agriculture, Salinas, Calif.

carbon; similar films of 2.5- to 3.5-year-old shrub contained about 88% pure hydrocarbon. Attempts to develop a process for recovering a,part of the rubber as latex and the rest as “worm” rubber were unsuccessful. (In the conventional recovery method, guayule rubber agglomerates, about the size and shape of a grain of rice, are called worms.) Production of high grade rubber by acid coagulation of the dilute dispersions, rather than by concentrating them t o obtain a 35% latex, appears to be applicable to the fully mature shrub but not to the younger shrub.

T h i s paper describes the progress made in developing a continuous process for recovering natural rubber as latex from guayule, Parthenium argentaturn. Experimental work was confined to batch treatment of a maximum of 150 pounds of shrub grown in California. The latex, containing from 35 to 50qo total solids, was obtained by centrifugal concentration of dilute latex disp-ersions produced by cutting and milling the lush shrub in water. Both the latex and the crude rubber obtained from the latex appeared to be of excellent quality as judged by chemical composition. Physical properties approach or are slightly inferior to those of Hevea for general use requirements.

Processing Steps. Freshly harvested unbaled shrub, or that stored for 2 weeks in closed containers, is processed before detectable coagulation of the shrub latex has set in. The shrub must be cut, crushed, or mashed in water or aqueous medium for maximum release of its latex as a dilute dispersion. Serum from subsequent concentration by a latex separator may be used instead of water, and the dispersion may be re-used to increase the concentration of the dispersed rubber. Initial size reduction by cutting is followed by more thorough disintegration by mashing the cut shrub in pebble mills for maximum release of latex from the rubber-bearing cells. A maximum of about 85% rubber is released from the shrub. Separation of the bagasse or coarse plant debris presents a practical problem as yet unsolved. It is believed that suitable equipment is available for this operation and that it could be used for subsequent washing of the bagasse to retrieve the dispersed rubber adhering to the bagasse before it is discarded as waste. The dilute dispersion must be clarified to remove appreciable amounts of fine solids or cell fragments. This was best accomplished in a Tolhurst clarifying-type centrifuge, but the economy of using this equipment on a large scale is highly doubtful. The clarified dispersion is next concentrated in two steps to give a 35 to 50% total solids latex. This operation requires a special latex separator for developing high centrifugal force. Efficiencies of 90 to 98y0were attained at feed rates of about 208 gallons per hour. Any future latex investigation should determine the feasibility of combining clarification and concentration in special separators of the solids-removal type.

T

HE war emergency prompted the investigation, beginning in 1942, of means for recovering latex from guayule, Parthenium argentaturn, as part of a program for improving the quality of natural rubber from this shrub. Spence initiated latex recovery investigation on guayule shrub some time prior to 1935, and the disclosures in his patent (6) constituted the chief source of information in this field. Although he showed that latex recovery from mature shrub is possible, it seemed desirable to determine the practicability of extraction as latex. The goal of this work was the development of a process for +,he extraction of the maximum quantity of rubber as latex from young (2- to 4-year-old) shrub grown in California. Spence processed relatively small quantities of 8-yearold shrub from which the leaves had been removed; consequently information on the efficiency of extraction from larger lots of young shrub under different conditions or processing techniques was needed. This paper reports the progress made in developing a more or less continuous process, using a maximum of 150 pounds of shrub, and includes much information of an exploratory nature dealing with the difficulties encountered. Investigations on possible processes for recovery of latex and solid rubber and recovery of acid-coagulated rubber from dilute dispersions are also discussed. Data obtained by batch processing show that a maximum of about 70y0of the rubber present in 3- to 4-year-old foliate shrub was recovered as a latex containing 35 to 50% total solids. A maximum recovery of about 80% is envisaged by more or less continuous processing with equipment adapted to the operations , involved. Rubber obtained from the latex appeared to be of excellent quality, judged by its chemical composition, comparing well with that of Hevea (Hevea brasiliensis) latex rubber, but in the physical properties of its vulcanixates, i t was slightly inferior for general use. There are indications that guayule latex could be used to make sponge rubber and dipped goods requiring high tensile strength, high elongation, and low moduli. Guayule latex rubber showed physical properties and chemical composition superior to those of regular commercial guayule rubber. When 8- to 10-year-old foliate shrub was processed as outlined, the cast film of latex rubber contained over 90% pure hydro-

MAXIMUM LATEX RECOVERY

Microscopic examination of freshly lifted, undamaged shrub indicates that approximately 95% of the rubber in the disconnected rubber-bearing cells of the plant is in latex form. Teasing freshly cut sections under water causes cloudiness a n 8 the formation of a dispersion containing particles that have Brownian motion. Gradual desiccation of shrub causes progressive coagulation to varying degrees depending upon storage conditions after lifting. Rupture of all the rubber-bearing cells could conceivably result in complete release of the shrub latex into a liquid medium, but practical accomplishment was exceedingly difficult. Processing shrub for maximum recovery thus involved a compromise between ideal and practical considerations. Foremost among the difficulties was bringing about the rupture of the cells without causing coagulation of the latex by the application of the attendant mechanical action. The large quantity of liquid required for relatively efficient disintegration of the shrub was an economically unattractive feature of the procedure. For example, 20 pounds of lush shrub containing about 108 pounds of water require about 200 pounds of added liquid for 864

May 1948

865

INDUSTRIAL AND ENGINEERING CHEMISTRY

processing. If the shrub contains 1 pound of rubber and all of it is released upon disintegration, the rubber concentration in the liquid is only 0.5% by weight. Complete recovery thus involves processing 200 pounds of liquid to get 1pound of rubber. This fundamental limitation may be partially circdmvented, but nevertheless it is real and may not be lightly dismissed. In addition to individual variation in rubber content of adjacent shrubs in a field, three main complications have retarded progress: (a) There is an innake difference in the mechanical stability and general processability between dispersions of the young (2.5- to 3.5-year-old) shrub and those of the fully mature (8- to 10-year-old) shrub, with the latter exhibiting greater stability; ( 6 ) seasonal variation in ease of processing is pronounced, and shrub processed during the months of June through September give most trouble; and ( c ) some unidentified substance in the leaves increases instability of dilute dispersions regardless of either season or shrub age. Batchwise processing of 20- to 150-pound lots of shrub had definite limitations but was adaptable to exploratory investigation and to available facilities and equipment. The continuous process shown in Figure 1is envisaged. Lush foliate shrub is Gut, mashed, crushed, or otherwise disintegrated in the presence of a relatively large quantity of water, after which the dispersed rubber is separated from the coarse plant debris, clarified, and subsequently 'concentrated in a centrifugal latex separator to produce a latex containing 35 to 50% total solids. Processing limitations and economic factors make it advisable to increase the rubber concentration in dispersion'; therefore, the flow diagram shows recycling of a part of the dilute dispersion as well as the use of serum from the separators to replace some of the t a p water in the initial size reduction by Gutting. Similar considerations lead to the inclusion of equipment for washing the plant debris or bagasse to retrieve dispersed rubber which would otherwise be lost. An alternate means of simultaneous clarification and concentration is also shown. Harvesting and Storage. The shrub, after either hand or machine digging (lifting), was hauled without baling to the laboratory where it was either processed immediately or stored in airtight containers pending use. Storage of shrub by immersion of either the roots or the entire shrub in water was found unsatisfactory. I n the former case loss of moisture and coagulation proceeded rapidly; in the latter case natural fermentation set in quickly, and processing after 2 to 4 days resulted not only in diminidhed release of rubber but also in increased difficulty in processing the dispersion. Storage in airtight containers for 3 to 4 weeks caused mold growth with resultant visible coagulation of latex. Subsequent processing resulted in diminished reIease of rubber to form a less stable dispersion. Partial desiccation and physical damage (branches broken before, during, or after lifting) coagulate some of the latex and thereby decrease the extraction efficiency. Rubber coagulated within the shrub was not recoverable as latex; consequently all shrub was handled so as to minimize coagulation without regard to the practicability of the methods employed. All results indicated the desirability of processing freshly lifted shrub, but storage for 2 weeks without measurable harmful .effect was feasible. The extent to which commercial lifting and baling practice would interfere with latex recovery was not determined, but somewhat lower recovery is certain. Initial Size Reduction of Shrub. Shrub was usually cut in a stream of 8 to 10 parts of water, serum, or aqueous solution in a conventional rotary-knife cutter through a 0.5-inch or 0.25-inch screen in preparation for more thorough disintegration by other 'means. Although no coagulation could usually be detected, it is likely that some coagulation of shrub latex actually took place. This procedure was adopted after other machines had unsuccessfully been employed for this purppse. These machines included a vertical-axis hammer mill, a garbage disposal crusher, a low and .high pressure cane mill, a meat grinder, and a meat or vegetable

LUSH W L E SHRUB SERUM

T

T

(n) CUTTER

I

I MAKE-UP SLURRY

1-1

I

I

4 WATER

(e) PEBBLE MILL

BAGASSE

1-

BAGASSE WASHER

- 1

h II

DILUTE DISPERSION FILTER (0) CLARIFIER

1

WASTE BAGASSE

I

I

(E) CONCENTRATOR

I

I WASTE

- --- - -

I

4 1 FINE SOLIDS

I

1 AND SERUM I CLARIFICATIDN

I

(F) RECONCENTRATOR

36-50% LATEX SERUM

Figure 1. Flow Sheet f o r Extraction of Latex from Guay u1e slicer. Cutter capacities were not established, but the stream of liquid prevented clogging of the screen, and the indicated capacities were high. Future work should include utilization of wet hammer-mill disintegration of shrub in a stream of water. During a considerable period of the work further disintegration or size reduction of the cut shrub was not practiced because of inability to concentrate dispersions prepared from foliate shrub without the formation of coagulated rubber in the latex separator. This impediment was later found t o be due in large measure t o separator design (4). During the interim a large number of ingredients were tested in a search for an ideal aqueous solution which would permit satisfactory concentration of the dispersion. Approximately 20 to 25 emulsifying, dispersing, stabilizing, and chemical ingredients were used in concentrations between 0.001 and 0.2% by weight of liquid within 6.5 to 9.5 pH range. Ammonium caseinate Ammonium oleate Ammonium tallate Aquarex D Calgon Emulphor ELA Emulphor ON Gardinol LS

Gelatin Lecithin Sodium caseinate Sodium oleate Tergitol Triton K50S Ultrawet A Ultrawet B

Ultranate 2 Ammonium hydroxide Disodium phosphate Sodium h droxide Sodium c&oride Triethanolamine Trisodium phosphate

The use of surface active agents was generally beneficial to stability and thus facilitated processing, but no combination tried was a panacea. The addition of 0.1% by weight of Aquarex D (sodium salts of sulfated higher fatty alcohols) or Emulphor ON, adjusted to a dispersion p H of about 9 (with ammonium hydroxide) , best satisfied the stability requirements. Fully mature shrub (7- to 10-year-old) could be processed the yearround in tap water. When using younger shrub (2- to 4-yearold) during the summer months, the addition of a stabilizing ingredient facilitated concentration. Foliate shrub must be cut or reduced in size in the presence of water or aqueous solution in preparation for further disintegration to allow maximum release of rubber latex. Dispersions prepared during the sum,mer months were particularly difficult

866


Vol. 40, No. 5

mancy. Table I lists the per cent dispersed rubber as \Tell as the per cent remaining in the washed bagasse for typical experimental runs with young Name of field Spenoe Guidotti Guidotti foliate shrub during the dormant season. 2.5 Age of shrub, years 3 35 Mill diameter, inches 27 27 53 The first two vertical columns show that an average % % In % % In of 74.1y0of the shrub rubber was released and that % % In Released Bagasse Released Bagasse Released Bagasse 14.47, remained in the discarded bagasse. About 89.5 12.1 14 8 13 2 2% of the shrub rubber was recorered as floating 79.5 12.6 85 0 63.2 10.7 83 37 12 01 worms. The total of the three figures is 90.57,, 68.5 16.8 80 19 71.5 17.1 84 0 13 9 leaving 9.5% of the shrub rubber unaccounted for. 72.2. 12.5 86 2 16 5 Phpsical limitations and unavoidable losses in con85.6 12.5 77.0 16.0 ducting the experiments batchwise tended to reduce 73.4 12.3 76.3 17.2 the rubber balance; consequently it is believed that 65.4 19.0 68.7 16.2 the rubber actually released into dispersion approxi86.7 11.9 mated 82%, with 16% remaining in the bagasse and 69.3 17.2 64.5 13.1 2% present as recoverable worms. 72.6 14.8 Similar data are shown for individual tests on older shrub from a different field with comparable a (although not always optimum) milling technique. The relatively small per cent of the shrub rubber Average 74.1 14.4 69.5 9.7 84.0 14.9 90.5 Rubber balance 81.2 100.9 in the bagasse is believed to be significant. RelaAverage.% rubber in tively greater release of dispersed rubber should have 10.6 shrub 8.2 10.5 a See footnote 2, page 874. been established. Improved experimental technique would give a truer rubber balance and Drobablv would establish a release of about 85% of the shrub to process; this difficulty was minimized by addition of stabilizing rubber with a loss of 11% in the bagasse. Test runs made more recently on 150-pound lots of the same agents. shrub, then 0.5 year older, gave more satisfactory data, as shown Milling of Slurry. Milling of the slurry, produced by cutting, for maximum release of shrub latex was best accomplished a t a by vertical columns 5 and 6. Milling was conducted in a mill ratio of liquid to dry solids of about 20 to 1 for 1 to 2 hours in 53 inches in diameter, and, although the loss of rubber in the stone-lined pebble mills. Duplication of results was unusually bagasse was high, the average established release was 84.0%. Optimum milling time in the large mill was not definitely estabdifficult, but it was definitely established that optimum milling lished; thus it is not improbable that a release somewhat greater conditions were more critical during the season of active growth and that leaves p e r se were responsible for processing trouble, could be established. MATURE-SHRUB MILLING. Milling of the older shrub was during any season of the year. carried out in the same manner as for the younger plants, except The term “release” will be defined because its value limits the amount of 35 to 5Oy0latex actually obtained from anv test run. that the optimum milling time was more critical. The release of rubber and processing of the dispersions were practically unShrub used for each expcriment containcd a known weight affected throughout the yearly growth cycle, presumably because 0: pure rubber as calculated from its weight and chemical analvof the relatively smaller quantity of leaves present on the mature SIS. A part of the rubber was released or exuded from the cells of the plant as latex dispersion upon cutting, and an additional shrub. The number of dead and damaged branches on the shrub, quantity was released by pebble milling. The total weight (by however, was relatively greater. Table I1 shows data for modanalysis) so released and dispersed in the aqueous medium is erately comparable test runs. called the amount released or amount dispersed, whereas the per The average per cent release of rubber for the two mills was cent dispersed in relation to the total in the shrub is called the per cent released or per cent dispersed. The per cent released is about the same, but rubber balances Tere low. With improved calculated on the assumption that the total weight of the discontinuous (rather than batch processing) technique, it is believed persion is equivalent to the weight of the Tater added to the that release approaching 8OY0 is attainable, with a bagasse loss of shrub during cutting and milling. Actually, it was not practical I6y0 and worm recovery of about 4%. to retrieve more than about 90% of the added water, but tests showed that washing the bagasse with water resulted in recovery Other mechanical means were tested for disintegration of the of almost all of the remaining 10% of dispersed rubber. cut shrub. A conventional paper beater was decidedly inadequate in that it caused rapid agglomeration of dispersed rubber to It was found that low water-to-shrub ratios of 6 to 1 or 12 to 1 form worms. Paper mill Jordans were tried with somewhat caused the formation of appreciable quantities of “worms” better but indefinite results, because of operational difficulties. before substantial disintegration of the cut shrub was accomA paste-type colloid mill equipped with coarse Carborundum plished. Adequate milling had the effect of reducing the amount rotor and stator presented feeding difficulty with the finest posof rubber in dispersion, while proportionately increasing the sible cutter screen. Partially milled slurry could be fed, but quantity of recoverable worms showing unusually high (about agglomerates of solid rubber formed in the throat which made 35%) resin content. Consequently milling of the slurry was a continuous operation of the machine impossible. necessary evil which was greatly minimized when about 20 parts Complete disintegration of the cut shrub for maximum release of water were used for each part of solid matter composing the of dispersed rubber was difficult to accomplish. Any means lush shrub. Milling was usually carried out in a stone-lined employed caused some of the shrub rubber to agglomerate and mill 27 inches in diameter containing 300 pounds of quartzite form worms of high resin content. Disintegration of the cut stones (initially nearly square with 3-inch sides) for a dry shrub shrub was best carried out in pebble mills with 17 to 20 parts of charge of about 14 pounds. A ratio of water to dry solids of water or aqueous solution. The maximum release of rubber was 1.7 to 1 or 20 to 1 gave greatest release for shrub cut through a about 84 and 74% for the 3- to 3.5-year-old and 9- t o 10-year-old screen having 0.5-inch round holes. shrub, respectively. YOUNG-SHRUB MILLING. Time of milling was less critical Separation of Coarse Shrub Debris or Bagasse. Although than for the mature shrub; 1.5 hours were considered optimum, data have been presented which deal with the per cent of the but extension to 2.25 hours was not harmful during shrub dor-

TABLE I. PERCENTRUBBER RELEASED AFTER CUTTIXG AND 2.5- TO YEAR SHRUB^

MILLIXG


M a y 1948

latex released from the shrub into aqueous media after' cutting

*

and milling, the bagasse or plant debris had t o be separated from the dispersion'and t,he latter clarified and analyzed before the per cent released could be calculated. Two means of separrition were chiefly used, either a wire basket make of '/*-inch mesh wire screen or an imperforate Tolhurst-type centrifuge. In the former case the slurry was poured into the basket and allowed to drain, with intermittent stirring after initial rapid drainage. The-mat of bagasse acted as a fine filter in retaining some of the dispersed rubber and thus gave erroneous results. The second method was preferable, since it gave a dispersion undiminished in rubber Concentration. Both methods gave a bagasse high in moisture content. Absorption or adsorption of the dilute dispersion on the exceedingly large surface area of the bagasse reduced the amount of dispersion readily recoverable for subsequent processing. The major part of this dispersed rubber was occasionally retrieved by washing the bagasse with water or serum and reseparating. In carefully conducted experiments, the dry solids recovered as bagasse represented from 50 to 5570 of those present in the foliate shrub used. Several exploratory runs were made on mature shrub slurry with an experimental continuous Bird filter. The principle of the machine seemed adaptable for use with slurries of the 3- to Cyear-old shrub; therefore the flow sheet includes two units, one for initial separation and one for reseparation after bagasse washing. A conventional flat vibrating screen could not be used satisfactorily because it became clogged quickly and passed a large quantity of suspended solids that were easily removed by the Bird filter. The separation of bagasse in the slurry after milling presented difficulties which reduced the amount of dispersion readily recoverable from the slurry. Separation in a Tolhurst-type imperforate basket centrifuge was satisfactory, but this method, if used on a commercial scale, would necessitate frequent interruptions for removal of solids; thus a continuous solids-removal type would be favored. Re-use of a part of the serum from the latex separator for bagasse washing and subsequent employment for cutting shrub is both practical and economically important because it increases rubber recovery. Clarification of Dilute Dispersions. Clarification of the dispersion, which must precede concentration in conventional latex separators, was accomplished in a Tolhurst-type clarifying centrifuge capable of holding 1 cubic foot of solids. The unit used developed a force about 400 times gravity and was equipped with a skimming lip. The maximum permissible rate of clarification (commensurate with the uninterrupted operation of the

TABLE $1. PERCENTRUBBERRELEASED AFTER CUTTING. AND MILLING9- TO ~O-YEAR SHRUB" Name of field Mill diameter, inches

Spence 27

% '

Released 84.5 84.3 79.5 81.0

71.0 78.5 77.0 81.5 69.4 61 .O 60.8 68.5 70.3 78.3 68.6 72.9

Average 74.0 Rubber balance 92.7 Average % rubber in shrub 12.6 See footnote 2, page 874.

Spence 53

% In Bagasse 12.4

ii :o

17.6 15.7 17.6 19.7 18.0 13.7 17.5 17.1 14.1 11.9 13.9 13.4 10.7 14.7

%

% In Bagasse

Released 81.0 70.7 74.7 74.1 76.5 67.1 61.3 77.2 65.7

72.0

..

14:1

11.5

13:1 17.1 11.8 12.5

89.4 12.6

13.4

867

commercial latex separator for a period of 45 minutes) was from 90 to 100 gallons per hour. Allowable space for reception of fine solids in the separator wm exceedingly small. Leaves of the shrub produce exceedingly fine suspensions of cellular tissue or fragments which are not removed by the separation of coarse woody matter. These fine solids represent only about 0.3% by weight of the unclarified dispersions, but the major portion must be removed before concentration of the dispersion by the latex separator because of the small space provided therein for receiving solids. Young-shrub dispersions foam excessively upon clarification in the Tolhurst unit equipped with a skimmer, and, unless pine oil (or antifoaming agent) is added, the foam persists for several hours. Mature-shrub dispersions produce less foam. However, the roots of the mature shrub contain latex particles which, when released as a dilute dispersion, are much less stable t o mechanical shock than those present in the remainder of the shrub. These relatively unstable particles agglomerate and gather on the leading edge of the skimmer and interfere with the orderly, continuous removal of the clarified dispersion. Chemical analysis of the agglomerated rubber indicates that it is markedly different from the more stable particles subsequently .recovered as latex, in th&tits pure hydrocarbon content, as determined by the usual bromination procedure, is as low as 25%, rather than about 90%. Clarification of dispersions was indispensable for continuous operation of the conventional latex separator. A Tolhursttype unit was generally used, but economic considerations would probably preclude its use for large scale operations. Clarification and concentration should be accomplished simultaneously in a centrifuge of special design which permits continuous removal of fine solids. (Referehce is made to improved or special separators designed for simultaneous discharge of a light and heavy aqueous dispersion, which makes it possible to recover and increase the conc6ntration of ingredients in either the light or heavy phase. Separators of this type have been used with marked success for several years in the production of starch.) Concentration of Clarified Dispersions. The purpose of concentration was t o produce a latex containing 35 to 50% total solids from the dilute clarified dispersion, usually containing from 0.6 to 1.5% by weight of rubber. The course of the experimental program during a large part of the investigation period was guided or controlled by limitations of operation with a small cream or milk separator of special bowl design. Toward the end of the work a commercial latex separator of similar design was successfully used. Data were obtained on the efficiency of separation of the latter a t various input or feed rates. At 200 gallons per hour input the normal efficiencyexceeded goyo, but it could. be increased to about 98% by appropriate re-use of serum as described below. The small cream separator was equipped with a special bowl 5 inches in diameter containing forty plates or separating disks and could be driven a t speeds of 6000 t o 11,000 revolutions per minute, which developed centrifugal forces of 2500 to 8500 times gravity. The 12-inch-diameter bowl of the commercial machine containing eighty plates could be driven a t 6500 and 7800 revolutions per minute, which corresponded t o centrifugal forces of 7200 and 10,400 times gravity: (Both separators were made by The DeLaval Separator Company.) The.slower speed was used for all efficiencydata mentioned. Concentration of dilute rubber dispersions was possible because the individual particles have a density less than that of the aqueous medium in which they are suspended. Efficient separation, however, was difficult, because of the extremely small size of some of the particles. They range in diameter from less than 0.2 to 3.5 microns, with their size-distribution curve showing a maximum at less than 0.5 micron. Efficient practical separation of spherical particles of the same assumed density (about 0.92) having a tenfold difference in diameter presents a difficult problem, as the largest particles migrate a t a rate 100 times faster


868

TABLE111. COXCENTRATION EFFICIENCY AT VARIOUSFEED RATES~ 3.6-Year Shrub Input Rate, Gal./Hour 100

a

150 200 250 See footnote 1.

% loss in serum 6.8 8.2 9.6 11.0

% effi-

ciency 93.2 91.8

90.5

89 .o

10-Year Shrub

% loss in serum 5.4 6.3 7.8 10.2

effioienoy 94.6 93.7 92.2 89.8

than the smallest particles, according to Stokes’ law. One of the largest rubber particles (which are believed to be spherical), however, presumably contains as much rubber as 1000 of the smallest particles; therefore efficient separation would depend upon an adequate number of the larger particles. A major difficulty initially prevented satisfactory continuous operation of either the small or the commercial latex separator and markedly delayed the progress of the work. Both machines were designed to deliver serum (heavy liquid) and cream (light liquid) from mechanically stable dispersions because of the difference in specific gravity between the liquid medium and the dispersed rubber. Delivery of cream should take place Butomatically when enough cream had separated near the center of the rotating bowl to produce a difference in static head sufficient to cause flow through an opening a t the top of the bowl. Guayule dispersions are relatively unstable, however, and, when dispersions were concentrated, the time required to separate a sufficient volume of the light liquid was too long, and the rubber particles agglomerated to form a mass of pasty or semisolid rubber. This necessitated discontinuing the operation and cleaning the bowl parts. The remedy for this difficulty consisted in controlling the time during which any rubber particle remained in the separator bowl. From a practical standpoint this involved sealing any possible liquid-discharge opening except the cream and serum outlets while providing for positive adjustment of the discharge rate of the latter. More specifically, while the input rate was maintained at a fixed value, the serum outlet was adjusted to deliver liquid a t a lower rate. This mode of operation caused a positive displacement of cream or latex near the center of the bowl, effectively prevented the formation of agglomerates of pasty rubber, and ensured continuous delivery of latex. The physical alteration of the bowl comprised the insertion of a rubber gasket between two bowl parts and required only minor alteration of one of the parts (4). . CONCENTRATIOX EFFICIEXCY AT VARIOUS FEEDRATES. Dispersions containing about o.7yO rubber were concentrated at various rates of feed. The tests were conducted with the commercial separator a t rates of 100 to 275 gallons per hour so as to effect a tenfold increase in the cream or rubber-rich liquid delivered by the machine. They were of short duration (about 8 minutes) and thus did not present practical test runs. Continuous delivery of cream was obtained when its rate was 10% of the input rate; when the rate was appreciably less than 10% pasty or solid rubber formed h the bowl. This observation confirmed similar experience in the operation of the small machine and proved that the innate instability of guayule dispersions was not completely eliminated under adverse operating conditions, despite the modification of the bowl. The minimum rate of cream delivery necessary to ensure satisfactory operation cannot be calculated but must be ascertained by actual trial. The minimum rate increases with increasing rubber concentration in the feed, and decreases when suitable stabilizing ingredients (enumerated above) are incorporated in the feed. Analysis of the dispersion entering the separator and of the serum after separation permitted calculation of the per cent loss of rubber in the serum. When the per cent loss of rubber in sera was plotted as a function of input rates, an approximately

Vol. 40, No. 5

linear curve resulted. The interpolated efficiencies (the per cent loss subtracted from 100) for both young-shrub and matureshrub dispersions are listed in Table 111. Thus, when a clarified dispersion prepared from 10-year-old shrub was fed at a rate of 200 gallons per hour, the efficiency of separation was established a t 92.2%. The serum, representing 90% by volume of the dispersion fed, contained 7.8% of the rubber in the dispersion’. The corresponding efficiency for the 3.5-year-old shrub dispersion was 90.570. Optimum feed rates of about 200 gallons per hour are indicated, and efficiencies can be improved, as is shown below. After the efficiencyat optimum input rate had been established for dispersions containing 0.7yc rubber, the question arose as to the effect of increasing the rubber concentration to about 1.570. When dispersions were prepared by utilizing the dilute dispersion for processing a second lot of shrub, the concentration was increased to about 1.5%. Average separator efficiencies a t 195 gallons per hour input were established at 91.1 and 93.3% for the young-shrub and old-shrub dispersions, respectively; it therefore seems safe to state that separator efficiency was not diminished by increasing the rubber concentration in the general manner suggested by the flow sheet. EFF.ECTOF RE-USEOF SERUMOK CONCENTRATTIOK EFFICIENCY WITH ~O-POUND LOTSOF SHRUB. When shrub containing 100 pounds of pure rubber was processed, a maximum of about 85% was released into aqueous media. As the unwashed bagasse retained about 107, of the added liquid, the clarified dispersion to be concentrated contained only about 76% of the shrub rubber. Usual experiment,al practice did not include bagasse washing; so concentration a t about 927, efficiency gave a final latex containing a maximum of 7Oy0of the hydrocarbon originally in the shrub. Recovery of all the rubber actually dispersed, by bagasse washing as indicated by Figure 1, should give a maximum yield of 7870. These facts had to be faced; consequently attempts were made to improve separator efficiency while maintaining the throughput of 200 gallons per hour. Re-use of serum from the separator to prepare a second dispersion for concentration seemed feasible and worth trying. When this was done with mature shrub, the release of rubber was undiminished; the serum apparently did not impose an added limitation. Separator efficiency for the first dispersion (prepared with tap water) was 91.37,. Corresponding efficiency for the second dispersion (prepared with serum from the first concentration) was 87.0%. At first glance this procedure appears harmful. However, the second dispersion contained about 8.7% of its total rubber as added fine particles; the net loss of rubber from the second lot of shrub was therefore 13.0% minus 8.7%, or 4.3%, corresponding to an efficiency of about 95.7% which should be compared with 91.3Yc in evaluating the effect of re-use of serum. This gives an average separator e,fficiency of 93.5% for the two lots of shrub. Following this improvement in efficiency, three lots of shrub were processed at daily intervals, serum from the previous day’s concentration being used on the second and third days. These conditions of re-use approximate those of continuous processing, because sufficient time is allowed for natural fermentation to set in. The data in Table IV show that repeated re-use of serum on successive days gave definite increase in separator efficiency. Here again the actual loss of rubber in the serum increased progressively, but at a diminished rate. Increment serum loss decreased to about 2.0% for the third lot of shrub, with a corresponding efficiency of about 98.0%. Conservative interpretation of the data indicates an average efficiency of slightly over 95.070 1 Since the resumption of the experimental work on guayule in August 1947 it has been established t h a t the method used for determining the rubber in the clarified dispersion and in the serum gave values which are Thus the efficiency figures reported t o o low and too high, respectively in this paper are actually lower than the t r u e values.

May 1948


869

for the three‘ lots of shrub, as compared with OF RE-USE OF SERUMO N CONCENTRATION TABLE IV. EFFECT 91.5% for the first lot. EFFICIENCY AT 195 GALLON PER HOUR INPUT EFFECT OF RE-USE OF SERUMON CONCENTRA10-Year Shrub 3.5-Year Shrub TION EFFICIENCY WITH 15o-POUND LOTSOF SHRUB. Rubber Loss in Rubber Loss in Efficiencies already cited were determined for Serum Serum Incre“/4 e 5 g effi- Actual ment, Inore-’ Shrub Aqueous Actual short periods of operation of the separator with ciency Yo Lots mebt, yo ciency Medium % ’ dispersions prepared solely by cutting but with First Water 9.2 ... 90.8 7.0 ... 93.0 no slurry milling. It seemed possible that sepaSecond Serum 12.4 3.2 96.8 11.0 4.0 96.0 Third Serum 14.8 2.4 97.6 12.7 1.7 98.3 rator runs of longer duration with dispersions Av. 5.0 95.0 4.5 95.5 prepared by milling for maximum release might behave differently; consequently 150-pound lots TABLE V. EFFECT OF RE-USEOF SERUM ON CONCENTRATION EFFICIENCY of shrub were processed for maximum release, (200gal. per hour input for operating period of about 45 minutes) and the dispersions were concentrated at about 10-Year Shrub 3.5-Year Shrub 200 gallons per hour as above. The serum from Rubber Loss in Rubber Loss in the first day’s operations stood overnight and was Serum Serum Actual InoreShrub Aqueous Actual Inore-‘ % effiused for preparing the second dispersion. The % ment, % Lots Medium % ment, % ciency pH of the serum after standing overnight dropped First 7.2 ... 92.8 9.1 ... 90.9 Water to about 5, but was increased to about 7 im10.2 3.0 97.0 Second 8.8 93.2 Serum 15.9 mediately after the second lot of shrub was cut. Av. 5 . 0 95.0 92.0 8.0 The volume of dispersion concentrated on each occasion was about 160 gallons, which sufficed for a concentration run of about 45 minutes. Table V filter, and’ the liquid effluent containing some rubber would be lists the pertinent data. used continuously for processing more shrub. The improvement in efficiency upon using the serum once was I n the second method the rubber concentration is increased by not so pronounced as indicated by Table IV for the younger providing for recirculation of unclarified dispersion through the tube mills until it equals 275, and thereafter recycling about two shrub. The amount of fine solids in 160 gallons of the dispersions thirds of the liquid after step C. Bagasse and serum disposition was usually slightly more than the allowable space in the sepawould be like that in the first method. rator bowl would accommodate; therefore the data do not reflect I n the third method the increase in rubber concentration would the maximum increase in efficiency as well as those of Table IV. be accomplished by treating successive lots of shrub with dispersion until the rubber concentration equals 2%. The same Nevertheless, improvement was established. Longer test periods total quantity of shrub arid liquid would be involved, but pracwith better facilities than were at the author’s disposal should tical processing would call for three cutting-pebble millingestablish ,maximum attainable efficiency and bring out any bagasse separating “lines” in series, each line being of approxilimitations associated with re-use of serum. mately one third the capacity re uired for the first and second methods. The slurry leaving t h e i r s t line would be separated by It is believed that the improved efficiencywas caused primarily a Bird filter to give an unclarified dispersion, and this dispersion by the known increase in water-soluble plant substances which would be used for cutting and milling shrub for the second line. raise the density of the aqueous medium, thus automatically inA repetition of this operation through the second and third line creasing the rate of separation according to Stokes’ law. Bacwould give a dispersion for clarification and concentration as terial and/or chemical changes in the serum may also promote in the second method. After bagasse washing the se arator serum (plus the necessary amount of fresh water) woulde! proclustering of. the extremely small rubber particles and thus conportioned equally to the three cutters t o maintain the milling tribute to improved efficiency. ratio. The increase in the number of individual pieces of equipA commercial latex separator can be used successfully for conment is unattractive, but it is probable that this method would give less difficulty than the second method. centrating guayule dispersions. The efficiency of separation decreased approximately linearly for increaaing feed rates, from Continuous processing is speculative, and the discussion should 100 to 200 gallons per hour, but the efficiency was disproporserve only as an appraisal of the possibilities. Actual tests tionately less a t a feed rate of 250 gallons per hour. At a feed would be required to establish the maximum amount of soluble rate of about 200 gallons per hour, the efficiency was approxiand colloidal ingredients that could be permitted without causing mately 90 and 92% for young-shrub and mature-shrub dispersions, processing difficulty. Subsequently, processing under optimum respectively. Utilization of the waste serum from the separator conditions would establish the actual separator efficiency and in the preparation of subsequent dispersions had the net effect of the necessary loss of rubber in the serum which must be disimproving the separator efficiency to about 98% at a feed rate charged as waste to maintain a solids-equilibrium in the system. of about 200 gallons per hour at a separator speed of 6500 revoluReconcentration of 7 to 15% Latex. As Hevea latices of tions per minute. RE-USE OF SEPARATOR SERUMON A CONTINUOUS PROCESSINQcommerce contain at least 35% by weight of total solids, the guayule latex after concentration was reconcentrated to give a BASIS.The increase in separator efficiency resulting from the comparable product. The necessary fourfold or fivefold increase use of serum in batch processing leads to a consideration of was attained by repassage through the separator after adjustment factors which would govern the practicability of continuous of the serum discharge orifices to deliver 20 to 25% of the input processing. The flow sheet (Figure 1) includes the basic conas latex. I n large scale operations one reconcentrator could ception of its use, although specific details remain to b: estabhandle the cream output from seven or eight separators a t comlished experimentally. Of the three general methods of using parable efficiency. serum the first assumes the concentration of a dispersion to about Reconcentration has also been brought about by the use of 0.7% in rubber, whereas the second and third involve increasing “creaming agents” as practiced with Hevea latex. The techthe dispersed rubber concentration to about 2%, so as to decrease nique is comparable and comprises adding a solution of amthe number of separators required for producing a given amount monium alginate to the 7 to 15% latex at a pH of about 9 and of rubber. heating to about 50” C. Upon standing overnight or longer the In the first method the dispersion produced after step C in the rubber particles rise, forming a sharp boundary between the flow sheet would be continuously clarified and concentrated. #concentrated latex and the serum which contains only a small The separator serum would then be mixed with the bagasse after amount of dispersed rubber. Figure 2 illustrates creaming. step C and reseparated by a second Bird filter. The waste bagasse The concentrated latex may be siphoned, or the serum may be would be washed with a minimum of fresh water in the Bird

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Vol. 40, No. 5


Figure 2.

Creaming Latex

withdrawn from the bottom of the container and the latex then run into another container. Each 100 pounds of dilute latex require about 0.1 pound of dry alginate, which adds about 0‘75 cent per pound to the cost of the dry Dubber obtained. The use of an extremely small amount (about 0.001%) of Calgon (sodium hexametaphosphate) promoted creaming and was desirable when treating dilute latex from young shrub processed during .the season of active growth. Figure 3 shows a typical three-year-old guayule shrub, gallon bottles of guayule (on the left), and Hevea latex. Above each bottle is a sample of sheeted wet rubber produced by coagulation with acid. Simultaneous Clari5cation and Concentration. Elimination of LL separate clarification operation by combining it with concentration appears to be a vital requisite to large scale operations. KOactual trials have been made, but the matter has been discussed extensively with the manufacturers of special types of separators which have been successfully used in both the starch and fish industries for continuous removal of fine solids. If adaptable to latex concentration, the dispersion, free of coarse solids after step C in the flow sheet, would be fed directly into a unit for simultaneous removal of fine solids and concentration of the dilute dispersion to deliver a 7 to 15y0latex and serum. PROCESSING VARIABLES THAT INFLUENCE LATEX RECOVERY OR LATEX QUALITY

As the chemical compositions of crude rubbers from the same source give some indication of comparative quality and can be partially controlled, composition will be discussed in the light of certain variables. Crude rubber was usually obtained from latex by allowing a thin layer to dry on a pane of glass at room temperature, but it could also be obtained by coagulation with organic or inorganic acids. I n this work, older shrub always gave a crude latex rubber higher in hydrocarbon and lower in both resin and ingredients insoluble in acetone and benzene. The average of a large number of comparable tests showed a resin content 5 to 8% lower for fully mature shrub in comparison with 3-year-old shrub regardless of whether or not leaves were manually removed before processing. No completely adequate study of crude-rubber composition’ with respect to seasonal change was attempted, but processing

2.5-year-old to 3.5-year-old foliate shrub during June, July, August, and September is considered impractical, mainly because of the instability of dispersions prepared during the season of active growth. Tests on hand-defoliated as well as foliate shrub showed that some constituent of the leaves contributes to the instability of the dispersion. Resin percentage was 2 to 4% higher in the latex rubber recovered from foliate shrub. DifFerence in Composition of Rubber in Roots and Branches of Mature Shrub. When the main stems of hand-defoliated shrub were severed just below the fist branches and the two plant fractions were processed separately, the latex rubbers showed significant difference in resin content. Moreover, dispersions produced from the branches were concentrated without difficulty whereas those from the roots caused the formation of some pasty, or semisolid rubber of unusually high (43%) resin content. Table V I shows that the latex rubber recovered from the branches was superior to that from the roots in chemical composition. Some of the dispersed root rubber was evidently highly unstable and contained a large quantity of resin, justifying the belief that the rubber in the roots of shrub of this age mas not identical with that in the branches. Influence of Dispersing Agents. Dispersing, emulsifying, and wetting agents were initially employed to enhance the mechanical stability of the dilute dispersions for satisfactory concentration in the latex separator. After the separators had been successfully modified t o permit satisfactory concentration, their use was discontinued, chiefly because they produced a rubber of lower hydrocarbon and higher resin contents. Table VI1 lists the resin contents of latex rubbers produced with a variety of ingredients when the aqueous processing liquid was void of and contained the dispersing agent (Aquarex D ) found to be most effective in enhancing the mechanical stability of the dilute dispersions.

TABLE

VI. CHEMICAL C O X P O S I T I O N O F LATEXRUBBERFROU ROOTAND BRANCH FRA4CTIOXSO F MATURE SHRUB Chemical Analysis of Dry Rubber, %

Resin Root fraction Branch fraction

RHC

Resin

Insolubles

RHC

80.0 88.0

19.1 10.8

0.9 1.2

0.12

0.24


May 1948

TABLE

VII.

D ON RESINCONTENT LATEXRUBBER

EFFECT OF AQUAREX

.

Aqueous processing liquid void of Aquarex D, but contained: Tap water only Disodium phosphate Disodium phosphate and gelatin Sodium hydroxide Ammonium hydroxide Animal gelatin Trisodium phosphate Ammonium caseinate Sodium chloride Average % resin Aqueous processing liquid contained Aquarex D and: T a p water only Trisodium phosohate Disodium phosphate Ammonium hydroxide Average yo resin

Shrub, 10 0 11.9 12 6 10:0 12 5

OF

g-Year 8 0 6 7 8 0 8.8 8 8

9 1 9 3

9 5

11 1 18.0

1810 19 8 18.6

8 4 15.7

16 0 17:6 16 4

Although the amount of rubber released in the experiments was independent of the ingredients added to the water, it is clear that the use of this particular dispersing agent (Aquarex D) greatly increased the resin content of the latex rubber. The exact function of the dispersing agent was not understood, but it was believed that it preferentially promoted the dispersion of resin. Data for the average resin content of the waste bagasse in these same experiments (Table VIII) support this belief. Use of the dispersing &gent gave bagasses low in resin content and rubbers high in resin content. Aquarex D must have preferentially dispersed resin when the shrub was processed, because the bagasse showed a greatly decreased resin content. The quantity of resin dispersed is assumed t o be proportionately greater, but the dispersions were not actually analyzed for dispersed-resin content in proof of this point. I n view of the above results it seemed advisable to ascertain the effect of the latex separator on the dispersed resin. Dispersion containing Aquarex D was analyzed for resin and rubber before and after passage through the small separator. Twentyfive per cent of the resin and 10% of the rubber were found to be present in the serum afterward, which indicates either a smaller

Figure 3.

871

diameter or a slightly greater density for the resin particles. This conclusion checks the experimentally determined specific gravities of guayule resin and guayule hydrocarbon, which are 0.984 and 0.915, respectively, a t 21" C. Thus it appears that the separator serves not only to produce a concentrated latex, but also t o decrease the resin in the latex rubber. Processing and shrub variations generally influenced the quality rather than the quantity of recoverable latex. Crude rubber in the roots of mature shrub was inferior in quality to that in the branches. Leaves decreased dispersion stability and increased the resin content of latex rubber. Processing 2.5-year-old t o 3.5-year-old shrub during June through September is not recommended. Dispersing agents enhanced the mechanical stability of dispersions, but gave a high-resin rubber. There are indications that guayule shrub makes and deposits hydrocarbon and resin separately, and in more or less pure condition, and that the presence of resin in the final rubber is caused by its admixture with the hydrocarbon during processing. PROPERTIESOFGUAYULELATEXRUBBER

Chemical Composition. The chemical composition of guayule latex rubber produced by alteration of processing technique served as a criterion in evaluating such changes. As the objective was the production of a high grade rubber, those changes which improved quality and appeared practical were favored and incorporated in the process. Latex rubber produced from foliate

TABLE VIII.

RESINCONTENT OF SHRUB AND FINAL PRODUCTS FROM PROCESSING

3-Yeas Shrub 9-Year Shrub No NO dispersing Dispersing dispersing Dispersing agent agent agent agent Resin content of shrub Yo 8.5 8 5 9.7 9.7 Resin content of bagassd, % 6.4 4 6 7.3 5.7 Decrease in % resin in bagame 25.0 46.0 25.0 42.0 Pure hydrocarbon in crude 87.9 80 4 90.6 82.6 rubber, % Resin in crude rubber.. %,I 11 1 18 6 8.4 16.4 Resin . 0.13 0.24 0.09 0.20 in orude rubber

G u a y u l e Shrub, Bottles of Guayule, and Hevea Latex

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872

INDUSTRIAL AND ENGINkERING CHEMISTRY

Vol. 40, No. 5

shrub was obtained by (1) film casting on glass, (2) coagulation with acids, and (3) coagulation with acetone; but method 1 was most feasible and generally adopkd. Satisfactory coagulation by method 2 required heating to 50" to 60" C., but the rate of coagulation could not be controlled. The chemical composition of rubber obtained from the same latex by the three methods was practically the same. The range listed in Table IX includes minor processing variables as well as seasonal variables not adequately investigated.

aging. Typical physical test data for unaged and aged guayule vulcanizates are given in Table XI. The vulcanizates were aged 48 hours in oxygen a t 300 pounds pressure a t 70"C., and the data show that they aged satisfactorily when 0.5% p,p'-diphenylphenylenediaminewas included in the formula. Diphenylguanidine was partially effective with respect to aging. More recent comparative tests with mature-shrub latex rubber and a good grade of Hevea smoked sheet (Honduras), using formula 5 , are listed in Table XII.

TABLE Ix. CHEMICAL COMPOsITION

Hot tensile strength was determined with a hot iron a t 230' F. after a hot tensile test for GR-S ( 1 ) . Abrasion was determined with a Bureau of Standards abrader, and results are reported as per cent of standard Hevea compound. Compression set was determined a t constant deflection. Rebound was measured with the Goodyear-Healy rebound pendulum at 75 O F. An angle of drop of 15" and test pieces, 1 by 1 by 2 inches, were used. Heat build-up was measured with a Goodrich flexometer.

OF GUAYULEAND HEVEA LATEXFILMRUBBER

Hydrocarbon, yo Reein, % Hnsolubles, % ' Ash a t 550° C., yo Nitrogen, % 'Copper, p.p.ni. Manganese, p.p.m. Iron, p.p.m.

Guayule Film Young Mature shrub shrub 86.0-88.0 89.0-91.o 10.0-12.0 7 .O-9. O 0.5-1.5 0.5-1.O 0.3-0.8 0,2-0,5 0.1-0.4 0.1-0.3 1-4 1-3 1-5 0.1-0.5 20-40 10-30

Hevea Film 88.0-92. O 6.0-8.0 2.0-4.0 0.5-0.8 0.5-0.6 4-6 1-5 50-70

The chemical composition of guayule film compared well with Hevea film rubber in pure hydrocarbon, ingredients insoluble in acetone and benzene, and ash and metal contents. The quantity of resin in guayule was greater and the total nitrogen was less. Guayule rubber containing twice the resin content shown could be produced, if required, but the added cost of the dispersing agent required might not compensate for the increased crude recovery. Guayule film rubbers are less firm and more easily distorted than Hevea films. Despite a favorable chemical composition they become tacky upon exposure t o indirect or subdued light. Tonox (p,p'-diaminodiphenylmethane) provides protection against development of tack of raw films not exposed to direct sunlight: exposure of films not containing Tonox to direct sunlight for three weeks doubles their original resin content. Tonox retards this increase in resin. Vulcanizate Properties. Typical guayule latex rubbers have been compounded, vulcanized, and subsequently tested on various occasions during the investigation. Three basic gum formulas and one tread io-mula, as listed, were generally used. The physical properties of the green or unaged vulcanizates of film rubber of guayqle and Hevea latices are shown in Table X. Investigation of optimum compounding formulas for guayule latex rubber was not undertaken, but the general character of the vulcanizates is apparent from the average data listed. The physical properties of the vulcanizates were greatly influenced by the formula used. Tensile strength was found to be generally independent of the age of the shrub as well as of significant variation in resin content. High tensile strength was developed only when sufficient accelerator and activator were included in the formula. The s'pecific effect of the accelerator was not apparent for Hevea film rubber as i t contains natural accelerators. Vulcanizates of guayule are characterized by lower moduli, higher ultimate elongation, lower permanent set, and slightly lower tensile strength than those of Hevea. Data for the tread stock formula are normal, showing increased moduli, hardness, and permanent set. The heat build-up (Goodrich flexometer) was smaller for the mature-shrub vulcanizate. More complete information on vulcanizate properties of young guayule latex rubber tread stock formulas has been published by Clark and Place ( 2 ) . Vulcanizates of guayule latex film rubber usually age poorly when subjected either to the oxygen bomb pressure test or to the accelerated oven method. Incorporation of diphenylguanidine (D.P.G.) or p,p'-diphenylphenylenediamine (J.Z.F.) in the compounding formula markedly improved vulcanizate

'

Chemical analysis of the guayule film rubber used in the comparative tests is given in Table XIII. The guayule vulcanizate showed very good permanent set, hot tensile, and abrasion resistance. It also showed relatively good aging properties. Rubber laboratories not connected with the Emergency Rubber Project have reported favorably on the utilization of guayule latex (private communications). One states that the latex seems adaptable for the production of good-quality sponge products, and that the normal tack of the film is very good for certain requirements. Another states that wet compounding of the guayule latex gave vulcanized films of high tensile strength (4400 pounds per square inch) showing high elongation (10500/0)

TABLEX. GREEN (UNAGED)VULCAKIZATE PROPERTIES OF GUAYULE AND HEVEA LATEXFILMRUBBER Ingredients Crude rubber D.P.G. Captax Stearic acid Zinc oxide Sulfur Carbon black E P C Wyex Tonox J.Z.F.

1 100

...

1.0 4.0 6.0 3.5

Formulas 3 4 100 100 ,.. 0.2 0.2 0.5 0.8 0.8 4.0 2.5 2.5 6.0 5.0 5.0 3.5 3.5 3.5 .. .. . 0.5 0.5 0.5

2 100

.. .... .. . .

0.5

Young Guayule Shrub 2.5 t o 3.5 Years 70 60 3300 3400 350 770 950 1750 790 710 42 39 Hardness, Shore 12 15 Permanent set, Vu Tensile product/100 294 275 Heat build-up: temp. rise, a F., 25 min. a t 0.175inch stroke

.. .. ..

...

5 100

...

1.0 4.0 6.0 3>5 50

...

0.3

Old 30a 20 20 3300 4100 3800 350 810 1070b 950 1910 1750C 780 730 42 61 38 1R 31 6 -" 283 335 280 100

Mature Guayule Shrub 8 to 10 Years Old 35a 25 20 70 60 Optimum time of cure min. a t 260" F. 3250 3500 4000 4400 3800 Tensile strength, lb./sq: in. 1050b 220 360 560 680 Modulus a t 500% elongation . 560 980 1700 1850 1810° Modulus a t 600% elongation 800 775 735 755 640 Ultimate elongation, 7" 56 42 45 41 40 Hardness, Shore 23 12 8 9 12 Permanent set % 300 310 335 375 280 Tensile produ&/100 Heat build-up; temp. rise, F., 25 min. 85 a t 0.175inch stroke Hevead Optimum time to cure,.min. a t 260° F. Tensile strength, Ib./sq. in. Modulus a t 500% elongation Modulus a t 600% elongation Ultimate elongation, % Hardness, Shore Permanent set, 70 Tensile product/100 a

Cured a t 287' F.

0

Modulus a t 400Y0 elongation.

30 4690 1200 2810 720 49 19 384

.. .

...

.. .. ., .. .. .. .

.., t .

20 4560 1350 3010 650 46 12 352

.. .... .. .. .. . .. ... .... ..

b Modulus a t 300% elongatjon.

d Rubber Reserve Company, normal latex (Hsuee bresiEiensis).

.. .. .. .. ..

.... ..

May 1948


873

a t optimum cure. The work TABLEXI. UNAGEDAND AGED VULCANIZATE PROPERTIES OF GUAYULE FILMRUBBER of the Guayule Rubber Extraction Research Unit was on Compounding Ingredients Crude rubber 100 100 100 the production rather than D.P.G. ... ... .100 .. 0.2 the utilization of guayule Captax 1.0 1.0 1.0 0.8 Stearic acid 4.0 4.0 4.0 2.5 rubbers, and possible uses of Zinc oxide 6.0 6.0 6.0 5.0 Sulfur 3.5 3.5 3.5 3.5 latex were not explored. * J.Z.F. ... 0.5 ... ... Guayule rubbers obtained UnUnUnUnfrom latex by c o a g u l a t i o n Physical Test Data aged Aged aged Aged aged Aged aged Aged with organic or inorganic acids Optimum time of &re, min. a t 260" F. 90 * 60 30 90 have been compounded and Tensile strength, lb./sq. in. 3400 2ooa 3500 2510 3600 3520 100 Modulus a t 500% elongation 650 450 ... 400 730 1180 vulcanized by some of the 760 . . . 4120 Modulus a t 600% elongation 1550 1890 1300 . . . 850 1720 2420 typical formulas mentioned. Ultimate elongation, % 750 790 750 750 . . . 600 700 Hardness Shore 40 44 44 40 76 40 ... G r e e n-vu 1c a n iz a t e t e n s i l e Permanedt set % 14 12 9 14 15 . . . 13 Tensile produ&/100 306 282 350 178 280 ... 310 strengths of the young-shrub latex rubber were about the a Test specimens did not permit obtaining datum. same as those for the film rubber. The mature-shrub rubber, however, invariably gave lower tensile strengths than established for the film rubber. yields of inferior-quality worms. Resort to parboiling the shrub It is generally believed that the quantity of resin in guayule for leaf removal previous to cutting solved the problem of obrubber governs to a measurable extent the physical properties of taining adequate yield of worms from the bagasse, but two its vulcanizates, but this could not be definitely established for other impediments were created. In the first place, parboiling latex-film rubber. It seems highly probable that guayule-latex halved the amount of latex released upon cutting, and in the rubber either lacks or is deficient in some beneficient ingredient second place the dispersion could not be satisfactorily concennaturally present in Hevea rubber. The degree or type of trated because pf the formation of solid rubber in the latex polymerization of the hydrocarbon may be different and thus separator bowl. . exert a major influence on the vulcanizate properties. Milling of Retted Latex Bagasse. Efforts were' made by the The chemical composition indicates that guayule latex rubber Microbiological Section of this laboratory to ret the latex bagasse, is of high quality. Physical tests on vulcanizates prepared by in the hope that both the yield and quality of the worms subseseveral formulas show that they possess tensile strengths which, quently produced upon milling would be satisfactory. The work though generally less, approach those of Hevea rubber. Accelerschedule did not permit sustained investigation, but observations ators or activators are required for development of maximum and data indicated that the customary problems associated with strength. The moduli are considerably lower than for Hevea, retting guayule shrub were exaggerated when dealing with latex and ultimate elongations are usually greater. There are indicabagasse. The removal of water-soluble substances, rubber, and tions that guayule latex could be used for the manufacture of resin were among the complicating factors. All results showed sponge rubber and certain other products where either high that parboiling and defoliation were essential for satisfactory elongation or good tack is desired. yield of good-quality worms. The results of a possible combination process have not been encouraging. Latex recovery was feasible with unparboiled COMBINATION LATEX AND WORM RECOVERY shrub, but Garboiling was essential for adequate yield of worms. Definite limitations in processing guayule shrub exclusively If the pronounced beneficial influence of parboiling on worm for latex make it improbable that recovery of as much as 90% recovery could be brought about without jeopardizing the of the shrub rubber will ever be commercially attainable. Denecessary latex operations, a combination process would appear velopment of latex processing as presented presupposes lifting more attractive. and transporting the shrub in loose form. It is likely that economic factors would demand baling after lifting, and this operation would cause breakage of the shrub and attendant coaguTABLEXII. VULCANIZATE PROPERTIES OF NATUREGUAYULFJ lation of some of the latex. Hence a process adaptable to reFILMAND HEVEASMOKED SHEET(HONDURAS) covery of a part of the rubber as latex and the remainder as worms Hevea Guayule Film Smoked Sheet would be more practical. Two general methods of attacking Unaeed Aged@ Unaeed Agedm the problem were employed, and though neither was successful, Optimum time of cure, min. a t 287O F. 25 30 a brief description of the efforts will be given. Tensile strength, Ib./sq. in. 3650 2160 4230 1700 Milling of Unretted Latex Bagasse. When lush foliate shrub Hot tensile strength, lb./sq. in. 2420 .. 2270 .. Modulus at 300% elongation 1360 .. 1920 .. was put through the rotary cutter with about 15 parts of water, Permanent set 19 31 Ultimate elondatfon, % 590 290 550 ib it was possible t o release about half of the shrub rubber as a Tear, lb./in. 900 410 855 239 dispersion which could be subsequently clarified and concentrated Abrasion, yo 160 .. 190 .. Compression set, % 43 .. 41 .* t o a 35 to 50% latex, the film rubber of which was equal in quality Rebound % 49 .. 58 *. Heat buiid-upb 85 .. 79 .. to that of the usual latex rubber. Subsequent milling of the Hardness, Shore 60 69 66 71 latex bagasse either before or after drying gave poor yields of a Test specimens aged 96 hours a t looo C . in oven. inferior-quality worfns. Milling the undried bagasse with coagub Same units as in Table X. lants (3)gave higher, but inadequate, yields of small soft worms, also of inferior quality. TABLE XIII. CHEMICAL ANALYSIS OF MATURE GUAYULE FILM: Treatment of lush foliate shrub by either immersion in water RIJBBER Hydrocarbon,' % 91.8 or allowing molds to grow before cutting imposed insurmountable Resin, % 7.1 Insolubles, % 1.1 difficulties in obtaining the latex from the dilute dispersions. Copper, p.p.m. 1.0 Moreover, milling the bagasses gav? poor yields of inferiorManganese, p.p.m. 1 .o quality worms. Hand defoliation of the shrub removed the Iron, p.p.m. 30.0 difficulties of producing the latex, but the bagasses still gave poor

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874

INDUSTRIAL AND ENGINEERING CHEMISTRY RECOVERY OF RUBBER BY COAGULATION OF DILUTE DISPERSIONS

In the recovery of rubber as latex the centrifugal separators used t o concentrate the dilute dispersions are expensive, require relatively skilled operators, and, despite their high efficiency, add to the cost of the product. Spence showed in his patent (6) that rubber low in resin content could be obtained by acidification of dispersions produced from defoliated 8-year-old shrub. The author confirmed this, but was unable to produce rubber equal in quality to commercial guayule rubber from dispersions of the 3-year-old shrub. The leaves, as well as the' age of the shrub, per se, adversely affect rubber quality. Moreover, dispersions must be unusually well clarified; otherwise the foreign solids in the dispersion will unduly contaminate the coagulated rubber. The general procedure used was to add mineral acid (either hydrochloric or sulfuric) to the clarified dispersion until a pH of about 3 was obtained. Subsequent heating to 70" to 100" C. produced an elastic clot or ball which was usually washed and sheeted on washing rolls and dried. Special treatments of the clarified dispersion, such as the addition of alkalies and/or autoclave digestion before the addition of acid, were tried. Satisfactory agglomeration of rubber to form clots was not attained when dispersions produced from either hand-defoliated or foliate 3-year-old shrub were coagulated. Five variations of the general procedure were used in coagulating mature-shrub (9-year-old) clarified dispersions. Addition Qf 1% sodium hydroxide by weight, folloTved by' autoclave digestion for 1 hour a t 120" C., previous to the addition of acid to pH 3 and reboiling, gave rubber of highest hydrocarbon and lowest acetone-insoluble and benzene-insoluble 'content. The bestquality rubber was produced from shrub processed in tap water. Its chemical analysis mas 90.5y0 hydrocarbon, 6.9% resin, and 2.6y0 acetone and benzene-insolubles. When compounded, by two of the formulas already cited, the physical properties of the vulcanizates were essentially the same as for latex rubber produced from the same shrub. Good-quality rubber was obtained by acid coagulation of well clarified dispersions of 9-year-old shrub. Three-year-shrub dispersions, Fhen similarly treated, gave a poor-grade rubber. COMMENT AND APPRAISAL OF THE LATEX PROCESS

Results of the investigation have demonstrated that rubber of high quality can be recovefed in the form of latex from both young and mature lush foliate shrub. An accurate appraisal of the ultimate cost of latex recovery cannot be made on the basis of the exclusively batch operations of this investigation. However, certain general observations can be made in regard to costs and to future work on latex recovery. The data from batch operations indicate that it is possible t o disperse 85% of the rubber available in the shrub, and then to recover 90% of the rubber from the original dispersions. This gives an over-all net recovery of about 76% of the shrub rubberz. Important in this connection are the facts that because of instability of dispersions, operations a t these recovery percentages would not be feasible during the growin'g season of the shrubJune through September-and would further be dependent on processing shrub lush from the field. These limitations weigh against the economic practicability of a factory designed and operated solely for the recovery of latex. The ultimate commercial development of the process is seen rather as a department in a factory, the chief production of which would be solid rubber. The latex facilities would be operated seasonally, with the main plant processing any shrub not in condition suitable for latex recovery. 1 Recent improvement in t h e method used for determining t h e amount of rubber i n dispersion has given values greater t h a n t h o s e b y the procedure used in determining t h e figures shown in t h i s paper. I t is probable t h a t t h e latter are t o o low.

Vol. 40, No. 5

Viewing from a comparative-cost standpoint the possibilities of latex as opposed to solid-rubber processing, latex recovery now appears less practical. An over-all recovery of only 76% of the latex involves a heavy dollar loss in shrub, at least double that estimated for possible solid-rubber recovery. In appraising possible costs in latex processing, the shrub cutting and separation of coarse solids appear to offer little difficulty and to involve no great expense. For the pebble-milling following cutting, however, a t the high water-to-shrub ratios necessary, the cost in terms of equipment and duration per unit weight of shrub is equal to or greater than that of the conventional pebble-milling process. The separation of fine plant solids from the dilute dispersions and the concentration of the dispersions by centrifugation present serious cost uncertainties. The commercial centrifuge used in the investigation, which concentrated tenfold a 2% dispersion, would handle approximately 32 pounds of rubber per hour760 pounds per day on a 100% time basis. In terms of equipment only, this is not prohibitive if operating and maintenance costs are low. Actual production, and centrifugal costs-bowl changing, cleaning, etc.-would depend on net operating time, which in turn would be dependent on the accumulation of fine solids and possible formation of solid rubber in the bowl of the concentrating centrifuge. The prospects of a commercially practicable cost of concentration by centrifugation would appear t o depend on getting dispersions containing a reduced amount of fine plant material and on the use of a machine that effects continuous removal of fine solids from the bowl. These solids, as noted, originate in great part from the leaves. Any method of defoliation before wet cutting and milling that would not affect detrimentally the dispersion of the latex would be of tremendous value, not only in clarification and concentration, but for increasing the concentration of the rubber in dispersion. Also, many difficulties in handling dispersions have been traced to the presence of leaves. The question of further research on latex recovery depends on the basic purpose of any future work on guayule. If work is t o be directed toward establishing guayule as a commercial farm crop, with cost a prime consideration, production of latex appears far less promising than that of solid rubber. If, however, future work is to be done on guayule as a domestic source of natural rubber in case of emergency and a latex is desired, then certainly the full possibilities of latex recovery have not been adequately explored. I n continuing development of the centrifugal concentration process for maximum latex recovery, the following would be of first importance: development of a method of field or factory defoliation which would not jeopardize full release of latex; and investigation of concentrating centrifuges in which there would be a continuous discharge of bowl solids. Among other points for investigation should be listed prominently: reduction of shrub in devices other than cutters and pebble mills; means of separating latex and finely divided plant material other than by centrifugation; and combination latex and solid-rubber processing. ACKXOWLEDGMENT

Many members of the Emergency Rubber Project research staff have contributed to this work, and the author wishes to express his appreciation of their aid and encouragement. LITERATURE CITED

(1) Braendle, H. A., Balden, E., and Wiegand, W. B., India Eubber World, 110, 645-6 (1944). (2) Clark, F. W., and Place, W.F. L., Ibid., 112, 67-72 (1945). (3) Jones, E. P., U. S. Patent 2,434,412 (Jan. 13, 1948). (4) Jones, E. P., U. S. Patent application, Serial No. 706,495 (Oct. 29, 1946). (5) Spenoe, D., U. S. Patent 2,119,030 (May 31, 1938).

RECEIVED May 19,1947.

Recovery of Rubber Latex from

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