CRUDE RUBBER PREPARATION Sheet Production by Continuous Coagulation of Hevea Latex E. B. ZEWTONL, W. D. STEWART2. L ~ E. D i. 'FILLSOY Malayan Research Laboratories, The B . E. Goodrich C o n ~ p c i nh~t ,t u l u Lrcnipur, Federated Mulay S t a t e s
w h e n fresh He\ea latex is acidified to pH 1.5 to 4.9, ab in standard estate practice, an elapsed time of 1 to 6 hours ib required for satisfactory coagulum formatiou. By the use of coalescence accelerators the time required for coagulum formation can be controlled. For instancc, it can be reduced to 1 minute or so, or e\en be eliminated altogether in the case of the most active agents. Representatibe of this t j p e of material are the straight-chain
T
saturated fatty acids, especiall? those of ten to fourteen carbou atoms, certain phenols, and some of the higher fatty alcohols. The former are added as soluble salts to fresh latex, which is then fed continuously together with a stream of coagulant onto a properly shaped conbeyer belt discharging the coagulum continuousl? into a sheeting batter?. AIachined wet sheet can be produced within 3 minutes or so after acidification of treated latex.
a few pounds, as is the caae'with iiiaiiy small holders (operators of less than 100 acres of rubber trees), coagulation is carried out in individual 1)ans of about 1 Imperial gallon capacity yielding 1.5 pounds of rubbcr sheet (dry basis). \There large volumes of latex a r c to \IO converted, as in estate practice, coagulation is run in rwtmgular tanks of 200 to 220 Imperial gallons capacity, B standuci size being 10 feet long, 3 feet n-ide, and 15 inches deep (Figuri, 1 ). Inside rlicse tankc; along their length, vertical slot? . l..5 inches so that, after the acid coagulant is mixed solid separating sheets or partitions may Iw iiiscrt et!. The acidified latex nhich subsequr~ritlycoagulates is t hu. effcctiv.cly shaped in dahs corivenient for handling. Cp\\-:ir[L of (Xighty 11f t1ii.w partitions per standard tank must be inserted, removed, and kept clean every time a coagulation is run. On some very large estates the method of fitting the partitions in the coagulation tanks is niodified so that only the opposite ends of alternate partitions touch the sides of the tank. With this system the coagulum forms in serpentine fashion in each tank. On removal of the partitions the coagulum can be lyithdrawn as a continuous ribbon and fed into a suitable sheeting battery. The process is continuous only in so far a s the machixjng of coagulum from an individual tank is concerned. The rate of. coagulum formation is not accelerated over that of the usual practice. After a visit to Malaya in 1939, Stevens ( I 7 1 commented t h a t the method of sheet rubber production had not changed basically since the days of the pioneers, and suggested that Tapping a Rubber Tree in RIalaja with a Jebong or a process of continuous Drawing Knife
H E \Tidespread use of dilute acid as the principal coagulant for Hevea latex in the rubber plantation industry is due to a suggestion of Biffen (9), who, during a n investigation of the type of smoke used in the Brazilian method of rubbcr preparation, found acetic acid to be the chief constituent of the aqueous condensate. The reaction of fksh Hevea latex to the addition of dilute acid? is quite $iffwent from t h a t of ammonia-preservtd latex as received in this country. (In this discussion the term "fresh latex" denotes latex not more than 12 hours old, counting from the time of tapping the trec. j K h e n ammonia-prcscrved lat cx is acidified, coagulation is essentially instantaneous a t th(, spot on the latex surface where the dilute acid fall$. But in I h c W P ~ , of fresh latex, flocculation occurs, and the mis, although thickened, can still be stirred with gentle action t o distribute the coagulant adequately throughout the entire batch. %lore than an hour later these floc3 begin to fuse into a solid coagulum, but it is not until some hours' standing, usually overnight in Malayan prac tire, that this coagulum is strong enough TO iyithsta.nd without, undue deformation the handling attendant on machining i t into wet sheet-the process of putting the slabs of coagulated lates through a series of close-set rolls (sheeting battery) t o squeeze out rubber-free serum. Where the daily output, of rubber amounts t o only 1 The B.F.Goodrich Company, Akron, Ohio. * Present address, The B. F. Goodrich Company, BoyceThonipson Institute, Yonkers,
N Y.
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August 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
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coagulation and sheet formation should be available where large volumes of latex are converted into sheet rubber. I n recent years patents have been issued describing rubbcr production b r continuous coagulation methods applicable t o fresh Hevea latex (18, 25) and to synthetic latices (4,15, 2 2 ) including the salt-acid coagulation method (24). CO.AGUL.ATIONO F H E W & L i m x
The importance of coagulation phenomena from theoretical as n-ell as practical viewpoints is evident on even casual examination of literature published since the turn of the present century. Pertinent to the present discussion is an interesting chain of studies started by U-hitby’s observation ( 2 3 ) Figure 1. 3Ialayan Sheet Factory that fresh latex which has been steriBackground, hulking tanks; aenter, coagulating tanka; foreground, sheeting hattery tized by heating does not form a solid coagulum or clot on acidification with ihe usual amount of acetic acid (to approximately p H 4.6 to 4.8). The latex particles merely clot forination in acidfied B liquid. The treatise includes a flocculate and cream upward but do not coalesce (19) into a clot series of reagents first used by that authoi, notably the ameven after 1 or 2 days’ standing. (The term “Coalescence” is monium soaps of oleic, palmitic, and stearic acids, and the amused in this discussion in the same way de Vries employed itmonia-soluble portion as well as the alcohol-soluble portion of the namely, “the transition of a loose mass of flocs into a fixed gel.”) acetone extract of crepe rubber, the latter being particularly Eaton and Grantham (6) observed t h a t some latex which they active. Until the n-ork discussed below n a s undertaken, no had heat-sterilized in an autoclave did form a clot on acidificainvestigator appears t o have applied coalescence accelerators tion, but Barrowcliff ( 2 ) pointed out’ t h a t as a result of their together with adequate control of p H t o the coagulation of fresh heating method, it is probable the temperature of the latex itself Hevea latex, either from the standpoint of theoretical interest never reached t h a t of the surrounding steam. The latter inor as a practical attempt t o devise processes for continuous sheet vestigator, xithout the use of pressure equipment, prepared a production. modified latex similar t o Whitby’s by slovily pouring fresh latex into a n equal volume of LTater at 90’ C. with subsequent cooling C O i L E S C E 3 C E ACCELERATORS IN FRESH L4TEX to room temperature. Latex so treated (Barrowcliff latex) lost The problem of accelerating the rate of coagulation of fresh its ability t o coalesce on acidification. H e also observed, howlatex was first attacked empirically (Table I). The vaiious ever, that the addition of a few drops of fresh latex to a small acids to be tested for coalescence activity were dissolved in amvolume of Barrowcliff latex restored this ability. Whitby ( 2 3 ) nionia and added to the latex diluted to a dry rubber content had suggested that the heating destroyed a specific enzyme es(D.R C.1 of 27,; this was then acidified with 27, acetic acid. sential t o coagulation, and Barroivcliff’s inoculation technique The general procedure of coagulation used in this study n a s Campbell’s experiments ( 5 ) led to a lent support to this vie!?. carried out as follows: similar conclusion. Eaton and Grantham ( 6 ) :on the other hand, 300 ml. of fresh latex Rere added ~IETHO OFDCOAGULATIOX. attributed the natural spontaneous coagulation of latex to bacto 300 ml. of t a p water containing the coalescence agent, and this terial action rather than to a specific enzyme, and it is noteworthy was then poured into 4000 ml. of t a p water containing 80 nil. of t h a t acidified Barrowliff latex will form a clot under tropical 2 q acetic acid with stirring. conditions after standing exposed t o the air for upxards of a week. Time to coalesce was taken as the inferval between addition of D e T’ries and BeumBe-Sieuwland (20) modified Barrowcliff’s the latex and the time a t which the flocculates actually began to method to give a more dilute latex, B liquid, and found it possible form a continuous clot vrhen the surface x a s moved n i t h a t o obtain from it a natural coalescence agent in this may (21). spatula. Time t o firm up was read n hen the coagulum no longer 3 liquid3 was inoculated with a felv drops of fresh latex and then adhered t o wetted fingers when slipped beneath the coagulum acidified to form a clot. The resulting coagulum, when dry, The p H was determined by glass electrode, and coagulation was was soaked in dilute aqueous sodium carbonate. Th6 cstract carried out a t 2ol, dry rubber content. so obtained could be used, interchangeably with fresh lates, When the latex flocs began t o stick together to form a conas a coalescence agent for B liquid. I t is of interest t o note that, tinuous clot, time n a s taken. This end point was a good deal with a given coagulum, successful extraction of coalescence agent more definite than might be predicted by one who has not obcould not be repeated indefinitely. Conceivably, the sodium served it. Readings could be reproduced within *lornc of the carbonate reacted with a portion of any free fatty acids present elapsed time, and even less on many occasions. K i t h the prei n the coagulum t o form sodium soaps, and these were the active ferred coalescence agents, a variation of *lo% in the time acagents. tually amounts to only 1 3 t o 6 seconds. \-an Harpen ( 7 ) has provided an exhaustive reviex u p to 1930 The d a t a in Table I show that, of the materials ttied, the soapof natural and synthetic agents capable of causing coalescenae or forming fatty acids possess greatest activity. While thc un3 Roil 9 volumes o f . Rater, a d d 1 volume of fresh l a t e r , boil 3 minutes, treated latex requires about 3 / 4 hour for clot formation, addition hold a t 1OOo C . for 5 minutes, a n d then cool t o room temperature. T h e of approximately 2m0 of the active fatty acids as soaps may reproduct is similar t o Barronclifl I n t e r in reaction t o acid a n d to inoculatioii ~ U C Pthis time to only a feiv seconds. x i t h fresh latex.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 39, No. 8
that such locally made soaps are entirely satisfactory in reducing
TABLE 'I.VARIOUSACIDS AS COALESCESCE ACCELERATORS time of coalescence to about 1 minute when they are used a t a Material Adipic acid o-Aminophenol sulfonic acid Azelaic acid n-Butyric acid n-Capric acid Chloroacetic acid Gallic acid Glycine Glycolic acid Lactic acid Lauric acid dl-Malic acid Naphthenic acid Oleic acid Phenylacetic acid Phtha!ic aqid Pyruvic acid Ricinoleic acid Succinic acid Control Coconut f a t t y acids (1% on rubber) Fresh latex
Quantity a per 100 G . Rubber 1.5 1.9 1.9 0.9 1.7 0.9
1.7
.
0.8 0.8 0.9
Time of Coalescence&, AIin.:Sec. >3:00 >3:00 >3:00 >3:00 0:lO >3:00 >3:00 >3:00 >3:00 >3:00 0:40 >3:00
.
2.0
1.3 1.3 2.8 1.4 1.7 0.9 3.0 1.2
3:OO 0:20 >3:00 >3:00 >3:00
pH oi beruln 4.33 4.50 4.68 4,il 4.70 4.50 4 36 4.60 4.25 4 60 4.78 4.11 4.75 4.77 4.58 4.50 4.69 4.87 4.40
concentration of 1 % (fatty acid basis) on the rubber. The procedure is as follow: St,ir hot caustic solution into hot oil to effeCT a good emulsion. Heat gradually to 120' C. (this takes 30 to 60 minutes) and use as 1% solution. Gelling during dilution is avoided by keeping the soap solution a t 60" to 70" C. Yield of fatty acids (if isolated) is 75 to 75.5 grams from 80 grams of palm oil. Coconut oil soap was prepared in the same way. Rubber produced with palm oil soap had less odor than that made with coconut oil soap. EFFECT O F VARIABLES 0%KATE OF COALESCENCE
COSCESTRATIOS OF C O A L E ~ C E AGEXT. These variation5 0:01 are shown in Table IV for whole coconut oil sodium soap alone >3:00 as well as for mixtures of this with sodium ricinoleate. As little as 0.25% of t,he former reduces time of coalescence t,o one third ... 1:20 4.75 that of the untreated latex which, in this series, is already more None About 45 : 00 4.70 sensitive than ordinarily encount,ered (probably because of a .a Coalescence agent5 added t o latex, on t h e basis of 0.01 molar equivalent higher-than-usual bacteria count,). Increased amounts of coaper 100 grams of rubber, dissolved in 207, aqueous ammonia (about 0.75 m ~ . per ) 100 grams rubber. lescence agrrit in the latex progressively decrease the time of coab Coalescence times of 3 minutes or less were regarded as preferable. lescence. Hence, the rate of coagulation can be varied over ir B G E S T S SHOWISG FAST ACCELERATION wide range ( a ) by choice of coalescence agent of suitable activit! TABLE 11. COALESCESCE Tirue Required t o and ( h ) by the amount of this agent used in the latex. The inCoalescence % by W t . Coalesce, Firm up, p H of clusion of lc'c, or so of fat,ty acid during the preparation of rubber Accelerator on Rubber min.:sec imn. :see aeruin for general purpose compounding is regarded as beneficial rather None (control1 30:00 60 :00 4.62 @-Xaphthola 0: 5 0:45 2:45 4.90 than detrimental. 1.0 0:45 2:oo 5.0 IKORGASIC IONS. A\ddition of barium, calcium, lead, magLauric acido 0.5 2:43 4:15 4 65 nesium, and zinc as acet'atea (17, each on the rubber) to the 1.0 0:55 2:15 4.81 1.5 0:40 2:oo 4 83 coagulant accelerated the rate of coalescence of fresh untreated Stearic acido 0.5 9:ld 15:OO 4 z0 latex, coagulation being run at 2% D.R.C. -4s shown in Table V. 1.0 7:30 12:oo 4., 9 1.5 2:15 5:OO 4.81 calcium, magnesium, and barium were active, the calcium esI,UX .oap flakei S o n e (control) 25:OO 38:OO 4.67 pecially so. In the presence of coconut oil soap, however, onl? 0.10 17:30 22:oo 4.72 the calcium ion shows enhanced activity. 0.25 5:45 8:30 4.85 0.50 2:45 3:oo 2.84 C . u x I n r KILTS. A s was shown in the preceding paragraph; 1.00 1:oo 2:15 4.85 1.50 0:40 2:15 2 94 the prrvnce o f calcium ions during roagulation with acetic acid Ril-inuleir acidt 0.25 1:oo ., 0.50 1.00 2.00
3.00 a .Added t o latex as aiiirnonium salts. b Added t o latex as sodium soap.
0:20 o:o2 0:01 0:Ol
. , ,
... .
.
I
COALESCEZCE AGENTS O F HIGH ACTIVITY
Some of the reagents shoning very fast acceleration are bet, forth in Table 11. The pH of the serum must be adequately controlled just as it should be in good estate practice There clean-cut, coagulation Tvith clear (not milky) serum is the rule. This means t,he pH of coagulation should be less than 5 , preferablx in the range of pH 4.6 to 4.9. A small increase in acid is required to neutralize the alkalinity contributed by the coalescence agent as added. Particular reference is directed to the remarkable activity of ric,inoleic acid. This material used in the range of 1 to 3% on the rubber causes inst,antaneous local coagulation where the acetic acid solution (as coagulant) strikes the lat,ex. Such coagulations gave milky serums because the coagulant could not be uniformly stirred in; but the coagulations using 0.5 and 0.25c; of this agent on the rubber were satisfactory in this respect. LOCALLY MADE SOAPS A S ACCELEKATORS
hlalaya is a large producer of both coconut oil and palm oil, the cheapest sources of fatty acids available in that locality. Their utility was investigated. The mixed fatty acids of each of these oils were isolated and, as sodium soaps, were tried as coalescence accelerators. I n another case the entire saponification mixture was employed for the same purpose. The coagulant in these tests F a s 270 acetic acid. Resuks recorded in Table 111 show
111. C O > I P A R I S O S O F CRUDE L l U R I C AND PALMITIL ACIDS, \THOLECOCOKUT OIL, A K D \THOLEPALMOIL AS SOAPS
TaBLE
v ,< by 1 1 3 .V D.R.C. a t Time t o . Coalescence \Tt. on H2SO:'A. Coagula- Coalesce, Fifm up, pH oi Rubber 1\11. tion inin. :-. .An acceptable method is the use of rotameters, one for the latex and one for the coagulant stream, fed from bulking tanks by gravity or under low air pressure. Tlie outlets should be arranged t o set u p turbulent flow to enswe adequate mixing of the coagulant a i t h the latex. COXTROL OF INDUCTION PERIOD.This step, in company with continuous feed, is essential t o the suecess of the mcthod. Control of coagulation rate is accomplished by addition of coalescence agents t o the latex prior t o its admixture n.ith coagulant, as has been described in foregoing sections of the discussion. It is entirely possible to obtain adequate acceleration of coagulation by addition of the coalescence agent t o the acidified latex, but pretreatment of the field latex is t h e preferred and simpler method. .Us0 preferred is the adjustment of the rate to give a firm coagulum in about 1 minute after admixture of the trcxtcd latex and coagulant. For general-purpose rubber, this rate may be obtained by the addition to the field latex of lr; of coconut oil soap alone or together with a smaller amount of one of thc, pticbnolie materials (Table VII) a s a fungicide. Although the coagulation may be made instantaneous, a s by the use of ricinoleic acid (Table 11), it is not desirable t o do so because of the difficulty then encountered of uniformly mixing in the coagulant, EXPERI~IESTAL RfAcH1h-E. The simplest method of forming the coagulum into a continuous ribbon is to flow the coagulating latex mixt,ure down a trough, but this is not a positive method. There is a tendency for the flocculated latex and the freshly set coagulum to adhere t o the aides of the trough and cause undcsirable accretions which impede smooth flow. This is especially true in cases where coagulation is run with low D.R.C. latex. The difficulty may be minimized by keeping the sides of the trough wet, with n-ater or by lining the trough with a very smooth surface which is readily \vetted by water. The best method, however, is t o run the coagulation on a horizontally moving conveyer belt (Figure 3). T h e outside surfacc of a n endless conveyer belt (about 8 inches wide in this case) is
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Val. 39, No. 8
was gradually squeezed between b o cloth-covered conveyer belts supported bv a series of rolls. This type of compressing machine was used because of interest in coagulation of latex at high dilution. Continuous sheeting batteries were commercially available before the x a r and doubtless,could be adapted to handle this type of coa.gulum when made from latex of about 15 to 25% D.R.C. DRYISG KETSHEET. This problem has received considerable attention in the past, and more recently by Piddlesden ( l a ) of the Rubbclr Research Institute. Continuous drying of natnralrubber n-et sheet is probably feasible if the sheet is thin enough. I n this laboratory, wet sheet x i t h a thickness of about 0.015 inch, made by a filtration process and seemingly more porous than machine coagula, has been dried in 8 t o 12 minutes at 10.5’ C. The method of continuously preparing and mechanically transporting coagula as here described is ideally suited to the handling of thin sheet as feed stock for continuous driers. PACKIXG.This matt,er, like that of drying wet sheet, has in the past received more than a modicum of attention. Thin sheet prepared by a continuous method could be rolled into cylinders, or, better (for st,acking), folded and cut into rectangular blocks, as is done with thin sheet a t the synthetic rubber plant designed by The B. F. Goodrich Company at Louisville. There seems to be merit in the idea of preparing parcels of a shape and size that can be easily handled by one man. This is the experience of the synthetic rubber industry and of manufacturers viho use that product. SUMMARY
Quantitative experiments in controlling the rate of coagulation of fresh Hevea latex are described, and the results have been applied in a process for the continuous preparation of sheet rubber on a laboratory scale. As with most continuous processes, this method is more applicable where large, rather than small, volumes of latex must be worked up. Its apparent advantages reside in a possible improvement in uniformity of product and a reduction in factory floor space for a given output, of rubber: Figure 4.
Continuous Sheet Rubber 7)Iachine with Sheeting Battery i n Foreground
covered n i t h a flesible sheet of vulcanized rubber which projects 5 or 6 inches beyond the edges of the conveyer belt. - i t the ends of the machine this special belt moves flat over driven rolls and from there is passed doxm a trough having sloping sides. When the belt enters the trough section, the rubber flanges arc bent upward to form a moving rubber lining. One end of this trough scction is sealed by a thin rubber sac shaped to the cross section of the trough. Contact with the sides of the trough is maintained by inflating the sac n-ith air or with a liquid, such a; calcium chloride brine. The sac is prevented from hulging nndr:r inflation pressure by flat metal plates on either side. Altcrnately, the seal may be made by feeding the conveyer bclt into the trough section a t a n angle. This particular machine had a trough w e tion about, 12 feet long, and the belt, moved at the rate of 6 feet per minute. For large scale production this rate would bc increased several times. Jn operation, a movable dam was placed in the trough Peetion just beyond the rubber sac. Streams of treated latex and coagulant were proportioned into the space so formed. When the space was filled t o the depth required, tlir machine Tyas started. After a travel of about 6 feet the latex first added began to coagulate. This solid coagulum formed the seal at the exit, end of the trough; at that, time the removable dam, no longer required, could be lifted out, and a continuous ribbon of coagulum emerged. MACHINING THE COAGULUM.This ribbon of coagulum vias fed into a continuous sheeting battery where excess serum was squeezed out, Figure 4 shows the battery actually used in the esperimental machine. The continuous ribbon of coagulum
LITERATURE CITED
hscoli, F. D., Trans. I n s t . Rubber I n d . , 21, No. 4 , 225 (1945). Barrowcliff.hl., J . SOC.Chem. I n d . , 37, 49 (1918). Biffen, I
