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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 18, No. 11
increase in consumption is maintained, we can expect the situation to some extent but would not wholly solve the world’s capacity to produce enough natural rubber to meet estimated shortage problem. the consumption to be greatly strained, and then much higher IC-Ninety-eight per cent of the natural rubber used today prices might rule. Such prices ought to stimulate greater comes from one species, Hevea brasiliensis, 94 per cent of production from tropical America and Africa of wild rubber which is furnished by planted crops and 4 per cent by wild and, if high enough, the shipments might again reach 70,000 trees in the forests of the Amazon Valley. Past experience tons annually. Even more might be forthcoming if the col- shows that, while other natural rubbers and reclaimed rubber lecting industry of the Amazon especially could be better can be used for special purposes and give as good service organized and enough capital made available to open up the as, and in some few cases better than, Hevea rubber, yet, more remote sources of supply. in so far as can be foreseen, the extent to which such rubbers 8-Such a n increase, together with the problematic in- can be substituted without deterioration in the quality of the crease in production of rubber from the native planta- manufactured goods is distinctly limited. The manufacturer tions of the Dutch East Indies, might be sufficient a t least of rubber goods exceeds these limits to meet the demand for to postpone the day of very acute shortage of natural a low-priced article or when he is compelled to do so by famine rubber. prices for Hevea. 9-Famine prices for natural rubber, if they come, will 11-While future developments may show that there are again reduce its consumption below normal, will stimulate other natural rubbers as good as Hevea, yet today Hevea research of the chemist to produce a better quality of re- rubber is not only recognized as the best rubber, but as a claimed rubber, and there is always the remote possibility planted product it can be raised at a cost cheaper than any of synthetic rubber. Such problems are up to the chemists. other natural rubber. Large quantities of wild Hevea can The possibilities in these two directions are so remote, how- be placed on the market only when the price of rubber is ever, that it would be folly to place too much reliance on high, much higher than the cost of raising it on plantations. absolute success-that is, a reclaimed rubber as good as The only absolute assurance that the consumer can have of a natural Hevea rubber, and synthetic rubber equal in quality sufficient supply of Hevea rubber to meet a normal increase and as cheap as that produced by Nature’s chemical labora- in consumption is to see to it that there is a sufficiently large tory. Partial success in these directions would relieve the planted acreage to furnish the quantities needed.
Recent Developments in the Preparation of Plantation Rubber By Henry P. Stevens 15 BOROUGH HIGHST., LONDON, S. E. 1,
H E R E are no radical alterations to be looked for in the preparation of plantation rubber in the future. As a whole, present methods are satisfactory; the product good and the general principles correct. The most to be expected is a n improvement in the details and preparation of sheet and crepe rubber of greater uniformity, as well as freedom from mold, spots, and other minor defects.
T
Coagulation
The most essential process for the conversion of liquid latex into solid rubber is the coagulation. The time is now past when the use of acid or other chemicals for this purpose can be condemned or even questioned. Nor can we go back to the laborious native process of smoking as practiced from the earliest times in the valleys of the Amazon. Such would be a retrograde step. As for the relative qualities of plantation and fine Para, it would be strange indeed if a crude native curing process yielded a superior or more uniform product than that produced by an up-to-date estate run on the most approved lines. The main deficiency in uniformity results from the large number of estates, some of which are not so u p to date as others, and the mingling of native-prepared with plantation-prepared so that the rubber manufacturer has no means of knowing or selecting suitable material except by a n actual test of the product offered to him. I n regard to acidity, it does not seem to be realized that the amount of acid used in coagulating latex is extremely small and that the acid is a, volatile organic acid. One part of acetic acid coagulates 1200 parts of latex standardized to 2 pounds dry rubber to the gallon. The presence of acid cannot be detected by the ordinary reagents in first-grade (standard) crepe rubber and sometimes not in sheet rubber, but it can always be detected in fine Para. From this evidence it
ENGLAND
might be concluded that acid is not harmful and may even be beneficial. As a fact, acid is harmful if used in large excess, particularly if a strong nonvolatile acid such as sulfuric acid is employed. I n the early days of plantation research various acids and acid salts were tried, but nothing more suitable than acetic acid was discovered and many were quite unsuitable, such as hydrochloric and phosphoric acids. The use of alum was banned by edict in the Federated Malay States. Alum is not so bad a coagulant as has been suggested. It does not damage the rubber, but results in a slower curing product. It is certainly not so harmful in this respect a s sulfuric acid, which was used to a large extent during the war when acetic acid was not procurable. Alum is also a n easier substance for a native to handle than a corrosive liquid such as acetic acid. The Dutch have not put any restriction on the use of alum; indeed, de Vries regards it as a very suitable coagulant for the native producer; its tendency to reduce t h e rate of cure being counterbalanced by the usual native procedure of allowing the rubber to mature. Recently the Rubber Association of America addressed a letter to the Rubber Growers’ Association of Great Britain dealing with the use of sulfuric acid as a coagulant and expressing apprehension of the probable harmful effect which would be produced on t h e curing and aging qualities of the rubber. The Rubber Growers’ Association has always discouraged the use of sulfuric acid as a latex coagulant and very little, if any, is used for this purpose at the present day.’ The writer’s objection to sulfuric acid is its influence on the rate of vulcanization of the resultant rubber. If the sulfuric acid is kept at a minimum, say, 1 part per 2000 of latex, the effect k not very marked-for instance, the time required to vulcanize a simple rubber-sulfur 9:1 mix is increased by about 10 or 12 per cent, 1
Bull. Rubber Growers’ Assoc., May, 1923.
November, 1926
IiYDUSTRIAL AND EiVGI,VEERI SG CHEMISTRY
but by increasing the proportion of sulfuric acid tenfald.the time of vulcanization may be increased 50 or 60 per cent. Figure 1 shows clearly the effect on rate of cure of increasing proportions of acetic and sulfuric acids, the uppermost curve relating to acetic acid and the lowest to sulfuric acid. The effect is very marked in this instance as the rubber used was "slab"-that is, unrolled sheet, which normally vulcanizes twice as fast as rolled smoked sheet. Figure 2 shows the effect of increasing proportions of sulfuric acid, alum, and acetic acid on the rate of cure of ordinary smoked sheet rubber. The continuous lines refer to rate of cure as measured by the vulcanization coefficient and the dotted lines as measured by elongat'ion at a constant load-that is, stiffness of t'he vulcanizate. Both method6 tell the same tale. The sharp upward bend of the curve for sulfuric acid indicates the very marked effect of a relatively small excess of coagulant on rate of cure. However, if the sulfuric acid is kept at a minimum the effect is not worse than would result from the use of alum or large proportions of acetic acid. Apart from the effect on the rate of cure no deleterious action as a result of using sulfuric acid for coagulation has been noticed. Samples of sheet rubber twenty years old coagulated with sulfuric. acid are in apparently good condition. As a fact, a trace of sulfuric acid enters into the coagulation process of most estates, as the combination of acetic acid and sodium sulfite or bisulfite must result in the liberation of a greater proportion of hydrogen ions than would result from the use of acetic acid alone. However, when we remember that the proportion of acetic acid required for coagulation amounts t'o less than 1 part per 1000 of standardized latex, it is obvious that the hydrogenion concentration must in any case be very small. Sulfuric acid is believed to be an unsuitable coagulant merely because of its strength-i. e., hydrogen-ion concentration-and not because of any specific action on the caoutchouc molecule. The same cannot be said of hydrochloric acid or phosphoric acid, the former producing tackiness in the rubber. More recently we have discovered that sodium silicofluoride is quite an efficient coagulant. It was hoped that its well-known fungicidal and preservative qualities would prevent mold from growing in rubber prepared with it, but this is not so, or a t any rate its mold-preventive properties are slight. On the other hand, it has decided advantages as it prevents the gaseous fermentation of latex. On certain days the coagulated rubber is honeycombed with minute gas bells arising from some variety of fermentation. The trouble usually occurs on damp mornings or after showers when the tree trunks are damp and the latex diluted with rain water. The infection is probably derived from molds or bacteria which are washed off the bark surface and so contaminate the latex. Infected latex is easily recognized because of its loss of mobility. Fresh latex of even high rubber content, such as 40 to 50 per cent dry rubber, is quite mobile and more like fresh milk than cream; but infected latex is more of a creamy or clott'ed consistency, although its rubber content may not exceed 20 per cent. Such latex usually has a low rubber content because of its dilution with rain .water. X microscopic examination reveals minute clots that are particles of coagulated latex coiisisting of from a few hundred to thousands of latex globules, so that the effect of the ferment is to cause a premature discreet coagulatioI1-only a portion of the latex consisting of free globules. There are, however, sufficient of theac to insure normal acetic acid coagulation, hut with the result already indicated. Such coagulum cannot be used for sheet-making, as the sheet would he unsightly. 1t.inust be worked up into crepe. If, however, sodium silicofluoride be substituted for acetic acid, gas formation is iixhibited and sheet free from bubbles can be obtained from this thickened latex. .The arnmtnt of sodium silicofluoride required for coagulation is very small, about 1 gram to 1500 cc. of standard-
1117
ized latex-that is, one-third per cent on the dry rubber produced. As a large part will be lost in the serum and wash waters, the small residue remaining could not be expected to have a very pronounced fungicidal effect. Sodium silicofluoride is a difficultly soluble substance, but it is found that coagulation is readily effected by stirring the dry powder into the latex. Apart from a saving in cost, a clean dry powder like sodium silicofluoride would appear to be an ideal substance for the native and could take the place of alum, which in the Federated Malay States he is not allowed to use. Sodium silicofluoride, like alum, produces a slightly slower vulcanizing rubber, but not to an extent which necessitates its segregation from rubber coagulated with acetic acid. Several of the largest rubber manufacturers in the United States tested sample cases of sodium silicofluoride rubber prepared in the ordinary routine on an estate in the Federated Malay States,
THE RETARDATION IN RATE OF CURE 0 SLAB RUBBER BY ACETIC ACID, SODIUM BISULFITE AND SULFURlC A C I D
P
-8Q
4 xox
E 6 Q b
e4
B z ct"
0 1 2 3 4 5 6 7 8 9 /O N 12 13 /4 / 5 / 6 /7 Pads o f Reoqenf per /ooo p a r h l'ar'ex Figure 1
and the writer had the pleasure of reading the very full reports which were submitted.2 Those who tested such rubber did not agree upon all points, but the general conclusion arrived at by all was substantially the same-namely, that rubber coagulated with sodium silicofluoride was quite suitable for their purpose and did not show a greater variation from rubber coagulated with acetic acid than was to be found between different samples of the latter. One other coagulant deserves mention. Many years ago, about 1908, the writer found that formic acid nas a inore effective coagulant than acetic acid and that usiiig the niiniiiiuin quantity for coagulation the rubber was -uhitantially the same in quality and rate of cure. At that time the o h e r i ation was of little interest ab the higher price of formic acid more than balanced its greater efficiency. More rectJiitly, however. forinic acid has been produced so cheaply in Holland and Germany by a contact proceis that it has now become a rii-al of acetic acid; the coyt per pound of the acids is about the same, but a pound of formic acid will coagulate approxi2
Summary in Bull. Rubber Growers' Assoc., September, 1925.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
mately twice as much latex as a pound of acetic acid. If this price ratio is maintained, a gradual replacement of acetic acid by formic acid for latex coagulation may be anticipated. Fungicides and Preservatives
For many years fungicides have been used, but only for application to the trees and not for adding to the latex. Our hopes of sodium silicofluoride for this purpose have been mentioned, and its prevention of bubbles in sheet rubber explained. These bubbles are frequently very minute so that they appear as a dull white patch in the sheet and give the appearance of imperfectly dried rubber. But our main trouble with sheet rubber is mold. Every one connected with the rubber factory is no doubt familiar with moldy sheets, which certainly have a very unattractive appearance and a fusty smell. The writer’s tests indicate that the figures given by such sheets are substantially the same as those obtained with clean sheets. This applies particularly to the light flufly molds resembling those which form on cheese and which are easily brushed off. I n such cases the sheets are drythat is, contain 1per cent or less of moisture-and there are no visible wet patches where the rubber shows dull or opaque to transmitted light. Where the sheets are moist and show a close damp mold growth, however, the rubber is generally slower vulcanizing than normal, particularly in a litharge mix. The reason is now known. The molds in their growth consume the small quantity of fatty acid present in the rubber, so that the resulting moldy sheets are deficient in fatty acids which are necessary to satisfactory vulcanization in the presence of litharge and certain other accelerators. Restore the balance by the addition of 1 per cent of stearic acid to your moldy rubber and the litharge mix then vulcanizes satisfactorily. It may be taken for granted that the actual rubber hydrocarbon is unaffected by the mold. Clean rubbers sometimes show a similar deficiency in rate of vulcanization, which can be rectified by the addition of a little fatty acid. This may also be due to mold, for mold may be present although invisible. It is the fluffy efflorescence which we see, while the minute threads of mycelia which penetrate the host are difficult to detect. We know that spotty crepe vulcanizes more slowly than sound crepe. I n case there is no efflorescence we should not be aware of the presence of mycelia were it not for the pigments which this type of mold (fusarium and penicilium sp.) produces in the course of its growth. If, then, we could prevent mold growth on rubber, we should not only avoid offering manufacturers unsightly rubber, but we should remove a t least one of the causes of variability. The problem of mold prevention has, therefore, been a prominent one before the Rubber Growers’ Association. For a long time attention was directed to improvement in the standard methods of preparation, in particular with smoke drying or “smoking” rubber. It is believed that smoking was originally tried with the idea of producing a rubber like native Brazilian fine Para. I n the early plantation days we were told that we could not do better than make rubber like fine Para, whose “nerve” was said to arise from the smoke used in its preparation. It was, therefore, natural that an attempt should be made to graft the smoking process on to the plantation method of coagulation; in fact, some people went further and prepared rubber by passing smoke into a tank of latex or coagulating with water saturated with wood distillation products. It is now known that smoking has little effect on quality of raw rubber; it certainly does not confer on plantation the supposed “nerve” of fine Para. It has been found, however, that smoking is a very convenient way of drying sheet rubber which otherwise is a rather slow process and it also has been found that smoked rubber is more resistant to mold than the airdried product. The smoking process was studied and controlled, particularly as regards type of furnace, fuel, and tem-
Vol. 18, No. 11
perature, but under no condition could immunity from mold be guaranteed even when the rubber was packed clean and dry. Smoked sheet samples packed in biscuit tins under ideal conditions arrive almost always free from mold, and the writer has examined many hundreds of experimental samples which have reached him in this manner. On the other hand, unsmoked sheets similarly packed usually arrive with a fine light surface mold and a distinctly moldy or fusty smell. I n recent years, therefore, the association has concentrated on experiments with fungicides. The failure to secure improved results with sodium silicofluoride has been described. Other mild fungicides and preservatives such as fluorides, boric acid, salicylic acid, benzoic acid, and so forth, were also found to be useless. The problem is not an easy one. An agent is needed which shall be effective in very small quantities and which shall not interfere with the vulcanizing properties of the rubber. Among numerous substances tried to date the most promising are p-nitrophenol and 3,5-dinitro-ocresol. A large number of experiments have been made with the former which can be used either by dipping the sheets in the solution before hanging to dry or by adding a small quantity with the acetic acid or sodium silicofluoride for coagulation. As little as 0.2 gram on 3000 cc. of-standardized latex-that is, the equivalent of 0.033 per cent on the dry rubber-is sufficient to prevent mold from forming on sheet rubber under ordinary conditions. It will probably be advisable to use rather more to be on the safe side, but the maximum would not exceed 0.1 per cent of the rubber and a large part of this will no doubt be lost in the serum. As p nitrophenol is an acid substance, the amount of acid used for coagulation can be somewhat reduced; for dipping a 0.1 per cent solution is strong enough. Several American manufacturers have been kind enough to undertake confirmatory tests on sheet and crepe rubber coagulated with acetic acid and pnitrophenol, the proportion of the latter used being approximately 0.1 per cent on the rubber. The results of tests of two manufacturers which have already been reported are favorable. The discovery of fungicides is important to the producer from another standpoint. One would imagine that in a tropical, forest-covered country like Ceylon and the Federated Malay States ample supplies of firewood for smoking would be available. This was initially the case when portions only of the estates were planted and there was plenty of surrounding jungle from which to draw. But it is not the case today. Firewood is now scarce and difficult to obtain, particularly in Ceylon and parts of the Malay Peninsula, such as the Klang district, where estates were planted many years ago and tlre now contiguous over a considerable area. Supplies are eked out with Hevea trees cut from the fields which are being thinned out, an operation which has to be undertaken with all areas from time to time as they mature. Nevertheless, wood is scarce and the Government, alarmed at the depletion, t,hreatens to take drastic steps and cut off supplies. The manufacture of sheet rubber is economical in machinery, as light rollers only are required and few estates could turn out the whole output as crepe with the machinery a t their disposal. Many of the smaller estates make sheet only and send their scrap to be worked up to crepe elsewhere. Therefore, estates could not well give up the manufacture of sheet rubber. Unsmoked sheet rubber, as already explained, is very prone to mold and could only be marketed at a loss. But p-nitrophenol or dinitro-o-cresol will enable unsmoked sheet rubber to be marketed in a clean condition and such sheet rubber will only differ from the smoked sheets in that it will be lighter in color. The washing process in the manufacture of crepe is a drastic one and most of the serum substances are washed out. Consequently there is insufficient nutrient matter to grow the
November, 1926
INDUSTRIAL AND ENGINEERING CHEMISTRY
fluffy surface molds and these are seldom if ever met with in crepe rubber. But this type of rubber may develop colored spots, to which reference has already been made. There is a great variety in color in these spots; some of them, such as the pink and yellow, are clear and bright. On cutting out these colored parts and making a vulcanization test using the uncolored parts as a control, it is found that the colored parts vulcanize 10 or 20 per cent more slowly using a 9:l rubberI5
20 25
4
30
900
35 40
45 50 55
sulfur mix. The spots are due to well-known types of microscopic fungi. Dr. Brown, of the Imperial College of Science in London, has obtained cultures of fusarium sp. and has succeeded in infecting clean rubber and producing a dull amethyst discoloration. The rubber manufacturer is naturally suspicious of this discolored crepe, although it is not so bad as it looks. This type of mold can also be prevented by the use of fungicides such as p-nitrophenol. Unfortunately, the crepe rubber has a tendency to darken or go yellow when p-nitrophenol is used with the acid for coagulation, and consequently it is not quite ideal for this purpose. The yellow color is not always apparent and it has been found that exposure to light intensifies it; subsequently, however, the color tends to fade again. Drying of Crepe
Most of the crepe rubber is air-dried-that is, hung in airdrying lofts depending on drafts and temperature produced by the natural heat of the sun on the walls and roofs; but on some estates, mainly in Ceylon, the crepe is dried artificially. Whatever way crepe be dried it must first be rolled thin; otherwise the time of drying would be too prolonged and the fungoid spots would develop, resulting in a slower vulcanizing rubber. Alternatively, the rubber might “mature” with the formation of an extra proportion of organic bases, which would result in a faster vulcanizing rubber. This would not disturb the planter, as the rubber would look all right, but it might not be so good for the manufacturer. For artificial drying the crepe is worked very thin and this means additional milling. Most of it then goes into hot air driers, where it is exposed for 1 to 2 hours to hot air. As the temperature rises to 90-95’ C . the surface of the rubber on the trays as they come out of the
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driers is sticky, or a t least soft and adhesive and unsuitable for marketing in this state. It is, therefore, taken to machines and rerolled to make the blanket crepe. This crepe is naturally softer and breaks down more readily when milled than the ordinary air-dried material. It is sought by rubber manufacturers for certain purposes, although it is probably a little inferior in quality to the air-dried product. Manufacturers do not always realize the difference between Ceylon blanket crepe and some of the reworked material of native origin commonly known as amber which is imported into this country so largely from Singapore, but which is seldom if ever met with in Great Britain. I n American journals there is a tendency to grade rubber into smoked sheets as being first quality and crepeamber or otherwise, as second quality. The writer is of the opinion, however, that properly prepared air-dried crepe is on an average of as good a quality as sheets and certainly more uniform. I t is, moreover, more expensive to prepare and gives a smaller yield from a given volume of latex. “Whole Latex”
By this term is meant rubber obtained in some form or another by the removal of moisture from the latex. There seems to be a mistaken idea very current that the preparation of fine Para is primarily an evaporative process and consequently methods df preparation dependent on evaporation should yield rubber of better quality than acid coagulation. Very little progress was made with the preparation of rubber by this method until the spraying process was introduced. As is well known, the evaporative method had already been applied successfully to milk, meat juices, and other foodstuffs for many years, but somehow the idea of using it for rubber latex was not applied commercially until pushed by an American with characteristic vigor and resources. As already pointed out, the yield of smoked sheet is greater than that of crepe from the same latex. The difference in yield between smoked sheet over crepe from the same latex may be put a t 1 to 2 per cent on the rubber, and is due to the difference in the thoroughness with which the serum liquors are removed. Almost everything water-soluble is washed out of crepe, but some serum remains in the sheets. Evaporated latex, whether obtained by spraying or any other means, contains all the serum constituents and consequently gives a substantially larger yield of dry product than is obtained by coagulation and washing. The increase in yield is about 10 per cent, which is mostly made up by the water-soluble 1methylinosite, an inert body in some respect resembling the sugars, and can only be regarded as a mere diluent in the rubber. Fine Para on analysis shows a composition similar to crepe or sheet rubber, proving that the native method of preparation is not evaporative but mainly coagulative. Thus, the acetone extract of Para rubber is approximately 3 per cent, that of plantation sheet and crepe rubber is the same, while sprayed rubber gives a figure of approximately 5. The aqueous extract of fine Para is about 0.15 per cent, that of plantation crepe rubber rather less, and of smoked sheet rather more; but of sprayed rubber it is up to 10 per cent or more. The percentage of protein (nitrogen X 6.65) in fine Para and plantation crepe and sheet is round about 3, but in sprayed rubber is over 4. The greatest difference is, therefore, in the aqueous extract. Production of Pale Crepe There has recently been a demand for very pale rubber, particularly for sole crepe, which could not be met by the usual methods-that is, by the addition of bisulfite to the latex. To meet these requirements resource has been had to a system of fractional coagulation originally described by de Vries. If, to the latex is added a substantial fraction only of the acid required for coagulation, a small part of the latex
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INDLlSTRIAL A N D ENGINEERING CHEMISTRY
coagulates. This first fraction is removed and the remainder of the acid added, when the bulk of the latex coagulates in the ordinary manner. The amount of acid required for producing the first coagulum varies with conditions and cannot be definitely fixed so that the operation requires particular care, but if satisfactorily carried out the first fraction will consist of approximately 10 per cent of the whole. The first fraction differs markedly from the second, being of a dark yellow color and having a different chemical composition. The second fraction, forming the bulk of the coagulum, is exceptionally pale in color, the coloring matters of the latex having been almost completely removed in the first fraction. A full description and differentiation of the properties of the two fractions may be found in the writings of de Vries. It will suffice to state here that the first fraction when worked up in crepe form contains a larger percentage of nitrogen and is also more difficult to mill. The second fraction is softer and differs less from normal pale crepe. There is no likelihood of second-fraction crepe being placed on the market in quantity or of it coming into the hands of the rubber manufacturers, for such rubber is obviously expensive to produce and is made in small quantity only for a special purpose. Its manufacture is rather to be discouraged, for the quality of the rubber from the manufacturers’ standpoint is not improved and it merely meets a somewhat artificial demand for an exceptionally pale product. Sheet Rubber
Historically, sheet rubber is a development from the biscuits, the earliest form in which plantation rubber was placed on the market. It is said that the pioneer planters, looking around for a suitable shallow receptacle in which to coagulate the first latex from their trees, employed soup plates, yielding flat circular sheets or biscuits which still occasionally appear on the brokers’ show tables. As the output of latex increased a type of vessel which would pack better and take less shelf room during coagulation was required and rectangular pans were introduced. The rectangular shape also facilitated rolling, the sheets were more convenient to hang and fitted better into the rectangular packing cases. But for an estate with a large output the number of pans required was large and involved a correspondingly large amount of handling. It was also difficult to insure correct measuring and thorough mixing of the acid in each pan. This led to an attempt to add the acid to the latex in bulk, ladling out the mixed latex and acid into the pans where it was allowed to coagulate. To do this successfully required great care in order to avoid excess of acid and too much handling. Otherwise the latex commenced to coagulate before the ladling out was completed. It might at hrst appear that there should be no difficulty in adjusting the proportion of acid to the minimum, but it must be remembered that the difierence in the amount of acid required to produce initial coagulation in, say. half an hour and two or three hours is very small, and also that the coagulability of latex varies to some extent under conditions which cannot be controlled. h o s t of the sheet prepared nowadays in the Malay Peninsula is tank-coagulated. The tanks are provided with separating boards, which are slid vertically into position in the tank after mixing the acid with the latex. These divide the tank into a number of compartments, yielding a corresponding series of cakes or slabs, which when rolled yield the sheets ot commerce. The introduction of tanks with separators, however, brought other difficulties. Vertical grooves had to be provided in the sides of the tanks to hold the separators and the crevices were ’difficult to k&p clean. I n z tropical country mold$ and b’acteri contamir%tion*df the addition of’acid and coagulation gives rise to &hebubhles to
Vol. 18, No. 11
which reference has already been made. Ceylon planters never took kindly to coagulating tanks, and it is believed that the great bulk of the sheet produced there is made in pans, either enameled iron or aluminum, and there is some talk of a reversion to pans in other countries. More recently, however, aluminum tanks have been successfully introduced and are creating considerable interest locally. If they stand up to the usage they are likely t o be generally adopted. Up to nom tanks have been usually built u p s f glazed tiles and the cement between the tiles is gradually dissolved away. Of course, aluminum is slightly attacked by the weakest acid, but it does not appear that the amount dissolved will seriously shorten the life of the tank. I n any case it would seem a backward step to revert to pans. Rolling
The general procedure for rolling rubber is well known. Sheet rubber requires very little rolling-in fact, excellent sheet may be made with hand-operated machinery, whereas crepe manufacture necessitates heavy washing mills and much power. This naturally tends to restrict an increase in output of crepe. I refer, of course, to what is termed standard or first latex crepe-i. e., crepe manufactured direct from the coagulated latex and not the so-called amber crepe. One of the difficulties in sheet manufacture is to prepare sheets of uniform size and finish. The original slab of coagulum is much thicker and shorter than the rolled sheet and regular treatment through two or three machines set a t different distances is necessary to prevent distortion and crumpling. The larger estates with adequate machinery probably do not have much difficulty in this respect, but smaller estates may. To provide a mechanical means for regularly reducing the thickness of the sheets there has been introduced the Tanah Besih kneading machines, the principle of which consists in passing the slab of coagulum between a series of rollers, the slab being carried forward by an endless band and being subjected to increasing pressure by successive rollers as it passes along, the final pair of rollers reducing the sheet to normal dimensions for hanging and smoking. Opinions on the efficiency or need for such a machine differ and the writer does not know of any at work outside the Dutch East Indies. The machine, although described by the inventor as a “kneadmachine,” is not a kneading machine in the usual English meaning of the term, but purely a rolling machine. Scrubbing and Smoking
It is usual after rolling to soak and scrub down the sheets in water. This has been the practice for some time, for after rolling a sheet sweats. The contraction or retraction of the rubber after it has been subjected to strain forces drops of serum to the surface and the soaking treatment results in a rubber less liable to mold and similar defects. The modification of this treatment has already been discussed, in which an antiseptic such as p-nitrophenol is added to the water in which the sheets are soaked. It is advisable to allow the excess of water to drip off and the sheets to surface-dry before hanging in the smoke house. Today this is frequently done on racks in the open. Sunlight has no effect on wet rubber during the few hours’ exposure and air-dripping in the open has been found to give better sheets. Improved types of sriroke houses have been introduced in recent years.J The modern saoke house is a compartment house to enable the sheet t o be replaced a part at a tnne without emptying the whole houw of h o k e every time dried rubber 1s withdraws and fresh wet sheets are hung in their on this subject See the.la ion RuBber.”
Of
I N D [is TRI.4 I, A S D E‘NGILVEERING CHEMISTRY
November, 1926 Packing
Rubber manufacturers are frequently eloquent on the subject of packing. Only one or two points need be mentioned here. Much of the trouble would be avoided if only the case of rubber could be delivered as packed in the East direct to the manufacturer. Ordinary, not to say special, care is taken in packing and the cases as they leave the estate appear as satisfactory as a;y other packed merchandise for export to Europe or America. Indeed, most of the cases are in excellent condition when they arrive a t the London docks. Then comes the opening for sampling and the hest of cases can never be satisfactorily coopered. KO doubt much the same applies at New York and other ports. Today many estates line their cases with “wrapper” sheets, R O that any
1121
dirt is retained by the outside sheets, but this does not help much if the case will not hold together. It is impossible to prevent rubber from taking up splinters from wood with which it has been in contact. The rubber, being plastic, forces its way between the fibers of the wood and when separated tears off splinters of wood with it, however smooth the wood is planed. The writer has suggested chalking the inner surface of the case before packing. The chalk can be held in place by a little soluble starch or similar adhesive. Rubber so packed was reported on favorably by one Amcrican manufacturer and the proposal has been ventilated but no action has been taken. Mat packing is strongly advocated in some quarters, but it is difficult to avoid massing of the rubber and there is the difficulty of sampling.
The Botany and Cultural Problems of Guayule By Wm. B. McCallum INTERCONTINENTAL RUBBERCo., SALIXAS,MONTEREY COUNTY,CALIF.
Botanical Considerations
H E number of plants known to produce rubber is upwards of one hundred, and the list is constantly increasing. TYhile these are fairly scattered throughout the plant kingdom, there is a tendency toward segregation in certain families, the euphorbias and the milkweeds affording a common example. I n most cases the amount of rubber in the plant at any one time is very slight. Even in Hevea, which practically supplies the world with rubber, the latter never constitutes, a t any one period. more than a fraction of one per cent of the total weight of the plant. Usually the rubber is in the form of a latex emulsion. This is developed in a system of minute tube-like glands that ramify through certain areas, more commonly the peripheral portions of the plant. This fluid, as in the case of most glandular secretions, is under a certain hydrostatic pressure, so that when the ducts are severed, as in tapping, a certain amount of the latex with its contents is forced out. Rut in some cases there is no definite glandular system and the rubber occurs in the general parenchymous cells of the plant, accumulating as solid rubber in the individual cells. This is the condition of guayule. This shrub belongs to the Compositne (sunflower) family, and to the genus Parthenium. The species is Partheniwn argentaturn, from the silvery gray appearance of the leaves. I t is a native of the plateau region of central and northern Mexico, and ranges in a broad zone from the State of Zacatecas to a little north of the International border. extending somewhat into the Big Bend region of Texas. It has never been found growing naturally outside of this area. I n the main, it confines itself to altitudes between 4000 and 7000 feet, and limits itself almost exclusively to the limestone hills and slopes. Rainfall, in guayule’s natural habitat, ranges from 7 to 14 inches annually. I was first employed in 1911 by subsidiaries of the Intercontinental Rubber Company to carry on in Mexico certain portions of the botanical study and propagation work that had been well started by Francis E. Lloyd, Shortly thereafter the revolutionary disturbances beoame so acute that our work in Mexico, together with factory operation, had to be suspended and the experimental plots on Cedros ranch and elsewhere were abandoned. Meanwhile we had all been greatly impressed with the potential importance of what we were trying to do and the officers of the company, with eyes open to the difficulties
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to be overcome, instructed me to salvage a large and representative seed supply and start over again in California; and this was done in 1912, thirty-six years after Sir Henry Wickham had taken the first Hevea seeds from Brazil and at a time when the world was beginning to realize the great importance of this historic event. Already the bulk of the world’s rubber supply was shifting from mild to cultivated sources with Hevea as the medium, and even then we thought we saw certain latent but inherent advantages of guayule over Hevea. Characteristics of Guayule Shrub
Being an evolutionary product of the desert, the guayule plant has the characteristics that adapt it to that arid environment. I n form it is an erect woody shrub, usually not exceeding 2 feet in height, and with a native dry weight of rarely more than 2 or 3 pounds. It is a hardy perennial and, under undisturbed natural conditions, lives probably forty or fifty years. It has small composite flowers which usually produce but little seed, but are potentially capable of producing abundant seed under suitable conditions. Like most plants of this type, its powers of vegetative reproduction are very weak, which added greatly to the difficulties of working out practical cultural methods. I n its natural habitat the growth is very slow. An average five-year plant from the seed will weigh less than a pound, although in occasional favorable conditions the weight is somewhat more. Natural reproduction is almost entirely by seed, but a majority of plants pulled during or just before the rainy season will resprout from the rootlets left in the ground. The seeds are very minute, one thousand weighing less than a gram. Ordinarily but little seed is produced, and in very dry years, none a t all; but in the occasional favorable years a large amount may be formed. The seed is quite fertile and retains its vitality for many years, but is incapable of germination a t once, and can do so only after a considerable lapse of time. One of the most important characteristics of the plant is its hishly developed capacity to withstand drought, remaining active and making a certain amount of growth with a very low water content. Guayule shrub will thrive and produce varying amounts of rubber under a broad selection of soil and climatic conditions, just as is the case with, say! potatoes or sugar beets, etc. However, as in the cases cited,
