Sidney Morgan

Almost without exception, the agent employed in the coagulation of plantation (Hevea) rubber is acetic acid, or in some cases formic acid. Under ordinary trade conditions supplies are always obtainable at reasonable prices, but during the recent War the question of possible substitutes was brought greatly to the fore. Fortunately the subject of coagulation and coagulants had been previously studied to such effect in laboratory practice, that there would have been small difficulty in prescribing agents other than acetic acid in cases of expediency. As far as our knowledge extends, all the possible substances which have the power of coagulating latex have been tested. They include mineral acids, organic acids, compounds known chemically under the general term of “salts,” alcohols, sugars, etc.The heading of this chapter must be seen to “beg the question,” inasmuch as it leads to the assumption that a coagulant (in the popular sense) is necessary to secure coagulation. In point of fact, methods are sometimes employed which depend upon no artificial coagulant to produce the desired effect. To these methods reference will be made later.

In this section it is proposed to describe briefly the more important agents which are used, or might be used, in effecting coagulation. In the class of those which are not in common use some could be used as expedients, while others are only of scientific interest.Acetic Acid.—There is no need to enter into a discussion of the merits of this agent. In practice it remains the cheapest and safest coagulant known at present.Formic Acid.—This agent is equally as safe to use as acetic acid, and as easy to handle. Bulk for bulk its coagulative power is higher than that of acetic acid. Its pre-war shipping price, when taken in conjunction with its coagulative power, was slightly below that of acetic acid, but local prices put the balance in favour of the latter. If prevailing costs put it on terms of parity with acetic acid, there would appear to be no reason why formic acid should not have a widely-extended use on plantations.Citric Acid, Tartaric Acid.—The acids of the extracted juices of most tropical fruits consist, to a large degree, of citric or tartaric acids. These can be used in place of acetic acid as satisfactory coagulants in case of emergency; but the questions of availability of supplies and of costs preclude their more general adoption.Oxalic Acid.—This is a satisfactory coagulant as far as observed effect is concerned. It produces a rubber paler than ordinary coagulants (without the use of sodium bisulphite), as it has the nature of an anti-oxidant.

It would not be a safe agent in the hands of coolies, as it is classed as a poison.Sulphuric Acid.—During the War, in a period of shortage of acetic acid and of high prices, this agent was used with success on some estates.

It scarcely need be remarked that it is a dangerous substance to handle, and that its employment must be accompanied by close European supervision.

At prevailing prices during the War it was very much cheaper than acetic acid, and even at the present reduced cost of the latter the advantage still lies with sulphuric acid.

It must be emphasised, however, that the abuse of this agent to any but the slightest degree is harmful to the resultant rubber. Hence its use would be sanctioned only in the absence of the commoner, and much safer, coagulants.

In view of the possible incidence of such an emergency, the following notes are given. It is impressed that strict adherence to the rules must be given.

Handling Sulphuric Acid.—(a) Always use glass or glazed earthenware vessels.

(b) Pour slowly and avoid splashing. Drops finding their way to clothing or other fibrous material will destroy it locally; and if thrown upon any part of the body may cause painful burns.

(c) When diluting this agent always remember to pour the acid into the water (i.e., the lesser into the greater), and never vice versa. Pour the acid carefully and slowly down the side of the vessel, and stir well.(d) Should strong acid be spilled, do not throw water upon it. A supply of sand or dry earth should be kept close at hand. Throw this upon the acid.

Storing Sulphuric Acid.—(a) Jars or cases should be handled as seldom, and as carefully, as possible. If the acid is contained in a case, the top should be plainly indicated.

(b) Stocks should be stored in a detached building which should not be damp. Jars or cases should not stand on a wooden floor if possible.

(c) See (d) above.

Buying Sulphuric Acid.—(a) Commercial acid of specific gravity 1·84 is the best of its kind. It contains impurities which are non-injurious to rubber preparation.

(b) It is always advisable, if possible, to buy the acid in small jars containing not more than 100 lbs. each. Smaller jars, with a content not exceeding 50 lbs., would be preferable.

(c) If the acid is bought in jars, it should be stipulated that the stoppers be covered with a plaster head, and that the containing crate or case should have prominent labels or marks indicating the top of the case.

Formula for Use of Sulphuric Acid.—It will be understood that as this formula has been calculated for working with latex, having a consistency of 11/2 lbs. dry rubber per gallon, it applies in a strict degree only to such latex. In other cases, where the dilution of the latex is not known, the formula will serve as a basis for experiment until the correct quantity has been discovered.

(Sulphuric acid of specific gravity 1·84.)

Note.—The directions must be followed carefully, and glass measuring vessels should be used if procurable.

(a) Measure out 1 pint of strong acid, and pour it carefully and slowly down the inner surface of a jar containing 20 gallons of water. Do not pour it directly into the water.

The heavy acid will sink to the bottom of the jar, and a good mixture must be obtained by stirring well.

(b) Of this solution (which is approximately 1 per cent. by weight), use 1 gallon to 20 gallons of latex.

Readers are doubtless now well aware of the corrosive action of strong sulphuric acid, and we scarcely need point out that even the dilute acid should not be kept in contact with the usual iron vessels found in factories. The mixing of solutions should be done in one of the glazed earthenware jars commonly in use.

The formula given above works out at approximately 1 part strong acid to 2,000 parts of latex (of dry rubber content 11/2 lbs. per gallon). The formula for using acetic acid with the same latex works out at about 1: 1,200. It will be apparent, therefore, that relatively sulphuric acid is a more powerful coagulant than acetic acid. In terms of dry rubber obtained from latex of the consistency indicated above—

1 lb. sulphuric acid will produce 300 lbs. dry rubber. 1 lb. acetic acid will produce 180 lbs. dry rubber.

With both acids selling at the same rate, sulphuric acid would be more economical in use; when its cost is less than that of acetic acid, which is the normal condition, the economic advantage in favour of sulphuric acid is augmented still further.

It may be found that the standard formula for sulphuric acid will not always give a perfectly clear remaining serum, even though an attempt is made daily to work to a uniform consistency for all latices. It is inevitable that the manipulation of the latices should be slightly in error on occasions, or that a small mistake might occur in preparing the solution of acid. Hence a clear remaining serum after coagulation may be secured less often than a slightly turbid serum. This is as it should be. The minimum quantity of acid may be adjusted so closely as to give such results. If a clear serum is obtained always, that should be an indication of continual excess of coagulant. Naturally, if a milky serum is always obtained, the reverse is the case.

As a last word on the subject, it may again be emphasised that the use of sulphuric acid is not advised, except in an emergency; and that the greatest possible care must be exercised in the observance of the strict formula for use.Hydrochloric Acid, Nitric Acid.—These mineral acids would prove more expensive than sulphuric acid. In addition they are much more uncertain in action. For example, the use of a certain excess of hydrochloric acid would not hasten coagulation, but would prevent it. Above all their effect, in excess, is deleterious to the rubber.Hydrofluoric Acid.—This has a strong corrosive action on porcelain or glass. Hence it has to be contained in bottles of gutta-percha or lead. It is mentioned here merely because some years ago it found a use as a coagulant, chiefly in Ceylon. It was sold in the form of a 10 per cent. solution under the name of “Purub,” and was the subject of a patent.

It is effective as a coagulant, and has also an anti-oxidant action, which was its chief recommendation when cheap and harmless anti-oxidants were not commonly known. It is comparatively expensive, and, as indicated above, difficult to handle and store. In short, it has nothing to commend it, in comparison with acetic or formic acids.Alum.—This substance has been used for years by native rubber producers as a coagulant. It fulfils the desired purpose, and its popularity was maintained because of the ease with which it could be stored and handled. Unfortunately, this facility often led to the use of an excess, and native sheets were often criticised as being brittle. Investigations have shown that alum, even in minimum proportions, has an appreciably harmful effect upon the quality of the rubber prepared by its use as a coagulating agent. Its employment by native rubber producers has now been largely superseded by acetic acid in some form.Pyroligneous Acid.—This is otherwise known under the names of “crude acetic acid” and “crude wood vinegar.” Owing to the shortage of acetic acid during the War, attention was directed towards the possibility of making an effective coagulant locally by what is termed the “dry distillation of wood“—i.e., the wood is not burned but heated in a retort. The enquiries could be placed in two classes:

1. Those which aimed at making the pure, strong acid of commerce.

2. Those which sought information concerning a crude coagulant (pyroligneous acid) on estates.

Regarding the first class, we can do no better than reproduce our remarks published in the April local report of the Rubber Growers’ Association for 1916—with the reservation that, on account of a threatened shortage of timber, a local scheme might not now be feasible:

Although it is clear that pure acetic acid is beyond the scope of an estate, crude pyroligneous acid has been produced on a varying scale in this country and in Ceylon. In the latter country some success was obtained by the distillation of coconut shells with comparatively inexpensive plant. In this country, wood-distillation was practised on a few estates, but improved facilities for obtaining pure acetic led to a termination of the experiments, although sufficient crude acid could then be made at a reasonable cost.

The pyroligneous acid obtained, is generally clear, after nitration, and of a dark brown colour. It has a peculiar odour reminiscent of smoked sheet-rubber, or of creosotic substances in general.

Its acid content depends chiefly upon:

Samples received from estates for testing purposes were found to contain equivalents varying from 2 per cent. to 10 per cent. of acetic acid.

They were all suitable coagulants when used in quantity calculated from the discovered acidity, but produced rubber darker than ordinary when air-dried. This effect was not of much importance in the preparation of smoked sheets, but to produce a pale crepe it was necessary to employ sodium bisulphite as an anti-oxidant.

This darkening in colour is to be ascribed to the presence of traces of phenols,[24] which are stated to exert an effect upon the rubber during and after vulcanisation.[25] This subject will be discussed in another section.

With this provision the crude pyroligneous acid which can be produced on estates, could be employed as a coagulant until such time as the price of glacial acetic acid was so low as to make the production of the crude acid non-profitable. This point would be determined from a knowledge of the cost of production per gallon, and the percentage of acetic acid per unit. For example, if the cost of production (including cost of timber for distillation, cost of fuel for heating the retort, cost of labour, etc.) was 60 cents per gallon of crude acid containing 9 per cent. of acetic acid, that would be equivalent approximately to buying glacial acetic acid at $30 per demijohn of 44 lbs.

Smoked Water.—A weak solution of pyroligneous acid may also be obtained by passing smoke through water. With this object in view, a machine was designed by the Federated Engineering Company of Kuala Lumpur. In this the principle of retorting was not employed. Smoke was produced by ordinary combustion in a compartment of the apparatus, and was drawn through water by the action of a high-speed fan worked by hand. A solution, equivalent in effect to a 2 per cent. solution of acetic acid, could be obtained at a comparatively cheaper cost than crude pyroligneous acid produced by dry distillation as it was then being practised. This was chiefly because of the wasteful methods of fuel combustion, in the latter process, in the heating of the retort.Chinese Vinegar.—This agent was found to be a satisfactory coagulant, and, a priori, there is no reason why it should not be suitable, as it is essentially a dilute solution of acetic acid.

The qualities sold were generally colourless, and were probably the result of acetic fermentation of rice.

Samples tested showed a varying content of acetic acid, ranging roughly from 3 per cent. to 8 per cent.; but on this basis of valuation it was found generally that the price bore no relation to the degree of efficiency.

It was advanced not only that the vinegar was an efficient substitute for glacial acetic acid, but that it was also cheaper. This latter claim was proved to have no foundation in fact, even at the high price of acetic acid prevailing during the period of stress. It is not likely, therefore, that vinegar can displace acetic acid, except as an expedient.Sulphurous Acid.—The anti-oxidant effect of sodium bisulphite and sodium sulphite is due to the liberation of the gas, sulphur dioxide. This gas dissolves easily in water, forming an acid solution called sulphurous acid.

This acid solution is an effective coagulant in fairly small quantity. Not only so, but it produces, in addition, the anti-oxidant effect noted in the employment of sodium bisulphite. It is thus possible to produce rubber varying in shade of paleness by means of a single solution.

In the event of sulphurous acid being used, it would be necessary to import cylinders of sulphur dioxide from which the solution could be prepared in factories each day. There would be no insurmountable difficulty in this, as it is only necessary to pass the gas through a series of closed vessels containing water. Enough solution could be prepared at one time for three or four days, but preferably the solutions should be as fresh as possible. Altogether there would seem to be possibilities in the use of sulphurous acid for preparing pale crepe rubbers, providing the cost is within comparable limits with the commoner coagulants at present in use, and that no adverse effect on the rubber can be shown to result. If the cost did not exceed the combined cost of acetic acid and sodium bisulphite, the employment of sulphurous acid solution might be worthy of consideration. There is one drawback to the use of sulphurous acid solution, and that lies in the proximity of the limits of the quantities necessary for coagulation and that which is in excess, and prevents coagulation. Thus, with ordinary field latex having about 20 per cent. dry rubber content, the minimum necessary for coagulation per 100 c.c. of latex is about 8 c.c. of a 1 per cent. solution. The maximum quantity possible for use is about 15 c.c. of a 1 per cent, solution, so that great care would have to be exercised in avoiding an excess of coagulant, otherwise coagulation would be effectually prevented.

It is believed that the preparation of rubber by this method is the subject of a patent secured by Messrs. Boake, Roberts, and Co., London.Sugars.—Coagulation may be effected by the addition of small quantities of sugars.[26] These are assumed to be effective by fermentative conversion into lactic and acetic acids. The presence of lactic acid is supposed to have a twofold effect:

Various Salts.—Of experimental interest only it may be recorded that coagulation has been effected by means of various chemical “salts“—e.g., calcium chloride, barium chloride, magnesium chloride, sodium chloride, aluminium sulphate, magnesium sulphate, sodium sulphate, etc. None of these has been found to have any practical application, except, perhaps, calcium chloride, which is used in small quantity as an accelerating agent in a special process of anaerobic coagulation, which will receive mention in the following chapter.

At one period during the War and the dearth of acetic acid, it was found that there were available in England large supplies of the acid sulphate of sodium (sodium hydrogen sulphate), which proved to be an effective coagulant. Experimental work gave satisfactory results, but no practical application resulted when supplies of acetic acid were again obtainable.Various Proprietary Compounds.—We have seen many proprietary coagulants advertised and pass into the limbo of forgotten things. They can generally be divided into two classes. The first embraces those founded upon a woefully incomplete knowledge of requirements. The second covers those which meet requirements, but for which exaggerated claims are made and excessive prices charged.

As as instance of a substance which fell under both classifications might be mentioned the case of “Coagulatex.” Pretentious claims were made, and it was emphasised that the liquid contained no vegetable acids. Acetic and formic acids might be quoted as examples of vegetable acids, and as these have been shown to be the most satisfactory coagulants now employed one fails to imagine where lay the value of the guarantee given by the advertisers of “Coagulatex.”

On analysis the liquid was found to consist mainly of sulphuric acid, against the indiscriminate use of which warnings have been given. Thus it was a dangerous substance for common use.

Furthermore, comparing the value with its sulphuric acid content, it was found that the price required for “Coagulatex” was roughly four times the contemporary cost of commercial sulphuric acid in the Federated Malay States.

Those in charge of estates should realise, therefore, that no proprietary coagulants should be adopted until a proper report of tests, and a comparative valuation, has been obtained from one of the research laboratories.Carbonic Acid Gas, Carbon Dioxide.—Now of only scientific interest, it may be noted that some years ago great claims were made for the use of carbon dioxide gas as a coagulant. In actual practice we were unable to effect coagulation by passing the dry gas into latex. It was suggested that the original investigators were misled by failure to secure a dry and clean gas. It would appear that probably the gas was prepared by the action of hydrochloric acid upon marble or limestone. Unless intervening “washers” and “driers” were used, the liberated gas, when passed into latex, would carry with it traces of hydrochloric acid, which would effect coagulation.Alcohol.—In the cheap form of methylated spirit, alcohol has been employed by us as a speedy coagulant for many years. Latex run slowly into alcohol coagulates instantaneously. The method has been in common laboratory use.

The employment of alcohol has also been made the part-subject of a patent process of coagulation, to which reference will be made in the succeeding chapter.Vegetable Extracts.—At various times experimental work has been directed towards the use of liquids of purely vegetable origin, such as the juices of tropical fruits, and of a waste product of tropical industry—the so-called “milk” (or water) of ripe coconuts.

In the former class there is usually a natural acidity, but in coconut water the acidity is chiefly the result of fermentation of the carbohydrate (sugar) constituents.

These substances were all found to effect a more or less satisfactory coagulation, but it is unlikely that they would be suitable for practical application on a large scale.

As being more directly related to the subject of coagulation in general than to coagulants in particular, a discussion of several special processes will be relegated to the ensuing chapter.