Every year appears to bring forth some new ideas in the mode of rubber preparation. Some of them are based in principle upon the oldest known method—i.e., the native Brazilian process of making “Hard Para.” Others strike a new note, and in a few cases the claims put forward are substantially confirmed by results. In other instances the claims are too pretentious, and discredit may be brought upon schemes which, although lacking in comparative success, are yet commendable for the ingenuity manifested.

To the present not one of these new methods has been able to compete to any marked degree in general practice with the established methods of ordinary preparation. A few continue to find local application, but most have either been abandoned or are gradually falling into desuetude.

We do not propose to discuss in fine detail all the various claims made on behalf of these special processes, or to enter into controversies. The aim is to present to the reader an outline embodying the main principles and advantages claimed.Da Costa Process.—Briefly, this was a method by which coagulation was effected with smoke. The smoke was generated by the combustion of wood in a special compartment, and was forced into latex by means of a jet of steam. It was really only applied to the preparation of coagulum intended for crepe form. The exact degree of coagulation effected was uncertain, and the final colour of the rubber precluded it from being classed as a modern No. 1 product.“Byrne Curing” Process.—This is a process for treating coagulum obtained by ordinary methods.It was the subject of a patent obtained by Messrs. E. J. and F. A. Byrne, and at one time had a considerable vogue on estates. The chief claim advanced was that the rubber produced was in all respects equal to Fine Hard Para, and could be shipped while still moist without detriment to the physical qualities.

The principle of the process was the treatment of coagulum, in either sheet or thick crepe form, with vapours produced by the volatilisation of two special fluids. This treatment was undertaken in comparatively small wooden sheds, in which the coagulum was placed. The “smoke” was conducted into the curing sheds from furnaces outside the building. The sheds were covered externally with “felt” material to prevent leakage of the vapours, and a very dense smoke was obtained.

The furnaces were specially designed, and consisted essentially of a “hot-plate” heated by a powerful kerosene blast-flame. On top of the machine were two reservoirs controlled by taps. In these were placed the special fluids which were released in definite proportion. The composition of the fluids was not divulged, but it is assumed that the principal ingredients were (a) wood tar products, (b) crude pyroligneous or acetic acid. The mixture of these, dropping on the hot plate at the correct temperature, spontaneously volatilised, to form dense whitish fumes, having an intense and not disagreeable odour of wood combustion. A duct led from the back of the machine into the curing-shed, where the vapours were distributed through perforations in the pipe.

The coagulum usually remained under treatment in the shed for three to four hours, and then was removed for ordinary air-drying. When taken from the curing-shed it had a pinkish colour, which later developed into a dark brown by a natural process of oxidation. The exterior of the rubber, on shipment, resembled the appearance of smoked sheets; while the interior, on cutting, was seen to be still white. As packed for shipping, the rubber contained from 10 to 15 per cent. of original moisture, for the usual sheet form, and even more when “slab” rubber was prepared.Originally either crepe or sheet rubber was made, but later the preparation of the crepe form was displaced largely by “slab” rubber. These “slabs” were really very thick sheets, which had been subject to only slight pressure.

Still later the preparation of the “slab” form was displaced by “loaf” rubber. This form was built up by winding together ordinary thin sheets which had been subject to the “cure.” Only slight tension was needed, during the operation of winding, to cause close adhesion of the component wet layers, and the final result was a “loaf” or roll dark in colour, and apparently dry when examined superficially. On being cut, even after an interval of months, the middle portion was still so moist as to be quite white.

In course of time it was discovered that all the claims made for the process could not be substantiated, and for various reasons (which need not be detailed) most of the estates which had adopted the scheme reverted to ordinary methods of preparation. At the time of writing few, if any, continue to work the process. It appears to be agreed, as the result of investigations, that in no degree does the process yield advantage over ordinary methods.Freezing Process.—A patent was secured a few years ago to cover a process whereby coagulation was effected by refrigeration.

Latex remained for several hours in the refrigerating chambers of an ordinary ice-making plant. The resulting solid mass, on being thawed, yielded a coagulum appearing in no way to differ from that obtained by ordinary methods of coagulation.

Provided the process exerted no influence for good or evil upon the quality of the resulting dry rubber, the value of it would appear to depend upon the relative cost of working, plus considerations of capital expenditure and depreciation on the plant. At the present time it would be difficult to imagine that the cost of preparation alone would compare favourably with that sustained by ordinary coagulative methods.

Furthermore, beyond the expensive refrigerating plant, the usual machinery of a factory would still be required if the ordinary market demands are to be met.Finally, it has not been found[27] that any advantage in the final physical qualities of the rubber is obtained by the employment of this process.

Wickham Process.—This process, invented by Sir Henry Wickham, aimed at the production of a rubber resembling Fine Hard Para. The principle employed was that underlying the preparation of the best rubber in Brazil—viz., coagulation of superimposed thin layers of latex by the action of smoke and heat.

In essential the machine employed consisted of a rotating drum into which latex and smoke entered. The result was the formation of thin “skins” of rubber which, coagulating in situ, formed a mass corresponding to “Fine Hard.”

That the rubber was fully satisfactory as to quality is acknowledged, but economically and in practical utility the process was unsuccessful, the rate of output being so low.Derry Process.—The invention of Mr. R. Derry, late of the Singapore Botanic Gardens, this in principle resembled the Wickham and other processes. It aimed at a mechanical imitation of the native method of producing Fine Hard Para.

In place of the rotating drum, an endless belt was used. This travelled over pulleys, more or less horizontally placed. The upper of these could be raised to varying height above the level of the other, and likewise could be so adjusted as to tighten the belt.

The under layer of the belt impinged, in its travel, upon the surface of a layer of latex contained in a shallow tray. The belt was operated by hand-power, and the height of the latex trays was adjustable.

The trays of latex were situated at the lower end of the machine which lay outside the smoking-chamber. It will be understood that the vastly major part of the total length of belt was always within the chamber.

Smoke was generated by combustion of wood in an external structure, was brought into the chamber by a wide duct, and was then distributed below the belt by means of perforated pipes.

The thin film of latex picked up by the belt was coagulated partly by the action of smoke constituents by evaporation due to heat. Assuming (1) that the belt was of adequate length, (2) that the rate of travel was not excessive, (3) that the latex was not too dilute, (4) that the temperature of the smoke was sufficiently high, (5) that the smoke was sufficiently dense and not too damp—then the process should be a continuous one.

It will be clear that success could only be obtained by a careful adjustment of all these factors. The latex must, necessarily, be of a fairly rich consistency (at least 21/2 lbs. dry rubber per gallon), but unfortunately there is considerable difficulty in maintaining such latex in a state of fluidity for the period demanded by this process, without loss of latex. Naturally, the addition of an anti-coagulant would retard the rate of output of the machine to a marked degree.

The layer of rubber thus formed on the belt was stripped off, and hung for further air-drying, as it still contained a fair percentage of moisture.

As a really practicable method for treating plantation latex, the process failed by reason of its low rate of output over a given interval. This alone was sufficient to condemn it, apart from the facts (1) that it was not shown to be a cheaper method than coagulation by acetic acid, (2) that the resulting rubber was not proved to be of superior intrinsic value to rubber prepared by ordinary methods.Spontaneous Coagulation.—All readers will be aware of the phenomenon of the curdling or souring of milk. The behaviour of Hevea latex, under certain conditions, may be taken to be analogous. Difficulty is experienced in maintaining fluidity—a difficulty which appears to vary in great degree according to locality, nature of soil, age of trees, the relative demand made upon the trees by the system of tapping employed, etc.It is sometimes found, before the latex reaches the store, that it may exhibit one of various stages of premature (spontaneous) coagulation:

Passing from this aspect of the question, it may be noted as peculiar facts that:

These observed facts are sufficient to indicate that there are apparently two distinct types of spontaneous coagulation, and that the latter takes place particularly where the latex is more or less out of contact with the atmosphere. We may, therefore, differentiate thus:

It is concluded[28] that there are present in latex, on collection in the field, two types of organisms. Those which work in contact with air (aerobic) show a tendency to prevent coagulation and to form an alkaline yellow slime on the surface of the latex. The others, which work in the absence of air (anaerobic), may, under favourable conditions, cause complete coagulation unaccompanied by any decomposition or offensive odour within a normal period. If air is rigidly excluded, the coagulum obtained is quite satisfactory for all purposes.

This type of coagulation, without the employment of a chemical coagulant, and under anaerobic conditions, was the subject of a patent granted in 1914 to Messrs. Maude, Crosse and others. The process has been in use on Cicely Estate (Perak) for some years. With subsequent slight modifications the apparatus consisted in essential of a tank with a loose cover. The flanges of the cover were sufficiently long to dip into a water-seal surrounding the tank. Thus the cover may rise and fall without an inrush of air.Coagulation, in fact, can be effected thus in any kind of air-tight receptacle; and experimentally the reader can obtain a satisfactory result by filling completely with latex the bottle which has a loose stopper.

Under the patent held the coagulum may be prepared either for crepe-making, or for sheets by a modification of the tank.

The crepe when dry does not have the bright appearance of the ordinary “Fine Pale” standard prepared with the aid of the anti-oxidant sodium bisulphite.

Unfortunately the addition of this substance to the latex in normal proportions is not possible under anaerobic conditions, as it is found to prevent coagulation, probably owing to its sterilising effect upon the anaerobic organisms.

To prevent the oxidation of the rubber in actual practice, the freshly prepared crepe is soaked in a solution of sodium bisulphite before hanging to dry. The resulting colour of the rubber is quite good.

It was shown by Eaton and Grantham that anaerobic coagulation is slightly uncertain in action. Owing probably to variations in the composition of the latices, or to the extent of infection by organisms, coagulation may one day be complete and on other days less satisfactory.They found further that, by the addition of small quantities of sugars, coagulation under both aerobic and anaerobic conditions was improved. The conclusion formed was that the addition of sugars created a medium favourable to the development of anaerobic organisms and unfavourable to those which cause decomposition of the natural nitrogenous constituents of latex.

This work was confirmed by Gorter and Swart,[29] who attributed the action to the conversion of sugar to lactic, acetic, and succinic acids by fermentation.

Denier and Vernet, whose work has already been mentioned, studied the presence of the organisms in latex, and succeeded in isolating one which, under anaerobic conditions, effects coagulation within twenty-four hours. Sometimes to produce complete coagulation it was found necessary to employ small quantities of sugars—e.g., 1 gramme per litre of latex (1:1,000).It is to be noted also that the addition of small quantities of soluble calcium (lime) salts to latex has much the same effect as the employment of sugars. Recent investigations[30] showed that the addition of 0·5 to 1 gramme of calcium chloride per litre of latex caused complete coagulation in closed vessels within twenty-four hours, a result agreeing with the findings of Barrowcliff.

On page 308 of the same publication, experiments on the effect of sugars are described, in connection with aerobic coagulation. Observations from a further set of experiments tended to indicate a direct connection between the effects of tapping and spontaneous coagulation. It is suggested that heavy tapping causes a diminution in the latex of those substances which act in some way as accelerating agents in coagulation—e.g., sugars. The smaller the proportion of these substances, the slower and less complete is natural (spontaneous) coagulation.Ilcken-Down Process.—This process is the subject of patents granted in 1915 to Messrs. Ilcken and Down. It has been in fair prominence, and has been tried experimentally on several estates and in public demonstration.

It is a coagulating process, and, in the original specification, employed as agents a mixture of alcohol (in the form of methylated spirit) and benzene (petrol), or alcohol with petrol and coal-tar naphtha. The mixture was injected in the form of a fine spray into the latex, contained in a tank specially fitted with paddles.

Later modifications covered the addition of a small quantity of glycerine; or, failing supplies of that substance, coconut oil.

Many advantages are claimed for the process, but most of them cannot be substantiated. The two chief claims are:

Regarding the first of these claims, it has been shown[31] that the rubber is not uniform in its behaviour on vulcanisation, and that its variability is similar to that of rubber prepared by other processes.

The second claim has been the subject of much controversy. Experiments made on estates under the supervision of, or in the absence of, the patentees have given conflicting results. When varying factors have been eliminated, the general conclusion was that no increase in weight of rubber was obtained.Private laboratory investigations led to a similar verdict, and Eaton[32] records a confirmatory finding. More recently the claims made for the process were investigated in Java[33] under varying conditions. Three series of experiments were made:

The agents used were (a) a mixture of alcohol and fusel oil, (b) alcohol and petrol (benzene).

In these experiments no advantage in weight of rubber was obtained by the Ilcken-Down process, and it would thus appear that the principal claim fails to be substantiated.

The general composition of the rubber was approximately the same as ordinary crepe obtained from undiluted latex. The rubber on vulcanisation was found to be normal in behaviour, and was similar to the controls.

The coagulum ordinarily is affected by oxidation, and does not produce a fine pale crepe. To remedy this defect the freshly prepared crepe is soaked in a solution of sodium bisulphite and sulphuric acid.

It may be noted that in the recent experiments coagulation was effected in vacuum in a specially designed wooden tank. From a study of the previous section on “Spontaneous Coagulation,” the reader will perceive that results equal to those obtained by the Ilcken-Down process can be obtained without the necessity of using such agents as alcohol, petrol, or fusel oil.Slab Rubber.—This type of preparation has been the subject of much discussion of recent years. There is nothing really special in the mode of preparation, and in its original form “slab” rubber is only a thick sheet which may be obtained by coagulation with acetic acid or other agents.

The coagulum, when removed from the serum, is subjected to comparatively slight pressure, and the “slab” thus made is either placed to air-dry at once, or may be subject to treatment in other liquids before drying.

The rubber is not allowed to remain until wholly dry, but is shipped while still containing an appreciable percentage of enclosed moisture.

It is claimed[34] that the production of “slab” rubber by standardised methods eliminates to a great degree the variability which at present characterises plantation rubber, and that a fast-curing medium is obtained. These claims will be discussed in later chapters dealing with the vulcanisation of rubber, and demand no notice in this section.

From the producers’ point of view, it may be noted that the preparation of slab rubber is a simple process, but not altogether as pleasant probably as might be desired, when undertaken in crude form.

The appearance of the partially dry slabs is unattractive, but that does not signify if the quality of the vulcanised product satisfies requirements.

For the average producer, the difficulty lies in having to meet the demands of the general market. Even, therefore, if one assumes that the intrinsic qualities of slab rubber are all that the claims advance, it would be necessary for the producer to be assured of definite and regular sales.

At present it would probably be fair to state that practically all the “slab” rubber being prepared is produced by those who are also consumers. They are thus in the enviable position of being able to satisfy their requirements as to the mode of preparation. Until such time, therefore, as there exists a regular demand for “slab” rubber in the general market, the vast majority of estates must proceed on ordinary lines of preparation.