The Preparation of Plantation Rubber Part 32

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 const.i.tuents by evaporation due to heat. a.s.suming (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 2-1/2 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 r.e.t.a.r.d 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 a.n.a.logous. 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:

(_a_) To all appearances it may be quite fluid, but a close examination shows it to consist mainly of a serum containing very minute particles of rubber in suspension (microscopic coagulation).

(_b_) In a later stage these particles coalesce to form larger "flocks" (macroscopic coagulation).

(_c_) The whole, or practically the whole, of the latex may have coagulated, forming one ma.s.s of rubber with a milky residual serum.

Pa.s.sing from this aspect of the question, it may be noted as peculiar facts that:

(1) A shallow layer of latex is less likely to coagulate spontaneously (_i.e._, without the addition of a coagulant) than a deeper volume.

(2) The shallow layer, and also the surface of the deeper volume (where exposed to air), on standing will be found to develop a superficial film of finely coagulated particles, yellowish in colour, and having an offensive odour due to decomposition of protein matter.

(3) While this partial coagulation is confined only to the surface of a shallow layer of latex, it will be found that below the surface film of the deeper volume a much more definite coagulation has taken place.

The coagulation will be practically complete, and the coagulum, apart from a spongy appearance, is normal in character. This coagulum is free from the offensive odour noted above.

(4) On testing the surface film of both the shallow layer and the deeper volume, it will be found to be _alkaline_ in character; whilst the lower liquid surrounding the main portion of the coagulum in the deeper volume of latex is of an _acid_ nature.

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:

(_a_) _In contact with air (aerobic)_: incomplete spontaneous coagulation, accompanied by yellowish slime, offensive in odour and alkaline in character.

(_b_) _Out of contact with air (Anaerobic)_: Practically or wholly complete. There is no offensive odour under normal conditions and the serum is acid in character.

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.

[28] "Preparation and Vulcanisation of Plantation Rubber" (Eaton, Grantham, and Day), Bulletin No. 27, F.M.S. Department of Agriculture, 1918; "De la Coagulation naturelle du Latex d'Hevea Brasiliensis" (Denier and Vernet), _Comptes Rendus l'Academie des Sciences_, No. 3, July, 1917.

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 f.l.a.n.g.es 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 quant.i.ties 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 const.i.tuents 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.

[29] Gorter and Swart, Bulletin No. 6, West Java Station.

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 quant.i.ties of sugars--_e.g._, 1 gramme per litre of latex (1:1,000).

It is to be noted also that the addition of small quant.i.ties 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 05 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.

[30] "Archief voor de Rubbercultuur," Nederlands Indies, 1920, 4, 273.

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 quant.i.ty 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:

1. The production of a uniform standard of rubber.

2. The obtainment from a unit volume of latex of a greater weight of rubber than can be obtained from an equal volume of the same latex by ordinary coagulation with acetic acid. It is to be inferred that the agents employed have the power of adding to the coagulum some of the substances which usually remain in solution in the clear serum.

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.

[31] "Preparation and Vulcanisation of Plantation Rubber" (Eaton, Grantham, and Day), Bulletin No. 27, F.M.S. Department of Agriculture, 1918.

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:

[32] _Ibid._