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 quant.i.ty. 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 pa.s.s 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 quant.i.ties 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 quant.i.ty 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 quant.i.ties of sugars.[26] These are a.s.sumed to be effective by fermentative conversion into lactic and acetic acids. The presence of lactic acid is supposed to have a twofold effect:

[26] "Preparation and Vulcanisation of Plantation Para Rubber" (Eaton, Grantham, and Day), Bulletin No. 27, F.M.S. Department of Agriculture; Gorter and Swart, Bulletin No. 6, West Java Expt. Station.

(_a_) As a direct coagulant.

(_b_) In its action upon certain organisms which, in the ordinary course of events, would delay or prevent coagulation. Although work on an experimental scale has been done, as far as we know no practical application has been made of the employment of sugars as coagulating agents.

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 quant.i.ty 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 pa.s.s into the limbo of forgotten things. They can generally be divided into two cla.s.ses. 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 cla.s.sifications 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 a.n.a.lysis 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 pa.s.sing 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 pa.s.sed 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 cla.s.s there is usually a natural acidity, but in coconut water the acidity is chiefly the result of fermentation of the carbohydrate (sugar) const.i.tuents.

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.

CHAPTER XX

_SPECIAL METHODS OF PREPARATION_

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 cla.s.sed 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 a.s.sumed that the princ.i.p.al 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 ma.s.s, 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.

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

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 ma.s.s 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.