The Preparation of Plantation Rubber :: CHAPTER VIII COAGULATION

CHAPTER VIII COAGULATION

Whether it is necessary to employ any coagulant, or whether latex should be allowed to coagulate naturally, will not be discussed at this stage. Neither will mention be made of any patent processes of coagulation which employ other than acid mediums. These subjects will be treated in a subsequent section of the book.Choice of Coagulants.—It is not proposed here to enter into a discussion as to the merits of the dozens of known coagulants. Suffice it to state that acetic acid, although the oldest general coagulant, still remains the best and safest at the present time. There is a deal to be said in favour of the use of another organic acid, formic acid. It is equally as safe as acetic acid, and quite efficacious; the only drawback is that, taking all things into consideration, it is very slightly more expensive. Acetic acid, therefore, will always be implied in this chapter when the word “acid” is used.Strength of Acid Solution.—In the old days it was the rule rather than the exception to find pure, undiluted acid used in coagulation. In many cases no harm resulted, for the simple reason that, owing to the large proportion of water in the latex, the acid was thereby very much diluted. The estates had to thank the over-dilution of the latex for the non-injury of the resulting rubber.

Some estates make up a stock solution of 1 part acid to 20 of water, and use this with success because of the fair amount of added water present in the latex.

It must be understood that what is being referred to now is not the absolute quantity necessary for coagulation, but the proportions—i.e., the respective volumes of acid and water in the solution of acid made up every day. That the strength of the acid solution, as well as the quantity used, has an effect upon coagulation can be easily demonstrated in the following way:

Take separate and equal lots of the same latex, and to each add the same quantity of pure acid, but in each case diluted with varying quantities of water. It will be found that coagulation is quickest where pure acid is employed, and slowest where the acid is most dilute. It will also be found that, providing the quantity of acid employed was sufficient for coagulation, the best and most uniform coagulation is obtained from the use of the most dilute acid, within limits. It will often be found that where pure acid has been employed coagulation is local—i.e., we have lumpy coagulation, and often a very milky remaining liquor. This is due to the fact that, as coagulation is immediate upon the spot which is first touched by the pure acid, a deal of the acid is enclosed within the rubber at that spot, and hence other portions of the latex are deprived of acid. It is in such cases that most air-bubbles are enclosed.

As the dilution of the acid solution is increased the mixing is more thorough and uniform. Coagulation is slower, and air-bubbles can escape to the surface.Method of Making Stock Solution.—Experiments have been repeatedly made in the laboratory with acid solutions of varying dilution, from pure acid down to 1 part of acid in 500 parts of water. While it has been found that a 1 in 5 solution can be used where the latex is very dilute (say, 1 part of latex to 5 parts of water), and a 1 in 20 solution may be used in fairly dilute latex (for crepe-making), it is undoubtedly a fact that for latex as generally “standardised” on estates a much more dilute solution of acid should be used—e.g., 1 in 100, or even 1 in 200, of water. It must be borne in mind that the quantity of acid necessary for coagulation is not changed, but merely the dilution. Let us take a concrete case to illustrate the point:

On an estate at present the stock solution is made up by diluting 1 pint of acid with 20 pints of water, and 1 gallon of this is necessary to coagulate 50 gallons of pure latex.It is desired to use a stock solution of 1 pint of acid to 100 pints of water. Evidently, therefore, 5 gallons of this stock solution contain only the same quantity of pure acid as 1 gallon of the old solution contained, and it will be necessary to add 5 gallons for every 50 gallons of pure latex. Thus:

1 to 20; 1 gallon necessary for 50 gallons pure latex.
1 to 100; 5 gallons necessary for 50 gallons pure latex.

It may be pointed out that the quantities worked out in the foregoing examples are not absolutely and mathematically correct, but they are quite close enough for all practical purposes.

It may be advanced by someone that if a dilute solution of acid, such as 1 in 100, is used the bulk of this stock solution (5 gallons to 50 gallons of latex) is very great, and might be injurious to the quality of the resulting rubber. A moment’s consideration will show that, after all, the volume of acid solution is only one-tenth that of the volume of latex. This can have no effect upon the quality of the rubber. Even dilution of the pure latex with half its bulk of water in the factory will have no effect upon the quality of the resulting rubber. It is to be remembered that, except in cases where the proportion of added water to latex is absurdly large, the main argument against putting water into the latex-cups is against the possible poor quality of the water rather than against the actual small quantity theoretically added. It is acknowledged that, where the water to be put into the cups can be guaranteed to be of good quality, no great objection would be raised against placing the smallest possible quantity of such water in the cups. But how many estates have such good water easily available to the coolies, and how many estates can be sure that only that smallest possible quantity would be used? It is a notorious fact that, even on estates where the quantity of water used was supposed to be a minimum, the proportion of water to latex in some cups often exceeded even three or four to one. In any case it may be stated as an elementary truism that the absence of water is more to be desired than water of doubtful quality.Quantity of Acid.—As a result of repeated experimental work it has been found that, for pure average latex, the quantity of acid necessary for complete coagulation, reckoned in parts of pure acid to parts of latex, is:

1 part pure acid; 1,000 parts average latex.

Where the latex is rather richer than average (above 30 per cent. dry rubber) probably a little more acid would be required, and similarly if the dry rubber content is lower the quantity of acid must be less.

It used to be a common belief that the more dilute the latex the greater the quantity of acid necessary, but this would only apply to cases of extreme dilution of latex.

As a matter of fact, up to certain limits of added water, the reverse is actually the case—i.e., the more water in the latex the less acid must be added, assuming that for pure latex the proportion of pure acid to latex is taken as 1 part to 1,000 parts. This was found to be the case up to dilutions of three or four times the volume of latex. To take concrete examples which will perhaps make the truth more clear:

Assuming we commence by making up our stock solution of acid by adding 100 parts of water to 1 part of pure acid, this gives us a mixture of 1 to 100. For 1 gallon of pure latex it would be necessary to add one-tenth of its volume of the above mixture—i.e., 16 ozs.

Suppose we take a gallon of pure latex and add a gallon of water, we now have 2 gallons of so-called latex. But we still have only 1 gallon of real latex present in the diluted latex, and it is only necessary to add sufficient acid to coagulate this gallon—i.e., 16 ozs.

Further, if 1 gallon of latex be diluted with 2, 3, or even 4 gallons of water it is still only necessary to add 16 ozs. of the acid mixture.

At dilutions beyond this limit, however, it is necessary to add a little more acid to obtain complete coagulation.

In the process of preparing sheet rubber it is very necessary to see that the minimum quantity of acid is used, otherwise visible defects are caused. But in coagulating latex intended for preparing crepe, where the rubber undergoes protracted washing on the machines, the presence of a slight excess of acid in coagulation is not calculated to cause any deterioration in the quality of the rubber. Advantage must not be taken of this statement to argue that more than a slight excess may be used without injury to the rubber, as it can be shown that the use of a large excess of acid results in an inferior rubber.Quantities Necessary for Modern Requirements.—It may be commended to the notice of the beginner that any further experimental work as to the quantity of acetic acid necessary for complete coagulation would only involve a waste of time and energy.

The general figure given in a preceding paragraph (1 part pure acid to 1,000 parts of latex) may be accepted as the rough basis for working. In modern practice, however, undiluted latex is usually diluted to a standard which may vary on different estates from 11/4 lbs. to 11/2 lbs. dry rubber per gallon.

Latices of these strengths can be coagulated at a ratio of 1 part pure acid to 1,200 parts of standardised latex; and this quantity need not be exceeded, except in cases where an appreciable amount of some anti-coagulant is present in the latex. The proportion may then be raised to 1 in 1,000.

If considered advisable the acid may be used in a 1/2 per cent. solution for sheet preparation; but in any case it is advised for the sake of uniformity that a 1 per cent. solution should be employed in the preparation of both sheet rubber and crepe rubber. In most modern factories, measuring vessels of various capacities are to be found, and it is always more satisfactory to have the solution made up in approximately correct strength at the rate of 1 oz. of pure acid to 5 pints of water. Often, however, on some estates European supervision of this work is not possible, and the preparation of the acid solution has to be left in the hands of a (more or less) skilled coolie. It is thus necessary to find some less fine, but still approximately correct, method of procedure. In the East the kerosene tin is in universal favour for the carriage of water, and there is no reason why it should not be utilised as a standard measure for preparing the dilute acid solution, providing it is not allowed to become rusty. The capacity of the tin is 4 gallons (640 fluid ozs.), so that a one-hundredth part would be approximately 61/2 ozs. It is suggested that this quantity should be measured out by means of a glass graduated vessel, and then that an aluminium cup should be cut down so as to hold the exact quantity.

This would reduce the making of a solution, sufficiently approximate to 1 per cent. strength for all practical purposes, into a simple operation of mixing pure acid and water in the ratio of one cupful of acid to 1 kerosene tin of water.

The actual quantity of solution required for the coagulation of any volume of standardised latex can be calculated easily from the ratio 1:1,200. As the strength of solution is 1:100 it will be seen that the quantity to be taken is always one-twelfth that of the volume of latex—e.g.:

(a) If the latex tank holds 90 gallons of standardised latex, 71/2 gallons of dilute acid solution are required.

(b) A tank containing 120 gallons of latex would need 10 gallons of the 1 per cent. acid solution.

It is assumed that all estates, not only in the preparation of sheet rubber, but also in the making of crepe rubber, always employ the system of standardising latex in order to obtain uniformity. They are ill-advised if they do not follow this practice; but in case average undiluted latex is treated in coagulation, the quantity of acetic acid to be used should be calculated from the ratio 1:1,000.

If the acid solution is to be employed in 1 per cent. strength, one-tenth of the volume of latex to be treated will indicate the required quantity of solution necessary for complete coagulation unless anti-coagulants have been used, when the quantity must be increased as experience directs. It will be recognised, of course, that undiluted latex may only be used in any case for the preparation of crepe rubber; or in some exceptional case, such as the special preparation of “slab” rubber.Care in Mixing.—It is essential that the mixture of dilute acid and latex should be thoroughly intimate. This can only be attained by careful manipulation, especially in the case of sheet preparation. Where crepe rubber is to be made it may be permissible to employ a solution stronger than 1 per cent., but it is not advised. The acid should be poured into the latex while stirring, and the agitation should continue for such a period as to ensure thorough mixing in all parts.It will be appreciated that in the preparation of sheet rubber this period may not be unduly prolonged, otherwise the latex will have begun to coagulate before skimming and the placing of the partitions in their respective slots can be effected. Furthermore, while in the preliminary treatment for crepe rubber, the formation of enclosed bubbles and surface froth is immaterial. For sheet preparation it is essential that the stirring shall be done so carefully as to try to avoid internal bubbles and to reduce surface froth to a minimum. For crepe-making a perforated board, with handle attached at right angles to the face of the board, may be used; but in shallow sheet-coagulating tanks, broad paddles (which may or may not be perforated) give good results as long as there is a sufficient number used to cover the area of the tank in reasonable time. Obviously also, where the area of any tank or compartment is of any appreciable size, the dilute acid solution should be poured in from various points so as to obtain a good even distribution. In some cases the acid is distributed from a sprinkling can, but this is a refinement which experience shows to be unnecessary. In actual practice, working on a tank measuring 12 ft. by 4 ft., no difficulty is found if coolies pour in acid solution from four points. The degree of success depends entirely upon experience and efficient supervision. This remark applies equally to the use of various devices, such as rakes with broad teeth, used as stirring implements. There is room for display of ingenuity in this direction, and it is found often that, while they are used successfully on one estate, they may be condemned on another.

Two Views of Dilution and Mixing Tanks.

Below, on the right, coagulating tanks. At the far end strainers. Each dilution tank is of equal capacity to the corresponding coagulating tank.

Use of Sodium Bisulphite.—Some few years ago a demand for pale crepe rubbers sprang up, and this demand has been maintained. The total quantity of pale rubber put on the market previously could only have amounted to very little, and that little was obtained by luck and various tricks in manipulation. It must be premised that if coagulation is allowed to take place, either naturally or with the aid of acetic acid, the resulting rubber will almost inevitably oxidise on the surface, except in the cases of very dilute or young latices. Even supposing that this darkening of the surface does not take place in the wet stage, it is often found that a rubber expected to dry to a pale colour does not fulfil expectations, and a dull neutral shade results. This darkening of crepe rubber may be attributed to a slow process of oxidation, which continues until the rubber is dry. From these remarks it will be seen that the process of oxidation is a natural one, and that any pale rubber formerly shipped was the outcome of circumstances outside the control of the estate, except in such cases where boiling of the coagulum, etc., was resorted to. The fact that one rubber happened to be a shade darker than another was absolutely no criterion as to the value of the rubber, but apparently the market thought, and still thinks, otherwise, although the actual necessities of manufacturers for a pale crepe to be employed in special processes must be comparatively small.

The prevention of this natural oxidation was a problem which exercised the minds of all responsible for the preparation of pale rubbers, and much time and thought were expended upon it. Various theories were propounded, and the chief conclusion arrived at was that the darkening of rubber was to be prevented by excluding all the light possible from the drying houses. To this end windows were to be kept shut, or else they were provided with ruby-coloured glass, which incidentally kept out the air. In spite of these precautions, little success attended the expenditure of so much energy and thought. It was absolutely necessary that some chemical agent should be discovered which would make the preparation of pale crepe possible for any estate. This chemical would have to fulfil several requirements before it could become popular:

1. It must be a simple substance capable of being easily handled.

2. It must be very soluble, so that solutions could easily be made up by inexpert workers.

3. It must be cheap.

4. It must be quite innocent of any harmful effect upon the quality of the rubber.

After months of investigation into the properties of other chemicals the writers decided that the only one which satisfactorily answered all requirements was sodium bisulphite. The writers make no pretension to any claim of having discovered the properties of this substance, which was a common chemical, and widely known. Even its action on latex was suspected before they engaged upon the work. These matters are only mentioned because the credit, if any, should be given to the laboratories of the Rubber Growers’ Association.

As soon as it began to be known on the market that sodium bisulphite was being used in the preparation of pale crepe, a great outcry was made, and estates were warned that no more rubber prepared in this way would be accepted. It was said that the chemical would destroy the “nerve” of the rubber,[2] and it was definitely stated that rubber prepared with this chemical was brittle. It must be remembered that brokers had some legitimate excuse in raising objections to the introduction of new and strange chemicals for preparing rubber, as they were quite without means of judging whether the rubber had suffered harm or not. Still, on the other hand, private tests had been made in conjunction with Messrs. Beadle and Stevens for fully eight months before the name of the chemical was mentioned in reports, and they had decided from the results of vulcanisation tests that the chemical was quite innocuous. Then, and only then, did we consider it advisable to recommend the use of sodium bisulphite in general estate practice. Owing to the initial prejudice against rubber prepared with sodium bisulphite, the results of our preliminary work were published by permission of the Rubber Growers’ Association.[3] The original instructions to estates regarding the proper employment of this chemical were given in the private reports issued by the Rubber Growers’ Association in 1911. At the present time it is probably accurate to state that it is now used by all estates preparing fine crepes. Representatives of manufacturers have sometimes given us to understand that the question of paleness of colour in such rubber is of no such importance as is impressed upon us as producers. While we are prepared to believe, we can only plead that from our point of view the supply arises from the demand. Such are the conditions governing the sale of rubber that, irrespective of the requirements of the ultimate user, we have to market rubber which is valued almost completely upon its appearance at the time of sale.

[2] Williams, International Rubber and Allied Congress, London, 1914.

[3] “The Employment of Sodium Bisulphite in the Preparation of Plantation Rubber,” Beadle, Stevens, and Morgan, India-rubber Journal, August 2, 1913.

As long as such conditions prevail estates must continue to adopt any device of proved harmlessness, in order to obtain the best possible price for their product, and not because we desire to continue a practice which some assure us to be unnecessary, and which, moreover, adds somewhat to the cost of production.Quantities of Sodium Bisulphite.—It must be premised that, although sodium bisulphite is employed on some few estates in the preparation of sheet rubber, we do not advise the practice. It is unnecessary, and may lead to some little trouble and delay in drying. In any case, sodium sulphite gives the results desired for sheet rubber (see following). It must be understood, therefore, that we are concerned here, in the case of sodium bisulphite, with its employment in the preparation of fine pale crepe only.

As the dry rubber contents of latices vary with the age of the trees, the general health of the trees, the seasons and general climatic conditions, the relative strain imposed by depletion of reserves through tapping, etc., it will be clear that the effect produced by a definite quantity of sodium bisulphite in any given volume of latex will also vary—i.e., the effect depends upon the potential amount of rubber present. A dilute latex needs less sodium bisulphite than a richer latex to produce the same effect in colour.[4]

[4] Incidentally there are certain occasions, as in the opening of areas of bark rested for long periods, when the latex is of a rich yellow colour. Sodium bisulphite will not “bleach” this colour, and it is well to remark again at this stage that the action of the chemical is only to avoid or arrest oxidation (darkening).

Hence it follows that if in any factory uniform quantities of the solution are used for any given volume of undiluted latices from different areas of the estate, the effect upon the dry rubbers will vary. This explains why some estates obtain different shades of rubber in their fine pale crepes.

The remedy obviously is to reduce the variation in latices by diluting them all to a standard rubber content as is done in sheet preparation. One is thus assured that the prescribed quantities of sodium bisulphite will meet requirements in every case, and that waste will be avoided.Working with a standard of 11/2 lbs. dry rubber per gallon the following formula should serve as a maximum:

Formula for Use of Sodium Bisulphite.

(a) Dissolve sodium bisulphite in water at the rate of 1 lb. to 10 gallons.

(b) Of this solution use 1 gallon to every 10 gallons of latex.

Making a Solution.—The making of a solution of the chemical would seem to be a simple matter, but to judge by the ill-effects sometimes observed in the dry rubber the simplicity of the operation appears to have been overrated. Great care must be exercised in preparing the solution, and the work should not be left to the few minutes preceding its actual requirement; such has been found to be the case in several factories, so that it is not surprising if the rubber is defective.

The powder should be added gradually to water with thorough stirring, which should be continued for five minutes at least. Even then there may often be seen at the bottom undissolved particles, sand, and other impurity. It is necessary, therefore, in such cases to decant the solution through a piece of cotton cloth before using. No solid particles should be allowed to enter the latex.Abuse of Sodium Bisulphite.—It is now generally recognised that the abuse of sodium bisulphite, in the form of an excess, leads mainly to delay in the period of drying and the production of an overpale rubber.[5] It is probable that few estates, if any, now experience trouble due to this non-observance of the rules and quantities laid down for use.

[5] “The Preparation of Plantation Rubber,” Morgan, 1913, p. 74.

Residual Traces of Sodium Bisulphite.—The prolongation of the drying period was attributed to the fact that traces of substances caused by the decomposition of sodium bisulphite remained in the rubber if the rubber were not sufficiently worked and washed on the rolls. These traces must have been very minute, but they were sufficient to retard the progress of drying. That much depended on the care exercised in washing is evident from the fact that samples prepared with varying quantities of the chemical show varying results on extraction. These samples were tested for the presence of sulphates. Of the series tested that sample prepared with bisulphite in the proportion of 1 part to 600 parts latex showed only a trace of sulphate present; while the one prepared 1:2,400 gave an equal quantity. Intermediate samples contained no trace of sulphate. On the whole, therefore, the presence of sulphate in crepe rubber is adventitious, and properly washed crepe prepared with moderate quantities of bisulphite may be taken as free from any residual quantities. Meanwhile there cannot possibly be any doubt of the advantages gained by the use of sodium bisulphite, and it would not be very wide of the mark if the statement were made that, in the event of this chemical being discarded, most contracts for pale crepe could not be fulfilled.Sodium Sulphite.—It would not be amiss to insist upon the point that while the nature of sodium bisulphite, as employed in the preparation of rubber, is anti-oxidant, sodium sulphite is employed chiefly for its anti-coagulant property. It is not used, therefore, in the making of crepe rubber, but is of service in the preparation of sheet rubber, where the aim is to keep the latex in good fluid condition as long as is necessary, and to retard coagulation slightly so that enclosed bubbles of gas or air may escape. FormulÆ have been given for its use in the field when required. On some estates this practice is not found necessary, but a quantity of solution is always placed in the bottom of the reception vessels prior to the straining of latex into them. Only a small quantity is used, and as a working basis the following formula may be adopted:

Sodium Sulphite: For Use in the Factory.

(a) Dissolve 2 ozs. of anhydrous sodium sulphite in a gallon of water.

(b) The gallon of solution, placed in the bottom of the reception jar or tank, is sufficient for the treatment of 40 gallons of standardised latex (11/2 lbs. dry rubber per gallon).

The warning previously given regarding the necessity for thoroughness in the preparation of solutions is here reiterated. Stirring should be thorough, say for five minutes, and if there is any sediment or undissolved matter the solution should be strained through cloth before using.

Where uniform jars or tanks are in use, the majority of which will contain uniform quantities of latex daily, the practice of using the chemical can be made almost fool-proof even in the hands of coolies. A calculation is made of the quantity of powder required for each vessel daily. The necessary number of lots is weighed out each morning and each placed in an envelope. The process is thus simplified by the fact that the contents of an envelope, neither more nor less, are required for each unit reception vessel. Even the weighing can be done by a coolie if he is given a counterpoise (of lead, for example) equivalent to the required weight.

It will not be found necessary to do any vigorous stirring of the solution with the latex, as the latter is strained into the solution and the continued addition of successive quantities is sufficient to give a good mixture.Use of Formalin.—Few estates now use formalin (formaldehyde) as an anti-coagulant. It must be acknowledged that when not abused there are points in favour of its employment in preference to sodium sulphite, but these are outbalanced by certain disadvantages. The argument may be stated thus:

Points for: (1) If made up freshly it is an effective anti-coagulant.

(2) Formalin being the solution of a gas in water, there is no residual substance left in the rubber to delay drying.

(3) Its use gives a bright clear rubber.

Points against: (1) Its cost at all times is greater than that of sodium sulphite.

(2) If the jar is not sealed there is loss by evaporation, thus increasing the cost.

(3) Its effect upon the rubber is uncertain. Even in normal quantity it is said to cause “brittleness” or “shortness.”

Certain few estates, however, have continued its use, and no trouble is claimed to ensue. The following formula is stated to give satisfactory results in the preparation of sheet rubber, when applied as in the preceding paragraphs bearing on the employment of sodium sulphite:

Formula for Use of Formalin (Formaldehyde).

(a) 1 pint of formalin is diluted with 5 gallons of water.

(b) Of this solution 1 gallon is required for 50 gallons of standardised latex.

In noting this formula the writer gives no recommendation regarding its use. Whatever may be the actual facts regarding the effect of formalin upon the vulcanisation of rubber, when used in minimum proportions, there can be no question concerning its injurious effect if used in excess. Beyond this the factors of cost and loss militate against its wider employment.

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