The Preparation of Plantation Rubber Part 24

It has already been remarked that it is possible for "spot" disease to develop in dry rubber which previously gave no evidence of the presence of fungi. The condition necessary to such an occurrence is supplied by the presence of moisture. Thus, to state instances which are by no means uncommon, if a box of rubber is allowed to remain exposed to rain, or is damaged by flood-water, or by sea-water during transit, or (sometimes) if the rubber is packed in a damp case, the crepe on arrival at its destination may be found to be affected to a degree dependent upon the extent of wetting and the duration of the wetting period.

No means are known by which these coloured spots, due to the growth of chromogenic organisms, can be removed from the rubber. Naturally, although they may be present in the darker lower grades of crepe, they are not so easily visible as in pale crepe. It follows, therefore, that every possible precautionary measure must be taken when pale crepe has to be prepared.

We are often asked whether it is possible for an infected piece of rubber to affect sound rubber hanging in the same building; and whether, in case of "spot" disease appearing, it is necessary to disinfect the drying-house.

In a general sense, the answer to both queries is in the negative. It has not been proved possible to transmit the disease from one piece of crepe to another, except by the closest possible contact and in the presence of an abundance of moisture.

A dry crepe, even when in close contact with an infected dry specimen, has not been found to be affected.

Unless, therefore, pieces of rubber are pressed together, under favourable conditions as to moisture, there has been observed no transfer of disease.

Similarly it has not been found that the presence of spotted rubber in one part of the drying-house has been responsible for an outbreak of disease in another part of the same building. Furthermore, after the removal of diseased rubber from the drying-shed, freshly prepared rubber may be hung on the same supports without becoming affected, and without any intermediate treatment of the wooden bars, providing the crepe is thin and weather conditions are good. In our experience, no case has been observed in which the disease has been communicated to freshly prepared rubber by reason of the previous presence of affected rubber. In our opinion, therefore, any scheme for disinfecting the interior of a drying-house, as a preventive measure against the spread of "spot" disease, is unnecessary.

All other things being equal, it is plain that much will depend, as to the incidence of coloured spots, upon the design and situation of the drying-house. Sufficient has been written in previous chapters to indicate the importance of these points as affecting the rate of drying, upon which hinges the possibility of the appearance of "spot" disease.

In conclusion, the chief points in any discussion of this subject may be summarised thus:

1. No coagulum should be left without working for longer than the ordinary period. Otherwise, the prevailing conditions are very favourable for the development of the disease.

2. Thin crepe only should be made. The quicker the rate of drying the less possibility is there of the coloured spots appearing.

3. Crepe should never be allowed to remain folded overnight, and batches of folded wet crepe should be placed on edge to drain off surface moisture. The rubber should be hung to dry as soon as possible.

4. Several species of fungi causing coloured spots have been recognised, and it has been proved conclusively that it is possible to infect latex and also fresh coagulum.

5. As far as our present knowledge goes, it appears that infection takes place chiefly, if not entirely, by means of the latex in the field-vessels. It may take place during transport also, or even during coagulation.

6. While it is certain that infection can be caused by contact, it has not yet been shown that infection of the finished wet rubber takes place in the drying-houses by means of air-borne spores--at least, under ordinary drying conditions.

7. There is reason to believe that no further infection takes place once the rubber is well into the drying stage, and that dry rubber is not infected even by contact. From this one might infer that, as long as rubber remains dry, infection cannot take place during the voyage to the port of consignment.

8. Coloured spots do not appear until the rubber is about half dry, because that period is necessary for the development of the fungus to that stage in its life-history when it excretes colouring matter. The fungus in its earlier and colourless stage may have been present from the time the latex entered the cup.

9. The natural habitat of the fungi would appear to be decaying vegetable matter in the field.

10. Finally, if it is found impossible to be rid of fungoid-spot disease after having exercised all care and observed all known precautions, nothing remains but to supersede the ordinary drying process by some system of quick drying, such as the vacuum-drying process or a hot-air draught system, in which the rubber dries so quickly that any possibility of appearance of "spots" is entirely removed.

SURFACE MOULDS OR MILDEWS ON CREPE RUBBER.--Defects of this nature are most uncommon in the higher grades of crepe rubber, but cases of affection in the lower grades are not rare.

It will be evident from all previous discussions that the incidence of these moulds must be due to an extremely slow rate of drying. The necessary conditions would be supplied by one or more of the following causes:

(_a_) Making the crepe too thick.

(_b_) Hanging the crepe in a badly ventilated or badly situated building.

(_c_) Occasionally by abnormally wet weather.

(_d_) Allowing piles of crepe to remain too long before hanging.

(_e_) Using excessive quant.i.ties of deteriorated sodium bisulphite. In short, any factor contributing towards a r.e.t.a.r.ded rate of drying may be responsible for the appearance of surface mildews. The last mentioned cause is of not infrequent occurrence. Knowing the chemical to be of poor quality, relatively more is used to produce the desired anti-oxidant effect. Unless the rubber is particularly well washed on the rolls, there remains within it a residue of sodium _bisulphate_, an oxidation product of the bisulphite. This is hygroscopic to some degree--_i.e._, it takes up moisture from the atmosphere. Hence drying is delayed, and even should mildews not develop the chemical may sometimes be seen on the surface of the rubber as a whitish "bloom."

The enumeration of the possible causes of mildews on crepe rubber is sufficient to indicate the necessary precautions to be taken, and the discussion will not be extended further.

TACKINESS IN RUBBER.--"Tackiness" is a term used to denote a deterioration of rubber which renders it sticky, and, beyond this, implies that some physical and chemical change in the nature of the substance has taken place. In fact, it is no longer "rubber," but an oxidation product containing much resinous matter. It does not behave as rubber, and hence its value is much depreciated.

With modern ideas of erection of factories to guard against the introduction of direct sunlight, it was hoped that this defect had practically ceased to exist. In one grade of rubber it would be expected that tackiness would continue to appear. Earth-rubber, often exposed to direct sunlight for a week, would naturally become tacky, and this tackiness cannot be avoided unless the earth-sc.r.a.p is to be collected more frequently. But in many cases even the higher grades of rubber show signs of tackiness. Experiments have been carried out at various times and in various places to determine the cause of tackiness. For some time the theory of bacterial origin was in favour, but none of the experimental results was convincing. Bacteria may be present in tacky rubber; but, on the other hand, many cases of bacteria in rubber have been observed in which there was no tackiness. Experiments were made by one of us some years ago with a view to testing the bacterial theory by inoculating latex with small pieces of tacky rubber. In opposition to the results which were stated to have been obtained, there was no spread of tackiness. Other investigators have obtained similar results. One writer proposed to explain tackiness as caused by excess of moisture. This perfectly simple explanation unfortunately displays only a profound ignorance of the subject, and does not take into account the fact that tackiness is incident in rubber after dryness has been reached. It need not be pointed out to planters in Malaya that wet sheets of rubber are often exposed to direct sunlight by workers of native holdings, with no resulting harm as long as plenty of moisture is present in the rubber.

TACKINESS THE RESULT OF A SLOW PROCESS OF CHANGE.--As stated above, tackiness does not appear until the rubber is dry, and even then it is to be noted that it is possible for tackiness to appear in rubber arriving in London, which showed no indications of tackiness when packed for shipment.

TACKINESS CAUSED BY TRACES OF COPPER SALTS.--Spence, as the result of investigations, has pointed out that none of the various theories put forward to account for tackiness--viz., the action of bacteria, premature putrefaction, oxidation, excess of moisture, the action of enzymes, etc.--have any basis in scientific proof, and believes that the cause of tackiness cannot be directly attributed to bacteria. It has been stated that the only known way of causing rubber to become tacky is to expose it to sunlight or heat. While agreeing that in the ordinary way this statement is correct as far as one rules out the employment of chemical substances, it must be pointed out that tackiness of the worst degree may be caused by the presence of traces of copper or copper salts. This point has already been touched upon in a preceding paragraph dealing with the defect of "green streaks" in pale crepe rubber.

In the course of laboratory experiments tackiness has often been induced by the use of traces of copper salts. The rate at which tackiness is induced appears to be dependent upon the amount of copper salt used, but once it begins, the rubber molecule is very rapidly broken down, and resins are formed. As the formation of resins is accompanied by the inclusion of oxygen in the chemical const.i.tution, it would be expected that dry rubber becoming tacky should increase in weight. This is found to be the case, and to give an idea of how this weight increases with the progress of tackiness, the results below may be studied.

It will be seen that the maximum quant.i.ty of copper sulphate used amounted to 0025 per cent, (approx.) upon the weight of latex taken. Now it is highly probable that only a fraction of this quant.i.ty was retained in the rubber on coagulation, the remainder being in solution in the serum.

Furthermore, as the rubber was well washed and worked down to thin crepe, _the total quant.i.ty of copper salt remaining in the dry crepe must have been exceedingly small_. Yet the effect is most marked and should impress upon all managers the necessity for guarding against any possible contamination caused by bra.s.s or copper.

----------+-----------------+-------------------------------------------- _Sample._ | | _Weight of Rubber._ +------+ +------+--------+--------+--------+---------- | | | After |Further |Further | Percent- |_Amount of Copper Salt._| When |Interval|Interval|Interval|age in In- | | Dry. |of Four |of Seven|of Three|crease in | | | Weeks. | Weeks. | Weeks. | Weight.

---+------------------------+------+--------+--------+--------+---------- | | Grms.| Grms. | Grms. | Grms. | 1 |002 grms. copper | | | | | |sulphate per 100 c.c. | 430 | 441 | 482 | 488 | 135 |latex | | | | | 2 | Ditto | 428 | 439 | 481 | 486 | 1355 3 |001 grms. copper } | | | | | |sulphate, per 100 c.c.} | | | | | |latex } | 962 | 987 | 1035 | 1036 | 77 |001 grms. copper } | | | | | |acetate, per 100 c.c. } | | | | | |latex } | | | | | 4 |0025 grms. copper | | | | | |sulphate, per 100 c.c. | 502 | 513 | 558 | 560 | 115 |latex | | | | | ---+------------------------+------+--------+--------+--------+----------

In view of the effect thus produced by the addition of traces to latex of a copper salt, and the observed effect on rubber of contact with copper salts, one may imagine the result produced some years ago when on an estate smoked sheets were washed with a solution of copper sulphate as a remedy for surface moulds!

With the exception of this chemical action we know of no other means by which tackiness is produced, beyond those of direct sunshine and heat.

Cases governed by these two causes are common on estates. They are confined chiefly to the lowest grades of sc.r.a.p rubber, when the component raw materials have been exposed to the sun for a period before being brought to the factory.

It is now comparatively rare to find cases of tackiness in the higher grades of crepe, and when they occur, one may look for evidence of gross carelessness in the admission of direct sunshine. Usually this means the failure of some individual to regulate window shutters according to the position of the sun in the sky. More rarely does it happen that tackiness may have been induced by placing thin crepe rubber too near the iron roof of the drying-shed.

Regarding the question as to whether tackiness may be communicated by direct contact, opinion appears to be divided. It has been stated that sound rubber left in contact with tacky specimens was found to be unaffected after two years. On the other hand, it is claimed that tackiness has been induced in a sound rubber by infecting it with small pieces which were tacky. In a preliminary article on the effect of copper and copper salts upon pieces of dried and sound crepe[21] it was noted, after one year, that tackiness had been communicated from the treated portion to the "blank" in contact. There is sufficient evidence to warrant the injunction that tacky rubber should be excluded from contact with sound rubber. If shipped it should be packed separately.

[21] Report I., 1916 (Sidney Morgan), Rubber Growers' a.s.sociation (Malaya).

Compounds have been put upon the market which a.s.sumedly claim to be cures for tackiness. These are merely palliatives, consisting of starch, talc, or chalk powders, which counteract stickiness.

NO CURE FOR TACKINESS.--At the present stage of our knowledge, there appears to be no cure for tackiness. Neither do we see the necessity for a cure when the phenomenon may be avoided by taking simple precautions, which may be briefly summarised thus:

(1) Any permanent openings through which it is possible for direct sunlight to enter, whether large or small, should either be totally closed or provided with some substance which cuts off the direct effect of the sunlight--_e.g._, ruby gla.s.s or ruby glazed cloth.

(2) Rubber should under no circ.u.mstances be placed near any source of heat.

(3) No rubber should be hung in a drying-room in such a position adjacent to a window or door that it is possible for sunshine to reach it, even should coolies neglect to obey rules.

(4) Instruments or vessels of copper or bra.s.s should not be used where acids are employed.

LACK OF UNIFORMITY IN COLOUR.--The complaint is far less real than it was a few years ago. The introduction by the Rubber Growers' a.s.sociation of the "Metrolac" led to uniform dilution of latices varying in rubber content.

Previously the only known method of obtaining uniformity in colour and appearance was that by which latices from all fields were mixed together in bulk. Even so the uniformity applied only to the one bulking operation, and any other day's results might show considerable variation from the first standard.

This does not take into account any observed differences in shade of colour attributable to natural oxidation which might vary in intensity from day to day. The introduction of sodium bisulphite as an anti-oxidant exerted a great influence upon the colour of pale crepes generally; but considerable variation would still have been notable but for the adoption of the scheme for uniform dilution, in addition to the use of small quant.i.ties of anti-oxidant.

On most estates it is now possible, with slight exceptions due to abnormal conditions, so to treat the latex that the pale crepes prepared on any one day differ in no perceptible degree from the product of any other day.