The Preparation of Plantation Rubber

by Sidney Morgan

In order to secure the desired effect the coagulum must be passed consecutively through the three types of rolls, and undergoes a varying degree of working in each.

Given the necessary equipment of machines, it is possible to make a good specimen of thin pale crepe if the coagulum passes through all the rolls a total of twelve times (or even less in exceptional cases). There is no intention of suggesting that this is possible on all estates. Clearly the number of times the rubber passes through the rolls will depend upon the total efficiency of the machines. This in turn involves such factors as (a) the size of the rolls, (b) the number of machines of each type, (c) the gearing of the pinions, (d) the speed of the drive, etc. Again, much depends upon the nature of the coagulum worked. A fairly soft coagulum will offer less resistance, and conversely a dense coagulum will require more machining.

It has been shown by the writers in previous publications that over-working of the coagulum has an effect on the vulcanisation of the rubber; and this has been confirmed by others.[10] Apart from this point, it should be recognised that over-working, beyond that necessary to produce a thin crepe of even texture, is to be deprecated, on the ground of economy, in working.

[10] Bulletin No. 27, Department of Agriculture F.M.S., April, 1918, “Preparation and Vulcanisation of Plantation Para Rubber,” Eaton, Grantham and Day.

Owing to the existing differences in equipment and speed of drive, etc., the regular practice of any one estate may be unsuitable for another. It remains, therefore, a matter of study for each estate to discover the minimum number of times which rubber should pass through the machines, consistent with the factors indicated above. In any case it may be assumed that if any factory cannot prepare a good crepe by passing the rubber, say, twelve times through the rolls, there is some deficiency in the machines, or of speed; the coagulum may be too hard, or the rolls may be badly worn.

Lower Grades of Crepe Rubber.—Even a few years ago it was plain that the lower grades of crepe (i.e., all grades lower than first latex rubber) were not sufficiently appreciated in the market. There was often a marked difference in price between a first-grade crepe and crepe made from naturally coagulated lump. This arose chiefly from lack of knowledge. It has since been recognised in some measure that no reason exists for such a wide difference in price, and more recently the margin between even the first-grade rubber and the lowest grade of scrap rubber has been a gradually diminishing one. Providing sufficient care is exercised in the preparation of the lower grades, one would expect to see but very small difference in prices between any two grades. It is true that adequate attention has been given to the preparation of the scrap grades only in comparatively recent years, and it is acknowledged that when high prices were ruling for first-grade rubbers sufficient attention was not generally given to the subject of the preparation of the lower grades.

Naturally Coagulated Lump Rubber.—The grade of rubber made from the naturally coagulated lump which forms in buckets and carts is usually of a mixed colour, due to the fact that the lumps oxidise very quickly. When they are allowed to remain overnight before being machined, it can be imagined that the colour of the dry crepe would be very dark, or would contain very dark streaks. Such is ordinarily the case, unless special precautions are taken.

Providing that the coagulated lump is free from bark, leaves, and leaf-stems, and certain other precautions taken, the difference in price between coagulated-lump crepes and first-grade crepes should be very slight. Too often, however, not sufficient supervision is given to the coagulated-lump rubber, and it is common to see it come into the factory containing leaves and bark. These should be picked out before the latex is strained, but obviously it would be better to ensure that they did not enter the buckets in the first place.

It would seem reasonable to suppose that if some means could be employed for preventing or checking the surface oxidation of naturally-coagulated lump rubber, there would be a corresponding improvement in the colour of the dry crepe. That such a method is practicable has been demonstrated on many estates. The lump when lifted out of the latex is allowed to drain for a few minutes, and is then (without squeezing) placed in a dilute solution of sodium bisulphite. A 1 per cent. solution is sufficiently powerful. It is not to be thought for a moment that by the use of sodium bisulphite any previous oxidation will be counteracted; all that is claimed for the treatment is that any further surface oxidation will be checked, and the rubber may be allowed to remain until the next day, for working, if it is so desired. It will probably be found that quite a quantity of latex has been expressed from the lumps by contraction, and acid may be added to obtain the rubber from this. On other estates the lump rubber is worked on the machines as it is received, and the resulting crepe is submerged in a weak solution of sodium bisulphite over-night. It is then rinsed in water and hung to drip before weighing and placing in the drying house. Under certain conditions some of the lump rubber darkens rapidly during transport to the store, and any such oxidised portions must be rejected if a uniform colour is to be expected in the crepe.

Following the procedure indicated above, some estates find it possible to prepare from naturally coagulated lump rubber a crepe which can be classed as No. 1 grade.