The Preparation of Plantation Rubber

by Sidney Morgan

The machines should be arranged as a battery, with the marking rolls at one end, so that the operations are consecutive. It is erroneous to imagine that heavy machines (such as those used in crepe preparation) are required. Light machinery only is necessary for sheet-making; but any available heavy smooth-roll machines in a crepeing battery may serve admirably for the purpose.

Marking.—Heavy machines are unnecessary for the purpose of putting a pattern on sheet rubber. If the rubber has been properly prepared a light pair of rolls is capable of exerting sufficient pressure to cause a good upstanding pattern.

Rolls are cut in various designs: some with “diamond” grooves on both rolls; some with grooves of varying width and depth encircling the circumference of the rolls, thus creating a “stripe” effect on the rubber; and some with diagonally-cut spiral grooves placed closely together. The last has the greatest vogue at present, while the first has almost gone out of favour. An objection lodged against the second design is that the edges of the grooves sometimes cut through the rubber, so that the dried sheet divides in strips. It would appear in such instances that either the coagulum was too thin and soft, or that the grooves had been cut too deeply and sharply. In any case the choice of a design is an arbitrary matter, and should depend upon the effect produced on the rate of drying and the general appearance.

The popular “close-cut spiral” roll is machined with varying measurements, but the usual design has grooves 1⁄8 inch wide by 1⁄8 inch deep and 3⁄16 inch apart.

Many estates have a particular “brand” cut in the middle of the rolls for purposes of identification. If this is done it is advised that the main grooving of the rolls be carried into the “branding” strip; otherwise grip will be lacking on this portion, and a certain amount of “cockling” of the sheets will result.

Sheets are often seen in which the potential effect of the grooving is reduced to a comparatively flat pattern in place of the desired ridges. The fault is generally attributed to the shortcomings of the marking rolls. While it is true that the grooving often deteriorates by friction-wear when the rolls are running “free,” experience generally decides that the deficiency in the appearance of the rubber should be attributed to faulty previous preparation rather than to the marking rolls. Sets of rolls have been changed often without justification or an improved result. It would always be well to be certain first that the trouble did not emanate from the fact that the coagulum had been previously rolled so thin and hard that the rubber could not be squeezed so as to fill the grooves. This has been found to be a common fault, and the general effect is to delay drying in spite of the thinness of the rubber.

Again, the trouble may have been due to an incorrect standardisation of the latex, generally in the direction of too heavy a density (too rich a latex) being employed. The original thickness of the coagulum would be normal, but owing to the abnormal rubber-content the effect of passing through the smooth rolls would be the production of a strip thicker and firmer than ordinary. If this firmness is appreciable the resistance of the rubber to the squeezing action of the marking rolls will result in a flat pattern—i.e., the grooves cannot be filled, and the ridges are low.

It is advised that all rolls used in the preparation of sheet rubber should be at least 18 inches wide, in order to avoid the appearance of thickened edges which delay drying.

Working with the correct standard of dilution of latex, and following the procedure indicated in the foregoing paragraphs, the dry sheet should not exceed an average thickness (over ridges and depressions) of 1⁄8 inch.

Preparation for Smoke-Curing.—It used to be the custom to allow sheet rubber to air-dry first for periods varying from one to several days. Naturally moulds were soon formed, and when the sheets were quite smoke-cured a mass of the dead moulds could be seen, if not over the whole sheet, at least in the corners of each diamond mark. It has been demonstrated in practice that there is no advantage in allowing sheets to air-dry partially before smoking. In fact, to obtain the greatest benefit from smoke-curing, sheet rubber should be placed in the smoke-house as soon as possible. The same effect of mould-growth may be noted if the wet sheets are placed in a smoke-house insufficiently heated.