The Preparation of Plantation Rubber

by Sidney Morgan

Rust-stains have also been caused in a few cases by the large knives which are used to cut up lumps of coagulum, or by allowing freshly coagulated rubber to come into contact with iron vessels in the factory.

A similar appearance has been traced in a few instances to contamination of the coagulum in transit by the dust of the reddish rock (laterite) employed in localities for road-making.

Oil-Marks.—The origin of oil-marks in crepe has already been described in [Chapter XIII]. The whole question resolves itself into one of cleanliness, moderation in lubrication, and supervision. The machines should be inspected every day, and once a week rolls may be swabbed down with a 10 per cent. solution of caustic soda applied by means of a piece of cloth fastened round the end of a stick. Immediately after this operation water should be turned on and the rolls set in motion, so that all traces of caustic soda are thoroughly removed. If possible, lubrication by oil should be substituted by grease lubrication through screw caps.

Particular attention should be paid to the back of the machines. None but the individual in charge of engines should be allowed to lubricate the machines, and he should be held responsible for any excess of lubricant.

As a rule oil-marks are restricted to the edges or the proximity of the edges of crepe, but sometimes the streak is to be found in the middle of the length. In such a case it is almost certain that the oil or grease has been picked up by the rubber in the tray. It sometimes happens, if the “liners” of the bearings are eccentrically worn, that a few drops of dirty oil or a particle of grease are squirted out to some distance. These usually find a resting-place in the tray, and the contamination may then appear in any part of the rubber.

It will be clear, therefore, that all trays beneath machines should be examined as the probable source of danger from contamination by oil and dirt. If the trays are as wide as or wider than the effective portion of the rolls, they should be discarded. In their place (except sometimes in the case of the macerating machine) all that is necessary is a movable piece of board, in width not less than from 4 to 6 inches shorter than the width of the rolls. Any oil or grease ejected from the bearings will thus be allowed to fall clear of the board; and defects due to oil streaks, etc., will be very much diminished, if not entirely obviated.

This point in connection with the damage possible by the existence of wide trays is commended to the notice of manufacturers of machines for plantations, as it is common to find that trays are made which contravene the rule prescribed by experience. In fact, trays on some machines have been so designed as to act as “traps” for all dirty matter exuding from the bearings. Not only so; they are sometimes made of such a shape and height that oil or grease lodging upon the edges act as a “wipe” to the rolls, thus increasing the possibility of contamination. Until this defect was investigated, it was common to note continued contamination of pale crepes in spite of all precautions taken in cleaning the rolls at frequent intervals. The trouble due to this cause is intensified when the same machines are employed for the preparation of scrap-rubber crepes and No. 1 crepe. Small pieces of scrap find their way towards the bearings and lodge on the edges of the trays. Unless a thorough inspection is made before proceeding with the working of the No. 1 (pale) grade, contamination may be continuous.

Dirt.—Streaks due to the presence of dirt (as apart from oil or grease contamination, or that due to pieces of oxidised scrap) are unusual, and when they do appear their origin seems to be somewhat of a mystery. It could scarcely be advanced that the dirt was picked up on the machines, as it is difficult to imagine where such dirt could come from. In one or two instances there has been fairly clear evidence that the dirt was contained in the coagulum, and the only explanation fitting the case is that it fell into the latex after straining and during the course of coagulation. On cutting open lumps of coagulum brought in from the field division, it has sometimes been noticed that dirt is included, and the foregoing explanation is the only reasonable one. How it was possible for dirt to get into the latex must be left for explanation to those better acquainted with the conditions under which the latex was coagulated.

Holes.—On some estates it would seem impossible, with the existing machines, to make really good crepe. The complaint is that, if thin crepe is attempted, it is invariably found to be “full of holes“; and as, apparently, the presence or absence of small holes in crepe rubber is a factor which influences buyers, this defect must be avoided at all costs. Why this matter of small holes in thin crepe should weigh so heavily with buyers is a matter which the writers are not in a position to explain. As a matter of fact, the presence of small holes is most generally an indication that the rubber has received the minimum amount of working on the rolls consistent with good washing. Further working would only be undertaken with the idea of so consolidating the rubber as to get rid of holes in order to meet the market scheme of valuation.

This is usually achieved by making a very thin crepe and rolling together two lengths when wet. The resulting crepe may be slightly thicker than ordinary, and the method employed may be usually detected by the appearance of the edges unless these are trimmed.