The Preparation of Plantation Rubber

by Sidney Morgan

Under existing conditions governing the sale and purchase of rubber, various “standards” are set up. These really have no bearing upon the intrinsic qualities of the rubber, and are concerned almost entirely with superficial attributes. They are necessary in the absence of any proper scheme of evaluation for the establishment of certain standards of comparison, which imply that the rubber is apparently clean, free from certain recognised defects, and has been carefully prepared—as far as can be determined by a superficial examination. Thus the question of “finish” has attained disproportionate importance, but must be respected when preparing rubber for sale.

Under ordinary conditions, thin crepe rubber, as it leaves the finishing machines, has what may be termed “deckled” edges. On many estates, in order to comply with market conditions, the edges of the wet crepe are trimmed, and the trimmings re-made into lengths of crepe. This is done under the impression that the market price is influenced by the evenness of the edges of crepe rubbers.

Again, it sometimes happens that, owing to “wear” of the rolls, the finished dry crepe may show a faint but distinct pattern of mark—a diamond or a horizontal bar. Since these are not accepted under the “standard” comparisons, rubber exhibiting these characteristics does not obtain the top market price. In other words, these innocent and innocuous marks are regarded as defects and penalised accordingly.

Enough has been written to show how very important becomes the question of finish. It will be acknowledged that the superficial qualities demanded in the “standard” market type can be reproduced by any estate having adequate machinery and ample facilities for drying and handling the rubber.

Methods of preparation and formulæ for the employment of chemicals are so well laid down that, up to the stage of machining, no difficulty need be encountered. But the standard type of finish in the dry crepes cannot be obtained unless the estate factory is fully equipped with the three types of rolls necessary—i.e., macerators, intermediate crepers, and smooth finishing-rolls. This subject has received full discussion in [Chapter XIII.], and is here only mentioned with the view of emphasising the point that no estate can be blamed for a lack of “finish” in crepe rubbers if the equipment of machinery is inadequate or in poor condition.

If, on the other hand, the factory has ample machinery for requirements, and a good finish cannot be obtained on the thin crepe, then it is time the rolls were attended to and changed, or that the ratios of the driving pinions were altered.

Dirty Edges.—It seems to be almost impossible to keep old machines clean, and it is equally difficult to keep the edges of crepe free from oil and dirt. Usually these dark edges are to be found on crepe which is rather wide, and it will be noticed that where wide crepe is made, unless special precautions are taken, the edges of the rubber often pass under the edges of the hopper and so pick up dirt and oil. On most machines it is a great mistake to attempt the preparation of wide crepe; nothing but narrow crepe must be made. To obtain this it is necessary to decrease the width of the hopper placed above the rolls. This can easily be effected by blocks of heavy hard wood, cut to shape and fastened in position.

Sometimes the dark edges of crepe are due to another cause. Rolls may be gradually worn in the middle, so that to obtain a good finish it becomes increasingly necessary to tighten up the screws which regulate the distance between the rolls. It thus happens that just at, and beyond, the edges of the rubber the rolls grind upon each other, and fine particles of iron and graphite are transferred to the rubber. In such a case it is evident that either the rolls must be “turned” or that a new pair of rolls must be substituted.

Iron-Stains.—One of the causes of iron-stain on rubber has been mentioned in the preceding paragraph. This particular kind of iron-stain must not be confounded with rust-stain, and gives a dark dirty colour. It results from the grinding together of the rolls, and is usually noticed in the finishing of fine pale crepe. For this operation it is necessary to screw up the rolls tightly, and it will be plain that, whenever the rolls are vacant of rubber, there is a tendency for them to grind upon each other, thus setting free fine particles of iron and graphite. In order to avoid this, one must be careful to see that between the working of each length of fine crepe the rolls should be occupied with another piece of rubber, which may be kept for the purpose. In some factories this trouble apparently does not exist, while in others the amount of wear on the rolls is surprisingly great, and the damage done to the rubber is excessive. The only way in which this difference can be accounted for is that there must be a great difference in the quality of the roll material. Some rolls seem to be excessively soft, and from these contamination by iron-stain is great. For this reason rolls are sometimes rejected, and there would appear to be an objection to any but chilled steel rolls for the final stage of finishing crepe rubbers.

Rust-Stains.—Rust-stains, on the other hand, throw the responsibility entirely upon the labour and supervision of the factory. Rust is formed upon the rolls when they are at rest, and any one passing pale rubber between the rolls before they have been thoroughly cleaned is guilty of culpable negligence. Even when apparently clean, a piece of lower grade rubber should be passed through the rolls several times so as to remove any slight trace of rust remaining.