Raw rubber, as produced on the plantations, is almost invariably subjected to the process of vulcanisation in the production of manufactured rubber articles as we know them. Previous to the advent of plantation rubber, the raw material was purchased by the manufacturer in a moist and impure condition; frequently the rubber was adulterated with sand, dirt, and even small stones. Consequently it was the invariable practice of the rubber manufacturer to wash the raw rubber and convert it into crepe, which was then hung and air-dried before use. The effect on the rubber, if of high grade, was more severe than the washing and crepeing process on the plantation, because the rubber was not a soft coagulum but generally dried on the surface and semi-hard. The power required was considerable, and the resulting crepe was consequently softer and more susceptible to heat than plantation first latex crepe. Much of the “wild” rubber was soft and tacky and inferior to “earth-scrap.”
Vulcanising in its simplest aspect consists in mixing the rubber with sulphur and heating the product under regulated conditions. The effect of heat on the inferior grades of “wild” rubber is very marked. A soft, sticky, and resinous material is transformed into a relatively tough and elastic product. The effect of vulcanising on the better grades is less marked, but immediately apparent. On the other hand, the effect of vulcanising is least apparent on first latex plantation grades, because in these we have a raw rubber prepared in a manner best suited to retain its natural characteristics.
The need of vulcanising in the process of manufacturing rubber goods became an axiom in pre-plantation days, and it is only quite recently that attempts have been made to utilise raw rubber directly, without vulcanisation, particularly for shoe soles. For this purpose a thick dense crepe has been found satisfactory. Smoked sheet rubber is not generally suitable, apparently owing to its microphysical structure. It is possible that the process of rolling in the making of dense crepe compacts the rubber particles, yielding a harder and more resilient product. The rolling must not be carried too far, or the “working” of the rubber will approximate to a preliminary mastication, and the product will be weakened.
The utilisation of crepe rubber directly has not yet been sufficiently tested to enable a definite conclusion to be reached as to its future scope, but it is obvious that for use in a raw state some modification in preparation may be advantageous. The present method—e.g., coagulation with acetic acid—does not yield the hardest and toughest rubber.
Hardness and toughness are actual drawbacks in the utilisation of rubber which is required for vulcanising. When the output of plantation rubber began to increase and to displace the inferior wild sorts, manufacturers complained of the increased power consumption of their machines. The power was required mainly to “break down” or “mill” the rubber preliminary to the mixing with sulphur and other ingredients. It is obvious that a material such as raw rubber cannot be mixed with powders such as sulphur with a pestle and mortar, or in any simple form of mixing machine. This difficulty was overcome by the earlier experimenters by immersing the rubber in a bath of molten sulphur. The latter was gradually absorbed and “dissolved” in the rubber, and the heat of the bath caused the dissolved sulphur to combine with the rubber to produce vulcanised rubber. The limitations of such a process are apparent. Thus the vulcanised rubber retains the form in which it was originally shaped. Moreover, other ingredients, such as mineral matters, cannot be dissolved or absorbed by the rubber in this manner. The method eventually adopted consisted in “breaking down,” “milling,” or “masticating” the rubber by passing it continuously between differentially geared steam-heated rollers. By this means a high-grade rubber is converted into a soft, plastic mass, which will “take up” sulphur, mineral matter, and other ingredients as desired. The mixing operation may be carried through on the same roller machine as was used for breaking down the rubber, or separate machines of other designs may be adopted. Details of the process will be found in books dealing with rubber manufacturing.[35] It will suffice here to explain that when rubber is kneaded between two hot rollers moving at different speeds the rubber forms a continuous band around the slower moving roller, and if the distance between the rollers be adjusted the excess of rubber held back by the nip of the rollers will form a “bank” or moving wedge-shaped mass on the top of the nip. This closes the space between the rollers, so that sulphur and powder placed on the rubber pass round towards the nip, and are there driven into the rubber. In this manner it is easy to mix, say, 10 per cent. of sulphur into the rubber without a single particle falling through. In technical mixes where large quantities of powders require to be mixed there is always some caking, and part of the powder falls between the rollers into a tray underneath. This is swept up with a broom and put back on to the rollers, the process being repeated until the whole of the ingredients have been incorporated.
[35] For instance, “India-Rubber and its Manufacture,” by H. L. Terry.
From this description it follows that, preliminary to mixing, it is necessary to thoroughly masticate or “plasticise” the raw rubber. Much of the “wild” rubber was of so inferior a quality that it very readily broke down, and but little mastication was necessary. It was soft and resinous, and readily took up the powders which were to be mixed with it. The better grades of wild rubber, such as Fine Para, were more difficult to break down, but not so difficult as most plantation rubber, because they had already received a preliminary “working” in the process of washing and crepeing, and we have already explained that such treatment takes more power than the crepeing of the soft moist coagulum on the plantations. The amount of “working” or “plasticising” produced in the rubber is connected with the power expended; the greater the expenditure of power, caeteris paribus, the greater the working effect on the rubber. Although the manufacturers possessed a relatively soft rubber in the form of washed Fine Para, it was customary in most cases to employ this rubber in conjunction with washed lower grades to produce a soft plastic material for further treatment. Now, however, the manufacturer has little else but plantation to deal with, and most of it more difficult to break down than washed Para crepe. This is the reason why a hard, tough rubber is no longer a desideratum with manufacturers, although originally taken as an indication of good quality. For the majority of purposes they want something which will break down easily. Hence if a rubber could be produced answering to these requirements, without loss of vulcanising quality, it would be preferred.
Having incorporated sulphur and other ingredients, the plastic mass is sheeted and run between layers of calico to prevent the superimposed sheets from adhering. From this “calendered sheet” the article, whatever it may be, is built up. The calender rollers are heated so as to keep the rubber compound plastic. There is a limit to the thickness of the sheet which can be produced. It is a difficult operation to perform satisfactorily so as to yield a smooth surface and a sheet free from enclosed air. When cool the rubber hardens and is readily handled. The object to be manufactured is then built up from the calendered sheet. Thus in the manufacture of a motor tyre the tread is built up on the casing or carcase by laying the sheets on the canvas and rolling these with a hand or power operated roller, so that they adhere firmly, the first layer to the canvas of the casing and subsequent layers to one another. This rough description will suffice to illustrate how important it is that the rubber when mixed should be plastic enough to give a smooth sheet, and to allow the sheet to be manipulated in building up the article in process of manufacture. The testing of rubber in regard to its plasticity and power to absorb finely divided mineral matter will be discussed in a [later chapter]. We may, however, point out here, that the mineral matter is not generally added as an adulterant, but because of certain specific properties it confers on the product.
To proceed with our outline of vulcanisation, we have now arrived at the stage at which the goods are built up and ready for vulcanising. For this purpose they are generally enclosed in some manner, either in metal moulds bolted together, or tightly wrapped in cloth, as, e.g., in the manufacture of inner tubes, hose, etc. In the latter case, you can detect the cloth mark on the finished product. Sometimes the rubber is spewed—that is, driven out of a barrel by means of an endless screw revolving in it. In this way rubber tubing, perambulator tyres, and such articles, may be made. More recently even tyre treads and the shaped rubber for band tyres (heavy solid tyres) have been extruded in this manner, for the process is much cheaper than building up a tyre from calendered sheet, and then cutting the mass to shape by hand. But for spewing the rubber mass must be very soft and plastic; this condition is not obtainable unless the raw rubber originally used can be made thoroughly plastic without damage. Nor can it be effected with a rubber mass containing much finely divided mineral matter, as this hardens the mixture.
For other purposes the rubber is swollen in a solvent, such as coal-tar naphtha, and subsequently masticated; the soft dough is then shaped or spread on cloth, and vulcanised after allowing the solvent to evaporate. Here, again, the properties of the raw rubber are of immense importance. Thus, the more plastic the dough, the less solvent required, and the less there is to drive off before vulcanising. The plasticity of the dough will depend on the plasticity of the raw rubber, and so forth. It is evident that the physical properties of the raw rubber are of great importance. They directly affect the manufacturing operations up to the vulcanising stage, and indirectly affect the results obtained on vulcanising.