A considerable proportion of plantation rubber is prepared on native estates without machinery of any kind. In these cases the latex is coagulated with a convenient coagulant, not necessarily a suitable one. Sulphuric acid and alum have been used for this pur pose although they affect adversely the behaviour of the rubber during manufacturing processes. The coagulum is lightly pressed by hand and whilst still wet is sent to a central factory where it is milled to crepe and sold as an inferior grade of rubber.
When Hevea latex is treated with any but very weak acids, the Brownian movement of the globules ceases, and they unite to form a coherent coagulum. Dehydrating agents such as alcohol and pro tein precipitants such as tannic acid also cause Hevea latex to coagulate.
In the presence of suitable proportions of an alkali such as ammonia or caustic soda, Hevea latex retains its liquid condition for many years, but in the absence of such additions bacterial action occurs, acids are developed and the latex quickly coagu lates. Disinfectants also have a preserving effect on latex.
As already indicated, undiluted Hevea latex usually contains of rubber. There are also present a number of other substances, amongst which by a remarkable chance are small quantities of compounds essential to the commercial applications of the product. When latex is coagulated with an acid only a portion of the accessory substances are coagulated with the rubber. The rest remains in the serum. The active non-rubber substances still mixed with the rubber are ample, however, to satisfy com mercial requirements. For this reason methods of preparing rubber by evaporating latex have not met with general approval. In such cases the pure rubber may only amount to 85% of the solid material, whereas commercial crepe and sheet prepared by acid coagulation may contain as much as 95%.
The following table shows the average percentage of the differ ent non-rubber substances in dried Hevea latex in comparison with the amounts present in crepe and sheet.
Amount present in Non-rubber constituent Dried latex Crepe and Sheet per cent per centProtein and nitrogenous matter . . 2 Constituents soluble in acetone . • 3 Mineral matter (ash) . . . . 1 0.3 1-methyl inositol traces Sugars traces Some of the constituents of the accessory substances are par ticularly active in accelerating vulcanisations and are therefore of great importance to the rubber manufacturer. A similar effect may also be produced by another of the accessory substances, viz., the ash which consists chiefly of potassium compounds.
Constituents soluble in acetone (a solvent which does not affect the rubber portion of the product) contain an appreciable amount of fatty acids such as oleic and stearic which dissolve and disperse some of the mineral powders mixed with rubber during com mercial operations. The acetone-soluble material also contains a substance (allied in chemical composition to the sterols) which is particularly useful in preserving vulcanized rubber goods against the effects of atmospheric oxidation, so that they remain supple and elastic for a longer period than they otherwise would.
Pure rubber (caoutchouc) is a compound containing carbon and hydrogen only, in the proportion corresponding to five atoms of carbon and eight of hydrogen (GIL). It belongs to the class of bodies known as terpenes and is related in chemical composition to the constituents of turpentine.
The specific gravity of rubber is a little less than that of water. It decreases regularly with increase of temperature except between 3o and 35° C when the decrease is greatly accelerated. At the temperature of liquid air rubber is transparent and brittle like glass. At o–io° C it is hard and opaque, but quickly reverts to a soft and translucent condition above 20° C. As the temperature increases the rubber becomes softer, stickier, weaker and less elastic. These changes are greatly accelerated at temperatures of 50-6o° C. At a little below 200° C rubber decomposes yielding liquid hydrocarbons of the terpene series.
When rubber is repeatedly pressed between rollers it becomes more plastic and sticky and less elastic. While in this condition large quantities of powders and plastic solids may be mixed with the rubber merely by repeatedly passing through rollers.
Rubber is insoluble in water and is unaffected by alkalis or moderately strong acids, but these substances may react with the non-rubber accessory substances present. Rubber is dissolved by benzol, petrol, carbon disulphide, chlorinated hydrocarbons, etc. It forms compounds with halogens, halogen acids, ozone, certain oxides of nitrogen, chromyl chloride and certain metallic halides. It is oxidised by nitric acid, potassium permanganate and hydrogen peroxide. It is also slowly affected by atmospheric oxygen, par ticularly in the presence of copper salts. It is reduced by hydrogen in the presence of a catalyst.
When rubber is heated at 120-160° C with sulphur it forms a product known as vulcanized rubber, which is stronger, more elastic and less affected by changes of temperature than the raw material. It is also insoluble in all the usual solvents. These changes are considerably modified by the amount of sulphur and heat applied and also (although to a lesser extent) by powders and other substances which may have been mixed with the rubber. With suitable adjustments it is possible to obtain from the raw materials a product which is as soft and elastic as an inner tube or as hard and brittle as a piece of vulcanite.