the formula C5H8. It thus possesses the same composition as the
hydrocarbon of gutta-percha and as that of oil of turpentine and
other terpenes which are the chief components of essential oils.
The properties of caoutchouc clearly show, however, that its actual
molecular structure is considerably more complex than is represented
by the empirical formula, and that it is to be regarded as the
polymer of a terpene or similar hydrocarbon and composed of a
cluster of at least ten or twenty molecules of the formula C5H8.
When solid caoutchouc is strongly heated it breaks down, without
change in its ultimate composition, into a number of simpler liquid
hydrocarbons of the terpene class (dipentene, di-isoprene, isoprene,
&c.), of which one, isoprene (C5H8), is of simpler structure than oil
of turpentine (C10H16), from which it can also be obtained by the
action of an intense heat.
When this volatile liquid hydrocarbon (isoprene) is allowed to stand for some time in a closed bottle, it gradually passes into a substance having the principal properties of natural caoutchouc. The same change of isoprene into caoutchouc may also be effected by the action of certain chemical agents. It may therefore be said that caoutchouc has been already artificially or synthetically prepared, and the possibility of producing synthetic rubber cheaply on a commercial scale remains the only problem. At present the change of isoprene into caoutchouc is mainly of scientific interest in indicating possibilities with regard to the conversion of the liquid globules of the latex into rubber and to the formation of rubber by plants. The exact chemical nature of caoutchouc is, however, not determined, and recent researches point to the view that its molecular structure may even be somewhat different from that of the terpenes.
The exact manner in which isoprene passes into caoutchouc is also not understood. These problems are, however, certain to be solved in the near future, and then probably caoutchouc may be formed in other ways than from isoprene.
The question as to whether synthetic rubber will ever be produced cheaply on a commercial scale is therefore the important one for those who are largely interested in the rubber-planting industry. No definite answer can be given to this question at the present time. Its settlement will depend in part on the cost of producing rubber from plants, which from their point of view it is to the interests of planters to reduce as far as possible. There are many substances produced by plants which can be synthetically prepared by chemical means, but, as with quinine, the process involved is too costly to enable the synthetic product to compete with the natural product. The chief properties of caoutchouc and its employment for technical purposes may now be considered.
Caoutchouc is not dissolved by water or alcohol, and is not affected except by the strongest acids. Alkalis have little effect on it under ordinary circumstances, although prolonged contact with ammonia results in a partial change. The best solvents for rubber are carbon bi sulphide, benzol and mineral naphtha, carbon tetrachloride and chloroform. These liquids, either alone or mixed, are employed in making the rubber solutions used for technical purposes. Vegetable and other oils rapidly penetrate caoutchouc and lead to deterioration of its properties. Sulphur when warmed with caoutchouc combines with it, and on this fact the vulcanization of rubber depends, and also the production, with an excess of sulphur, of the hard black material known as vulcanise or ebonite. Caoutchouc is a soft elastic resilient solid. In this respect it differs from gutta-percha, which, like caoutchouc, is derived from the latices of certain plants. The technical value of caoutchouc chiefly depends on the extent to which it is capable of being stretched without breaking, and the extent to which it at once returns to its original dimensions. Caoutchouc is a bad conductor of heat and electricity, and alone or mixed with other materials is employed as an electrical insulator.
When caoutchouc is heated slightly above the temperature of boiling water it becomes softer and loses much of its elasticity, which, however, it recover es on cooling. At about 150°-200° C. caoutchouc melts, forming a viscous liquid which does not solidify on cooling. This viscous liquid is present in small proportion in some commercial rubbers owing to overheating during their preparation. It appears to be the principal cause of stickiness or the tacky ” condition of some rubbers, which considerably depreciates their commercial value. There is some evidence that “ tackiness ” may be induced by a kind of fermentation which takes place in crude rubber.
At higher temperatures the viscous liquid suffers decomposition with the formation of various liquid hydrocarbons, principally members of the terpene series. Similar products are also formed by heating gutta-percha which closely resembles caoutchouc in its chemical structure.
Rubber slowly absorbs oxygen when exposed to air and light, the absorption of oxygen being accompanied by a gradual change in the characteristic properties of rubber, and ultimately to the production of a hard, inelastic, brittle substance containing oxygen. Ozone at once attacks rubber, rapidly destroying it. If ozone is passed into a solution of rubber in chloroform the caoutchouc combines with a molecule of ozone forming a compound of the empirical composition C5H8O3. When this compound is acted on by water, hydrogen peroxide and levulinic aldehyde are formed, the aldehyde being subsequently oxidized by the hydrogen peroxide, forming levulinic acid. The hydrocarbon of gutta-percha yields similar results and is therefore closely related to caoutchouc. The study of the action of ozone on caoutchouc has thrown new light on the complex question of the chemical structure of this substance, and discloses relationships with the sugars and other carbohydrates from certain of which levulinic acid is obtained by oxidation.
Caoutchouc, like other “unsaturated” molecules, forms compounds with chlorine, bromine, iodine and sulphur.
Commercial Treatment of Rubber.
In the industrial working of indiarubber, the various impurities present in the crude “ wild " rubber (bark, dirt and the principal impurities derived from the latex, except resin) are removed by the following process: The lumps of crude caoutchouc are first softened by the prolonged action of hot water, and then cut into slices by means of a sharp knife—generally by hand, as thus any large stones or other foreign substances can be removed. The softened slices are now repeatedly passed between grooved rollers, known
Fig. 8.—Roller of Washing Machine.
as washing rollers (fig. 8), a supply of hot or cold water being made to flow over them. Solid impurities speedily become crushed, and are carried away by the water, while the rubber takes the form of an irregular sheet perforated by numerous holes. The loss on washing ranges from 10–15 % with “fine Para” to 40 % with other “wild” rubbers. In the future this washing of “wild” rubber may be conducted in the tropics, thus furnishing the manufacturer with rubber which, like “plantation” rubber, need not be subjected to this process in the factory. The washed product contains in its pores a notable proportion of water, which is removed by hanging the rubber for some days in a warm room. It is now ready either for incorporation with sulphur and other materials, or for agglomeration into solid masses by means of the masticating machine—an apparatus which consists of a strong cylindrical cast-iron casing, inside which there revolves a metal cylinder with a fiuted or corrugated surface. Some of the rubber having been placed in the annular space between the inner cylinder and the outer casing, the former is made to revolve; and the continued kneading action to which the rubber is subjected works it into a solid mass, something like a gigantic sausage. Before commencing the mastication it is generally necessary to warm the apparatus by means of steam; but as the operation proceeds the heat produced requires to be moderated by streams of cold water flowing through channels provided for the purpose. The inner cylinder is generally placed somewhat eccentrically in the outer casing, in order to render the kneading more perfect than would otherwise be the case.
To convert the masticated rubber into rectangular blocks, it is first softened by heat, and then forced into iron boxes or moulds. The blocks are cut into thin sheets by means of a sharp knife, which is caused to move to and fro about two thousand times per minute, the knife being kept moistened with water, and the block fed up to it by mechanical means. Cut sheets are largely used for the fabrication of certain classes of rubber goods-these being made by cementing the sheets together with a solution of rubber in' naphtha or benzol. Most articles made of cut sheet rubber would, however, be of very limited utility were they not hardened or vulcanized by the action of sulphur or some compound of that element. After vulcanization, rubber is no longer softened by a moderate heat, a temperature of 160° C. scarcely affecting it, nor is it rendered rigid by cold, and the ordinary solvents fail to dissolve it. It must, however, be distinctly understood that it is not the mere admixture but the actual combination of sulphur with indiarubber that causes vulcanization. If an article made of cut sheet be immersed for a few minutes in a bath of melted sulphur, maintained at a temperature of 120° C., the rubber absorbs about one-tenth of its weight of that element, and, although somewhat yellowish in colour from the presence of free sulphur, it is still unvulcanized, and unaltered as regards general properties. If, however, it be now subjected for an hour or so to a temperature of 140° C., a combination occurs, and vulcanized caoutchouc is the result. When a manufactured article has been saturated with sulphur in the melted sulphur bath, the heat necessary for vulcanization may be obtained either by high pressure steam, by heated glycerin, or by immersion in a sulphur bath heated to about 140° C. In this last case absorption of the sulphur and its intimate combination with the rubber occur simultaneously. Cut sheets, or articles made from them, may be
