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Rh Approximate World Consumption of Rubber, 1920.

United States ....

260,000 tons 50,000 20,000

10,000 '

8,000 16,000

Great Britain and Colonies .... France

Italy

Japan. . . . . ..

Other Countries

Total

364,000 tons

Synthetic Rubber. From the time when India rubber began to be important in the arts, its synthetic production was the dream of the inventor. Analysis of rubber with a view to ultimate synthesis was made between 1835 and 1840 by Dalton, Liebig, Himly, A. Bouchardat and Gregory. A more systematic attempt to isolate and examine the products in crude caout- chouc distillate' was made in 1860 by Greville Williams, the English chemist. He obtained isoprene,C 5 H 8, a hemiterpene, a fluid boiling at 37 C., and a hydrocarbon now known as dipen- tine, boiling at 170 to 173 C., which he named "heveene." An important step towards the production of artificial rubber was that taken by Gustave Bouchardat of the Paris School of Phar- macy in 1879, when, in studying the action of hydrochloric acid on isoprene, he noted the formation of a substance having the same percentage composition as isoprene, lacking chlorine, possessing elasticity, insoluble in alcohol but soluble in ether and carbon bisulphide like natural rubber, and yielding on dis- tillation the same hydrocarbon as caoutchouc. Sir William Tilden, the English chemist, in 1882 observed the polymeriza- tion of isoprene and that it could be converted into true caout- chouc with certain chemical reagents. In 1884 he obtained iso- prene by passing the vapours of turpentine through a hot tube. In 1887 Prof. Otto Wallach of the university of Gottingen noted that isoprene undergoes polymerization on exposure to light with the production of a rubber-like mass, and Tilden in 1892 showed that such material could be vulcanized with sulphur. The synthesis of isoprene and, as a consequence, that of caout- chouc, was accomplished] in 1897 by Prof. Euler-Chelpin of the university of Stockholm. In 1909, due to the rapidly mounting cost of natural rubber, greater efforts were made to produce the artificial kind on a commercial scale, the problem being attacked in England by W. H. Perkin, his assistant Weizmann, and Francis Matthews; by August Fernbach in France; and in Germany by the Bayer and Badische companies. In 1884 Tilden suggested that not only isoprene but its homologues should be capable of similar polymerization. Now these bodies, chief among them butadiene, form the basis of methods for obtaining synthetic caoutchoucs. Dr. Fritz Hofmann and Dr. Carl Coutelle, chemists in Germany, in 1909 devised a process for making absolutely pure isoprene, converted it; into rubber by heating it in a closed tube or in the presence of other sub- stances, and sent the sample to Prof. C. D. Harries, of Kiel University, who pronounced it true rubber. In 1910 Prof. Harries showed that isoprene could be converted into rubber by heating it in a closed tube with glacial acetic acid. He had in 1905 determined the chemical constitution of natural rubber. The German scientists did not confine themselves to isoprene but experimented successfully with the homologous hydrocar- bons suggested by Tilden. Harries and the English investigators, Matthews and E. Halford Strange, noted independently that polymerization proceeds at great velocity in the pres- ence of metallic sodium and the resulting rubber differs much in its properties from that produced by mere heating. German chemists observed different results when polymerization by sodium was carried on in an atmosphere of carbonic acid. A later process in Germany was based on the use of ozonizers on sodium hydrogen peroxide as catalysers. Some of the synthetic rubbers are soluble, elastic, and may be readily vulcanized; others possess only some of these qualities. They are obtained from butanes, dimethylbutanes, and from isoprene, and in each of the three classes are to be found standard ozonide, carbonic acid and sodium rubbers.

Despite this wide range of materials with their possible use in the arts, the making of synthetic rubber is still a minor industry as com-

pared with the production of natural rubber and the manufacture of goods therefrom. In the manufacture of the hydrocarbons of the isoprene series for synthetic rubber there are such large quantities of by-products that their removal or utilization presents a problem more difficult than the production of the artificial rubber; hence competi- tion with natural rubber is very unlikely. Synthetic rubbers lack the durability of natural rubber, possibly because they lack the resins, albumen, etc., which act as protective colloids to lessen the vulnerability of the natural article. Then, too, for a wide range of needs, synthetic rubbers cannot be substituted for natural rubber because the latter product is a uniform vegetable substance, not a mixture like the artificial product. While synthetic rubber must be greeted as a chemical triumph, it is not an industrial success, and must still be classed commercially with the more or less haphazard production of alleged rubber substitutes prepared, often by honest inventors and manufacturers, from oils, gums, cellulose, or in fact anything that will produce a waterproof plastic.

Reclaimed Rubber. In few other industries is conservation such an important factor as in rubber manufacture. Nearly all kinds of worn-out vulcanized goods are collected and the basic material recovered to be compounded, manufactured arid vulcanized again into new articles that compare favourably with those made from new gum. To so many uses is devulcan- ized or reclaimed rubber now put, that its annual consumption fully equals that of new crude gum. Experiments early demon- strated the value of " reclaim," and while the more conserva- tive long looked askance at the utilization of " refuse rubber;" buyers of goods made wholly or in part from the regenerated material found that for most purposes the goods were practi- cally as serviceable as those made directly from fresh gum. The element of cost, too, played an important part in popularizing reclaimed rubber, as articles made of it could be produced and sold for much less than those made with new gum only, and to a considerable degree the price of the crude gum has been kept from rising too high by the ample supply of the reclaimed. As the advantages of reclaimed rubber became better appre- ciated, and as through the activities of rubber chemists and manufacturers the quality of the product was improved, it became an important factor in the industry. To meet more satisfactorily the fast-growing demand, large companies with world- wide connexions and specialized equipment soon sup- planted the small reclaimers. Such concerns employ not only the most modern machinery but also maintain research and analytic laboratories for control of the processes, for standard- izing the products, and for the study of reclaiming and com- pounding problems.

The first attempt to reclaim rubber commercially was that made in the early 'fifties when Hiram L. Hall, the pioneer manufacturer in Massachusetts, boiled powdered vulcanized rubber in water and then sheeted it. Francis Baschnagel, an early American experi- menter, next patented a method for devulcanizing rubber, finely ground, by exposing it to live steam. An important later develop- ment was the destruction of fibre in the ground material by means of acids, chiefly sulphuric, for which processes over 50 patents were granted, which incidentally became the subject of much litigation. The acid process was of use chiefly in the reclaiming of worn-out footwear or " dry heat " goods, but was not of great value in re- covering other waste. The alkali process, patented by Arthur Hud- son Marks, an American manufacturer, solved the latter problem. In this, caustic soda was used to destroy the fabric and incidentally it proved to be the most effective agent in desulphurizing the mass. The entire removal of not only the free sulphur from vulcanized rubber (which modern reclaiming accomplishes) but also of the sulphur which during curing unites chemically with the crude rubber, is the goal towards which experimenters were striving in 1921. Notable progress in this direction had been made in England by Dr. David Spence, who used an accelerator, aniline-potassium, but in solution in excess of aniline. He claimed not only the dissolution of the waste rubber but the liberation in soft rubber of from 78 % to 90 % of the combined sulphur, and the changing of the latter into an insoluble alkaline sulphide. In hard rubber 73 % of the combined sulphur was said to be similarly reduced.

Vulcanization, or curing, is effected generally by either the heat cure or the cold cure. In the first-named method either steam or heated air is employed. A wide range of rubber goods, either in moulds wrapped with strips of cloth, or imbedded in pans of French talc to preserve their shape, is very efficiently cured with live steam in various types of vulcanizers. For many other needs the dry-heat cure, in which the goods are placed in a hot compartment without either wrapping or mould protection^hajs