INDUSTRIAL A N D ENGINEERING CHEJUSTRY
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Vol. 18, No. 11
Artificial Rubber during the War in Germany' By C. C. Burgdorf ALBANY,N. Y.
T
WO questions are always asked in connection with the manufacture of artificial rubber in Germany during the war. First, what is the real truth about the manufacture of artificial rubber? Were certain quantities made, and was it really rubber? Second, can synthetic rubber compete with the natural product in quality and price? At the end of the war ten tons of rubber were being made daily, part of which was used directly in the industries, and part mixed with natural rubber. Available Information
As a starting point, we knew that isoprene, and its homologs erythrene and dimethylbutadiene, polymerized to give products similar to rubber. Technical ways to obtain these hydrocarbons had been found by Fritz Hofmann, head of the department of the Farbenfabriken, now professor a t Breslau, and his collaborators, among whom Coutelle should receive first mention. The board of directors of the Farbenfabriken vorm. Friedrich Bayer & Co., to encourage the research work, had offered a sum of money equal to several times that of the yearly earnings of a high-class chemist as a special remuneration for a technical method for the manufacture of rubber. The numerous patents taken out on this subject indicate the number of experiments made in the laboratory. It was, however, a long way from the first colloidal product obtained from isoprene until a product suitable for use in therubber industry was produced. Experimental Plant
An experimental plant to manufacture dimethylbutadiene was kept running at a time when the price of rubber was high. When prices returned to normal its manufacture was discontinued, and the installation partly dismantled, and partly used for other purposes. Research, however, was continued. During the war, substitutes for rubber became urgent. It was badly needed for tires, for cables, and last, but not least, for storage batteries in submarines. For these, hard rubber boxes were necessary and all substitutes tried had been found unsatisfactory. At last the Hagener Accumulatorenfabrik made experiments with some rubber which had remained in the apparatus ever since rubber had been made in our experimental plant. In order to test the quality of these boxes, they were filled with lead and acid and put on a see-saw moved by mechanical means. As the hard rubber made of artificial rubber withstood these tests, it was decided to manufacture it on a large scale. Plant Manufacture
Material was extremely difficult to procure, and it was, therefore, not an easy matter to enter upon the plant manufacture of this product. Furthermore, valuable material could not be spoiled with experiments. The problem was given to Messrs. Ionas and Tschunkur, as well as to certain chemists chosen for their long manufacturing experience. At first it was thought that the production of dimethylbutadiene would be the most difficult, but aside from difficulties with construction of the apparatus, its manufacture went along smoothly, since well-defined chemical compounds,
* Received
September 15, 1926.
which were either liquid or could be melted, were always used. Chemically, it was, therefore, a question of yields and purity of product. Mechanically, the most important factor was keeping the apparatus tight in order to avoid losses by fire and explosion. DIMETHYLBUTADIENE-Dimethylbutadiene was practically the only intermediate used. This was obtained from acetone by reduction with aluminum, which gave the aluminum salt of pinacol and small amounts of isopropenol. By hydrolysis aluminum hydroxide and pinacol are obtained, which latter, in turn, on losing two molecules of water, gives dimethylbutadiene and as a by-product, by unusual rearrangement, pinacolin. I N s T A L L - ~ T I O N - - T ~ ~entire installation consisted of fourteen units, established with the most economical material possible. Copper and tin could only be obtained in small amounts. Throughout the whole plant iron valves and plug-cocks were used. Even the gaskets were made of low-grade material, which was all that was available. No specifications could be set for equipment. The chemist had to be satisfied with whatever was available. Yet despite these difficulties the apparatus was always free from leaks, even though boiling and very volatile liquids were handled. An idea may be formed of how tight the apparatus was when it is understood that in the distillation a loss of 500 kg. per charge was considered as high, with 250 kg. as normal. As each charge was 70,000 kg. (18,000 gal.), that meant an average loss of less than 0 5 per cent. The plant worked well from the first day, no changes of any importance had to be made, and the expected capacity of production was obtained immediately and later exceeded. AcFiroNE-The acetone used was a t first obtained by distillation of wood. Stocks from this source were soon depleted, and so were the quantities of imported acetate of lime. Production of acetic acid by fermentation from grain and potatoes was not possible, as these foodstuffs were needed by the population. For the same reason the production of acetone by direct fermentation (bacillus macerans) was impossible. We started, therefore, from calcium carbide, from which acetylene was made, and then, by treatment with a mercury salt, acetaldehyde, which in time was oxidized to acetic acid. Acetone was obtained from acetic acid by catalysis, and was superior in quality to that obtained from the distillation of wood. Thus, synthetic rubber was prepared entirely by synthesis, starting from coal. Polymerization
For a 10-ton daily production of artificial rubber 300 to 500 tons of volatile liquids had to be handled. This was done without a fire or explosion. The work was made more hazardous owing to frequent aerial attacks. The reactions for producing the twice unsaturated hydrocarbon are clearly understood. We knew the by-products and could check every stage of the reaction by analysis. We knew nothing about the theory of the polymerization, and all our knowledge is purely empirical. A number of scientists claim the honor of obtaining colloidal rubber by polymerization of the different butadienes, with or without condensing agents a t high and low temperature, but say nothing, or nearly nothing, about vulcanization of these products for technical purposes. As a matter of fact, the first batches made in our experimental
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1926
plant by hot polymerization always came out different, although there seemed to be no changes in conditions. Spontaneous polymerization in the cold, according to Kondakoff, was very irregular, sometimes going slowly, sometimes quickly. The product was sometimes white and crumbly, a t other times partly viscous. An enormous number of experiments were necessary to find out what conditions had an influence on polymerization and on the vulcanization of the product. Polymerization required several months for completion. Asthe demand was urgent, we could not wait for the results of one series of experiments before starting others. It was necessary to start several hundred experiments a t a time. Successful results were obtained by the time the plant was in construction, so that we could start the polymerization as soon as the first batches of dimethylbutadiene came through, The reaction, however, showed up differently in 750-gallon kettles than in the glass apparatus used for experiments. One day the reaction went so quickly that the pressure went to 90-100-110-120 pounds per square inch in a kettle, which had been tested to only 60 pounds pressure. Two courses were open: one to open the safety valve and let material badly needed by the Army go into the air, or to risk an explosion. The pressure, however, after a night of anxiety, finally subsided and no damage was done. Experiments with different conditions of polymerization continued throughout the entire time, The plant was running very well and practically every batch was up to our specifications. Quality
The value of the artificial rubber has to be determined by the quality of the vulcanized product. Vulcanization had to be carried out by new methods, and the best conditions for vulcanization of this product had to be determined by experiment, as was the case with natural rubber. The artificial rubber had to be used with reclaimed rubber and factis. It took several decades to find out the conditions for obtaining the high qualities now produced starting from natural rubber. The by-products found in natural rubber do not, of course, exist in artificial rubber. These by-products, however, probably serve to stabilize the rubber and also to act as catalytic agents during tho vulcanieation. Hence it was necessary to find stabilizers for the artificial product and also to study further the action of accelerators. Patents taken out on these subjects show the intensity of the research work which has been done. All the artificial rubber was mixed with stabilizers or “conservators”-mostly strong organic bases having a low-vapor tension-and with accelerators. The number of accelerators tried a t that time was very great. Another difference in the treatment is made necessary because natural rubber is formed from isoprene and contains five atoms of carbon for two unsaturated bonds, whereas the artificial rubber contained six carbon atoms. There is, therefore, one-sixth more carbon in the compound, having the same number of unsaturated bonds. As a rule, in organic chemistry higher homologs of the same class of compounds have higher melting and boiling points, so we find in the case of artificial rubber prepared from dimethylbutadiene the maximum of elasticity a t a higher temperature than in natural rubber. Likewise, it becomes hard and brittle and breaks sooner. As a matter of fact, it was necessary to jack up all motor cars provided with full tires of artificial rubber if they were standing for a long time in cold, icy weather. I t is necessary, therefore, to make additions to artificial rubber in order to make it softer and more elastic a t the normal temperature. As a general rule i t can be said that the vulcanized prod-
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uct obtained from artificial rubber was good enough for the average technical use of soft rubber, although not so good as the product obtained from natural rubber. For hard rubber our polymerization gave excellent results. By vulcanization, goods of high elasticity and of excellent dielectric behavior were obtained, which for certain purposes were no doubt superior to those obtained from the natural product. Artificial rubber as a chemically pure hydrocarbon is free from ash and other impurities, and for electrical purposes is more uniform and reliable. For dental work it was preferred to natural rubber. Competition
The question as to whether artificial rubber, as made in Germany during the war, could be competitive with natural rubber in quality and in price, has partly been answered by the fact that interest ceased when rubber prices became normal. Since that time noticeable progress has been made, both in quality and yield. To obtain effective competition, further study in the manner of polymerization is necessary, raw material has to be cheap to be available or to have the possibility of producing artificial rubber in unlimited amounts. Although no exact figures for world consumption of rubber per year are available, 300,000 tons is probably not far from right. Not many raw materials could be bought in such large quantities without raising the price. The capital needed for a rubber plant producing 3000 tons, or less than 1 per cent of the world’s consumption, would be 10 to 15 million dollars, to which the plant necessary for the raw material (acetone) would have to be added. As has been stated, the polymeriaation required several months, so the working capital, too, had to be relatively large. Possibilities
An advantage of organic synthesis is that a large number of products may be obtained by similar methods. It is, therefore, possible to choose products possessing certain properties in a very high degree. At the beginning of the dyestuff industry the main purpose was not to replace the natural dyestuffs, but to find uses for certain new products, surpassing the known dyestuffs in shade, brightness, affinity to the cotton fiber, etc. A small production of high-priced products permitted further investigation, improvements in the manufacture of raw materials, and later the practically complete replacement of natural dyestuffs by synthetic products. In our case it would be necessary to study all the products capable of polymerization and work out carefully the conditions of polymerization, to study further the best conditions for vulcanization, and to try to find the best utilization for products with special qualities. If, in this way, application for certain high grades of synthetic rubber could be found, it could serve as a basis for manufacture, which would mean more research and new discoveries. Much has been done already. Experiments without number have been made and checked, both for polymerization and vulcanization. It is, however, only a start and more work must be done. Many problems have already been solved by technical chemistry and many laboratory syntheses have become technical reactions. Certainly synthetic rubber will replace natural rubber for special uses, and the writer hopes that some time in the future it will replace it practically entirely, not only with products of equal but of better qualities. Doubtless later on we will know not only one “rubber” but a series of different rubbers appropriate for the various technical uses.
