German Buna with a military mobilization flavor
German synthetic rubber, also known as Buna, from World War II is a very interesting story not so much a scientific discovery, but rather the creation of a complex technological chain for the production of the necessary semi-finished products used in the manufacture of the final product.
Despite much written about the Buna, its history is littered with omissions for reasons that remain unclear. Perhaps some of the documents were lost, and those involved failed to leave their memoirs or comments. Indeed, the two main developers of the Buna died shortly after the war: Walter Bock in October 1948, and Eduards Chunkur (a native of Livonia and a graduate of the Riga Polytechnic Institute) died in 1946.
The strange twists and turns of the German Buna
Although the name buna itself is derived from the abbreviation of the words Butadien and Natrium, metallic sodium was not used at all in this technology.
IG Farbenindustrie, of course, produced butadiene rubbers obtained by polymerization in the presence of metallic sodium and potassium, but these were produced in small quantities and for auxiliary purposes. Buna 32 and 115 (viscosity index) were produced using metallic sodium; the former as a softener for polymers produced by the emulsion method, while the latter, like Buna 85, produced using metallic potassium, was used to produce ebonite.
The basic technology used an emulsifying agent known as Necal BX, which contained sodium sulfate and a small amount of sodium chloride. Walter Bock, in his early work on synthetic rubber, departed from the path already trodden by S. V. Lebedev and, in late 1926, succeeded in accelerating the very slow polymerization of isoprene in an aqueous emulsion by adding a small amount of peroxides, achieving a time of up to 25 days. A German patent was granted for this method on January 15, 1927.
This is where a question arises, one to which the literature provides no clear answer. As an experienced chemist, it would have been easy for Bock to reproduce the same technology that S.V. Lebedev was simultaneously refining—polymerization in the presence of metallic sodium. But for some reason, Bock chose his own path from the very beginning.
The further development of the use of butadiene, and then of a copolymer of butadiene and styrene, also remains unclear, since styrene, of course, added wear resistance to bunion, but also seriously complicated the technology of preparation for vulcanization.
Not to mention that butadiene itself was produced not from ethyl alcohol, as was the case with Soviet technology, but from acetylene, which is obtained from calcium carbide.
The result is a rather curious technology, seemingly tweaked from the very beginning by someone, gently but insistently demanding, "Find another way." The German Buna has a distinctly military-mobilization flavor, but, as far as I know, it hasn't been studied in detail. Although it's hard to vouch for, as it's impossible to skim through all the literature.
In short, between 1927 and 1929, Bok and Chunkur, having developed the emulsion polymerization method, moved on to refining the technology for copolymerizing butadiene and styrene. Much time was spent finding the right proportions, and only on June 21, 1929, was a patent granted for butadiene-styrene synthetic rubber, or Buna S. On July 11, 1929, a patent was granted for a name that didn't reflect the essence of the technology.
Then came the Great Depression, which sent the global market plummeting, including the price of natural rubber. IG Farbenindustrie supposedly lost interest in the technology and cut back on research funding, but with Hitler's rise to power, interest in synthetic rubber was revived. This is the generally accepted version; some researchers even quipped, "Hitler as a catalyst."
However, if we look at the resulting results, it turns out that a technology was created specifically adapted for large-scale industrial production in Germany and using German raw materials. And it was largely completed before Hitler became Reich Chancellor.
I'm just curious who was so insightful as to supervise the work of two chemists. Incidentally, there is a suitable candidate. He was Lieutenant General Karl Eduard Wilhelm Groener, an officer of the German General Staff. During World War I, he was head of the General Staff's field railway department, responsible for troop transport and supplies. He then headed the Military Food Department under the government, and then the War Department in the Prussian Ministry of War. At the end of the war, he commanded the 25th Reserve Corps and, in 1918, directed German occupation policy in Ukraine. After the war, he served as Minister of Railways in the republican government, then, from 1923 to 1928, as head of the statistics department in the Ministry of War, and then, from 1928 to 1932, as Minister of War and even Minister of the Interior. This general was well versed in military mobilization and undoubtedly understood the importance of synthetic rubber for military transport, having experienced firsthand all the problems of supplying troops during the war. Such instructions could have come from him.
Butadiene
So, the technology consisted of three main stages. The first was the production of butadiene and styrene, each using a specific technology. The second was copolymerization. The third was processing and preparation for vulcanization.
The stage of obtaining semi-finished products consisted of two separate technological lines, each of which consisted of sub-stages.
The butadiene production line included calcium carbide production, acetylene production and butadiene synthesis from acetylene.
Calcium carbide production was a well-established, but highly energy-intensive, technology. Coke and quicklime were delivered to the plant in Schkopau, where, in low-pit furnaces equipped with self-baking carbon anodes developed by the Norwegian designer Wilhelm Söderberg for smelting aluminum, the coal was fused with lime at temperatures of 2000-2200 degrees Celsius, producing liquid calcium carbide. After cooling and mechanical crushing, the calcium carbide was loaded into acetylene generators, where it reacted with water to produce acetylene.
From here on, the Schkopau plant was already in socialist times, 1953. But basically, minus some alterations and restoration work, it was the same as it was during the war.
Then came the synthesis of butadiene. The first stage was the hydration of acetylene. First, acetylene was passed through large steel towers containing mercury sulfate (a highly toxic substance), from which acetaldehyde—a gaseous, flammable, and explosive substance—was released.
The second stage is aldol condensation. Acetaldehyde was fed into stirred reactors with a water jacket and a powerful cooling system, as it generated a lot of heat and the reaction needed to be carried out at a temperature of 10–20 degrees Celsius. These reactors were cascaded one after another to ensure maximum gas condensation. Aldol is a thick syrup.
The third stage is the production of butanediol. This was achieved using powerful reactors made of thick forged steel. At a temperature of 100 degrees Celsius and a pressure of 296 atmospheres, in the presence of a copper-chromium catalyst, 1,4-butanediol is produced—a viscous liquid that solidifies at 20 degrees Celsius.
Finally, the fourth stage was the dehydration of butanediol. This was accomplished using a cracking furnace with a coil filled with coke coated with phosphoric acid as a catalyst. At a temperature of 280 degrees Celsius, butanediol was converted to butadiene. The furnace emitted a mixture of butadiene, water vapor, unreacted butanediol, and various byproducts.
After all this, the butadiene was purified and refined. First, the gas was passed through a cooling tower filled with water, where butanediol and a number of high-boiling components condensed and were pumped out at the bottom of the tower. The gas containing butadiene was cooled, compressed, and sent for further purification.
The butadiene-containing gas then passed through two distillation columns, each 30-40 meters high and containing 60 trays. In the first column, lower-boiling components such as acetylene and ethylene were removed from the gas. In the second column, butadiene and butene isomers were distilled.
After this, the butadiene gas was purified of residual aldehydes by washing it in a washing tower with sodium bisulfite. The gas was then passed through a drying tower, where the remaining water was absorbed by calcium chloride. Only then could the butadiene be cooled, compressed, and stored.
Styrene
The second styrene production line. To obtain another component, styrene, benzene, produced in large quantities during coal coking, was used in Germany. It arrived at the plant in a ready-to-use form.
To process benzene, ethylene was produced from acetylene, which was taken from acetylene generators in an adjacent line, by hydration with hydrogen. This was done in a battery of large vertical reactors, each containing tubes filled with diatomaceous earth coated with a palladium oxide catalyst. The space around the tubes was filled with pressurized water, as the reaction generated significant heat and needed to be carried out at temperatures between 200 and 250 degrees Celsius.
The Schkopau plant received hydrogen via a special hydrogen pipeline from the Leuna Werke, where it was produced from water gas, which was then passed through a carbon monoxide hydrogenation reactor. The mixture of water gas and steam, at a temperature of 400-500 degrees Celsius and in the presence of a chromium-iron catalyst, was almost entirely converted into a mixture of carbon dioxide and hydrogen. This mixture, under a pressure of 28 atmospheres, was pumped into a water-filled washing tower, where the carbon dioxide dissolved in the water. Since carbon monoxide destroyed the palladium catalyst, the hydrogen for the Schkopau plant was purified in a special reactor under a pressure of 200 atmospheres, passing it through a solution of copper acetylide in ammonia.
A mixture of acetylene and hydrogen was pumped into the hydration reactor from above, with the gas flowing downward. The resulting gas was cooled and then passed through a washing and separation column, from which the ethylene was sent for further processing, and the remaining acetylene was returned to the next cycle.
Benzene alkylation took place in a reactor filled with liquid benzene, to which aluminum chloride and a small amount of hydrochloric acid were added in powder form. The reactor was heated to 90-95 degrees Celsius, and ethylene was fed from below under a pressure of 1-2 atmospheres. The resulting product was liquid ethylbenzene.
A mixture of benzene, ethylbenzene, and polyethylbenzene was pumped out of the reactor's overhead and sent to a decanter, where the catalyst, mixed with hydrochloric acid to form a heavy oily liquid, was separated and pumped back into the alkylation reactor. The mixture was then passed through two wash columns (one with water, the other with a dilute sodium hydroxide solution) to remove traces of acids and chlorides. Following this, distillation took place in three distillation columns: the first distilled off the benzene and recycled it, the second distilled off the ethylbenzene, and the third mixed the residue with fresh benzene and sent it to a separate reactor to produce ethylbenzene.
Next, ethylbenzene had to be dehydrogenated to produce styrene. A tubular reactor, the tubes of which were made of special chromium-nickel steel, was used for this purpose. Ethylbenzene in the vapor phase was mixed with water vapor superheated to 700 degrees C in a ratio of 1:10 or 1:15 and fed into the tubes. The tubes contained a catalyst—zinc oxide and alumina pressed into granules. As the tubes passed at a temperature of 600-620 degrees C, hydrogen was released from the ethylbenzene, producing styrene. The hot gas was fed into coolers, where the hydrogen was first separated (returned to the alkylation process), then condensed water, and finally a mixture of organic compounds consisting of 35-40% styrene, as well as unreacted ethylbenzene, benzene, and toluene.
This mixture was distilled in a high-vacuum distillation column to prevent both the styrene from exploding and its spontaneous polymerization. This was successful in the vacuum column, and after adding hydroquinone, which prevented spontaneous polymerization of the styrene, it was fed into a polymerization autoclave.
Only the Germans could dare work with a mixture of explosive gases—hydrogen and styrene—heated to 600 degrees Celsius. Such production requires exceptional care, because a careless tightening, a faulty seal, or a barely noticeable crack—and the gas will leak out, mix with air, and explode with glee.
Polymerization
The polymerization was carried out in a cascade of six to eight autoclaves with stainless or enameled steel stirrers, each with a capacity of 10 to 20 cubic meters. A mixture of 75% liquefied butadiene and 25% liquid styrene, along with distilled water, was added to the first. Then, the emulsifier (nekal, mentioned at the beginning), the reaction initiator (potassium persulfate), and the regulator (sulfur compounds, such as diproxide) were added. Each reactor was heated to 50 degrees Celsius and maintained at a pressure of 3-5 atmospheres. Because the reaction generated significant heat, the reactors were water-jacketed.
The mixture was circulated through all the reactors with constant stirring until snow-white particles of artificial latex formed. Regulators were periodically added to the various reactors. Monitoring the process was easy because samples could be taken from the autoclave at any stage. The process itself was continuous: new portions of raw materials were added to the first autoclave, and portions of the finished product were removed from the last.
The reaction was carried out to approximately 60% of the required material consumption, because beyond this point, the long chains of rubber molecules began to link together into a network, forming a gel unsuitable for tire production. When the reaction needed to be stopped, sodium dithionite was injected into the water-rubber mixture as it was withdrawn from the final reactor, instantly stopping the polymerization reaction.
After this, the butadiene and styrene, which remained in large quantities, had to be removed from the mixture. Butadiene was removed in a vacuum degasser, where it was quickly evaporated from the liquid, pumped out, compressed, and returned to the cycle. The mixture was then sent to a stripping column, where steam removed the styrene, which was also collected and returned to the cycle. Finally, table salt was added to the remaining mixture, which separated the rubber from the water. The rubber was then separated, washed, and dried, yielding a product suitable for further processing.
The product of deliberate effort
So, how do you like all this?
Firstly, such a complex, multi-stage technology couldn't have been achieved by chance. It was the product of long, focused efforts, persistently focused on a single goal. Such a technology required not only chemists who developed the polymerization process itself, but also a chief designer and technologist who assembled such a complex production chain consisting of many stages, each requiring its own uniquely designed apparatus. This technologist had to set the design requirements for these apparatuses. He was pure genius.
Secondly, this is undoubtedly a military mobilization technology, which relies almost entirely on coal and its derivatives, as well as lime, of which Germany had a fair amount. Chemists and engineers would have been happy to make it simpler, but apparently there was a requirement that all raw materials be purely German. No matter the cost. In the event of another war, there will be no imported raw materials.
Thirdly, Hitler only briefly considered this topic when this complex technology was in the final stages of its development and transformation into an industrial technology. When he complained in a secret memorandum in August 1936 about the inability to solve the problems of synthetic rubber, IG Farbenindustrie certainly had good reasons for such excuses. This technology is difficult to describe, let alone implement in all the required detail and launch.
The management of IG Farbenindustrie, especially Karl Krauch, undoubtedly understood that their method was still crude and needed further refinement. After the release of superheated steam, the parties apparently reached a reasonable compromise. IG Farbenindustrie, in accordance with Hitler's demands, launched the plant as quickly as possible, but also launched intensive work to improve the technology. In fact, the Schkopau plant produced 2110 tons of Buna S in 1937 and 3994 tons in 1938, indicating that this was still a pilot production facility. In 1939, output increased to 20173 tons, marking the start of large-scale industrial production.
Furthermore, the technology for manufacturing rubber products from the resulting rubber needed to be refined. To this end, in 1939, a central research laboratory was established in Leverkusen, where IG Farbenindustrie had a pilot plant, at a cost of 10 million Reichsmarks. It employed 35 doctors of science, 150 engineers and technicians, and 300 workers. This core scientific team addressed numerous issues related to the production and processing of synthetic rubber.
So, it is worth concluding that the Germans have not told even half the story of the origin and production of their buna, keeping silent about many important and interesting details.
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