German synthetic rubbers, tires and special rubber
The topic of synthetic rubbers in stories World War II is one of the most underrated. While some have written about oil, along the lines of "it's a war of engines," nothing is usually said about rubber and the rubber made from it, without which no type of military equipment, no vehicle, could have traveled anywhere or participated in any battle.
From my perspective, I can say that World War II was a war not only of motors and oil, but also of synthetic rubber, which accounted for the overwhelming majority of raw material consumed in the production of rubber, tires, insulation, and other diverse products in the major warring countries. And this wasn't just a matter of autarky, as is incorrectly said about Germany, but also the fact that natural rubber no longer met the needs of the warring powers in either quantity or quality. To understand this issue better, we must look not only at synthetic rubber itself but also at its grades and their applications. There were mass-produced grades, primarily used for tires, but also specialty grades, essential for, for example, aircraft manufacturing.
As an example, we take the German Buna stamps, because they are described quite well in the literature without any kind of omissions, pseudo-secrecy or distortions.
Buna S and the intricacies of tire production
The first and main commercial synthetic rubber was Buna S, produced by emulsion copolymerization of divinyl and styrene. However, due to slightly different formulations and degrees of polymerization at different plants, different grades were introduced. The Schkopau plant produced Buna S1, and the Güls plant produced Buna S2. During the war, these two plants produced almost all of this type of Buna. For example, at the peak of production in 1943, Germany produced 117,6 tons of synthetic rubber, of which 110,5 tons were Buna S, of which S1 accounted for 67,7 tons and S2 for 34,6 tons.
The main areas of application were automobile tires, since Buna S had higher resistance to thermal aging and abrasion compared to natural rubber, and electrical cable insulation, due to its high heat resistance and aging resistance.
German Buna S differed from American divinyl-styrene synthetic rubber in that it was very rigid and, before being used for vulcanization, underwent a thermoplasticization process—controlled heating to 130 degrees Celsius for 30-40 minutes, which increased its plasticity. This process was already being used in tire factories by companies such as Continental, Fulda, Dunlop, and Phoenix, and was the first stage in converting the rubber into a finished product.
Plasticity was needed for roller processing, when carbon black was added to the rubber. Carbon black was applied to rubber sheets and rolled through rollers, which pressed the carbon black into the rubber mass.
Since Buna S became very hot during rolling on rollers, the process itself took place at a temperature of 110-130 degrees, and could begin to vulcanize spontaneously, it was lubricated with oil, usually linseed, and the rollers themselves were cooled from the inside with water.
The rubber sheets were cooled slightly, after which they were coated with sulfur powder and other vulcanizing agents, and then rolled again through the rollers.
Another important property of rubber was its adhesiveness, the ability of its sheets to adhere to one another. Buna S lacked this quality, which significantly complicated tire manufacturing. A raw or "green" tire made from natural rubber could be simply glued together from several strips, to which cellulose fibers, reinforcing the tire's interior, were glued and rolled on rollers. Buna S did not allow for this.
The Germans developed a different technology. They didn't have enough cotton, a cellulose fiber, so they replaced it with viscose, a fiber derived from wood. However, viscose had a fairly high moisture content (typically 12-15%), absorbed moisture intensively, and therefore bonded poorly with buna. The viscose fiber was first impregnated with a mixture of latex (latex is a very fine emulsion of rubber in water; the Germans produced latex from all types of synthetic rubber), resorcinol, and formaldehyde in a special immersion bath. The fiber was then coated on both sides with thin sheets of buna and passed through calender rollers. The result was a strip into which the viscose fibers were pressed.
The next step was to create a tire rim reinforced with steel wire. Strong carbon steel wire doesn't bond well with rubber. Before the war, the wire was coated with a copper-zinc alloy, which, when vulcanized, formed a strong bond with the gray rubber. However, during the war, due to copper and zinc shortages, lead coating or phosphating of the wire were used for the same purpose, which, however, degraded the bond quality.
Next, several strands of wire, typically between 4 and 12, were passed through an extruder, where a very hard mixture of rubber, carbon black, and sulfur, ready for vulcanization, was applied to the wire. In the next stage, the rubberized wire was formed into a rim on a rotating mold. A wedge-shaped rubber strip, which played a crucial role in the formation of the tire, was then glued to this rim.
Then, a strip of fiber was placed inside the mounting drum, with the edges protruding inward. The rubber-edged rims were carefully positioned according to the dimensions. Now it was time to glue the strip to the rims.
This is where the problem of the stickiness of synthetic rubber arose, as it barely bonded. The Germans used a phenol-formaldehyde resin called koresin as an adhesive, which they produced by condensing isobutylphenol with acetylene. Workers applied this resin to the parts to be bonded using a syringe and pressed them together using a heavy hand roller. The work required skill, as the hard buna often failed to bond, and the tire blank could pop out of the drum.
Once the tire was mostly bonded, a thick strip on the outside, or "tread," was glued onto it and rolled on with a roller.
This is what the glued-on "green" tire looked like.
Finally, the "green" tire was placed in a double-sided steel mold, which had a profile cut out for the tire's outer surface. A thick rubber hose was inserted into the mold and inflated with water or steam at up to 20 atmospheres of pressure to ensure the tire was pressed firmly into the mold. The mold was heated to 140-150 degrees Celsius, where the vulcanization process of the "green" tire occurred.
This is a vulcanization mold. The photos aren't German, but they're what they are. For some reason, the most important and interesting things in the industry aren't photographed.
Korezin... we skipped over it, huh? Isn't that interesting? We could talk a lot about strategic decisions, about the genius of a commander, we could argue about the thickness tank Armor and gun calibers. The fact is, if the Germans didn't have 100 tons of koresin per year, the Wehrmacht would be practically incapacitated due to a lack of tires and, consequently, vehicle transport. This small factory, staffed by a few chemists, a dozen engineers, and fifty skilled workers, produces a chemical without which defeat in the war is almost inevitable.
It's not as if the Germans were completely satisfied. The situation forced them to intensively improve the quality of their Buna. In the summer of 1942, when a shortage of linseed oil was discovered, German synthetic rubber factories were forced to develop a new grade—Buna S3—made using dipropoxide, caustic alkali, and a mixture of fatty acids derived from the products of the Fischer-Tropsch synthesis.
The new product proved to be somewhat superior to previous grades. Buna S3 was well suited for the production of automobile tires, as it offered the same strength and wear resistance but had better tack, simplifying the tire forming process. According to German data, tires made from Buna S3 were 35% better than the highest-quality natural rubber tires. At the very end of the war, Buna S4 was developed—a modification of the S3 grade with greater ductility, which could be processed on rollers without prior thermoplasticization. Little is known about it, as very few were produced.
Special grades of buna
Buna S solved the main problems with rubber by replacing natural rubber in tire production, especially since its supply, even smuggled, had ceased completely by mid-1943. However, natural rubber, obtained by reclaiming old tires and other natural rubber products, was still used in tire production for the V-belts glued to the rim.
But later, due to specific requirements for rubber, it became necessary to create specialized grades of synthetic rubber. These were produced in relatively small quantities, but were of exceptional importance for the production of military equipment.
Among them was Buna SW (soft), a flexible synthetic rubber with a low styrene content, which was used to make products that operate at low temperatures, in particular, aviation Cables and gaskets. At -40°C, Buna SW had a tensile strength of 350 kg/cm² and could also elongate by 400%. Natural rubber completely lost its ductility and strength under such conditions. This was of paramount importance for aviation, especially high-altitude aviation.
We open the hood of the Bf-109 and immediately the rubber hoses, tubes, cables catch our eye... Each plane probably had at least 30 kg of various rubber, that is, synthetic rubber of different brands.
Buna SR is a blend of two grades: Buna S and Buna R (a variant of Buna S, but with a polymerization degree increased to 90-95%, instead of the usual 55-60%). This blend was distinguished by its high frost resistance and was used to manufacture car tire inner tubes, as well as rubber parts for ships, submarines, and torpedoes.
Buna SS is a synthetic rubber with a high styrene content (53% instead of the usual 27-30%). This made it less wear-resistant, elastic, and frost-resistant. However, it was easily calendered and had the highest insulating properties of all German Buna grades. Therefore, its applications included the production of inner tubes and electrical cable insulation.
There were two more special brands: Buna SSE for the pharmaceutical industry and Buna SSGF for the food industry.
In 1943, the following special grades were issued: Buna SW – 193 tons, Buna SS – 334 tons, Buna SSE – 118 tons, Buna SSGF – 337 tons.
There was also another line of synthetic rubber produced for special purposes. Using the same copolymerization technology with active mixing and the addition of various substances regulating the polymerization process, the Germans developed two grades of divinyl and acrylonitrile copolymers, initially known as Buna N and Buna NN, and later renamed Perbunan and Perbunan Extra, respectively.
These polymers and the rubber made from them proved highly resistant to oils and solvents, leading to the production of specialized rubber products such as gaskets, rubber suspensions, shock absorbers, clutches, and so on. These synthetic rubber grades were also used to manufacture belts for coal mine conveyors and special tires for aircraft tail wheels. In 1943, the Leverkusen plant produced 3341 tons of Perbunan and 221 tons of Perbunan Extra.
In addition, all of the above-mentioned grades of synthetic rubber were used to produce latex, which was used to manufacture adhesives, rubberized fabrics, and various specialty products. Until the very end of the war, German chemists worked on new grades of synthetic rubber, not only divinyl-styrene but also those made from other components. They conducted research and testing, producing impressive experimental samples that later became trophies of victory and contributed to the further development of synthetic rubber.
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