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As traced back to the first half of the 19th century, attempts at synthetic rubber production were simply to reproduce the original material due to the rubber shortage following World War II. Synthetic rubber production processes differentiate between producing the rubber, which can be done via solution polymerization and emulsion polymerization and creating the final product with the help of techniques such as molding and vulcanization.

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What is synthetic rubber?

Generally speaking, synthetic rubber is any artificial elastomer. In technical terms, it is a chain of polymers synthesized from petroleum byproducts. Unlike natural rubber, synthetic rubber can withstand high temperatures, as well as be more durable and resilient. There are about twenty different chemicals used for synthetic rubber production, which yields a large number of synthetic rubber types, for instance, neoprene, EPDM, SBR, butyl rubber, fluoroelastomers, silicone and Buna N rubber.

Solution and emulsion polymerization – the two synthetic rubber production methods

The synthetic rubber production process starts with combining naphtha and natural gas resulting in monomers such as styrene and isoprene that undergo either emulsion polymerization or solution polymerization.

Solution polymerization is a reaction process in which a monomer is dissolved in a solvent, either cyclohexane or hexane, and polymerization is carried out by adding alkyl lithium as an initiator. Polymerization is executed at the temperature of 50-100°C and the period lasts for 60 minutes. The coupling agent used in this process is silicon tetrachloride. Furthermore, the flash desolvation is carried out by adding an anti-aging agent once the temperature reaches 90~100 °C and concentrating the glue at 20%. The flashed concentrated glue liquid is then transferred to the buffer tank and blended. Finally, the remaining solvent in the product is eliminated by using a squeeze-type liquid separator.

On the other hand, emulsion polymerization is commonly used for copolymer rubbers. The emulsion is created by adding water and an emulsifier prepared from rosin acid and sodium fatty acid soap in a proportionate mixture of monomeric butadiene and styrene. Although, at first, the polymerization temperature is 40-50 °C, in later stages it is controlled at 5-10 °C, which is perfect to start the polymerization reaction which is terminated once it reaches the 60% conversion rate. The post-treatment includes heating and dehydration, similarly to the solution polymerization process.

Manufacturing processes of synthetic rubber: compounding, mixing, shaping and vulcanization

Once synthetic rubber is produced, it comes in the shape of large bales which have to be formed into products ready for everyday use. There are four manufacturing steps: compounding, mixing, shaping and vulcanizing. The compounding phase is used to add chemicals that react during vulcanization to stabilize the rubber polymers and prepare them for their intended use.  The mixing step has two stages: mixing the additive into the rubber and adding the chemicals for vulcanization once the rubber is cooled.

The technique used during the shaping stage depends on the desired final product. For instance, for tire production, the preferred method is compression molding, which is one of the three molding methods alongside transfer molding and injection molding. The final process, vulcanization, occurs during the molding stage. Vulcanization manipulates the cross-connections between the polymers. If the desired product is elastic and soft, there should be fewer cross-connections between the rubber polymers.

For the harder rubber, the cross-connections should be increased. This phase includes the addition of chemicals such as sulfur and an “accelerator” to hasten the interlinking reaction, as well as the addition of “activators”, usually zinc oxide and stearic acid, the key to creating sulfur interlinks. Vulcanization is the essential step to achieving the durability and adaptability of rubber.

Styrene-butadiene rubber, the natural rubber alternative

Styrene-butadiene rubber, abbreviated as SBR, is one of the most common types of synthetic rubber, mainly due to its inexpensive production, as well as its resistant properties. By virtue of its comparable characteristics, it can be used as a natural rubber alternative. Therefore, SBR is a predominant material used in tire and other automotive parts manufacturing. Based on its production method, we can differentiate between Emulsion SBR and Solution SBR.

Carried out at 50 to 60°C, the hot emulsion process results in a branched polymer, which reflects characteristics such as low mill shrinkage, good dimensional stability, and good extrusion. On the other hand, about 5° is sufficient for cold emulsion SBR which has superior mechanical properties and better abrasion resistance. Finally, produced by termination-free anionic solution polymerization in a hydrocarbon solvent, solution SBR has tensile strengths and low rolling existence.

Processing steps involved in synthetic-rubber making

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