Method to degrade synthetic tire rubber revealed by researchers

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Researchers from Martin Luther University Halle-Wittenberg (MLU) and the Leibniz Institute for Plant Biochemistry (IPB) in Germany have found that enzymes are able to degrade synthetic polyisoprene.

The specific conditions necessary for this to happen were created and exploited by the researchers. Polyisoprene is the main component of natural rubber, a material used for the production of car tires. Until now, the degradation of polyisoprene was only possible with a composition similar to natural rubber. The MLU and the IPB believe that the research could provide important information for a move towards a circular economy.

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Natural rubber is used for the production of polyisoprene, which is then used to manufacture different types of rubber. Polyisoprene is a long chain molecule formed by linking hundreds or thousands of smaller isoprene molecules.

“Various bacteria are able to degrade natural polyisoprene using enzymes,” explained MLU chemist Vico Adjedje.

With global demand for rubber products exceeding the existing stock of natural rubber, the raw materials used for these products are produced primarily by chemical synthesis. The natural and synthetic versions share similar properties but have several differences in the structure of the molecules they are composed of.

With the help of their research teams, Prof. Dr. Wolfgang Binder from MLU and Junior Prof. Dr. Martin Weissenborn from IPB discovered a method to break down artificially produced polyisoprene using an enzyme called LCPK30.

“We are the first to have successfully put polyisoprene into a form that the enzyme can also work with,” Binder said. In doing so, the researchers drew inspiration from nature.

“Our hypothesis was that the synthetic polyisoprene had to be present in an emulsion for the enzyme to work properly,” Adjedje added.

The researchers explain that milk, which consists mainly of water and fat, provides a typical example of an emulsion. It forms globules of a few micrometers and its fine distribution in water makes the milk cloudy. Like grease, polyisoprene is practically insoluble in water. Nature manages to distribute it evenly in water as milky white latex milk, which is harvested from rubber plantations and then processed into natural rubber.

The researchers were inspired by latex milk and then managed to evenly distribute the polyisoprene produced by synthesis in water, using a specific solvent. The enzyme conformed to the artificial emulsion and therefore remained intact during the reaction time, breaking down the long molecular chains of the polyisoprene into much smaller fragments.

“A lot happens to the starting material before it becomes a finished tire: the chains of molecules are chemically cross-linked to change the mechanical properties,” Adjedje said. “Plasticizers and antioxidants are added. The latter in particular present a problem for the enzyme because they attack its structure.

In the future, the researchers aim to break down other similar substances found in car tires and say the results could also encourage a recycling economy.

“We could further process degradation products into fine chemicals and fragrances – or [to]reproduce new plastics,” Binder said.

The researchers used LCPK30 as it is found in nature. Weissenborn and his research team are currently working to optimize the enzyme so that it becomes less sensitive to solvents and triggers other reactions.

The study appeared in the Green chemistry newspaper.

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