IPB scientists , together with partners from MLU, have succeeded for the first time in degrading synthetic rubber enzymatically. Synthetic rubber has become an indispensable alternative to natural rubber in modern life. Its manifold use in engines, sports equipment and everyday objects, but above all as a component of car tires, requires the production of several million tons per year. As much as it is needed - it is difficult to break down. In particular, a lack of hydrolyzable functional groups in the polymer backbone makes it difficult to recycle synthetic rubber. Up to now, old tires have been disposed of mainly by mechanical shredding. The resulting rubber particles can be used as low-quality fillers in new rubbers. Other efforts to degrade synthetic rubber chemically involve rare metals as catalysts or flammable highly toxic solvents.
While natural rubber, which is obtained as latex from the Brazilian rubber tree Hevea brasiliensis, is composed of isoprene units that exist exclusively in the cis form, its synthetic counterpart consists of a mixture of cis- and trans-1,4-polyisoprene. Nature itself has taken care of the degradation of natural rubber. Rubber oxygenases, produced by rubber-degrading bacteria, initiate the oxidative cleavage of the polyisoprene backbone, and the resulting cleavage products serve as a carbon source for the microorganisms. Whether these bacteria or their enzymes are also suitable for enzymatic degradation of synthetic cis-trans-polyisoprene has not yet been investigated.
For their synthetic rubber degradation experiment, the Halle scientists used a polyisoprene with a cis-trans ratio of 56 to 27 and the latex clearing protein LcpK30, a rubber oxygenase from the strain Streptomyces sp. K30. The biggest hurdle to efficient enzymatic degradation of synthetic polyisoprene, the chemists hypothesize, is its poor solubility in water, which significantly reduces the enzymatic access to potential cleavage sites. Application of surfactants as emulsifiers was not an option, because they might denature the enzyme. However, the Halle scientists achieved surfactant-free emulsification of the synthetic rubber by low-frequency ultrasound and addition of a hydrophobic co-solvent in water after sonication.
After the emulsions were produced, various parameters such as buffers, other solvents, stirring speed, temperature and enzyme concentrations were successfully optimized. For the first time, the Halle chemists managed an enzymatically break down of polyisoprene into oligoprenoids with 1 to 13 intact isoprene units. At the same time, they compared the degradation rates of their approach with those of conventional methods and showed that the rubber oxygenase LcpK30 can tolerate trans-isomers of polyisoprene. In addition, modifications to the enzyme led to an increase in its thermostability and solvent resistance.
According to the authors, their work could serve to establish biotechnological processes and dispersion methods for the degradation of polyisoprene and may lead to a significant paradigm shift for future reprocessing strategies of synthetic rubbers. Since 2017, natural rubber has been classified as a critical raw material by the European Commission, making the development and optimization of chemical or biotechnological recycling processes for scrap tires, natural rubber and synthetic polyisoprene even more urgent.