Tyre recycling that treads new ground The majority of rubber recycled from car tyres is currently used for relatively low-quality applications, rather than in the production of new tyres. A more effective method of recovering rubber from used tyres could help close the loop between recycling and production, as Ir. Hans van Hoek and Associate Professor Wilma Dierkes explain. Although a significant proportion of the rubber in used tyres is currently recycled, only a relatively minor amount is used again in the production of new tyres. This is a topic which is the focus of a lot of attention in global research, as countries seek to use raw materials more efficiently, not least because of environmental issues. Based at the University of Twente in the Netherlands, Dr. Wilma Dierkes and Ir. Hans van Hoek are working on the Closing the Loop project, an initiative aiming to help overcome the issues which currently limit the use of recycled rubber in car tyre production. “After
Closing the Loop
Closing the loop: re-use of devulcanized rubber in new tires Dr. Wilma Dierkes University of Twente ET/MS3/ETE Drienerlolaan 5, 7522NB Enschede T: +31(0)53 489 47 21 E: w.k.dierkes@utwente.nl W: https://www.utwente.nl/en/et/ms3/researchchairs/ete/ W: http://www.windesheim.nl/onderzoek/ onderzoeksthemas/technologie/kunststoftechnologie Dr. Geert Heideman, Associate Professor of the professorship of polymer engineering at the university of applied sciences Windesheim, is one of the members of the supervising group of this project. Ir. Hans van Hoek is also a member of his research team. Dr. Wilma Dierkes, Associate Professor of Sustainable Elastomer Systems in the Faculty of Engineering Technologies of the University of Twente, is the principal investigator of this project. Ir. Hans van Hoek is a PhD candidate in the Faculty of Engineering Technology at the University of Twente. Over the last 15 years, he has worked as a teacher in mechanical and control engineering courses, while he also has experience in marine engineering on both motor and steamturbine-driven ships. His day to day job is teaching at the university of applied sciences Windesheim, Zwolle, the Netherlands.
Compact laboratory setup for crosslink analysis.
a successful project using EPDM rubber, in this project the focus is on re-using rubber from tyres for road vehicles. We are looking at the de-vulcanisation process and aim to make the de-vulcanizate more widely applicable,” says Ir. Hans van Hoek, the project’s main researcher.
The results of the project so far are very promising, with the pure de-vulcanizates showing a tensile strength of 8 Megapascal and good characteristics in terms of strain-at-break and other important parameters. Devulcanisation The de-vulcanisation process is much more sensitive than rubber reclamation, another method used to recover waste material. With reclamation the main polymer chains in rubber are broken, whereas in the de-vulcanization process they remain intact and only cross-links are disconnected. “During vulcanization you get mono-, di- and polysulfidic cross-links. The de-vulcanisation process does not de-crosslink the monosulfidic cross-links,” explains Ir.
Surface of vulcanized blend of 50/50 %wt of de-vulcanized rubber and virgin rubber.
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van Hoek. Researchers aim now to optimize process conditions with a view to improving the quality of de-vulcanized rubber. “My first target was to increase the tensile strength of the revulcanized de-vulcanized rubber,” says Ir. van Hoek. “The minimum tensile strength we have obtained now is about 8 Megapascal.” The de-vulcanized rubber will always be used in a blend with virgin compounds. “When you mix the de-vulcanized material with virgin rubber, a low tensile strength of the revulcanizate will have a significant influence on the tensile strength of the blend,” points out Ir. van Hoek. “That’s why our primary focus was on the tensile strength.” Researchers are now exploring issues around blending of virgin and de-vulcanized rubber, aiming to identify means to optimise the quality of the eventual mixture. A tyre is of course subjected to
significant stresses and strains on the road, part of the natural wear and tear on a car, that’s why the rubber of car tyres has to meet rigorous standards, which of course apply too to the blend of virgin rubber with de-vulcanizate. The results of the project so far are very promising, with the revulcanized pure devulcanizates showing good characteristics in terms of tensile strength, strain-at-break and other important parameters. An important further step is to consider whether this can be scaled-up to industrial levels, to provide an efficient means of producing good quality recycled material. “Companies are interested in using this on a larger scale,” stresses Dr. Wilma Dierkes, the project’s Principal Investigator. This work is driven to a large degree by environmental concerns. An efficient, reliable method of compounding de-vulcanized rubber with virgin materials could have a significant impact in this respect, yet meeting the quality requirements of the final material remains the focus of Ir. van Hoek’s research. “We are not yet ready with the whole process and there is still more work to do,” he continues.
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