New master’s thesis identified the best circular economy methods to recycle end-of-life tires

In his master’s thesis for Tampere University, Mikko Sirén explored the most potential technologies to turn scrap tires into secondary raw material and value-added products.

The research project started in May 2020 from an initiative of Black Donuts that desired up-to-date research on the recycling of discarded tires as part of their circular economy initiatives. The company constantly cooperates with customers, technology suppliers, and universities to enhance the greener future of the tire industry.

Mikko Sirén studied the feasibility of using pyrolysis in recycling end-of-life tires.

– The ideas of circular economy are inherent in our business philosophy, as we consider every stage and aspect of tire production and consumption. End-of-life tires and their re-use possibilities are a key topic at the moment for both our customers and us, Technical Director Martti Päivinen says.

Studying circular economy as his minor, Mikko was eager to dive deeper into the suggested research topic. Black Donuts gave him free reins to do his research. In terms of the research context, some additional detective work was needed.

– The world of tires was totally new to me, so it was both challenging and interesting to learn more about tire industry, Mikko says.

– And valuable for us, thanks to Mikko’s fresh and new ideas, Martti appraises.

After having an understanding of the plausible processes for recycling, Mikko decided to concentrate on two of them in his evaluation: devulcanization and pyrolysis. Although both methods offer an environmentally attractive way to transform waste into something useful, there was no consensus about their economic and environmental feasibility for the tire industry.

Mikko Sirén studied the feasibility of pyrolysis in recycling end-of-life tires

Devulcanization means reprocessing the vulcanized rubber back to its original formulation. The process causes a breakup of certain chemical bonds so that the rubber can be mixed again.

– Devulcanized rubber can replace natural and synthetic rubber, so it has good potential to increase productivity due to savings in raw material purchases. Consequently, this will also decrease the level of total emissions, Mikko explains.

Benefits of pyrolysis

Pyrolysis, on the other hand, is an endothermal process where organic material is degraded in the absence of oxidizing material. In practice, when tires are pyrolyzed, they break down into carbon black, gas, oil, and metal residue.

– Pyrolysis means a 100 per cent recycling of tires. After separating the materials, it is practically possible to recycle everything to make new products.

As valuable by-products, the process generates gas and oil which can be used to support the energy production of the factory. For Mikko and the BD team, the most interesting subject for future research is carbon black.

– Next, we are going to explore the quality of the pyrolytic carbon black more deeply in order to evaluate its most efficient usage in tire construction and production, Martti reveals.


In his analysis, Mikko concluded that pyrolysis as a more flexible, less risky, and less expensive method would be optimal for the special needs of a tire factory.

While operating devulcanization would practically require its own organization, incorporating pyrolysis into current production is less resource-dependent in terms of people and space.

All that is required is an extra space of around 1500m2 plus an additional source of discarded tires, as it is unlikely that own production would produce the required amount of input material.

According to Mikko, pyrolysis has numerous applications of interest to green technology. Its benefits are two-fold.

– While pyrolysis may have a smaller direct impact on the emissions than devulcanization, it contains huge potential as a tool to support circular economy. It not only provides an outlet for waste streams but also produces renewable fuel sources by upgrading used tires to value-added products, he concludes.