High-performance sports shoes are usually manufactured from a complex combination of materials. This affects both production and recycling. In the ZiProMat project, researchers at Fraunhofer have found an alternative approach. A sole made of programmable monomaterial performs several functions and is also made of the same material as the upper. The concept combines high-tech, sustainability and recyclability.
The sole of high-quality sports shoes has different properties in different zones. For instance, the heel is extra well cushioned to absorb the shock when hitting the ground. The midsole is quite soft to promote smooth rolling, and the forefoot is slightly firmer. This requires a combination and merging of different material properties.
High-tech sports shoes
Researchers at Fraunhofer have now developed a technology where the sole consists of a single material that combines different functional zones. By designing the material structure with a specific configuration of ribs or ridges, the Fraunhofer researchers can actually programme the behaviour of the material under load.
Cooperation with Puma
In collaboration with sportswear manufacturer Puma, a team from the Fraunhofer cluster of excellence Programmable Materials CPM has already demonstrated the feasibility and benefits of this technology. Researcher Christof Hübner from the Fraunhofer Institute for Chemical Technology ICT says: "We combined our expertise in materials science with Puma's market-oriented product knowledge, which allowed us to demonstrate that the fundamental concept of programmable monomaterials can also be usefully applied in high-quality everyday products, such as sports shoes."
Circularity
The Fraunhofer CPM institutes have enlisted additional support for the follow-up project ZiProMat, which focuses on circular programmable materials. They are collaborating with experts from the Fraunhofer top cluster Circular Plastics Economy (CCPE). The aim is to find ways to recycle the sole of sports shoes together with the upper by manufacturing both parts from polyester-based materials. New applications should also be developed, with a particular focus on further optimising the recycling process.
Thermoplastic polyester
The researchers use a thermoplastic copolyester elastomer as the monomaterial for the sole. The upper and outer textile material of sports shoes is usually also made of polyester. "This offers the possibility of including a much larger part of the shoe in a sustainable raw material cycle," says researcher Maximilian Wende from the Fraunhofer-Institut für Verfahrenstechnik und Verpackung IVV.
Ribs or ridges
The soles are manufactured from sheets of copolyester. These are heated and thermoformed using a vacuum to pull them into a mould that gives the sheet its predefined structure with ribs or ridges. The sheets are then stacked and fastened together. This gives not only the outside of the sole, but also the inside the special structure responsible for its mechanical properties.
Two recycling methods
The research team uses two processes to develop innovative recycling methods. In the solvent-based process, the materials undergo a targeted, selective dissolution process, separating foreign particles. The resulting product is high-purity polyester. The pyrolysis process, on the other hand, breaks down polymers into smaller molecules. Suitable separation methods and chemical processes can then be used to recover high-quality basic chemicals such as terephthalic acid for the chemical industry.
Pure polyester from old sports shoes
"We have succeeded in obtaining pure polyester from old sports shoes made of very different materials. "Wende is delighted: "Recycling and high quality no longer have to contradict each other."
A complete value chain
The collaboration between Fraunhofer CPM and CCPE is a crucial part of the project's success. "The collaboration between different institutes, each with their own specialised expertise, allows technologies and processes to be combined to cover the entire value chain," Hübner explains. The researchers are working on expanding the range of programmable functions for high-performance sports shoes. There are also plans to apply this technology to other products, such as work shoes.
Technology platform
The ultimate goal of the ZiProMat project is to establish a technology platform for the use of programmable materials and the development of pioneering recycling methods. The platform should be applicable for various products and incorporate all used materials into a closed recycling loop as much as possible.
Opening photo: The sole made of copolyester elastomer consists of many individual layers. The functional zones are defined by the structure of ribs and ridges. Each individual layer exhibits this structure. (Photo: Fraunhofer CPM)
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