The launch of Infinergy by BASF is the debut of the world’s first expanded thermoplastic polyurethane (E-TPU). It is being used in Adidas’ new “Energy Boost” running shoe.
BASF’s Infinergy closed-cell particle foam is said to show good resilience, high durability and abrasion and chemical resistance. (photo: BASF)
The latest addition to the BASF foam plastics family combined the company’s technological expertise in particle foams with its experience with thermoplastic polyurethanes. The result is the world’s first expanded thermoplastic polyurethane (E-TPU), which BASF is now marketing under the name Infinergy. The first product making use of this entirely new, closed-cell particle foam is the “Energy Boost” running shoe from Adidas, the sports equipment manufacturer. Developed in partnership with BASF, the new shoe went on the market earlier in 2013. With Infinergy, BASF now offers a broad portfolio of particle foams, from rigid (EPS) to soft and stretchy plastic (E-TPU).
Development
Infinergy began with BASF’s existing Elastollan TPU, which was expanded at the company’s Ludwigshafen headquarters, using a novel procedure. The process retains the benefits of TPU and adds the typical properties of foams. As a particle foam, Infinergy has a low bulk weight, with a density of about 110kg/m3, and a moulded part weight of between 200 and 320kg/m3 after processing on standard moulding machines. That places the new foam somewhere between expanded polystyrene (EPS) or polypropylene (EPP), which are generally lighter, and the heavier elastomeric PU foams. The closed-cell structure means that Infinergy absorbs little water; less than 2% by volume in 24h. It also possesses high breaking elongation (100 – 150%, depending on density), tensile strength (approximately 600kPa) and abrasion resistance, along with good chemical resistance.
The first product making use of this closed-cell particle foam E-TPU is the “Energy Boost” running shoe from Adidas. (photo: Adidas)
Good recovery behaviour
Infinergy’s good recovery behaviour is due in part to the foam’s closed-cell structure. BASF says that it is the most elastic particle foam currently available. Tests under ISO 8307 (ball rebound test) and under DIN 53512 (using a pre-set pendulum hammer) show that the rebound height for E-TPU can be as much as 55%, which is significantly higher than other particle foams like EPS (less than 20%) and EPP (30%).
Infinergy demonstrated retained resilience under continuous load, in a high-frequency fatigue test using dynamic loads at five cycles/s and a constant pressure of 250kPa. After 40,000 load cycles,the thickness of the test piece of E-TPU was 37mm (starting figure: 40mm). The comparator expanded polyethylene (EPE) sample remained permanently compressed and the thickness of the test piece was reduced to about 9mm. This means that Infinergy returns almost all the energy that is applied to it. The material remains highly elastic and soft over a wide temperature range. Dynamic mechanical analysis found that it remained soft, pliable and stretchy at temperatures of minus 20°C.
BASF’s E-TPU is being used in Adidas’ new running shoe. (video: BASF)
Processing options and possible areas of application
With the help of crack splitting and pressure filling, Infinergy can be processed on the same moulding machines as EPP. Pre-foamed particles are pressed together under hot steam and bonded together in a number of stages. Because PU binders adhere well to Infinergy, processing techniques such as gluing and foam sealing can be used, which enables large-scale processing.
Infinergy can be used in any applications requiring a combination of low weight, high durability and good mechanical properties across a wide temperature range – for example, in the sports sector or for technical applications in logistics. Possible uses include floors for sports halls, bicycle inner tubes and upholstery. Infinergy could also replace rubber as a cushioning element, where a low-weight material is needed.
At Adidas, the forces and deformation that occur during running were simulated in order to calculate the wear and the energy return. (photo: Adidas)
Boosting the energy and comfort of runners
Runners constantly talk of the need for “good cushioning”. In running shoes, this is the function of the midsole: in just a few milliseconds, it absorbs the kinetic energy generated by the runner as the foot lands and returns some of it while the foot is pressed down. High elasticity and deformability in the material used will increase the efficiency of this process. In the past, runners had to choose between hard, elastic competition shoes or very soft training shoes with a lot of cushioning. In less than three years, Adidas and BASF resolved this dilemma with the development of Adidas Boost technology, which is the world’s first application of the Infinergy material.
During testing in the Adidas laboratory, the forces and deformation that occur during running were simulated in order to calculate the wear and the energy return. The shoes were tested across a temperature range from -20 to +40°C. Tests in the biomechanics laboratory examined the stability of the running shoes and the effect of the products on the performance of athletes, using the VO2max test which measures the maximal oxygen uptake by an athlete. The comfort, durability and how they felt to wear were tested by working directly with runners at all levels of ability. Patrick Makau, the marathon world record-holder, was involved in the development of the running shoes. Compared with ethylene-vinyl acetate (EVA), Boost is said to show three times better temperature resistance in its deformation behaviour; less hardening in the cold and less softening at high temperatures. Its durability under cyclical dynamic loading is also said to be significantly better than established EVA cushioning systems.
Visible success
Energy Boost was one of the most successful market launches for a running shoe in the history of Adidas; it has sold about a quarter of a million pairs of the innovative shoes worldwide since February 2013. The shoe was voted “Best Debut” by the world’s leading running magazine, Runner’s World, and Dennis Kimetto set a new track record in this year’s Tokyo Marathon while wearing Boost shoes.
