Germany-headquartered BASF introduces the world's first expanded thermoplastic polyurethane (E-TPU), Infinergy.

The new closed-cell particle foam has found its way into footwear application.

The new "Energy Boost" running shoe from Adidas, which the sportswear manufacturer has developed in close partnership with BASF, has utilised Infinergy’s mechanical properties, said Gerd Manz, Senior Innovation Director, Global Brands Adidas AG.

“The outstanding running characteristics of the new Energy Boost running shoes are due mainly to the combination of softness and high elasticity in the midsole, “ said Manz.

He explained that during the materials testing in the Adidas laboratory, the forces and deformation that occur during running were simulated on test machines in order to calculate the wear and the energy return. To guarantee maximum performance in all seasons and all climate zones, the shoes were tested across a temperature range from -20 to +40°C.

According to Martin Vallo, Sales Elastollan Footwear, BASF Polyurethanes GmbH, Infinergy has a low bulk weight, with a density of about 110 kg/cubic metre, and, after processing on standard moulding machines, a moulded part weight of between 200 and 320 kg/cubic metre.

That puts the new foam somewhere between expanded polystyrene (EPS) or polypropylene (EPP), which are generally lighter, and the heavier elastomeric polyurethane foams. Infinergy also absorbs little water, less than 2% by volume in 24 hours, due to its closed-cell structure. Like the TPU on which it is based, it is also characterised by very high breaking elongation (between 100 and 150% depending on the density), tensile strength (approximately 600 kPa) and abrasion resistance, combined with good chemical resistance.

Moreover, a main feature of Infinergy is its excellent recovery behaviour, which is due partly to the closed-cell structure of the foam. This makes it the most elastic particle foam currently available on the market. Tests of the resilience elasticity under ISO 8307 (the 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 about 55%. This is therefore significantly higher than other particle foams like EPS (less than 20%) or EPP (30%).

Infinergy does not lose its excellent resilience even when under a continuous load: during a high-frequency fatigue test using dynamic loads at five cycles per second and a constant pressure of 250 kPa, the material performed about 75% better than EPE. After 40,000 load cycles, the thickness of the test piece of E-TPU was still 37 mm (starting figure: 40 mm), whereas the EPE remained permanently compressed and the thickness of the test piece was reduced to about nine mm. This means that Infinergy returns almost all the energy that is applied to it.

Furthermore, Infinergy – unlike other foams – also remains highly elastic and soft over a wide temperature range: dynamic mechanical analysis has shown that, even in extremely low temperatures of minus 20 degrees Celsius, Infinergy is still very soft and stretchy and does not go stiff.

To process Infinergy, a crack splitting and pressure filling enables it to be processed on the same moulding machines as expanded polypropylene (EPP). In a number of stages, the pre-foamed particles are pressed together under hot steam and bonded together. In addition, because polyurethane binders adhere so well to Infinergy, other processing techniques are possible such as gluing and foam sealing of the particles. This procedure enables large-scale processing.

Infinergy can be used anywhere where customers require a combination of low weight, excellent mechanical properties and good durability 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, anywhere where a low-weight material is needed.