Technology & Breakthroughs

First expanded TPU in markets

The product is marketed as Infinergy. Photo: BASF/adidas

BASF has produced the world’s first expanded thermoplastic polyurethane (E-TPU), which it is now marketing under the name Infinergy. The first product making use of this entirely new, closed-cell particle foam went on the market recently: the new “Energy Boost” running shoe from adidas, which the sportswear manufacturer has developed in close partnership with BASF. This means that BASF is now able to boast a broad portfolio of particle foams, from rigid foam (EPS) to soft and stretchy foam plastic (E-TPU).

To produce Infinergy, the well-established BASF thermoplastic polyurethane (Elastollan) is expanded at the Ludwigshafen headquarters, using an innovative procedure. In the process the benefits of thermoplastic polyurethane (TPU) are retained, but to these are added the typical properties of foams.

Being a particle foam, Infinergy has a low bulk weight, with a density of about 110 kg per cu m, and, after processing on standard moulding machines, a moulded part weight of between 200 and 320 kg per cu m. That puts the new foam somewhere between expanded polystyrene (EPS) or polypropylene (EPP), which are generally lighter, and the heavier elastomeric polyurethane foams. Thanks to its closed-cell structure, Infinergy also absorbs little water, less than two per cent by volume in 24 hours. Like the TPU on which it is based, it is also characterised by very high breaking elongation (between 100 and 150 per cent depending on the density), tensile strength (approximately 600 kilopascals) and abrasion resistance, combined with good chemical resistance.

One of the main features 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 per cent. This is therefore significantly higher than other particle foams like EPS (less than 20 per cent) or EPP (30 per cent).

Infinergy does not lose its 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 kilopascals, the material performed about 75 per cent better than EPE. After 40,000 load cycles, the thickness of the test piece of E-TPU was still 37 mm (starting figure: 40 millimeters), 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.

With the help of crack splitting and pressure filling, Infinergy can be processed on the same moulding machines as expanded polypropylene (EPP).