The German chemicals company BASF (Ludwigshafen; www.basf.com) has developed what it says is the world’s first expanded thermoplastic polyurethane (E-TPU) and is marketing it under the brand “Infinergy”. The first commercial product made with the closed-cell particle foam is the “Energy Boost” running shoe developed by BASF in cooperation with sportswear manufacturer Adidas. To produce Infinergy, the thermoplastic polyurethane “Elastollan” is expanded at Ludwigshafen in a process said to preserve the benefits of TPU while adding the typical properties of foam.

With a low bulk weight and a density of about 110 kg/m³, after processing on standard moulding machines Infinergy has a moulded part weight of 200-320 kg/m³. This positions the new foam somewhere between the generally lighter EPS or EPP foams and the heavier elastomeric PU foams. Thanks to its closed-cell structure, it is claimed to absorb less than 2% water by volume in 24 hours. Like the TPU on which it is based, BASF says the new foam is also characterised by very high breaking elongation (100-150%, depending on the density), tensile strength (around 600 kilopascals) and abrasion resistance as well as good chemical resistance.

One of the main features of Infinergy, according to BASF, is its “excellent recovery behaviour,” which, in part, reflects the closed-cell structure of the foam. This, the group says, makes the E-TPU the “most elastic particle foam currently available on the market.” What’s more, it boasts a rebound height of up to 55%, “significantly higher” than EPS (less than 20%) or EPP (30%) particle foams. and in high-frequency fatigue tests remained permanently compressed, thus returning almost all the energy applied to it. Even at extremely low temperatures of minus 20°C, BASF says Infinergy “does not go stiff.”

With the help of crack splitting and pressure filling, the new E-TPU can be processed on the same moulding machines as EPP, with the pre-foamed particles pressed together under hot steam and bonded together. Because polyurethane binders adhere so well to Infinergy, BASF says other processing techniques such as glueing and foam sealing of the particles can also be used.

Other than shoes, potential uses for the new foam include floors for sports halls, bicycle inner tubes and upholstery. Infinergy also might replace rubber as a cushioning element where a low-weight material is needed, BASF believes. In biomechanical tests, the “Boost” technology is said to have proved superior in many ways when compared with ethylene-vinyl acetate (EVA) cushioning systems predominantly used by the shoe industry. In fact, it is claimed to show three times better temperature resistance in its deformation behavior as well as improved durability under cyclical dynamic loading.