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News

E-TPU polyurethane particle foam revolutionises running shoe

BASF Aktiengesellschaft : 16 August, 2013  (Application Story)
Using BASF’s Infinergy foam, Adidas has developed the Energy Boost, a thoroughly new running shoe with unique spring and cushioning properties. Its outstanding feature is the midsole, the central element of every running shoe. It is made from a new particle foam which absorbs the shock impact on the foot during jogging, while simultaneously cushioning the foot. The high rebound effect of the material provides the runner with an energy return not offered by any other running shoe.

Today’s sports shoes are genuine high-tech products compared to previous versions. Worldwide, runners spend an estimated EUR15 billion on their equipment. Even people who only go jogging occasionally now look for lightweight soles that provide optimal shock absorption and enhanced running comfort.

The many different materials used to manufacture the shoes make them lightweight and stable. These materials enable the shoes to be tailored precisely to suite the style of running, the customer’s expectations and the various running disciplines. All this can be confirmed with a look around the development laboratories of sports shoe manufacturers: test subjects run over sensitive pressure measuring plates which analyse and evaluate every step. High-speed cameras film the movement patterns and the heel-to-toe motion of the feet in the tiniest detail. All this information goes into the subsequent shoe design, which enables athletes to achieve new personal bests – and also offers recreational runners optimal running conditions.

Infinergy is made of expanded thermoplastic polyurethane (E-TPU). “E-TPU is manufactured by foaming the starting material, TPU granules,” explains Dr Uwe Keppeler, material and process developer at BASF. “After pretreatment with pressure and heat, the individual granules measuring up to 5mm each are blown up like popcorn.”

In the process, its volume increases tenfold to produce oval foam beads with tiny, closed gas bubbles inside. “These sealed air cells make the lightweight foam beads very elastic and provide the desired rebound effect. The individual beads can be imagined as being like tiny footballs: The more air they contain, the better they bounce and rebound back,” says BASF research scientist Dr. Frank Prissok.

For every midsole, Adidas needs about 2500 of these small beads. To convert them into the desired form, they are treated with hot steam, a process in which the outmost layer of the beads melts slightly, causing them to bond into a stable shape. The internal air cell structure remains unaffected by this process.

Bottom Left: Midsole of 5-10mm
fused foam beads made of Infinergy

Bottom Centre: Cross section of foam
bead with air bubbles
measuring 30 to 300 microns

Bottom Right: Molecular structure
of the expanded thermoplastic
polyurethane with stiff and elastic
elements

 

 

 

 

 

 

A midsole made from Infinergy can be compressed to an extreme degree - by about half of its volume at a pressure of 2bar. This property is particularly effective in absorbing the shock impact on the foot. As soon as the compressive impulse subsides, the foam returns to its original shape at lightning speed. The sole therefore absorbs the runner’s energy, but then returns a large amount to the runner. This rebound elasticity of the individual Infinergy beads literally transforms the running shoes into energy stores.

“When the foot is pushed off the ground, the force expended is largely returned to the athlete. This results in completely new and improved running behaviour. Many runners have told us that the shoe feels like it is almost alive,” explains Gerd Manz, Senior Innovation Director Global Brands at Adidas.

BASF’s specialty foam is not only lightweight, but also remains elastic over a wide temperature range. The particles make sure that the Energy Boost shoe also retains its positive properties at -20C.

Unpuncturable tires

In the future, BASF’s foam could make the “flat-less” bicycle tire that many cyclists are dreaming of become a reality. Infinergy is also potentially suitable as flooring for running tracks. Other applications are envisaged in the automotive industry.

Cross section of a single foam bead:
The scanning electron microscope image shows
tiny air bubbles (measuring 30 to 300 microns)
enclosed in the BASF plastic.

Design Pointers

More comfortable jog Unique foamed material delivers spring and cushioning properties
High rebound effect Special air cell structure of welded foam beads returns energy
Light and flexible Expanded thermoplastic polyurethane opens up new applications
Combines softness and resilience Two characteristics
previously thought irreconcilable
Materials testing Heel strike returns energy
much better than alternatives
Temperature stability 3x temperature
resistance compared to conventional foam
Foam softer than previous materials ...but cell structure
remains intact for much longer

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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