As stretchy as rubber but also light and springy

June, 30, 2013 - In plastics research, there are various ways of developing innovative and potentially successful new materials. Additives help to give familiar substances new properties, while mixing polymers can lead to some interesting blends. Sometimes progress comes from using processing techniques that were previously reserved for other materials: when it came to the latest addition to the BASF foam plastics family, the company's technological expertise in particle foams was combined 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 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).

Materials expertise + processing know-how = innovation

To produce its TPU Foam, BASF thermoplastic polyurethane (trade name = 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, BASF´s TPU foam has a low bulk weight, with a density of about 110 kilograms per cubic meter, and, after processing on standard molding machines, a molded part weight of between 200 and 320 kilogram per cubic meter. 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, the polyurethane foam also absorbs little water, less than two percent by volume in 24 hours. Like the TPU on which it is based, it is also characterized by very high breaking elongation (between 100 and 150 percent depending on the density), tensile strength (approx. 600 kilopascals) and abrasion resistance, combined with good chemical resistance.

Good recovery behavior is retained - in heat and cold

One of the main features of the TPU Foam is its excellent recovery behavior 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 percent. This is therefore significantly higher than other particle foams like EPS (less than 20 percent) or EPP (30 percent).

TPU Foam 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 kilopascals, the material performed about 75 percent better than EPE. After 40,000 load cycles, the thickness of the test piece of E-TPU was still 37 millimeters (starting figure: 40 millimeters), whereas the EPE remained permanently compressed and the thickness of the test piece was reduced to about nine millimeters. This means that Infinergy returns almost all the energy that is applied to it.

Furthermore, TPU Foam – 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, TPU Foam is still very soft and stretchy and does not go stiff.

Many processing options - many possible areas of application

With the help of crack splitting and pressure filling, TPU Foam can be processed on the same molding 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.

BASF´s TPU Foam 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. TPU Foam could also replace rubber as a cushioning element, anywhere where a low-weight material is needed.

Boosting the energy and comfort of runners

When you talk to runners, they always express a 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 to the runner while the foot is pressed down. High elasticity and deformability in the material that is used will increase the efficiency with which this happens.

Previously, however, runners had to choose between wearing hard, elastic competition shoes or very soft training shoes offering a lot of cushioning. In less than three years, by working together, adidas and BASF have succeeded in resolving this dilemma. In a close partnership, the two companies developed the adidas BOOST™ technology – the world's first application of TPU Foam material.

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. 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.

In tests in the biomechanics laboratory, the adidas product developers tested 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. For example, among others, marathon world record-holder Patrick Makau was involved in the development of the Boost running shoes.

These tests showed the BOOST™ technology to be superior in many ways when compared with the ethylene-vinyl acetate (EVA) cushioning systems that are predominantly used in the industry. No other midsole on the market generates more rebound (energy return). Compared with EVA, BOOST™ shows three times better temperature resistance in its deformation behavior – less hardening in the cold and less softening at high temperatures. Its durability under cyclical dynamic loading is also significantly better than with EVA cushioning systems.

However, what nearly all the runners who tested the Energy Boost shoes highlighted above all was the very special feeling produced by the shoe when they were running. Test runners at adidas found it to be innovative and inspiring.

Source: BASF