LIGHT-FOOTED FOR LONGER WITH A NEW MIDSOLE MATERIAL
21 April 2005 - Bayer MaterialScience AG

The development team has succeeded in reducing the density of the foam to 230 kg/m³ and thus making it 30 percent lighter than standard polyurethane foam.

“The lower density makes Bayflex® Active Lightweight the material of choice for the sports and leisure shoe segment,” says Dr. Erhard Michels, an expert in shoe technology at Bayer MaterialScience. “When developing the product, we naturally made sure that the typical benefits of polyurethane were retained despite the reduction in weight.”

After all, polyurethane is well known for giving shoes exceptional wear comfort due to its excellent damping properties, good resilience and balanced energy absorption. Despite its low weight, the polyurethane mid-sole performs the cushioning function on its own without the need for any extra air or gel cushions. The material's ability to absorb and transfer energy ideally supports the flexing action of the foot and ensures fatigue-free walking and running – a particularly important quality characteristic for sports shoes.

In addition, athletes want shoes that last. A runner will wear conventional shoes out after only 800 to 1,000 kilometers. After 200,000 simulated steps in Bayer's testing laboratory, the new soling material still had 68 percent of its compressibility, which is a record in this weight category and also ensures that the shoes retain their damping properties over long distances.

The lightweight polyurethane also performs impressively in production. Apart from a few minor machine modifications, it can be processed in the same way as conventional polyurethane. Moreover, it is suitable both for the economical direct molding process and for unit sole production. In direct soling, the material's excellent adhesion properties simplify the production process because it bonds permanently and without adhesives to leather, thermoplastic polyurethanes or polyurethane at the initial foaming stage. The polyurethane midsole also makes for greater freedom in shoe design.

It can be easily colored by color injection shot by shot, and yields soles with accurate mold detail reproduction. Traditionally, shoe manufacturers have used ethyl vinyl acetate (EVA) for the soles of lightweight leisure and sports shoes, but, although the material weighs less than conventional polyurethane because of its lower density, it is more difficult to process and quickly loses its cushioning properties due to compression and shrinkage.

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