DuPont: Grommet strips made of Zytel® nylon and friction-reducing fluoroadditive Zonyl® enhance power and precision in tennis

Innovative grommet strips made of a particularly low-friction modified grade of DuPont™ Zytel® nylon 66 (www.dupont.com) help deliver even more power to tennis rackets from HEAD (Kennelbach/Austria) and allow for even greater shot precision. In order to reduce the friction coefficient of the base material, the manufacturer uses DuPont™ Zonyl® MP 1300, a fluoroadditive which – just like other DuPont™ Teflon® products – belongs to the fluoropolymer product family from DuPont. The grommet strips will be initially used in the ‘MicroGel Extreme’ range of tennis rackets for professional and serious amateur players, which were launched during the first quarter of 2009. Plans are in place to extend their usage to other racket models.

It is in this context that HEAD has entered into a licensing agreement with DuPont Fluoropolymer Solutions, effective 1 January 2009. It allows the sports equipment manufacturer to use the Teflon® brand in the marketing of its rackets – a brand which is amongst the most famous in the world and is synonymous for minimal friction and excellent sliding properties.

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Zonyl® fluoroadditive is a finely-ground polytetrafluoroethylene (PTFE) powder, which – when added in small quantities – can improve the tribological properties of engineering thermoplastics such as nylon (PA) and polycarbonate (PC). Moreover, the grade used in this application reduces deposits on the mold and enhances the surface quality of the molded part. During production of the grommet strips, the additive is fed directly into the injection molding machinery in powdered form, where it is mixed in the cylinder with the Zytel® ST801 base material. The ideal concentration of 1 wt. % for this application was set by polymer specialists at HEAD working in close cooperation with DuPont material experts. “The use of this fluoroadditive ensures that the strings glide more easily through the deviating points in the grommet strips, which means that less energy is lost through friction when the racket hits the ball,” explains Ralf Schwenger, director R&D racket sports at HEAD. “At the same time, the racket’s so-called sweet spot, in other words the surface area which produces the greatest acceleration and precision during shot-making, is extended. This means that the player is still able to play the ball powerfully and precisely even if it hasn’t met the center of the racket – an important factor in today’s tennis, which is played at a greater tempo and with more power than ever.”

The impact-resistance modified, unreinforced PA 66 Zytel® ST801 (ST = super tough) has been the material of choice for grommet strips for many years, due to its combined benefits of very good mechanical properties and excellent processability. “The grommets themselves are exposed to very high mechanical and dynamic forces when the racket is strung. This requires the material to be both strong and resistant to impact, in order to avoid the grommets breaking and damaging the delicate strings,” continues Schwenger. “We have had good experience with Zytel® ST801 in the past. Moreover, we use the same material for the so-called bumper – the part at the end of the racket frame which protects it from mechanical damage such as chips and grazes. In order to provide the polymer with the required level of abrasion- and impact resistance for this application, it is strengthened using aramid fibers.”

DuPont de Nemours (Deutschland) GmbH, Neu-Isenburg, Germany

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