It’s downhill all the way for polymers in the world of skiing technology. Mind you, the same could be said for any of the advanced materials now being used in the skis, boots, bindings and other associated equipment that allows skiers to weave their way down the mountains. Far from falling out of favour, polymers are being developed for an increasing number of applications as ski and boot manufacturers strive to make the journey from top to bottom quicker, safer and more comfortable for skiers and snowboarders alike.
DuPont has been working with ski equipment manufacturers for more than 25 years, developing resins to suit the demanding conditions imposed by a rapid ride over icy humps and freezing temperatures. The company has come up with new materials to meet the needs of winter sports equipment manufacturers as they invent innovative ways to ski, such as the snowboard.
One of the latest forms of acrobatic ski is called the Snowblade, which has been developed by the Salomon group for skiers wanting to perform all sorts of acrobatic feats. The ski is only 90cm long and its monocoque construction combines many different components in such a way as to make the ski feel more ‘lively’ and reactive to the slightest movement of the skier’s feet. These components need to he made from tough materials, and DuPont's Delrin resin fits the bill.
Delrin – Acetal Resin
The security lever on the Snowblade, which locks the boot onto the fixation, is made from Delrin 107, a UV-stabilised acetal resin with very high viscosity. This property gives the material very high impact resistance at temperatures down to -20°C as well as high flex and tensile strength. Two other pieces on the Snowblade are also made from Delrin - supports that are screwed to the body of the fixation to house the vibration dampers. These not only provide the required mechanical strength, despite being very thin, but can also be brightly coloured to fit in with the design of the ski. Delrin resin can be produced in a wide range of colours - an essential requirement given the bright designs that are now de rigeur on the slopes. Salomon has also recently introduced a range of skis called ‘Carve’ skis, which have an enlarged ski-tip and end to help the skier execute tighter turns. Delrin is being used for the blocks used to adjust the bindings.
Properties of Delrin
Delrin has many applications in areas other than winter sports, being a high performance resin. However, its combination of high crystallinity and low glass transition temperature make it a very useful material for winter sports applications and for sporting goods in general. It has a balance of toughness and stiffness properties that make it more metal-like in its mechanical behaviour, and it offers high fatigue resistance - an important property in ski-wear applications in which a binding, say, is repeatedly put under stress during the course of a single downhill run.
Another key property of Delrin for ski applications is its resistance to moisture - the material is unaffected by spending hours covered in snow and ice, figure 2. It is resistant to other chemicals too, and has low wear and a good coefficient of friction. Of course, these properties can be enhanced or tweaked for particular components of ski equipment. Tougheners can be added to give higher impact resistance, pigments and UV stabilisers may be used to introduce and maintain the desired appearance, glass fibres can be added for higher stiffness and Teflon or Kevlar additions will improve the material’s wear and friction characteristics.
Zytel – Polyamide Resin
Another key material being used in many winter sports applications is Zytel, DuPont's polyamide 6 and 66 resin. Salomon is using Zytel in its snowboards for the base of the fixation that accommodates the ski boot and houses the entire locking system. DuPont worked with Salomon to produce a grade of material that offers a flex modulus of 9000 MNm-2 after moisture conditioning, and very good mechanical properties down to temperatures of -20°C.
Like Delrin, Zytel can incorporate many additives, pigments and modifiers. In Salomon’s snowboard, glass fibres give the material higher stiffness and tensile strength, and yet despite their presence, the material still has an attractive surface appearance and so does not need painting. Other options with Zytel include the addition of tougheners for improved impact resistance or Kevlar for better wear properties.
Hytrel – Thermoplastic Elastomer
A third DuPont material, offering different properties and so attracting different applications to Zytel and Delrin, is Hytrel. Hytrel is a thermoplastic elastomer ether ester, and grades are available with intermediate properties between those of thermoplastics and those of elastomers. The material offers very good flexibility at low temperatures, which is vital for components of ski boots and cross-country ski shoes, as well as toughness, elasticity and good impact, fatigue and creep resistance.
Examples of Material Usage
By working with equipment manufacturers, DuPont has developed grades of these materials and others to provide a range of benefits for many different ski components. A few examples are shown here.
Figure 1 shows a touring binding manufactured by Fritschi AG of Switzerland. Delrin 107 is used in all the blue parts and in the white heel holder, while Zytel 80633 is found in the heel base-plate, the front base-plate and the head. The materials gave Fritschi more design freedom and lowered the weight of the binding compared to previously used materials. Both Delrin and Zytel give the binding its much needed long life under the demanding conditions of cross-country skiing.
Figure 1. A touring binding from Fritschi AG using Delrin and Zytel.
Delrin is also used in a shoe guide for a cross country ski binding produced by Salomon. The material was chosen because it can be printed by sublimation techniques, allowing the company to emblazon its logo on the part. Additionally, Delrin has excellent ‘non-stick’ properties, so less snow accumulates on the component this might otherwise prevent the boot sole and the shoe guide from ‘mating’ and reduce the performance of the skier.
Figure 2 shows another set of components for a binding produced from Delrin 107, again for Salomon. The body of the binding, two wings, two springs and a flexible blade are all formed from the material, giving better settings and improved contact between the ski shoes and the bindings.
Figure 2. Ski binding components.
Stiff Ski Boots
Figure 3 provides an example of how Hytrel can be formulated with Kevlar to make stiffer ski boots. The boot is made by Rossignol-Lange, who wanted increased stiffness so that the skier’s ankle is held more firmly, allowing them to better control the skis. Kytrel-Kevlar is used in the external component shown and gives the desired improvement in stiffness without increasing the thickness or the weight of the boot. The material was also formulated so that it could be injection moulded to produce this and other components.
Figure 3. Stiff ski boots.
Flexible Cross Country Ski Boots
Hytrel also features in Salomon’s ‘Optima Ultra Light’ ski boots. The material, selected because of its good flex fatigue, impact resistance and modulus at low temperature, is used to make the front strap of the boot, and gives the wearer greater comfort, especially when bending forwards.
Figure 4 gives a final couple of examples of the uses of DuPont’s materials in boots, this time in cross country ski boots. These Salomon boots are used for top level competition and so must offer extremely high performance. Hytrel is used to produce the blue component shown - the material allows the two functions of collar and spring to be incorporated into one part, which has good fatigue resistance and the necessary high modulus at low temperatures. The red part, a component of the yellow boot, is made from Delrin. This was tested at the Lillehammer Winter Olympics in 1994. It reduces the effort required from the skier during the forward and backward movements of the shoe and the ski through enhanced ‘springiness’.
Figure 4. Cross country ski boots.
DuPont’s polymers also turn up in a number of other applications, from snowboard boot fixations and step-in housings and bindings to teaching devices designed to help beginners keep the tip of their skis together. So if you are out on the slopes this winter, whether beginner or expert, the chances are that parts of your skiing equipment will rely on DuPont’s materials for their performance - something for which your ankles and feet will be very grateful.