Skis have long been made of materials which exhibit a low coefficient of friction to provide a fast and smooth gliding surface. Theoretically the sole of a ski should be as smooth as possible and should consist of a material not eroded by icy snow in such a manner that the surface becomes corrugated, causing increased friction forces. The material must also exhibit an extremely low surface tension.
Materials such as polypropylene, polymethacrylate and polytetrafluoroethylene would be expected to be suitable as gliding materials owing to their low surface tensions and correspondingly low coefficients of friction. However, these materials would suffer if used to make skis or ski soles in that they are not durable enough to not be marred by ice, snow and sometimes dirt, gravel or debris skis slide against during use. Thus, the use of these materials has not been of much interest in manufacturing skis and ski soles.
Polyethylene, on the other hand, is a material having useful characteristics in ski and ski sole applications, which stem from a high resistance to oxidation and good mechanical properties including elasticity modulus, high tensile strength, and high breaking load, all factors that increase with increasing molecular weight. However, polyethylene exhibits a surface tension of about 31 to about 33 dynes/cm, and thus the use of wax coating on the ski sole is recommended.
Polyethylene of high molecular weight (HMW) and of ultra-high molecular weight (UHMW) are used for skis and ski soles due to the good mechanical properties and sufficient hardness they exhibit. Despite some desired properties, even these higher molecular polyethylenes suffer from deep scratches and corrugations caused by friction on icy surfaces. Ski wax is used in order to eliminate the corrugations and irregularities on the sole surface, to lower the surface tension of the ski sole surface, and to provide a lubricant between the sole surface and the snow-covered ground.
In general, ski waxes are composed predominately of linear paraffins which, owing to their chemical structure, are partially soluble in the polyethylene. This solubility, however, decreases as the degree of polymerization is increased, and in polyethylenes of high and ultra-high molecular weight, the solubility of paraffins is reduced drastically such that the wax does not strongly cling to the ski sole but tends to be easily rubbed off. The impregnation capacity of the ski soles by the paraffin wax depends essentially on absorption exhibited due to the physical nature including porosity of the material. Polyethylenes of high molecular weight tend to have low impregnation capacities due to the high amount of polymerization which deleteriously affects porosity of the material.
To improve wax retention on HMW and UHMW polyethylene ski sole surfaces, particular ski waxes were developed such as "CERA F" wax, a perfluorinated paraffin, available from various suppliers including Miteni of Italy and Hoechst Aktiengessellschaft of Germany. Coatings of the CERA F wax exhibit surface tensions of about 16 to about 18 dynes/cm, as opposed to about 28 to about 30 dynes/cm for normal paraffin wax. However, CERA F is not sufficiently soluble in polyethylene to cling well.
Ski soles have also been made of HMW polyethylene and graphite. Graphite functions as a solid lubricant and is a hard material exhibiting a low surface tension and a low coefficient of friction. The static coefficient of friction (.mu.) of polyethylene on polyethylene is 0.2 whereas .mu. for graphite on graphite is 0.12. The addition of graphite to the polyethylene: lowers the coefficient of friction of the ski sole from a value of .mu.=0.2 for polyethylene to a value of .mu.=0.12 for graphite; renders the ski sole and ski sole surface harder and more homogeneous in depth rendering the sole less susceptible to abrasion; helps maintain the same degree of gliding after the surface has been marred, scraped or corrugated during use; and produces a non-porous surface benefiting from the mechanical properties of the graphite which acts as a lubricant. Despite the advantages of mixing graphite with polyethylene to form ski soles, the mixture material nonetheless fails to meet an ever increasing demand for even faster and smoother ski sole surfaces.
Fluorographite is a solid lubricant obtained by direct fluorination of graphite with elemental fluorine. The coefficient of friction for fluorographite is 0.08, as compared to 0.12 for conventional carbon graphite. Fluorographite has been used in lubricant compositions containing fats, perfluorinated oils and TEFLON powder.
Fluorographite has also been incorporated in various polymer materials alone or associated with other lubricating or reinforcing materials. The polymer materials include is polyamidic resins as taught in Japanese Patent publication 91/252,453 (Nov. 11, 1991), ethylene-tetrafluoroethylene copolymers as taught in Russian Patent No. 899,597 (Jan. 23, 1982), epoxy-silicones as taught in Russian Patent No. 1,031,993 (Jul. 30, 1983), and fluoropolymers and polyacetals as taught in Japanese Patent publication 87/54,753 (Mar. 10, 1987). However, none of these references disclose incorporating fluorographite into high molecular weight polyethylene to form a composition which exhibits an extremely low coefficient of friction and which is strong enough to be used as a gliding surface of a ski sole.
The present invention provides polymeric compositions having extremely low coefficients of friction and surface tensions. The present invention also provides durable, long-lasting, scratch-resistant, wax-less ski soles which can withstand wear under skiing conditions and which are made from a polymeric composition having an extremely low coefficient of friction and surface tension which are far below the values achieved from incorporating conventional carbon graphite into polyethylene. The present invention also provides methods of making such compositions and for making skis and ski soles.