Ski wax

Before a ski can slide, it must overcome the maximum value static friction, ${\displaystyle F_{max}=\mu _{\mathrm {s} }F_{n}\,}$, for the ski/snow contact, where ${\displaystyle \mu _{\mathrm {s} }}$ is the coefficient of static friction and ${\displaystyle F_{n}\,}$ is the normal force of the ski on snow. Kinetic (or dynamic) friction occurs when the ski is moving over the snow.[8] The coefficient of kinetic friction, ${\displaystyle \mu _{\mathrm {k} }}$, is less than the coefficient of static friction for both ice and snow.[9][10] The force required for sliding on snow is the product of the coefficient of kinetic friction and the normal force: ${\displaystyle F_{k}=\mu _{\mathrm {k} }F_{n}\,}$.[11] Both the static and kinetic coefficients of friction increase with colder snow temperatures (also true for ice).[10]

Snowflakes have a wide range of shapes, even as they fall; among these are: six-sided star-like dendrites, hexagonal needles, platelets and icy pellets. Once snow accumulates on the ground, the flakes immediately begin to undergo transformation (called metamorphism), owing to temperature changes, sublimation, and mechanical action. Temperature changes may be from the ambient temperature, solar radiation, rainwater, wind, or the temperature of the material beneath the snow layer. Mechanical action includes wind and compaction. Over time, bulk snow tends to consolidate[12]—its crystals become truncated from breaking apart or losing mass with sublimation directly from solid to gas and with freeze-thaw, causing them to combine as coarse and granular ice crystals.[13][14] Colbeck reports that fresh, cold, and man-made snow all interact more directly with the base of a ski and increase friction, indicating the use of harder waxes. Conversely, older, warmer, and denser snows present lower friction, in part due to increased grain size, which better promotes a water film and a smoother surface of the snow crystals for which softer waxes are indicated.[15]

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  Dendritic snowflake—micrograph by Wilson Bentley.
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  Platelets and needles, two alternate forms of snowflakes.
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  Fresh, dry snow with newly formed bonds, showing a grain boundary (top center).
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  Cluster of ice grains in wet snow at a low liquid content—grain crystals range 0.5 to 1.0 mm.

Colbeck offers an overview of the five friction processes of skis on snow. They are the: 1) resistance due to plowing of snow out of the way, 2) deformation of the snow over which the ski is traveling, 3) lubrication of the ski with a thin layer of melt water, 4) capillary attraction of water in the snow to the ski bottom, and 5) contamination of the snow with dust and other non-slippery elements. Plowing and deformation pertain to the interaction of the ski, as a whole, with the snow and are negligible on a firm surface. Lubrication, capillary attraction and contamination are issues for the ski bottom and the wax that is applied to reduce sliding friction or achieve adequate grip.[15]

Typically, a sliding ski melts a thin and transitory film of lubricating layer of water, caused by the heat of friction between the ski and the snow in its passing. Colbeck suggests that the optimum water film thickness is in the range between 4 and 12 μm. However, the heat generated by friction can be lost by conduction to a cold ski, thereby diminishing the production of the melt layer. At the other extreme, when the snow is wet and warm, heat generation creates a thicker film that can create increased capillary drag on the ski bottom.[15] Kuzmin and Fuss suggest that the most favorable combination of ski base material properties to minimize ski sliding friction on snow include: increased hardness and lowered thermal conductivity of the base material to promote meltwater generation for lubrication, wear resistance in cold snow, and hydrophobicity to minimize capillary suction. These attributes are readily achievable with a PTFE base, which diminishes the value added by glide waxes.[16] Lintzén reports that factors other than wax are much more important in reducing friction on cross-country skate skis—the curvature of the ski and snow conditions.[17]

A variety of glide waxes are tailored for specific temperature ranges and other snow properties with varying wax hardness and other properties that address repellence of moisture and dirt. The hardness of the glide wax affects the melting of the snow to lubricate its passage over the surface and its ability to avoid suction from meltwater in the snow. Too little melting and sharp edges of snow crystals or too much suction impede the passage of the ski. A tipping point between where crystal type dominates sliding friction and moisture content dominates occurs around 26 °F (−3 °C). Harder waxes address colder, drier or more abrasive snow conditions, whereas softer waxes have a lower coefficient of friction, but abrade more readily. Wax formulations combine three types of wax to adjust coefficient of friction and durability. From hard to soft, they include synthetic waxes with 50 or more carbon atoms, microcrystalline waxes with 25 to 50 carbon atoms and paraffin waxes with 20 to 35 carbon atoms.[18] Additives to such waxes include graphite, teflon, silicon, fluorocarbons, and molybdenum to improve glide and/or reduce dirt accumulation.[19]

Glide wax can be applied cold or hot. Cold applications include, rubbing hard wax like a crayon, applying a liquid wax or a spray wax.[20] Hot applications of wax include the use of heat from an iron, infrared lamp, or a "hot box" oven.[21]

The role of glide wax is to adapt and improve the friction properties of a ski base to the expected snow properties to be encountered on a spectrum from cold crystalline snow to saturated granular snow. Modern ski bases often are made from ultra-high-molecular-weight polyethylene (UHMWPE). Kuzmin asserts that UHMWPE is non-porous and can hold neither wax nor water, so there is no possibility for filling pores; furthermore, he asserts that UHMWPE is very hydrophobic, which means that wet snow does not appreciably retard the ski and that glide wax offers little additional ability to repel water. He notes that clear bases are more durable and hydrophobic than those with carbon content.[1] The same author asserts that texture is more important than surface chemistry for creating the optimum balance between a running surface that's too dry (not slippery enough) and too wet (ski subject to suction forces). In warm, moist snow, texture can help break the retarding capillary attraction between the ski base and the snow.[21] Giesbrecht agrees that low wetting angle of the ski base is key and also emphasizes the importance of the degree of surface roughness at the micrometre scale as a function of snow temperature—cold snow favoring a smoother surface and wetter, warmer snow favoring a textured surface.[22] Some authors question the necessity to use any glide waxes on modern ski bases.[23][24]

Ski wax may contain chemicals with potential health affects including per- and polyfluoroalkyl substances (PFASs). Levels of perfluorinated carboxylic acids, especially perfluorooctanoic acid (PFOA), have been shown to increase in ski wax technicians during the ski season.[32][33][34]

When skiing, the friction between the snow and skis causes wax to abrade and remain in the snow pack until spring thaw.[35] Then the snowmelt drains into watersheds, streams, lakes and rivers, thereby changing the chemistry of the environment and the food chain. PFASs in ski wax are heat resistant, chemically and biologically stable, and thus environmentally persistent.[36] They have been shown to accumulate in animals that are present at ski venues.[37] The International Ski Federation (FIS) announced to introduce a ban on PFASs in waxes in all competitive ski disciplines from the winter season of 2020/21.[38]