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 1.11 The Boundaries Between the Individual Reservoirs Are Not Sharp

The three reservoirs described above form a useful working model, not three perfectly separated layers.

In reality, continuous transitions exist between them.

Wax filling a deeper groove in the ground structure is geometrically located above the polymer surface, but it may simultaneously be retained very strongly by mechanical interlocking.

At some locations, the thin surface film may be only adsorbed, while in a neighbouring amorphous region it may already partly penetrate beneath the geometric surface.

A microcrack or an interphase gap around an additive particle may simultaneously function as:

• a surface depression,
• a mechanical wax reservoir,
• an entry pathway into the polymer surface zone.

At different locations on the ski base, all three reservoirs may therefore coexist in different proportions.

1.12 Practical Importance of Dividing the Wax into Three Reservoirs

Dividing the wax into three reservoirs makes it possible to define the objective of scraping and brushing more precisely.

First reservoir

The macroscopic surplus should be removed almost completely. Leaving it in place would cover the structure and impair the function of the surface.

Second reservoir

The thin film should be reduced selectively. The ground structure must be opened and loose or excessively thick residues removed, but it is not self-evident that the optimum objective is to remove all wax from every mechanically accessible part of the surface.

Third reservoir

The wax within the polymer surface zone should remain preserved. Its removal is not part of the desired cleaning of the ski base, but rather a possible secondary consequence of abrasion of the UHMWPE itself.

The fundamental question in brushing is therefore not merely:

How quickly does the brush open the ground structure?

An equally important question is:

At what point does the brush stop removing technological surplus and begin reducing the functional film or the wax-enriched polymer surface zone itself?

This question will be examined in the following chapters dealing with manual and rotary steel brushes.

 

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 1.8 First Reservoir: The Macroscopic Wax Layer Above the Surface

The first and most clearly visible reservoir consists of wax that remains as a continuous layer above the original ski-base surface after ironing.

Its thickness may be locally relatively high, particularly where a large amount of wax has been applied or where the wax has not been distributed uniformly by the iron.

Wax filling the deeper parts of the ground structure also belongs to this category.

This is not wax absorbed within the polymer matrix. It is wax lying above the local UHMWPE surface and mechanically filling:

• grooves,
• depressions,
• grinding marks,
• open surface defects,
• spaces between protruding fibrils.

This layer is largely a technological surplus.

If it remained on the ski base, it would cover or significantly restrict the function of the ground structure. It could increase the real contact area, impair the removal of water and contaminants, and form a soft surface susceptible to the mechanical retention of dirt.

It is therefore intended to be removed primarily with a plastic scraper and by subsequent brushing.

The macroscopic reservoir is not equally thick everywhere. Its thickness varies along the X- and Y-axes according to the local topography, the amount of applied wax, the direction in which the iron was moved, and the geometry of the structure.

1.9 Second Reservoir: A Thin Wax Film Within the Surface Microstructure

After the macroscopic surplus has been removed, a very thin wax layer remains mechanically and adhesively bonded to the surface.

It is located in particular:

• on the peaks and walls of the ground structure,
• in fine scratches,
• between fibrils,
• in surface defects,
• in open microscopic depressions,
• at interfaces between differently oriented UHMWPE regions,
• around certain additive particles and agglomerates.

This second reservoir is no longer merely a thick technological surplus.

It may contribute significantly to the resulting:

• frictional properties,
• wettability,
• surface energy,
• resistance to contamination,
• initial glide performance.

It is, however, difficult to define precisely which part of this film is still excessive and which part is already functional.

Brushing must remove wax that blocks the ground structure, but it should not automatically mean removing all wax from the peaks, walls, and microroughness of the surface.

The second reservoir is the most vulnerable to excessive brushing because it is directly mechanically accessible to the tips and side surfaces of the brush filaments.

1.10 Third Reservoir: Wax Within the Uppermost Polymer Zone

The third reservoir consists of wax that has penetrated beneath the geometric surface into the uppermost part of the UHMWPE during heating.

This wax is not stored in ordinary open pores. It is located primarily:

• in surface-accessible amorphous regions,
• in partially accessible transitional regions,
• at interphase boundaries,
• in manufacturing and surface defects,
• in microcracks,
• in areas disturbed by grinding.

Its concentration is not uniform either across the surface or with depth.

Along the X- and Y-axes, regions capable of absorbing relatively more wax alternate with areas whose surfaces consist predominantly of crystalline material or additive particles and therefore have a very low absorption capacity.

Along the Z-axis, the wax concentration is likely to decrease rapidly from the surface into the material.

This reservoir is not normally accessible to the scraper or brush as free wax. A steel filament cannot simply “comb” it out of undamaged polymer.

It may, however, remove it indirectly if the filament begins mechanically removing the uppermost UHMWPE layer in which the wax is stored.

Even a very small loss of polymer may be significant in this respect if most of the absorbed wax is concentrated within the first few hundred nanometres of the surface.

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 1.6 Effect of Additives on the Ski Base’s Capacity to Absorb Wax

Various additives and solid lubricants are commonly incorporated into UHMWPE intended for ski bases, including carbon black, graphite, and other carbon-based materials. The use of carbon particles and solid lubricants in ski-base materials is well documented in the technical literature.

Additives may affect wax absorption through several mechanisms acting simultaneously.

1.6.1 Replacement of Part of the Polymer Volume

Additive particles occupy volume that is no longer formed by UHMWPE.

A particle of carbon black, graphite, or another filler cannot absorb wax through the same mechanism as the amorphous polymer phase. As the additive content increases, the volume of polymer matrix that is potentially capable of absorbing wax decreases.

From this basic volumetric perspective, the following tendency can be expected:

The higher the additive content, the lower the maximum volumetric capacity of the ski base to absorb wax into the UHMWPE.

1.6.2 Modification of Polymer Crystallisation

Additives may act as heterogeneous nucleation sites and promote the formation of crystals. At higher concentrations, however, they may simultaneously restrict the motion of macromolecules and inhibit the growth of crystalline regions.

Both of these opposing effects have been described for carbon fillers: heterogeneous nucleation as well as restricted crystal growth caused by reduced chain mobility.

The effect of an additive on wax absorption is therefore not simply a function of its weight fraction. It also depends on whether the particular additive:

• increases or decreases crystallinity,
• changes the size of the crystalline lamellae,
• creates a rigid interphase zone around the particles,
• disrupts the continuity of amorphous regions,
• creates new defects or gaps at the particle–polymer interface.

1.6.3 Interphase Region Around Particles

An interphase zone forms around an additive particle in which the arrangement of the polymer chains may differ from that in the unrestricted polymer matrix.

Depending on the quality of bonding between the particle and the UHMWPE, this region may be:

• denser and more rigid,
• less mobile,
• or, conversely, defective and contain microscopic gaps.

A well-bonded rigid interphase region may restrict wax penetration. A poorly bonded interface may locally create space for the mechanical retention or capillary entry of wax.

It cannot therefore be assumed that every additive and every interface always reduce wax absorption in the same manner.

1.6.4 Practical Overall Relationship

When otherwise comparable ski bases are considered, a high proportion of solid additives can generally be expected to reduce the total volume of polymer matrix capable of absorbing wax.

The practical rule “the more additives, the less wax” is therefore a physically reasonable starting assumption.

It must, however, be understood as a general tendency rather than an absolute linear law. The resulting wax-absorption capacity is also affected by:

• the type of additive,
• particle size,
• particle shape,
• degree of agglomeration,
• spatial distribution,
• quality of bonding to the UHMWPE,
• the additive’s influence on crystallisation,
• the formation of defects during sintering.

Microscopic analyses of actual ski-base materials show that carbon particles may be distributed relatively homogeneously within the matrix, while agglomerates and other particulate inclusions may also be present.

1.7 Depth of Wax Penetration into the Ski Base

The ski base cannot be understood as an openly porous material that absorbs wax to depths of tens or hundreds of micrometres during hot-wax application.

Wax molecules enter primarily the uppermost part of the polymer, which becomes sufficiently heated during application and contains accessible amorphous, transitional, or defective regions.

Penetration is restricted by several factors:

• the very short period during which the ski base is heated,
• the relatively low temperature compared with the melting point of UHMWPE,
• the high molecular weight and density of the polymer,
• the high proportion of crystalline phase,
• the limited diffusive mobility of wax molecules,
• the rapid decrease in temperature with depth,
• the lack of connection between some amorphous regions and the surface.

For a working technical model, it may be assumed that during normal hot-wax application, the great majority of the absorbed wax is concentrated within an extremely thin surface region.

The following may be considered as an approximate working range:

• the main proportion of absorbed wax within approximately the first 250 to 350 nm,
• occasional or significantly lower penetration to depths on the order of up to 1 μm.

These values must be understood as estimates of the characteristic depth of the surface zone, not as a sharp boundary.

There is no interface above which the polymer is uniformly saturated with wax and below which no wax is present. The wax concentration is likely to be highest immediately at the surface and to decrease rapidly with depth.

The actual penetration depth will depend in particular on:

• the molecular size and distribution of the wax components,
• the viscosity of the molten wax,
• the temperature of the ski base,
• the duration of heating,
• the number of repeated applications,
• the density and crystallinity of the UHMWPE,
• the interconnectivity of amorphous regions,
• the content and distribution of additives,
• the condition of the surface after grinding.

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 1.3 Which Regions Can Absorb Wax

Under normal conditions, wax molecules cannot freely penetrate perfectly ordered crystalline regions of UHMWPE.

In the crystalline phase, polymer chains are tightly packed and there is insufficient free volume between them for the easy diffusion of paraffin-wax molecules. Crystalline regions therefore represent effectively closed or very poorly accessible parts of the material with respect to wax penetration.

Wax can penetrate primarily into:

• amorphous regions,
• the less ordered parts of transitional regions,
• interphase boundaries,
• defects and voids created during sintering,
• microcracks and surface imperfections,
• regions disturbed by grinding or use.

The statement that amorphous and transitional regions can accept wax does not mean that all such regions are equally accessible.

Part of the amorphous phase may be enclosed between crystalline lamellae and may not be connected to the surface by a diffusion pathway. Transitional regions are also more structurally constrained than the fully amorphous phase. Their capacity to absorb wax may therefore be lower and depends on the local density and mobility of the polymer chains.

For practical wax impregnation of the ski base, the decisive factor is therefore not merely the total proportion of amorphous material, but primarily the proportion of amorphous and transitional regions that are:

• connected to the surface,
• sufficiently mobile at the application temperature,
• geometrically accessible to the molecules of the wax being used.

1.4 Relative Proportions of the Crystalline, Amorphous, and Transitional Phases

UHMWPE is a highly crystalline polymer, but the exact proportions of the individual phases are not universal.

They depend in particular on:

• the type and molecular weight of the original UHMWPE,
• the pressing and sintering conditions,
• the cooling rate,
• the presence of additives,
• the orientation of the material,
• subsequent thermal and mechanical processing.

In the conventional two-phase description, only crystalline and amorphous regions are usually distinguished. This model is, however, too simplified for describing wax absorption, because a significant proportion of the polymer chains may exhibit intermediate mobility between the perfectly crystalline and fully amorphous states.

For a working model of the ski base, it is therefore more appropriate to consider three groups:

• the crystalline phase,
• the transitional or rigid-amorphous phase,
• the more mobile amorphous phase.

It is also necessary to distinguish between:

• the volume fraction of the individual phases,
• the typical size of the individual domains,
• the total interfacial area between them.

A material may, for example, have a high proportion of crystalline phase while simultaneously possessing a very fine lamellar structure. In that case, a large total interfacial area may exist between the crystalline and amorphous regions.

Conversely, a material having the same overall degree of crystallinity but larger crystalline domains may have a smaller total area of transitional regions.

From the perspective of wax absorption, it is therefore not sufficient to know only the percentage crystallinity. The size and spatial arrangement of the crystalline lamellae, amorphous regions, and transitional interfaces are also important.

1.5 Relationship Between Domain Size and Wax Absorption

The size of the individual crystalline and amorphous regions may affect the ski base’s ability to absorb wax in several ways.

Larger crystalline regions

Larger and more perfectly ordered crystalline regions create more extensive areas that are difficult for wax to access. If the total interfacial area is simultaneously reduced, the number of pathways through which wax may enter the polymer surface zone may also decrease.

Smaller crystalline lamellae

At the same degree of crystallinity, a finer crystalline structure may create a larger total interfacial area between the crystalline and amorphous phases.

This may increase the amount of transitional material. It does not, however, automatically result in greater wax absorption, because some transitional regions may be rigid, dense, and poorly accessible to wax molecules.

Larger interconnected amorphous regions

More extensive and mutually interconnected amorphous regions may create easier diffusion pathways from the surface into the material.

Small isolated amorphous regions

An amorphous region enclosed between crystalline lamellae may be practically inaccessible to wax, even though it is included in the total proportion of the amorphous phase.

The decisive parameter is therefore not merely the amount of amorphous material, but also its:

• size,
• interconnectivity,
• distance from the surface,
• molecular mobility,
• connection to interphase and defective regions.

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 1. What Actually Remains on the Ski Base After Hot-Wax Application

To describe correctly how manual and rotary brushes remove wax from a ski base, we must first explain where the wax is actually located after hot-wax application.

The wax does not remain on the surface as a single homogeneous layer. After ironing, cooling, and scraping, it is present in several geometrically and physically distinct regions. The individual parts of this wax “reservoir” differ in their accessibility to the scraper and brush filaments, and they also play different roles in the resulting properties of the ski base.

For the following discussion, it is useful to distinguish three basic wax reservoirs:

1. a macroscopic wax layer above the ski-base surface,
2. a thin wax film retained within the surface microstructure,
3. wax that has penetrated into the uppermost polymer zone of the UHMWPE.

This division cannot, however, be understood solely as a variation in the direction perpendicular to the surface, that is, along the Z-axis. The actual ski-base surface also changes continuously across the plane, along the X- and Y-axes. Consequently, the thickness of the surface wax, its adhesion, and the possibility of its penetration into the polymer may differ from one location to another.

1.1 The Ski Base Is Not a Homogeneous Solid Surface

A ski base made from sintered UHMWPE is not a perfectly homogeneous material. UHMWPE is a semicrystalline polymer in which the following regions alternate on the nanometre scale:

• crystalline regions,
• amorphous regions,
• transitional or intermediate regions between them.

The existence of these three distinct populations of polymer chains—crystalline, intermediate, and amorphous—is supported, among other methods, by NMR measurements of molecular mobility in UHMWPE.

Crystalline regions consist of relatively regularly arranged sections of polymer chains. The chains are tightly packed and their mobility is significantly restricted.

Amorphous regions have a less ordered structure. They contain more free volume between the chains, greater molecular mobility, and a higher degree of polymer-chain entanglement.

Transitional regions are located at the interface between a crystalline lamella and the fully amorphous matrix. The chains in these regions are partially oriented and constrained by the crystalline phase, but they are not ordered sufficiently to form a perfect crystal lattice.

These regions do not form simple, clearly separated blocks. On the nanometre scale, crystalline lamellae are interspersed with amorphous and transitional regions. In studied UHMWPE films, for example, nanodomain layers with thicknesses on the order of several tens of nanometres have been described, showing that the individual structural regions may alternate over extremely small length scales.

1.2 Variations in the Ski Base Along the X-, Y-, and Z-Axes

The ski-base surface must be understood as a three-dimensionally variable environment.

Z-axis: changes with depth

In the direction perpendicular to the surface, the following regions are encountered progressively:

• free or continuous wax above the original surface,
• wax retained within the ground structure and surface microroughness,
• a UHMWPE surface zone affected by grinding, thermal processing, and use,
• deeper polymer material that is mechanically less disturbed.

The amount of wax that may have penetrated during normal hot-wax application generally decreases rapidly with depth.

X- and Y-axes: changes across the surface plane

Changes parallel to the surface are equally important.

As one moves across the ski-base surface, the following properties continuously change:

• the relative proportion of crystalline, amorphous, and transitional regions,
• the orientation of the polymer lamellae,
• the density and size of microdefects,
• the amount of loose fibrils,
• the presence of carbon black and other additives,
• the local surface topography produced by grinding,
• the degree of thermal and mechanical damage.

Even at a constant depth, there is therefore no perfectly continuous and chemically homogeneous layer.

At one location, the exposed surface may be dominated by a relatively inaccessible crystalline phase, while only several tens or hundreds of nanometres away there may be an accessible amorphous region, a transitional interface, a microcrack, or a space surrounding an additive particle.

The result is a spatially highly non-uniform capacity to absorb wax.

The ski base should therefore not be described as a container with uniformly distributed pores. It is more accurate to imagine it as a three-dimensional mosaic of regions having different densities, molecular mobilities, and levels of accessibility to wax molecules.