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.