Hand or Rotary Brush - That Is the Question? Service People Version - Wax Layers - Part. No. 1/3

 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.