What are the main weaknesses of the existing UHMWPE ski base types? – Part I.

It is well-known that competition skis are equipped with UHMWPE ski bases where PE with ultra-high molecular weight = ultra-long molecular chains is enriched with different additives, first of all carbon black.  

UHMWPE is a type of material which is difficult to be processed. The longer the molecular chains, the more problematic the process-ability. One of the few possible processes is sintration. PE in granular form is mixed with soot and other additives and sintered with help of heat and pressure to a coil which is cut to a continuous strip which is grinded and flamed

 

Here the first issue of the existing UHMWPE ski base types originates. The coil has normally a diameter of ca. 1,5 m, the steel form is cooled down from edge to centre resulting in faster cooling of outer coil areas and slower cooling of inner coil areas. Cooling speed is directly influencing the fraction of crystalline and amorphous areas or ordered and disordered molecular structure. Molecular grid arrangement (crystalline and amorphous) is directly influencing the mechanical features and ability of the ski base material to create free cavities = to take additionally applied gliding waxes.

Only amorphous and transition fraction can take additonally applied wax. Crystalline fraction cannot be entered by any wax at all.

 

Each ski base surface revealed by each grinding is an original with a unique arrangement of crystalline, amorphous and transition fractions.

Which components of the ski base are the most important

 

 

Even if specialized magazines and articles mention many different additives and ingredients used in ski bases of competition skis, finally we will find out, if we look in much more detail, that there are only two main components which influence the most important features of each ski base type, especially now after fluor ban.

 

First is the UHMWPE itself which defines the features of the matrix. The matrix itself can get touch in contact with snow surface, that’s the reason why PE is used, because currently there is no other polymer with such a low coefficient of friction and so high hydrophobicity available. With respect to other components the matrix is responsible for the wear resistance and fixation of additives.

 

For wear resistance the most important figure is the length of molecular chains, the higher the length of molecular chains, the better the wear resistance. Reaching the value of 10.000.000 the polymer can be hardly processed.

 

Additive fixation in UHMWPE is problematic. The most additives do not form strong chemical bonds with the base matrix. Additives are bound in the matrix either by very weak chemical bonds or by mechanical retention. Especial close to the surface where the contact snow and ski base surface take place especially the mechanical retention decides.

 

The most important additive used in modern ski base types is soot or carbon black. The importance is given by the amount used in the competition ski base types which reaches up to 20 % of weight fraction in Nordic skis and up to 40 % of weight fraction in Alpine skis and by the way how carbon black is fixed in the PE matrix.

 

Some carbon black is incorporated directly in the PE matrix, but most of carbon black is filling the free space or cavities between the PE corns which are sintered together using heat and pressure. Especially the second type of soot fixation is responsible for high values of E-modulus which decides how ski base changes elastically under load.

 

What is the main difference between cold and wet ski base?

Competition skis or racing skis are normally offered with three different types of ski base: cold ski base for cold, hard and abrasive snow conditions, wet ski base for wet, soft snow conditions with water film in the interface and universal ski base for transmission snow conditions between cold and wet.

Ski base compositions differ in the used additives to comply with different snow conditions.

Soot or carbon black in cold ski base types is used to increase hardness, wear-resistance.

Graphite in lamella form in cold ski base types is used to improve gliding features and electric / heat conductivity.

Metal oxides in cold ski base types are used to increase hardness and wear-resistance.

Molybdenum disulfide in wet ski base types is used to improve water and dirt repellency.

Wax like additives in wet ski base types are used to improve gliding features.

Are the different additives used in cold and wet skis the main difference between cold and wet ski base types?

I would say NO, they aren’t...

I would say that the main difference between cold and wet ski base types is the different ratio of crystalline and amorphous regions or fractions and the different wax intake capacity of those fractions.

UHMWPE which is the main material used for production of racing ski base types is a semi-crystalline polymer which consists of crystalline - for waxes and most types of additives inaccessible - regions and amorphous regions where the most additives are allocated and where the additionally applied wax is connected with the ski base. Crystalline and amorphous regions are normally separated by transitional regions which are - compared to micron-sized crystalline and amorphous regions - in nano-meter scale.

Cold ski base types have the majority of crystalline regions which are harder, compacter and more wear-resistant, but cannot absorb any additives and waxes (thus they are more or less pure UHMWPE), wet ski base types have a higher fraction of amorphous regions which are softer, modified by additives, contain cavities to absorb waxes...

 

This is the main difference between cold and wet racings skis :-)