Ski Base Basics

The photo shows the molecules of an UHMWPE ski base. The wavy sections, or lamellae, are the crystalline sections.

The base material on high-quality cross-country skis is made of Ultra High Molecular Weight Polyethylene (UHMWPE). The basic chemistry is simple, the monomer ethylene (C2H4) that is made into extremely long chains of 100,000 to 250,000 monomer units. Glide waxes are mostly made from the same monomer with the softer/warmer waxes often being single-chain molecules, or in the case of very hard glide waxes branched chains (usually less than 50 carbon atoms).

This is remarkable stuff! Very slippery, highly resistant to abrasion and impact damage, highly water repellent, and extremely low chemical reactivity (low affinity). No wonder this is the magic stuff used in joint replacements.

Time to get one thing out of the way.

UHMWPE bases do not have “pores”, they do not “soak up” or “absorb” glide waxes. This base material is what is called a semi-crystalline solid – the extremely long CH chains are squeezed together under very high pressure until the chains align in the most compact arrangement possible. Think of this, that pencil “lead” and a diamond are essentially the same, just that the molecular component is arranged differently.
Why does this matter? Because glide wax cannot “go into the pores”, or “soak into” the spaces in between the long chain UHMWPE molecules of the crystalline portion.

But there is a small portion that isn’t crystalline. Imagine a bunch of spaghetti noodles – the dry noodles can be held together tightly in a bundle and there’s no space in between each noodle, but mixed here and there imagine some cooked noodles. These cooked noodles would be loosely arranged in a disorganized clump, although squeezed tightly together they are still randomly arranged. Upon heating up, the dry noodles – representing the crystalline UHMW PE molecules – wouldn’t change, but the randomly arranged mess of squiggly noodles can expand. The squiggly randomly arranged noodles representing the non-crystalline UHMWPE molecules is termed an amorphous zone. Thus, a “semi-crystalline solid”.

This tells us a lot about how to prepare our ski bases.

But performance can be noticeably and significantly improved by adding wax. The proof is from empirical evidence, if an untreated UHMWPE base was fastest by itself, it would be used that way in international competition. But although untreated bases have been in the mix for testing for race performance, to my knowledge only once was this the best choice. That outlier situation had snow that was heavily contaminated due to pollution from a coal burning factory. Keep this in mind as it will come to play later.

Most of the speed provided by ski wax is done by a very thin layer of wax on top of the base, and in this regard the use of easy to apply liquid glide waxes will provide satisfactory performance for most skiers. For those seeking higher performance there is great value in using heat to apply glide wax. There are two objectives in using heat to apply glide wax. The most obvious is to apply that thin film of glide wax on top of the base. The second reason is to modify the relative hardness of the base, and to improve durability of the top layer.

Heating the base with melted wax will excite the base molecules, and the amorphous zones are far more responsive than the crystalline zones. The resulting expansion of the molecules in the amorphous zones allows the melted wax molecules to “blend in” (this is neither a chemical bond nor chemical solution), and once cooled the base will have different properties than an untreated base.

This may explain why skiers have different experiences about the durability of liquid glide waxes. The crystalline portion of the base is slippery, and glide wax just doesn’t stick very well to it for a long time. With short chain alkane glide wax in the amorphous zones, the top layer (liquid or hot applied) has something chemically similar to adhere to.

But far more important is that the hardness of the base has been changed. This isn’t an absolute where the hardest glide wax will always be better. Warmer glide waxes are far more slippery but also have high penetrability, glide waxes for cold conditions are much harder and have low penetrability but also not as water repellent nor as slippery.

As an example, there was a World Cup race in Finland where the temperature hovered just above the legal FIS limit (it was about - 4 °F). The ski techs only got the skis to glide well after hot waxing/scraping/brushing five times! Each hot wax application of the needed very hard polar type of wax mixed with and gradually diluted the glide wax already in the amorphous zones.

Hot Waxing does and don’ts

UHMWPE is usually listed as having a melting point between 130 and 145 °C. Glide waxes have recommended iron temperatures from 110 °C and 180 °C. And the materials in a ski will be damaged once 90 °C internal temperature is reached.

Sounds like an impossible task!

Back to chemistry, the response of UHMWPE under heat is a bit more complicated. The very long laminar sections of the crystalline zones are far less “excitable” under heat compared to the amorphous zones (look up “Van der Waals force if you want to geek out). The amorphous zone reacts very quickly to heat and start to expand at lower temperatures, perhaps as low as 120 °C (there’s another result at about 130-140 °C called “glass transformation”, but we’re getting too deep in the geek with that).

The task of hot waxing is therefore not so scary! Simply move fast – if the iron has melted the wax, then the job is essentially done. The wax itself may even become an insulator; in a recent demonstration I placed my hand on a ski base only 5 seconds after melting on a glide wax with a 155 °C melt point.

If the objective is to apply a top film, a single iron pass is probably sufficient (if the wax was melted and flowed easily). If you’re not sure, just wait for the ski to cool down before another pass.

If the objective is to modify the base property, this is where multiple iron passes or wax applications are necessary. Again, very simply just let the ski cool down before the next iron pass.

Scientific method is cool.

Twenty years ago, a common hot wax method was recommended to keep the wax in a melted state for as long as possible. The assumption was that this promoted a deep penetration of wax into the “pores” of the ski base.

I was no different, but after destroying a couple of pairs of skis, or finding that the result was slow skis, I asked myself “how does wax deep in the “pores” have any interaction with a snow crystal?”. Simple to look up the chemistry of UHMWPE, and slap myself out of my ignorance.