Sometimes, plastic ski boots break for no apparent reasons. I remember my days when I was in the ski boot business and we had warranty calls for “exploded” polyurethane (PU) ski boots shells. In December 2021, I bought a new pair of Nordica boots and after using it 21 times, it just broke at the toe.
Earlier this week a French friend of mine showed me how some Nordica boot shells after years of storage could be broken like one would break a large Easter egg chocolate in tiny parts. All that prompted me to investigate the technical origins of that mechanical breakdown. The culprit is called hydrolysis and it’s both a very common and frustrating form of failure for plastics, particularly PU, that are stored for long periods under certain conditions.
Hydrolysis is a chemical reaction where water (hydro-) breaks (-lysis) chemical bonds. In the context of PU, we have a polymer made up of long chains of repeating units, linked by specific chemical bonds (often called ester or urethane linkages). Water has a huge influence on this material. Ambient air humidity slowly attacks and breaks the ester or urethane bonds within the polymer chains. When the bonds break, the long polymer chains that give the plastic its strength and flexibility become shorter and shorter.
As the chains degrade, the material loses its mechanical properties. It becomes brittle, loses its elasticity, and its strength dramatically decreases. Eventually, the material becomes so brittle and weak that even a small stress (like putting on the boot, or even just temperature fluctuations) can cause it to shatter into many small pieces, much like a dried-out, old piece of chocolate. While water is the primary agent, several factors accelerate this degradation, like higher temperatures that increase the rate of chemical reactions, to the point that storing ski boots in a hot attic, garage, or shed will significantly speed up hydrolysis.
Even moderate warmth over many years will contribute to its deterioration. Humidity plays an active role. The more moisture in the air, the more readily water molecules are available to attack the polymer bonds. Add the time element to that slow process (we’re talking years, like 5 to 15+ years depending on the specific plastic formulation and storage conditions). Then this will vary with specific PU formulations as not all PUs are equally susceptible. Some formulations are more resistant to hydrolysis than others, which is why certain ski boots hold up better than others.
Comes also exposure to ultraviolet (UV) Light. Sunlight and UV rays oxidize and further weaken the polymer, especially if boots were stored in an attic or garage with important temperature swings. Even in storage, UV exposure from fluorescent lights or brief outdoor use, creates free radicals that attack the PU’s molecular structure. If direct UV isn't the main cause for these problems, general environmental exposure can also play a minor role in degradation. PU often contains plasticizers (softening chemicals) that leach out over time. As they evaporate or migrate, the material hardens and cracks.
You might also notice a greasy film on old boots—this is degraded plasticizer seeping out. The problem is PU because of its specific properties. Boots need to be stiff but also have some flex and they must be solid enough as well as able withstand cold temperatures that make PU more brittle. If stored in a cold garage, cycles of freezing/thawing sped up the breakdown. Finally it seems that from the 1980s to the early 2000s many PU formulations were prone to hydrolysis.
Modern boots often blend TPU (thermoplastic PU), which slows degradation. The choice of PU, combined with typical storage conditions (often in cold/warm garages, or attics, where they experience huge temperature fluctuations and varying humidity), makes them prime candidates for hydrolysis over extended periods. Morale of this story:
Take much better care of your ski boots so they don’t blow up from under you while you’re launching down Corbet Couloir at Jackson!
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