Midsole Midlife Crisis

2017 / March / 02 / All the Tricks /

very now and then our service team receives a call from a customer concerned that the sole of their boot has come off. In some cases, it’s crumbled to pieces. This problem can even occur in boots that have hardly been used. Why is this? The answer: It’s a problem with the polyurethane used to make the midsole. And the problem is called hydrolysis. We’d like to try to explain…

They sit in a box in the cellar or the loft waiting to be worn – your mountain boots. In some cases, they might still look brand new. The sight of a pair of classic leather boots can be enough to make you want to start planning your next trip to the mountains. However, leaving home with a pair of old boots that have not been worn for a long time might mean coming back with your soles in pieces. Why is this? Hydrolysis is the culprit. Hydrolysis is the chemical breakdown of a compound due to reaction with water. In our case, we’re talking polyurethane (PU). Polyurethane is used to make midsoles (the component in the middle of a boot between the outsole and the upper/ insole). It can gradually absorb moisture over time and deteriorate. The polyurethane gets harder, more porous and, at some point, starts to crumble. In extreme cases, it can disintegrate completely. And if this happens, the sole might well fall off your boots.

Polyurethane can have either an ester or an ether basis. Both compounds have their pros and cons and offer different amounts of cushioning, durability, elasticity, impact resistance, UV and microbial resistance. Most manufacturers of high-quality mountain and trekking footwear use ester-based PU. It’s the most suitable material – despite hydrolysis. So, all footwear manufacturers have to deal with the problem of ageing polyurethane. The annoying thing about hydrolysis is that it’s not visible from the outside. Deterioration takes place from within. Even new looking boots may have a crumbling construction. We strongly recommend taking your old boots on a test run before you wear them on a long trip, especially if they’ve been sitting around in the loft or the cellar. A visual inspection is not sufficient. In shoemaking circles and user forums, it’s often said that hydrolysis is slowed down if boots get used. At Hanwag, we sometimes use the term Standschaden (storage deterioration). It’s an issue we are working on. We plan to follow up on it in for the future.

Either way, mountain boots and trekking footwear belong outdoors, not on the shelf. Proper storage can help prevent hydrolysis. Boots are best kept in dry, well-ventilated conditions. In addition, they should never be exposed to high temperatures. Both of these factors significantly speed up the process of hydrolysis. It’s also important to avoid contact with muck and manure (microbes) – especially long-term exposure. If you happen to walk through a cowpat – it’s better to wash it off when you get back. There are significant differences in when hydrolysis sets in. At the moment, we estimate that it starts around six to seven years after a midsole is manufactured. It’s worth considering that the manufacturing date of your sole might well be some time before the purchase date when you bought your boots. In addition, supply time for the sole, storage of the sole and manufacturing at Hanwag plus delivery to retailers and further storage can all add up. When you buy a pair of Hanwag boots in an outdoor shop, they could have been on the shelf for a few months. If you snap up a pair of last year’s model in the sales, then the boots might even have been standing about in the store for a couple of years. Take this into account when you try to work out how old your boots are.

How to Protect Your Boots from Hydrolysis

  • Store your boots in a dark, dry and well-ventilated area.
  • Hydrolysis is not visible from the outside.
  • Keep your boots away from sources of heat.
  • Test your boots out before you wear them on a long trip.
  • Clean your boots regularly.
  • Wash off muck and manure with water at the end of a trip.

Should you find that your boots are affected by hydrolysis, all is not lost. Inferior boots are made using a strobel construction and cannot be resoled. However, Hanwag footwear is made with a cemented construction and can easily be resoled – with a brand-new midsole and outsole.

Resoling: This is How We Do It

Generally speaking, uppers last longer than soles. Why buy a new pair of boots or shoes, when you can have your old ones resoled (including if the sole is damaged due to hydrolysis)? Simply bring your old boots to a Hanwag specialist retailer. They’ll send them in to us to be resoled. Your retailer will be able to tell you how much it will cost. Unfortunately, it’s not possible to send them to us directly. You’ll be surprised how good they look once we’ve finished with them. And how well they continue to fit you… Find out more on our website.