Aging/Hydrolysis of the PU sole ➤ Reasons and Background

Here we answer the following questions:

1. Why can the PU sole detach/decompose?
2. How long does my PU sole last?
3. Why do we use PU soles in our shoes and clogs?
4. What can I do to make my sole last as long as possible?
5. What can I do if my sole is already detaching?

Shoes are made to be worn, and the sole is the part that suffers the most. Besides the wear of the tread sole, there are other reasons why a sole replacement may become necessary.

This refers to cases where the PU sole can dissolve from the flexible woody wooden sole.

This phenomenon repeatedly causes surprise because it particularly affects older shoes that are little to hardly used. For shoes that are regularly worn, the problem occurs much less frequently or much later.

On this page, we want to explain what diffusion of PU soles is about, what you should consider, and how we can support you in case of emergency.

Why can the PU sole detach?

If the sole of your woody shoes detaches from the wood or starts to crumble, it is very likely the aging process of PU rubber. Many a beloved woody shoe has fallen victim to this phenomenon.

If you look into the background, it becomes clear that the topic is much more complex than initially assumed. Because the degradation process of the sole depends on a variety of factors. The well-known hydrolysis is only one of these factors.

Reason #1: Hydrolysis
Hydrolysis describes the chemical degradation process of materials based on polyurethane (PU), triggered by moisture. As a result, the sole loses its flexibility and ultimately, the shoe loses its sole. Particularly relevant in hydrolysis is the absolute humidity during storage.

Reason #2: Bacterial infestation or chemicals and acids
Another factor that accelerates the decomposition effect of PU is bacterial infestation or contact with acids. The bacteria settle in the open-pored structure of the PU foam and also attack the polymer structure of the PU. Often, contact with cow dung or saltwater is enough if it has not been carefully washed off over a longer period. (See further below in the text: What can I do to make my sole last as long as possible?)

Reason #3: Heat, UV radiation, and ozone
If shoes are dried too quickly or too hot, it increases the risk that the PU will also be affected. The material dries out and loses its elasticity. The same applies to high UV radiation but also ozone. This should be considered when choosing the place where the shoes dry. (Further information can be found below in the text: What can I do to make my sole last as long as possible?)

How long does a PU sole last?

The period during which the sole can separate from the wood depends on a variety of factors.

Our experience has shown that with regular use and proper maintenance, the earliest time when a decomposition effect can occur is about 5 years after production. However, many customers only send their shoes to us for resoling after 10, sometimes even 15 years. If you actually have problems with your shoes within the first two years after purchase, please contact us directly. We will try to provide you with the best possible support to help you.

Why is PU used as sole material at woody?

The special material property of PU ensures that the cushioning property of the sole remains very consistent, across the entire surface and over many years, despite intensive use. At the same time, PU is particularly dimensionally stable, meaning it withstands physical stress well, not just walking on asphalt. To complete this package of material properties, PU is also lightweight and flexible. This is exactly why the use of PU is very popular, for example, in mountain boots, hiking shoes, and sneakers, not just at woody!

At woody, we use the highest quality PU available on the market for durability in our shoes. Through close collaboration with our suppliers, we are committed to continuously improving sole technology. Due to over a million satisfied customers and the technically outstanding, positive material properties, the use of PU at woody will continue.

Like any material, PU is not immune to aging, so after some time, aging signs will also appear in woody shoes with PU soles.

What can I do to make my sole last as long as possible?

You can follow these tips to delay the diffusion process in shoes with PU soles. These recommendations apply equally to all other shoe models and sole variants to maximize lifespan.

• Walking: Our experience has shown that shoes regularly in use are less often affected by sole loss than shoes that have not been worn for a long time. This is because the plasticizers in the sole remain supple, and the aging process is delayed.
  
  
• Storage: Dry storage in a dark, well-ventilated room is ideal for the durability of our woody shoes. Dryness here refers to absolute humidity, including (relative) air humidity. (warm air can hold more moisture)
  
  
• Care: Have you walked through a field with fresh dung or manure, or done a mudflat hike with your shoes? We recommend cleaning the shoes thoroughly afterward. The acid in slurry or saltwater significantly accelerates the hydrolysis process. The same applies to soil, clay, or peat soils. Further care instructions can be found here (Link to cleaning & care)
  
  
• Drying: Do not dry your shoes on or near a direct heat source (heater/wood stove). The heat damages the sole material. The leather also becomes dry and cracked under direct heat exposure.
  
  
• Avoid exposing the shoes to direct sunlight or strong heat for extended periods (e.g., for drying). UV radiation has a significant impact on the material consistency.

What can I do if my sole detaches?

If the sole of your shoe detaches from the wood or starts to crumble, you don't have to dispose of the shoe right away. As long as the upper material is still in good condition and it still makes sense, we can equip most shoes with a new sole. This way, you can continue to use your beloved shoes, save the investment in a new pair, and the environment will also be happy. On our website, you will find all the necessary information (Link to sole replacement - resoling)

For further information, contact us at woody@woody.co.at or via the form below.