Polyurethane soles: Properties, uses and challenges

Polyurethane (PU) soles are used for shoes because they are lightweight, durable and comfortable. They are flexible and provide great shock absorption, making them ideal for both safety and casual footwear. PU soles have a wide range of uses, from leisure footwear to heavy-duty industrial and military footwear.

Why do PU soles become brittle?

PU soles degrade over time through a process called hydrolysis, a reaction with water that causes chemical compounds to break down. The moisture does not need to be in liquid form, exposure to humidity is sufficient to start the breakdown process. Hydrolysis causes PU soles to lose their elasticity, at which point they become brittle and begin to crumble. While the process is slow, soles may break down suddenly when a critical point is reached.

The rate of hydrolysis is affected by:
• Air humidity and temperature – PU becomes brittle more quickly in hot and humid conditions.
• Storage environment – Long-term storage in a humid and warm environment accelerates hydrolysis.

How do you prevent PU soles from becoming brittle?

1. Store your shoes in a dry and cool place.
- Avoid storing your shoes in a damp environment, such as a basement or garage.
- Do not store your shoes in airtight plastic bags or boxes.
2. Allow your shoes to breathe and dry thoroughly.
- After wearing your shoes, allow them to air dry at room temperature.
- Remove the insoles during storage to allow any moisture underneath to evaporate.
- Do not dry your shoes next to a heat source, such as a radiator or drying cabinet.
3. Wear your shoes regularly.
- Long-term storage accelerates the degradation of PU.
- The material remains more flexible if you wear your shoes from time to time.

How do you tell if soles have become brittle?

If your shoes have been in storage for a long time, check their condition before wearing them:
• Gently press on the sole – if it feels hard or is cracking, the PU has started to break down.
• Bend the sole – if it breaks or peels off in layers, the shoe is no longer safe to wear.
If the soles have already become brittle, usually the only way to repair them is to have the manufacturer recast the soles using the original mould.

Summary

It is natural for PU soles to become brittle, but with proper storage and use the process can be significantly slowed. By taking care of your shoes, you can extend their service life and avoid unexpected damage.

Sources:
- SATRA Bulletin, July-August 2012, p. 17
- SATRA Bulletin, March 2014, p. 19
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