One of the main technical challenges with polyurethane is overcoming an ageing deterioration known as ‘hydrolysis’.
Hydrolysis is the chemical breakdown of the PU polymer and the resulting physical breakdown of the sole by the attack of water (usually in vapour form), occurring over a period of several years – even when the shoes are in store.
This process is accelerated by warmth and high humidity. It will, therefore, happen more quickly in tropical climates, but also in confined spaces (such as in lockers) if footwear is put away damp.
In the most advanced state of hydrolysis, the sole will lose all its physical strength, thus cracking or crumbling.