For many years (from the 90s to today) a new material was imposed on rubber, a lighter and more comfortable material: polyurethane.
The result was lighter and more flexible soles that not only absorb shocks, but are also extremely resistant, which is why polyurethane is ideal for footwear, but they had a much shorter expiration date, between 5 and 10 years (depending on the conditions in which they have been stored). This did not happen with rubber soles, so if you have always bought rubber soles or another material, this problem will seem impossible or you will attribute it to cheap materials, when it is not.
One of the main technical challenges with polyurethane is to overcome an aging deterioration known as “hydrolysis”. Hydrolysis is the chemical decomposition of the PU polymer and the resulting physical rupture or crumbling of the PU sole by the attack of water, usually in the form of steam, which occurs over a period of several years, even when the shoes are stored!
This process is accelerated by heat and high humidity. Therefore, it will happen more quickly in tropical climates, but also in confined spaces, such as lockers, if the shoes are stored wet. In the most advanced state of hydrolysis, the PU sole will lose all its physical strength, so it will crack or crumble.
There are two types of PU soles. There is a PU sole with a polyether base and a PU sole with a polyester base. In polyester-based polyurethanes, a separation of the polyester chains (hydrolysis) occurs in the case of prolonged storage and/or under conditions of high temperatures and high humidity. The consequence is a reduction in the mechanical resistance properties of the material: traction, tear, abrasion resistance and flex resistance. In the soles it translates into that it “crumbles”, “falls apart”.
the most used polyurethane (polyester polyurethane) can suffer that hydrolysis 4 years after being manufactured and that the current sporadic use of footwear (not continuous) further aggravates the problem.
Hydrolysis is a chemical reaction of double decomposition that appears when water or moisture is combined with many substances.
Customers who suffer this type of problem see the product ruined in a rather unpleasant way: crumbled inside the storage box, with a dusty touch or with the sole almost disintegrated or separated in the middle of an excursion.
The expiration of polyurethane soles is determined by the use of the shoes and the storage conditions (cool place, without humidity or sudden changes in temperature) but as a general rule about 5-6 years after purchase (think that the sole material is manufactured before, then comes the manufacture of the shoe and its distribution, put on sale and finally your acquisition). The hydrolysis process is accentuated in shoes that have not been used for a long period of time. It is important that you keep in mind that the durability of the sole is from when it is manufactured, not from when you buy and/or use it.
