Polyurethane (PU) will gather humidity by hydrolysis in storage if not used regularly, especially if stored airtight, and will crumble in half an year.

It’s used as sole material in sportswear, Scandinavian outdoor and military gear, and several Indian shoe manufacturers. Especially the military boots are funny, because they literally crumble off the shelf.

Also used as vegan / synthetic leather and has to be dried properly if wet.

Take good care of your gear.