Polyurethane is normally an open/semi close cell and flexible foam. This material can be made in a variety of densities and hardness.
They are divided into two big families i.e. esters and ethers.
The Polyurethane (PUR) are materials with an open cell that have good properties of acoustic absortiion. The oil and water resistance are well-known characteristics of esters and ethers, respectively.
The difference between these two types of foam is basically the performance with water and oils. For parts that will be in touch with water you must use Ester and for oils Ether.