There appear to be many misconceptions about the differences between polyurethane and polyethylene foam. Here are some facts to help guide you in understanding how these foams differ in their specifications
What is polyethylene foam?
Polyethylene is a firm, closed-cell foam. The polymer molecules are cross-linked providing more rigidity, and therefore this foam is often referred to as "crosslink" or "crosslink polyethylene" foam. The crosslink nature of this foam type causes it to try very hard to rebound to its original shape quickly, which has the effect of introducing a little bit of "bounce" in addition to offering shock absorption. Due to its rigidity, polyethylene does not distribute fall energy broadly. You could consider this a "shock repelling" foam.
What is polyurethane foam?
Polyurethane foam is a softer, open-cell type of foam that is made in varying degrees of firmness. Because the cells are open and not cross-linked, air can flow throughout the foam just like water can flow throughout a sponge. This characteristic makes the foam softer and gives it the ability to absorb shock energy better. It does not try to rebound as quickly as polyethylene, and it distributes the energy of the compression more broadly. This foam is generally used in products that need to provide more crash protection and are therefore thicker and softer.
Is polyurethane and polyethylene ever mixed in the same product?
Yes. There are some products in which the two foams are used in conjunction. The softer polyurethane will be used for the bulk, bottom, core of the product, while a thinner, firmer, polyethylene layer is added as the top surface only. This combination creates a thin firm top and tall soft bottom. When this combination experiences an impact like a foot landing, it provides sufficient firmness to prevent ankle rolling. However, the entire softer bottom layer compresses at the same time, creating a cushioned effect. This is particularly helpful when you normally land or stand on the mat, but want to be able to "soften the blow" when crashing onto the mat. Two products that often utilize this combination are deluxe inclines used in training facilities for development of more advanced skills and deluxe martial arts mats to provide a bit of extra cushion for frequent "take down" training.