What's the Difference Between Polyurethane and Polyethylene Foam?
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 and how that relates to gymnastics products manufactured using one (or both) of these foam types.
Is there a difference in quality between these foams?
Quality does not factor into the difference between these two types of foam. They have different characteristics and are therefore used for different gymnastics or martial arts applications. One is not better or worse than the other; they are different.
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.
This foam is traditionally used in gymnastics products that are relatively thin (1" to 2") and therefore need to be firm throughout. It provides a combination of light cushion and fall protection, while still providing a firm footing when landing on it. This foam is generally used in applications where you land on your feet or hands, and want to avoid twisting your ankles or wrists. It is normally found in standard folding mats, gymnastics flooring such as carpeted foam rolls, martial arts flooring, and portable, slatted, cheer flooring such as flexi rolls. It is also sometimes used as a top layer over softer foam in order to provide better footing while the lower, softer foam adds some shock absorption.
What is polyurethane foam?
Polyurethane 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 (4"-32" thick) and softer. It provides more cushion in situations where you may be landing on your body rather than on your feet or hands. Common products that utilize this foam are throw, practice, landing and stunt mats, inclines, and other items used to provide protection against injury from falls.
How do we measure polyurethane firmness?
First of all, what we want to look at is firmness, not density. The two are often incorrectly thought of as the same thing, and they can correlate, but they are actually different characteristics. Density tells us an item's weight per unit of volume, but that doesn't necessarily indicate its firmness. When looking at gymnastics foam, what we are interested in is firmness, which will tell us how soft or firm the foam feels to us when stepping or falling onto it. Foam firmness is measured using a unit of measure called Indentation Load Deflection, commonly referred to as ILD. In gymnastics applications, you will generally see ILD numbers for polyurethane ranging from about 24 to 75. The higher the number, the firmer the foam. The lower the number, the softer the foam. Therefore, a 36 ILD foam is pretty soft, while a 75 ILD foam is significantly firmer, but still softer than crosslink polyethylene foam.
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.
Are different ILD levels of polyurethane ever mixed in the same product?
Yes. Many of the thick mats (landing, practice, pit, crash/stunt) will utilize combinations of higher and lower ILD foam in different layers. Again, generally the higher ILD foam (firmer) will constitute the top layer, while the lower ILD (softer) foam will be used as the bottom layer. The top layer is not as firm as polyethylene foam, but will provide a bit more footing than if the entire mat were made of the bottom, softer foam.
For example, you may see practice mats where the bottom 50% is 36 ILD, while the top 50% is 75 ILD. Coming off a vault, this will help your feet to stick the landing, but will cushion your fall and protect you if you land on your back or tailbone. On the other hand, when a stunt person is thrown out of a saloon window, they will never even attempt to land on their feet. The idea here is to cushion the fall when the entire body hits the mat. Therefore, a 24" thick stunt mat may be made entirely of very soft, 24 ILD foam with additional air chambers, and because it's so soft, so you need a lot of thickness to absorb the impact.