The amount of specialized vocabulary you come across when buying running shoes can be daunting. There are so many terms, many of them technical, that it can be difficult to tell what is critically important, what is marketing jargon run amok, and whether a proprietary term from one manufacturer means the same thing as a fancy word from another.
Nowhere is the terminology more confusing than when naming foams and rubbers…materials where shoes differ the most from each other. One such term is EVA.
So what, exactly, is EVA? And what is the relationship between this material and running shoes? Should runners choose footwear using this material?
To clarify some questions and maybe give you some flashbacks to chemistry class, here are the basics on EVA and its application in shoe manufacturing.
What Is EVA?
Ethylene Vinyl Acetate–EVA–is a copolymer, or the joining of two plastics in the same polymer chain. In this case, the two plastics are vinyl acetate and ethylene. EVA is sometimes known as PEVA or poly-ethylene-vinyl acetate.
EVA can produce materials that are rubber-like in flexibility and softness. Beyond that, EVA can be glossy or clear, and resistant to stress-cracking, UV radiation, and damage resulting from very high or very low temperatures. EVA is extremely useful in manufacturing, well beyond the shoe industry. Often, EVA is pitted against vinyl polymer and rubber as a material for electrical applications.
The Three Kinds of EVA
There are generally three kinds of EVA, and they differ in the amount of vinyl acetate they contain.
- Vinyl-Acetate Modified Polyethylene
This variation, more similar to polyethylene than to EVA, has a low proportion of vinyl acetate–roughly around 4%. Vinyl-acetate modified polyethylene is processed like a thermoplastics material and is usually considered to be non-toxic.
- Ethylene-Vinyl Acetate
The second kind of EVA is the thermoplastic ethylene-vinyl acetate co-polymer, which has a medium content of vinyl acetate, ranging between 4% to 30%. It is a thermoplastic elastomer material that is not vulcanized. This kind of EVA shares some of the properties of rubber.
- Ethylene-Vinyl Acetate Rubber
Last, ethylene-vinyl acetate rubber is an EVA co-polymer with a high concentration of vinyl acetate–more than 40%.
Application of EVA in Shoe Manufacturing
If you spent more of chemistry class staring at your shoes and wishing you were out running than you did thinking about ethyls and vinyls, you might know that EVA is the most common cushioning material in athletic footwear midsoles. It’s found in basketball, running and most other kinds of athletic footwear because the material can resist compression and is lightweight. It is also easy to form and easy to manufacture in a rainbow of colors. EVA is used for cheap sneakers and the most expensive athletic footwear alike.
Why Have EVA Midsoles?
Those who have seen the word EVA describing shoes ask what EVA is, but also why it is so important in footwear.
EVA is used mostly for the midsole (or the layer sandwiched between the outsole and upper). The midsole is the part responsible for cushioning and energy return, which aid in comfort, efficiency, and protection. When well-constructed, the midsole follows the foot line, so EVA’s maleability makes it an obvious choice.
Stability is also an essential feature midsoles provide. This is why midsoles have to be constructed from materials that are capable of handling pressure, either from the weight of the user or the surface they are running on. Again, EVA has ideal characteristics.
EVA for the Soles
Contrary to popular belief, EVA is not just used for midsoles. These days, the use of EVA in shoe manufacturing is more varied than before, and advances in materials have put EVA in new parts of your favorite shoes.
Take the outsole and insole of shoes, for example. And not just your running shoes; you’ll find EVA in your household slippers, too.
Why? For one thing, EVA soles are resistant to corrosion caused by sea water, fats, oil, or chemicals (certain types of rubber degrade quickly when in contact with oil). It is also considered safe and environmentally friendly, due to its anti-bacterial and non-toxic properties. It is also relatively non-polluting.
As mentioned, EVA is great for cushioning and vibration reduction due to its high resistance, tensile strength, and toughness. It insulates well in cold temperatures, which is why EVA commonly appears in hiking boots as well.
The Downside of EVA for Shoes
EVA (and really any foam, from shaving cream to the meringue on your pie) is made up of thousands of small bubbles trapping air. This is the attribute that allows for its great shock absorption and cushioning qualities, making it the number one option of athletic shoe manufacturers. However, those tiny bubbles are also the downside of using EVA for shoes.
Over time, no matter how well-made the foam, bubbles pop or compress. This is why most users, especially runners, complain that their shoes feel flat eventually. And this is why most shoe manufacturers specify a certain lifespan for running shoes (400 miles is a commonly-cited figure). EVA material has a short life.
Manufacturers of high-end and expensive athletic footwear have found a solution to make EVA last longer by using molding. This process entails compressing the material using a pressurized mold, so that the midsoles end up forming a thick skin, which makes the EVA soles more durable. This molding process is quite expensive, which also makes the footwear made from molded EVA pricey.
What to Look for in EVA shoes
The hardness of EVA is measured in degrees, and ranges from 5 to 70. Runners should look for EVA midsoles in the 40 to 65-degree range, as lower than 40 degrees is (a) not stable enough and (b) makes the footwear prone to faster compression and deterioration. On the other hand, a density of more than 65 degrees is too firm to be comfortable for most runners.
The standard density for basketball and running shoes is roughly 55 degrees.
Bottom Line: Shoes with EVA are Great
When people buy running shoes, they look at the design, price and, to some extent, the materials. Arguably, not every shopper is discerning enough to take the time to understand how the content of the footwear affects its comfort and performance.
EVA is a material favored by many shoe manufacturers, particularly athletic footwear makers. This is because the material has many great attributes that provide support, stability, comfort and insulation. Contrary to popular belief, midsoles are not the only part of shoes made from EVA, as some shoe makers also choose this material for the insole and outsole as well.
If you want athletic footwear that lasts long, then consider buying one that uses a molding process to configure the EVA. After all, the main disadvantage of this material is that it does not last long, which makes the shoe feel flat after some time. Luckily, shoemakers have found a way to make EVA soles last longer with molding. For the right pair of shoes that help you achieve peak performance, this is worth the investment.