Material Matters: Eva Foam
Modern day athletic shoes are filled with foam – quite literally. Foam is used to provide padding around the collar, to cushion the footbed, it can be used to line, and even make, uppers. Today, one of the major uses for foam in the sneaker world is to make midsoles. When you read the specs of the latest and greatest runners to hit the market the abbreviations and acronyms can seem like a foreign language, EVA, PU, SBR – what does it all mean? Let’s take a look at the humble building block that is foam.
It all starts with a basic form of rubber or plastic. People have known about synthetic polymers that could create rubber since way back in the 19th century, when the industrial revolution fuelled a need for new materials alongside the new industry of industrial chemistry. The earliest form of rubber foam came in 1929 when Eric Owen, a chemist working for Dunlop Rubber, simply whipped latex in much the same way that you’d prep some cream for your sponge cake. Soon the value of what we call polymeric foam, which is foam made from polymers, became evident and the innovation began to flow.
When you look at a pair of shoes, they’re likely to contain two different kinds of foam – open cell and closed cell. The properties of each kind of foam can vary widely depending on the basic ingredients, which can be blended in different ratios resulting in varied density, breathability and something called compression set ratio – referring to the ability of a foam to bounce back to its original shape. Foams with high compression set are no good for things like midsoles, as they’ll end up crushed in a few days, but foams with too low a compression set will be unable to mould to your feet over time.
Open cell foam is a spongey, porous material that will allow air and water into its structure. Open cell foams are great for uppers as they allow your foot to breath easily. A plastic foam known as KF, a type of polyurethane or PU, is often used to make up the comfy, padded parts of shoes like the collars and the tongues. Another kind of PU foam is often used as a lining material, very thin layers can be bonded to the panels of the upper to reduce wrinkling and to allow the seams to sink into the surface, as raised stitches are more likely to be snagged and break. Memory foam is a kind of PU foam with an extra ingredient added to slow its springy return rate, but it does have an unfortunate tendency to freeze solid in cold weather.
The most common foam in sneakers is a closed cell foam called EVA, or ethylene vinyl acetate. EVA is perfect for midsoles and has been used all the way back to Bill Bowerman’s 1974 Waffle Trainer but can still be seen in sneakers like the modern Lunarlon models that are at the forefront of Nike’s current offering. EVA is what’s known as a copolymer, a combination of two different plastics, and can vary widely in its ratios. Higher quality shoes will have around 40 percent vinyl acetate and 60 percent ethylene, the lower the amount of vinyl acetate, the more brittle and likely to crumble the midsole will be.
EVA plastic can be ‘foamed’ in a couple of different ways, each of which introduces bubbles into the raw ingredient. When the plastic is in a semi-liquid state a blowing agent can be introduced as a solid. When the combination is heated, the blowing agent will expand and turn into a gas, leaving bubbles all throughout the plastic. This can be done in a large heat press to produce sheets of the material, although these days it’s usually done with an injection machine that uses a hydraulic ram to compress and heat the EVA before it’s quickly released, allowing the foam to expand – this is how Crocs are made. The other method uses pellets of EVA mixed with pellets of a blowing agent. The combined ingredients will be compressed in a mould, where the pressure will heat them up, before they’re slowly cooled. The resulting foam from these methods will still need to be re-heated and formed into its final shape before it can be used as a midsole.
Once we have the material, it can be formed into shape in a few different ways. First it can be cut out of a sheet, as with older styles like the Nike Cortez, which has a single piece and a wedge attached to the outsole before the edges are buffed for an even finish. One of the more common methods for modern sneakers is cold pressing. After the sheets are cut into a rough shape they’re heated up and pressed into a mould, which is cooled and gives midsole their details. In 1993 Saucony were the first to create a dual density moulded midsole by combining different foams for areas of stability and cushion – now this is pretty much industry standard.
Phylon midsoles are made from EVA by using a compression moulding technique. Expanded EVA, the foamed stuff, will be loaded into a mould, which is then put into a heat press. While it’s in the press, the air bubbles in the foam will expand even more, which will result in a lighter, more springy midsole. It’s kind of like a twice-cooked foam. Nike use Phylon as the base for all of their Lunarlon technology and have pioneered ways to produce dual density soles using this technique.
Some of the other kinds of foam that have taken the sneaker world by storm have been featured on Material Matters in the past. The Boost technology employed by adidas uses an expanded TPU, or thermoplastic polyurethane, which is created in pellets and moulded together. Neoprene is another form of foam which is prized for its elastic qualities, making it perfect for snug-fitting uppers.
It may not seem like the most glamorous material but foam has undoubtedly formed the foundation of the modern runner. Next time you slip on a comfy, supportive sneaker spare a thought for our old mate EVA and the wider foam family.