An introduction to EVA
introduction
EVA is perhaps the most common sole material known for its softness and extreme light weight. In this post we examine its composition and the means used to achieve a finished outsole.
Composition
EVA (Ethylene-Vinyl Acetate) or also known as Phylon, is a copolymer of ethylene and vinyl acetate. The ratio of these two materials in the EVA compound determines its hardness. The vinyl acetate content makes up around 15-30% and its what gives the material it's rubberlike properties. These properties can be further increased by mixing SBR (Styrene Butadiene Rubber) or natural rubber. EVA is stabilized with peroxides which also cures rubber in EVA/rubber blends. Other ingredients in the product include dyes and propellants. A gas-generating chemical used during the curing process to create closed-cell microcellular bubbles in the structure. A closed cell structure means that the cells are not interconnected and therefore the material is not spongy and does not absorb water.
How EVA is made
EVA foam is produced via a process called compression molding. This process involves heating EVA pellets until they become molten and then injecting them into a mold where they cool and harden into the desired shape. Compression molding is a very efficient way to produce EVA foam and can achieve very precise dimensions and designs.
Use cases
The low density, resilience and softness of the material combined with great shock-absorbing properties make it a popular material to use in running shoes or other casual type footwear. However higher density compounds of EVA can be achieved and is sometimes used as an alternative to resin rubber, for example used in court shoes. EVA is also widely used in the manufacturing of flip-flops due to its low cost and easy to shape properties.
Disadvantages of EVA
EVA does have some disadvantages compared to other materials. EVA is not as durable as rubber and will break down over time, especially if exposed to sunlight or high temperatures. EVA is also not as grippy as rubber which can make it less suitable for use in certain types of footwear such as hiking boots.
Conclusion
EVA is a very popular material for use in footwear soles due to its light weight, flexibility and low cost. EVA does have some disadvantages but overall it is a good choice for many different types of footwear.
