EVA stands for Ethylene-Vinyl Acetate. When you mentioned “eva” in the context of a “knitted upper,” you were likely referring to EVA foam, a common material used in the midsoles and sometimes outsoles of shoes.

Here’s what makes EVA foam so popular in footwear and other applications:

Properties of EVA Foam:

  • Lightweight: It’s a very light material, contributing to the overall lightweight feel of shoes.
  • Excellent Cushioning: EVA foam provides good shock absorption, which is crucial for comfort and reducing impact during activities like walking and running. Its closed-cell structure contributes to this.
  • Flexibility: It’s a flexible material, allowing the shoe to bend and move with your foot.
  • Durability: EVA foam is reasonably durable and can withstand regular wear and tear.
  • Water Resistance: Its closed-cell structure makes it water-resistant, preventing absorption.
  • Chemical Resistance: EVA foam is resistant to many chemicals, oils, and fuels.
  • UV Resistance: It has good resistance to ultraviolet (UV) radiation, which helps prevent degradation from sunlight.
  • Thermal Insulation: EVA foam offers some level of thermal insulation, helping to keep feet warm in cold conditions.
  • Easy to Mold and Shape: It can be easily molded and cut into various shapes and sizes, offering design flexibility.
  • Stress-Crack Resistance: EVA foam exhibits enhanced stress-crack resistance, ensuring it remains in good condition even under demanding conditions.

How EVA Foam is Used in Shoes:

  • Midsoles: This is the most common application. The EVA foam in the midsole provides the primary cushioning and shock absorption. Different densities and formulations of EVA can be used to achieve varying levels of firmness and responsiveness. Some manufacturers even market their proprietary EVA-based foams under different names (e.g., Nike’s “Phylon”).
  • Insoles/Footbeds: A layer of EVA foam is often used in insoles to provide additional cushioning and support directly under the foot.
  • Outsoles: While less common as the primary outsole material due to lower abrasion resistance compared to rubber, EVA can be used in outsoles, often in combination with rubber in high-wear areas to improve durability while maintaining lightweight and cushioning properties.

In relation to a “knitted upper”:

The knitted upper provides a breathable, flexible, and often lightweight structure that wraps the top of the foot. The EVA foam in the midsole then handles the underfoot cushioning and impact protection. This combination is frequently found in modern athletic and casual footwear, aiming to provide both comfort and performance.