Comparison of physical properties and durability tests of PU and EVA used in shoe midsoles

Introduction to Shoe Midsoles

When it comes to choosing the perfect pair of shoes, comfort and durability are key factors that consumers consider.
A significant component contributing to these factors is the midsole, which acts as a crucial cushioning layer providing support to the foot.
Two of the most common materials used in shoe midsoles are Polyurethane (PU) and Ethylene Vinyl Acetate (EVA).
In this article, we will explore the physical properties and durability tests of PU and EVA, and analyze how each material performs in various conditions.

Understanding PU and EVA

Polyurethane (PU)

Polyurethane is a versatile polymer that is widely used across various industries, including footwear.
PU is known for its excellent durability and flexibility, which makes it highly suitable for shoe midsoles.
This material is capable of maintaining its shape and cushioning properties even after extensive use.
Moreover, PU offers good resistance against abrasion and impact.

Ethylene Vinyl Acetate (EVA)

Ethylene Vinyl Acetate is another popular choice for shoe midsoles.
EVA is appreciated for being lightweight and offering great cushioning properties.
Due to its closed-cell structure, EVA can provide excellent shock absorption, making it ideal for activities that involve high impact.
It is also easy to mold, which allows for innovative shoe designs.

Physical Properties of PU and EVA

Cushioning and Support

One of the primary functions of a midsole is to provide cushioning and support.
PU stands out for its elasticity and ability to regain its original form after compression, offering stable and consistent support over time.
On the other hand, EVA provides excellent initial cushioning and is valued for its softness.
However, EVA is known to wear down faster compared to PU, potentially leading to reduced support.

Weight

Weight is another crucial factor, particularly for athletic and sports footwear.
EVA is significantly lighter than PU, making it the preferred choice where reducing the shoe weight is a priority.
The lightness of EVA provides an advantage in activities requiring agility and quick movements.

Temperature Resistance

Both PU and EVA react differently to temperature changes.
PU generally shows better performance in varied temperatures as it maintains its structural integrity and flexibility.
In contrast, EVA may become hard under cold temperatures, impacting its cushioning properties.
In hot conditions, EVA tends to become too soft, which could affect the stability of the shoe.

Durability Tests of PU and EVA

Abrasion Resistance

Abrasion resistance is a critical measure in durability testing.
PU is highly resistant to abrasion, meaning it can withstand rough surfaces and intensive use without significant wear and tear.
EVA, while offering good abrasion resistance, does not match the level provided by PU.

Compression Set

The compression set indicates how a material performs under continuous pressure.
PU excels in this test, as it recovers its original thickness faster after pressure is removed.
This makes it better for long-term use.
EVA performs well initially but tends to compress and lose its shape over time.

Flex Crack Resistance

Flex crack resistance measures how a material deals with repeated bending and flexing.
PU’s structural flexibility ensures that it resists cracking even after extended use.
EVA is more susceptible to cracking as the material ages, especially when used in footwear for high-impact activities.

Environmental Impact

Considering the environmental impact of materials is becoming increasingly important.
PU production involves complex chemical processes, and while it offers great longevity, its environmental footprint is more significant due to difficulty in recycling.
EVA, though more environmentally friendly in its production, tends to wear out faster and may require more frequent replacements, leading to more waste.

Conclusion

Both PU and EVA have their unique advantages and limitations, depending on the intended use of the shoe.
PU offers superior durability, abrasion resistance, and stability, making it a better choice for high-performance and rugged footwear.
EVA is ideal for lightweight shoes where comfort and initial cushioning are priorities, although it may not be as long-lasting.
Ultimately, the choice between PU and EVA comes down to balancing these properties to meet the specific needs of the footwear and user preferences.
Understanding these factors can guide consumers in making informed decisions when selecting shoes that offer both comfort and durability.