Common types of elastomers in midsoles of sports shoes

The midsole is the core component of a pair of sports shoes. If you compare a pair of sports shoes to a car, the upper is the body, the sole is the tire, and the midsole is the suspension system. Its importance is self-evident.

For consumers, the midsole of shoes can provide good stability, cushioning and resilience, so as to protect the feet well. For major shoe brand manufacturers, the midsole of the shoe is the most technologically effective part that affects sports performance, which can directly affect their core competitiveness.

Elastomers are used in shoe midsole materials, so what are the commonly used elastomers in shoe midsoles? Let's take a look together.

1.TPU

TPU is the abbreviation of thermoplastic polyurethane elastomer, which is a block copolymer composed of diisocyanate, chain extender and polyol.

Among them, the soft segment composed of polyols exhibits flexibility and toughness, while the diisocyanate acts as the hard segment to impart hardness and rigidity to the material, and the hard segment is crystallized as a physical cross-linking point, making the TPU exhibit high elasticity.

Therefore, TPU has significant advantages such as high tensile strength, high elongation, and low long-term compression set rate. The foamed TPU, or ETPU, will have good resilience. Generally, the rebound value of the foam can reach 60%.

The most typical example of the application of shoe materials is Adidas' Boost technology, which uses BASF's ETPU.But ETPU processing design is difficult, limiting the design freedom, resulting in a single style, other shoe brands are not very enthusiastic about ETPU midsole.

In general, as a commonly used shoe material elastomer, TPU's explosive growth is due to the huge output and disruptive influence of the Adi Boost series on the one hand, and its excellent performance still attracts many upstream and downstream shoe material manufacturers to conduct research. .

For example, traditional physical foamed ETPU will have the defect of low hardness. By controlling the foaming conditions, compounding TPUs with different hardness, and introducing other elastomers with low density and good elasticity, ultra-light and high-resilience with increased hardness can be obtained. Elastic ETPU shoe midsole, compared with ultra-light and high-resilient EVA products, also has the characteristics of higher rebound rate, lower compression set and better tensile strength.

2.EVA

EVA is a random copolymer composed of ethylene and vinyl acetate. Vinyl acetate (VA) is used as a flexible segment in the EVA molecular chain with a mass content of 26%-40%. Ethylene is used as a crystalline segment. The overall performance The softness and high elasticity.

The emergence of EVA materials also subverted people’s understanding of shoe midsole materials. The resilience of pure EVA foam is generally 40-45%, which is far better than PVC, rubber and other materials. Easy to process and other characteristics, so that it has been widely used in shoe midsoles.

However, EVA-based foam material products have poor aging resistance, flex resistance, elasticity and wear resistance, and the filling capacity of EVA foam materials is limited.

In addition, the emergence of ETPU also makes people put forward higher requirements for EVA materials, such as lower hardness, higher resilience, low compression deformation, etc., so the combined use of EVA rubber and plastic has become a hot research topic for various manufacturers. .

Rubber and plastic are used together, EPDM, POE, OBCs, TPE, such as SEBS oil-extended elastomers will be blended for use, and use EPDM rubber, POE high elasticity, OBCs soft crystallinity, TPE flexibility, etc. to achieve the purpose of blending modification , So as to improve the EVA foaming performance, the resilience can generally be increased to 50-55%, or even higher.

3.EPDM

EPDM is a copolymer of ethylene, propylene and a small amount of non-conjugated diene. Its main chain is composed of chemically stable saturated hydrocarbons and only contains unsaturated double bonds in the side chain. Therefore, it has excellent aging resistance, such as ozone resistance, heat resistance, and weather resistance.

And its application in shoe midsole is mainly EVA rubber and plastic. Because it contains ethylene component, it has good compatibility with EVA. It can improve the crystallinity of EVA to a certain extent, thereby enhancing the recovery of EVA foam. It is flexible and brings aging resistance such as ozone resistance, heat resistance, and weather resistance. The dosage is generally 15-25phr.

4.SEBS

SBS is a styrene-butadiene-styrene block copolymer. SEBS is a saturated hydrogenated product of SBS. They are all styrene thermoplastic elastomers.

SBS has good slip resistance, low temperature resistance, insulation, processing performance, and high tensile strength. After hydrogenation, the UV stability, oxidation resistance and thermal stability of SEBS are relatively strong, and the impact strength is also greatly improved.

At present, it is also mainly used for EVA rubber and plastics in shoe midsoles, which can bring good flexibility, make EVA more rubbery and lighter, and its dosage is generally 15-35phr. In addition, SEBS has very good oil absorption and good filling properties, which can be one of the means to reduce costs.

5.POE

POE is a thermoplastic elastomer of random copolymerization of ethylene and octene. It belongs to a class of polyolefin elastomers. It is an elastomer material synthesized by Dow Chemical with metallocene as a catalyst with narrow molecular weight distribution and uniform short-chain branch distribution. , The grade is ENGAGE.

This elastomer has been used in the shoe material industry since 2000. At present, the application in shoe midsoles is mainly EVA rubber and plastic. It is lighter than EVA and can bring good tensile strength and better touch. , It has better cold resistance, aging resistance, ozone resistance, and chemical resistance. The dosage is generally about 20phr.

6.OBCs

OBCs are semi-crystalline thermoplastic polyolefin elastomers synthesized based on Dow's patented Chain Shuttling System technology, under the brand name Infuse.

It is an olefin block copolymer with light weight, good resilience and soft texture. The resilience of pure OBCs material can reach more than 65%.

OBCs have good compatibility with EVA and POE. Basically every EVA rubber and plastic formula will have these three combinations. The mass of OBCs is generally 10-20phr, which is used to improve the performance of the overall shoe midsole foam material.

In addition to acting as a modifier in the combined use of EVA rubber and plastic, OBCs can also be used as the main component of shoe midsoles, with a content of up to 80phr. For example, UA (Underma)'s HOVR midsole technology is based on OBCs. Compared with the traditional EVA foam material, it can give more feedback energy after being impacted, and at the same time, the HOVR will recover better after being deformed under the dynamic compression test.

7.TPAE

TPAE elastomers have been popular for high-end shoe materials in the past two years. Because they are more expensive, although they are popular, the market usage is not particularly high.

TPAE elastomer is the common name of polyamide-based thermoplastic elastomer. The English name is Thermoplastic polyamide-based elastomer, abbreviated as TPAE, which refers to a thermoplastic semi-crystalline type composed of a rigid polyamide hard segment and a flexible polyether or polyester embedded soft segment. Copolymer.

For TPAE elastomer materials, the polyether soft segment gives it excellent flexibility and high resilience performance, and the polyamide hard segment gives it sufficient rigidity and excellent low-temperature impact resistance.

Judging from the related shoe models of sports brands in the past two years, TPAE elastomers can be made into shoe midsoles in a manner similar to ETPU, that is, through bead foaming and remolding, or sheet materials can be foamed and cut into midsoles. It can be blended with EVA as a compound ingredient.

The above are the types of elastomers commonly used in shoe midsoles, which are for reference only. If you have different opinions or supplements, welcome to discuss with us!