ETPU Material: The 'popcorn Revolution' That Redefines Sports Shoe Sole Technology

1.What is ETPU? The Transformation from Molecular Structure to "Popcorn Granules"

1. Basic Definition and Chemical Characteristics

ETPU (Expanded Thermoplastic Polyurethane) is a product of TPU (Thermoplastic Polyurethane) modified by supercritical physical foaming technology.

·The original characteristics of TPU: TPU is composed of alternating hard segments (rigid segments such as MDI and BDO) and soft segments (flexible segments such as polyester/polyether polyols), combining the elasticity of rubber with the strength of plastic.

·Leap after foaming: Through high-pressure gas injection, closed honeycomb shaped micropores (pore size of about 100-300 microns) are formed inside TPU, reducing density by 50% -70% and increasing resilience by more than 40%.

2.Why is it called "popcorn material"?

Morphological analogy: Unfoamed TPU particles are solid particles with a diameter of about 3-5mm. After foaming, the volume expands to 5-8 times, and the surface is covered with pores, resembling popcorn (see Figure 1).

Core difference: Unlike traditional EVA foam materials, the pores of ETPU are independent closed cell structures, which avoid water absorption deformation and have stronger resistance to compression deformation.

2.How is ETPU manufactured? Unveiling the 'black technology' of supercritical foaming

1. Detailed explanation of supercritical foaming technology process

①Raw material pretreatment: drying and dehydration of TPU particles (moisture content should be less than 0.05%);

②Gas permeation: Place TPU in an autoclave and inject supercritical CO₂/N₂ (temperature>120℃, pressure>150Bar), and the gas dissolves in the gaps between TPU molecular chains;

③Instantaneous pressure release: Quickly release pressure, causing gas to rapidly expand from supersaturated state, forming uniform micropores;

④Steam compression molding: Foam particles are poured into the mold and heated with high-temperature steam at 120-140℃. The surface of the particles is partially melted and bonded, and cooled to form the sole of the shoe.

2. Technical difficulties and breakthroughs

Bubble uniformity: Deviation in pressure and temperature control can cause the merging or rupture of pores (accurate to±1Bar,±2℃);

Adhesive strength: Micro fusion welding should be formed between particles to ensure the integrity of the sole (steam heating time error<5 seconds).

3.Performance advantages of ETPU sole: data and scenario interpretation

1. Cushion rebound elasticity: scientifically verified "poop feeling"

Laboratory data:

Energy return rate: ETPU can reach 65% (EVA about 45%, rubber about 30%);

Impact attenuation: ETPU absorbs over 60% of the impact force (ASTM F1614 test).

User scenario: When running on the ground, the ETPU air hole is compressed → stores energy → releases rebound force when leaving the ground, reducing knee joint wear (see Figure 5 for simulation of impact force distribution).

2. Lightweight: Breaking the Paradox of "Weight Durability"

Density comparison: ETPU (0.15-0.25g/cm³)<eva(0.25-0.35g m    ³)<rubber (1.1-1.3g="" m="" ³);  

Practical case: Using ETPU in the midsole of basketball shoes can reduce weight by 80-120g, which is equivalent to the weight difference of carrying 2 AA batteries inside the shoes.

3. Low temperature resistance: The secret to not hardening at -30℃

Principle: The soft segment (polyether type) of TPU maintains molecular chain flexibility at low temperatures;

Test comparison: Under -20 ℃ environment, EVA hardness increased by 300%, while ETPU only increased by 50% (ISO 7619-1 standard).

4. Wear resistance: a long-lasting battle beyond rubber

DIN abrasion test: ETPU volume wear is less than 40mm ³, rubber is about 60mm ³, EVA＞100mm³；

User experience: After 1000 kilometers of marathon running, the ETPU sole pattern retention rate is greater than 80% (see Figure 6 for wear comparison).

5. Environmental Value: Future Potential from Recycling to Degradation

Physical recycling: ETPU soles can be crushed and re foamed (recycling rate>90%);

Biobased TPU: Some manufacturers have launched ETPU containing 30% plant-based materials (such as BASF Cellasto) ® ECO）。

4.Commercial Application of ETPU: From Sports Shoes to Everything

1. Sports shoe field: Giant technology branding

adidas Boost： The first brand to commercialize ETPU on a large scale (each pair of shoes contains 2500 ETPU particles);

PEAK "state pole": ETPU+adaptive gel composite material, realizing "soft when soft, elastic when hard";

ANTA "Nitrogen Technology": Supercritical foaming ETPE (ETPU upgraded version), with a rebound rate increased to 80%.

2. Cross border application cases

Medical orthopedic insoles: Utilizing the cushioning and customizability of ETPU to assist patients with plantar fasciitis (see Figure 7 for medical scenarios);

Car seats: The BMW i3 electric car adopts ETPU seat cushions, which are shock-absorbing and breathable;

Military explosion-proof equipment: ETPU energy absorbing layer is used for bomb disposal protective clothing, absorbing 80% of shock wave energy.

5.The Future of ETPU: 3D Printing, Intelligence, and Carbon Neutrality

1. 3D printed ETPU shoe sole

Technical advantage: No need for molds, personalized sole customization can be completed within 72 hours (such as Reebok Liquid Speed);

Challenge: The printing accuracy needs to reach 0.1mm level to avoid collapse of the pore structure.

2. Intelligent cushioning system

Conceptual design: ETPU midsole embedded pressure sensor, real-time adjustment of area density (see patent schematic diagram in Figure 9);

Commercialization progress: Under Armour has launched ETPU smart running shoes that can be connected to an app.

3. Sustainable development path

Biobased ETPU: BASF and Adidas collaborate to develop ETPU with biomass content ≥ 50%;

Chemical recycling: Decompose ETPU into monomers and synthesize TPU again (laboratory stage).