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In recent years, the global footwear market has been saturated, and the competition between mid-to-high-end brands is also extremely fierce. Shoe sizes featuring various new concepts and new technologies are constantly being introduced, which has further spawned a large demand for foamed materials in the footwear industry. High-performance polymer foamed shoe materials have even become the core of product solutions for many terminal brands. This trend is particularly pronounced in the field of sneakers.
Usually a pair of sports shoes is divided into three parts: upper (Upper), midsole (Midsole), outsole (Outsole).
The role of the midsole is to provide cushioning and rebound, absorb the impact generated during exercise, provide protection and a gentle and comfortable foot feel. It can be said that the midsole is the soul of sports running shoes. The material and foaming technology of the midsole are the signs that distinguish the core technologies of major brands.
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EVA--the earliest foam shoe material used
Ethylene-vinyl acetate copolymer (EVA) is the earliest foam material used in shoe midsole. The resilience of pure EVA foam is generally 40-45%, which is far better than PVC, rubber and other materials, and has a lighter material. , easy processing and so on.
In the field of shoe materials, there are generally three types of chemical foaming processes of EVA: traditional flat large foaming, small in-mold foaming and injection cross-linking foaming.
At present, injection cross-linked foaming is the mainstream process of shoe material processing factories.
In order to further improve the physical properties of EVA products, various manufacturers will also add EPDM, POE, OBCs, TPE such as SEBS oil-extended and other elastomers to the EVA raw materials for blending. The flexibility of TPE can be modified to achieve the purpose of modification. For example, after adding POE elastomer, the resilience of the product can usually be increased to 50-55%, or even higher.
In general, the chemical foaming technology in the field of shoe materials in my country has been relatively mature. Thanks to the price advantage of EVA foaming materials, at present, EVA chemical foaming shoe materials will still be the main choice for mass sports shoes. The latest technology and more cost-effective materials are also constantly breaking the game.
2
ETPU--a game-breaking "physical foaming" technology
In 2013, Adidas launched the Boost midsole technology shoes with "popcorn" (E-TPU) as the core, and caused a sensation in the entire shoe material field. Since then, physical foaming represented by BASF's "popcorn" (E-TPU) foaming technology has also developed rapidly in the field of shoe materials.
TPU is the English abbreviation of thermoplastic polyurethane elastomer, which is a block copolymer composed of diisocyanate, chain extender and polyol. The soft segment composed of polyols exhibits flexibility and toughness, while diisocyanate acts as a hard segment to impart hardness and rigidity to the material, and the hard segment acts as a physical cross-linking point after crystallization, making 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, namely ETPU, has a popcorn shape after molding, and has excellent resilience and deformation recovery ability. The rebound value of the foam falling ball can usually reach 60%.
Supercritical fluid microcellular foaming principle
We call the liquid whose temperature and pressure are above the critical point as supercritical fluid. Supercritical fluid is a structure between liquid and gas. It fills the entire space like a gas, and its viscosity and diffusion coefficient are the same. It is closer to a gas, but its density is similar to that of a liquid.
Supercritical fluids are generally dominated by CO2 or N2, because CO2 and N2 are chemically stable and inexpensive, non-toxic and harmless, and are environmentally friendly. Secondly, the critical pressure and temperature of supercritical CO2 and N2 are relatively mild, easy to achieve, and have good solubility for polymers, strong diffusion ability, and easy control.
The process of supercritical fluid microcellular foaming can be divided into 4 stages:
01 Make the supercritical fluid enter the polymer matrix to reach a saturated state to form a polymer/gas homogeneous system;
02 A sudden temperature rise (step-by-step heating method) or a sudden pressure drop (rapid pressure relief method) makes the gas in the homogeneous system reach a supersaturated state, that is, a thermodynamically unstable state, which causes bubbles to nucleate;
03 The gas quickly diffuses into the bubble core, and the bubble gradually grows;
04 Rapid cooling to complete the shaping of the cell structure. After this process, countless invisible bubbles are formed.
▲After the polymer foam beads are obtained, there are usually two molding processes for the subsequent shoe materials: steam molding and PU bonding. Affected by factors such as environmental protection and weight reduction, the current molding process is basically based on steam molding
Compared with the traditional foaming process (the cell density is usually 10^4~10^6 cells/cm3, and the cell diameter is 50~200mm), the cell density of the microporous material obtained by the supercritical fluid foaming technology is average. It is 10^9～10^15 pieces/cm3, and the cell diameter is less than 10mm.
Therefore, for foam materials with the same density, the microcellular materials obtained based on supercritical fluid foaming technology have higher cell density and smaller cell size, and show better mechanical properties: the existence of cells does not It will not reduce the strength of the material. On the contrary, when the crack expands under stress conditions, the micropore can passivate the crack tip and prevent the further expansion of the crack. .
Moreover, with the continuous improvement of technology, in the field of shoe materials, the applicable polymer matrix of supercritical fluid microcellular foaming technology has also expanded from the original thermoplastic polyurethane (TPU) to thermoplastic polyester elastomer (TPEE). , One or more mixtures of block polyetheramide elastomer (PEBAX), ethylene-vinyl acetate copolymer (EVA).
For example, Nike’s ZoomX series uses PEBAX foam midsole, the 361° Q-bounce super running shoe midsole is made of EVA+TPU, Jordan’s PRO midsole is TPE, and Skechers’ Hyper Burst midsole is EVA.
In the past two years, the development of supercritical fluid microcellular foaming technology in China has been getting faster and faster, but the investment cost of related equipment and technology is high, while the domestic related equipment and technology started relatively late and need to be further improved, and there are low processing efficiency and And other issues.
However, based on the advantages of "lighter", "more elastic" and "more wear-resistant" of supercritical fluid microcellular foaming materials, major brands in the industry have generally made it clear that in the future, supercritical physical foaming materials will be used to replace existing chemical foaming materials. market.
3
PEBAX--a new star in the future
PEBAX Elastomer is an extremely high performance thermoplastic polymer, a block copolymer composed of rigid polyamide blocks and soft polyether blocks.
These unique polymers maintain the traditional concept of combining the toughness associated with polyamides with the elasticity of polyethers. During repeated bending, PEBAX elastomers can provide extremely efficient energy recovery.
Wearing PEBAX midsole technical shoes can reduce the energy loss coefficient to a lower level, and has strong cold shock resistance. In addition, PEBAX elastomer has the lowest weight in the thermoplastic category, which is 20% lighter than many low-density polymers, and the rebound rate can reach 70%-80%, and the firmness will not decrease at all.
However, this material is not perfect. If you have PEBAX shoes in your hands, you will find that even the new midsole still has a lot of wrinkles. This is due to the poor support between the molecules of PEBAX. In addition, the cost and rejection rate of PEBAX foaming technology are relatively high, resulting in high cost. The scrap rate was so high that Nike could use his scrap to make another shoe.
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