In the foaming processing of polymer materials, according to the production mechanism of bubbles, it can be divided into two categories: chemical foaming and physical foaming.
Supercritical foaming is the 1 kind of physical foaming, full name is supercritical fluid microcellular foaming. The physical properties of supercritical fluid are between liquid and gas. Under certain temperature and pressure, it has the properties of gas and liquid. It can diffuse as easily as gas and can be easily infiltrated as liquid. The basic principle of supercritical foaming is to use the dependence of the solubility of gas in the polymer on pressure and temperature, so that the polymer mixture system after supersaturated with supercritical fluid enters a thermodynamically unstable state during the cooling process, inducing the formation of gas nuclei, and then obtaining a microporous structure. Chemical foaming is added to the polymer material in the chemical foaming agent for heating to explain the release of gas and foaming, the whole foaming process is similar to the fermentation of pasta, is a traditional and mature foaming process, is still widely used in the production of ordinary midsole.
Compared with traditional chemical foaming, supercritical foaming technology has obvious advantages, which can achieve the two goals of light weight and high elasticity of the midsole at the same time. Supercritical foaming can produce small and uniform bubbles, so that the density of the midsole is lower, thereby reducing the weight of the sneakers and reducing the burden on the user's feet. Supercritical foaming can evenly distribute the bubbles in the midsole material, and the formed buffer structure can effectively absorb the impact, provide better energy feedback, reduce the pressure of joints and muscles, and also improve the compression resistance, so that the midsole is more durable.
The characteristics of supercritical fluid foaming technology include:
Fully utilizing the fast diffusion rate and high solubility of supercritical carbon dioxide in polymers.
During foaming, the polymer is in a semi-solid state, and the melting zone allows for the growth of pores, while the unmelted zone provides melt strength and maintains the pore structure.
Extremely rapid pressure relief induces the formation of a very high nucleation rate, ensuring the formation of a cell structure with micro-nano size and high pore density.
In addition, supercritical foaming does not produce some gases harmful to the environment or human body like chemical foaming, and has the advantages of environmental protection and cleaning.
However, supercritical foaming requires much more equipment and process control than chemical foaming, so the production cost is higher.
At present, there are mainly 4 kinds of raw materials for supercritical foaming: EVA, TPU, TPEE and PEBA.
EVA (ethylene-vinyl acetate) is the earliest application, the most mature, the most extensive foam shoe material, the performance is more common, durability index is relatively backward, after a long time wearing easy to be "flat", thereby reducing the rebound, cushioning performance.
TPU (thermoplastic polyurethane) has excellent rebound performance and strong durability, but its own weight is large, and it will heat up after being subjected to pressure deformation for a long time. TPU is divided into aromatic and aliphatic. Overall, the aliphatic is superior in performance to the aromatic.
TPEE (thermoplastic polyester) can be regarded as an upgraded version of TPU. Generally speaking, it is not much different from TPU in terms of rebound, durability and other indicators, but significantly improved in terms of lightweight indicators.
PEBA (polyether block amide), also known as nylon elastomer, has the highest level of resilience, lightweight performance and durability that does not lose TPU and TPEE. It is the standard midsole of top sneaker. The cost of PEBA is high, and PEBA is commonly used to mix EVA, TPU and other materials for foaming to reduce costs.
