Supercritical foaming molding is a physical foaming molding technology, as well as a microporous foaming molding technology, which replaces organic foaming agents with supercritical carbon dioxide or nitrogen gas and performs foaming under certain pressure and temperature. This process has no residual formamide, is odorless and non-toxic, provides long-term protection for both consumers and producers, does not use crosslinking agents, can be melted and recycled, and is environmentally friendly.

 

We have previously sorted out the shoe material midsole supercritical foam materials, including EVA, TPU, TPE, PEBA, etc. Some of them are formed by compression molding supercritical foam, while others are formed by kettle pressing supercritical foam. And today we are organizing the foam forming of supercritical sheet materials.

 

The characteristics of supercritical fluid sheet 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 fast pressure relief induces the formation of extremely high nucleation rates, ensuring the formation of bubble structures with micro nano size and high pore density.

 

 

 

The following is an analysis of the foaming performance of TPU, TPEE, and PEBA supercritical sheets.

 

1、 TPU foam

TPU (thermoplastic polyurethane elastomer) is a polymer material made by reacting and polymerizing diisocyanate molecules such as diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI) with high molecular weight polyols and low molecular weight polyols (chain extenders).

Schematic diagram of TPU molecular chain structure

Using board foam, with a size of 2 * 1.3 meters and a density of around 0.15, its biggest feature is its rebound performance. Although it is not as eye-catching as PEBA, its compression performance is excellent, with a temperature measurement within 30% and a stable rebound performance of over 65%.

 

 

2、 TPEE foam

Figure TPEE molecular structure formula

After foaming, the density of the board is generally between 0.11-0.3, and the hardness of the main material is between 25D-50D. Different application types choose raw materials with different hardness. The rebound of TPEE is higher than that of TPU, at over 70%.

 

3、 PEBA foam

Thermoplastic polyamide elastomer, also known as nylon elastomer, abbreviated as TPAE in English, is essentially a linear block copolymer composed of polyamide hard segments and polyether or polyester soft segments. It has the characteristics of high tensile strength, good elastic recovery, high low-temperature impact strength, and excellent low-temperature resistance. There are various types of TPAE based on the different components of hard and soft segments. At present, the vast majority of nylon elastomers on the market are polyetheramide block copolymers, also known as poly (ether block amide) and abbreviated as PEBA. Therefore, PEBA and nylon elastomers are common names.

Figure: General formula of PEBA structure

The biggest feature of PEBA is its lightness and elasticity. The normal density is 0.07-0.8, but it can also be made into 0.09-0.10. The rebound performance of the ball is very excellent, reaching 73% -75%.

 

4、 Application

TPU, TPEE, and PEBA are three thermoplastic materials that can be thermoplastic processed and reused. Applied to shoe materials, it provides the possibility of whole shoe recycling and is environmentally friendly. Using physical foaming of the board instead of chemical foaming, it is non-toxic, odorless, slip resistant, wear-resistant, and scratch resistant.