The Past and Present of Supercritical Foaming Technology

01

Supercritical foaming technology

▲ technical background

Have you found that the midsole materials of today's running shoes, whether high-end racing shoes or long-distance training shoes, show the characteristics of light weight and excellent cushioning and springback performance, and also look quite similar in appearance? Behind this, it is actually the huge progress in midsole material technology that has promoted the overall improvement of the performance of running shoes. In the past, we often saw popcorn like midsole materials on Adidas boost series running shoes, but now, in addition to Adidas, more and more international and domestic brands have adopted this kind of popcorn material, thanks to the continuous innovation and breakthrough of midsole material technology. At present, supercritical foaming materials have become the mainstream technology in the industry, leading the new trend of running shoe midsole materials.

▲ technical principle

Foaming materials is an important means to reduce their density and expand their functional applications. Foam materials have been widely used in many fields, such as daily necessities, packaging, sports equipment, medical equipment, automobile manufacturing and construction, due to their light weight, heat insulation, sound absorption, seismic resistance, high specific strength and low price. The emergence of supercritical foaming technology has promoted the development of this trend and become the cutting-edge technology in the current industry.

Supercritical fluid foaming technology, as a rapid rising method in the past 20 years, has become an effective way to prepare microcellular plastics. The microporous structure involved in this technology is quite similar to the well-known popcorn structure. The principle behind it lies in the unique properties of supercritical fluid. When the temperature and pressure exceed the critical point at the same time, the liquid will enter a supercritical state. In this state, the fluid has the characteristics of both liquid and gas. It can occupy space as easily as gas, showing low viscosity and high diffusion coefficient, while maintaining the density similar to liquid. Such characteristics make supercritical fluid an ideal medium in the foaming process, thus realizing the efficient preparation of microcellular plastics.

▲ material advantages

The foaming process can be summarized as four key stages: 1) the supercritical fluid is introduced into the polymer matrix until it reaches saturation, and then a homogeneous system of mutual dissolution of polymer and gas is constructed. 2) Through the rapid rise of temperature or the sudden release of pressure, the gas in the homogeneous system is supersaturated, that is, it enters the thermodynamic unstable state, and then triggers the formation of bubble nuclei. 3) The gas rapidly diffuses into the bubble core, which makes the bubble pore gradually increase. 4) Then, the temperature is cooled rapidly to ensure that the bubble structure is solidified, so as to complete the manufacture of popcorn products.

02

1. application examples of running shoe midsole materials

1.1 ▲ Adidas boost series

In the application of midsole materials for running shoes, the commonly used polymer matrices mainly include ethylene vinyl acetate copolymer (EVA), thermoplastic polyurethane (TPU), thermoplastic polyester elastomer (TPEE) and block polyether amide elastomer (PEBAX). These matrices can be used alone or combined into a mixture.

Adidas boost series, this famous running shoe, has an impressive midsole design. The midsole material has an obvious sense of particles, and the touch is similar to foam, thanks to its predecessor infinergy foam microspheres. This kind of microsphere is made of thermoplastic polyurethane (TPU) raw material particles through a special foaming process, which brings excellent elasticity and comfort to running shoes. Boost material is carefully developed by the well-known BASF Ag in Germany. Adidas has successfully created the boost series of running shoes by using BASF's latest foam microsphere infinergy technology. Its excellent elasticity and shock absorption performance make it continue to be popular in the market.

Despite the fierce market competition, the boost series of running shoes still maintain its classic status. As a running shoe, its durability and comfort are still trusted by many runners. Infinergy technology mainly relies on the unique properties of foamed thermoplastic polyurethane (e-tpu), which not only reduces the weight, but also provides excellent cushioning and rebound effect.

1.2 ▲ Nike PEBAX foam

Nike PEBAX ultra-high performance elastomer is known as the most popular midsole material at present. With the glorious history of Nike from 4% to next%, this new type of nylon elastic foam material gradually came into people's vision.

After supercritical foaming process, Nike's unique zoomx material was born. This material not only has high flexibility and excellent resilience, but also has excellent performance in lightweight and low-temperature impact resistance. PEBAX ultra high performance elastomer is not only known as one of the best performance midsole materials, but also the standard of many top racing shoes, although its high cost limits its application in the second top shoes or training shoes.

1.3 ▲ relationship between carbon plate and foaming material

In the past two years, the popularity of carbon board running shoes has continued to rise, and many people have overemphasized the efficacy of carbon board. However, if there is no supercritical foaming material represented by PEBAX, the role of carbon plate will be greatly reduced, and this relationship can be described as "skin without hair".

Specifically, although the carbon plate can show excellent resilience when bending, its texture when not bending is very hard. Therefore, after adding carbon plates to the midsole structure, it is necessary to match high-quality cushioning materials to reduce the hard impact of carbon plates on the feet. In this way, it can not only ensure that the compliance of the shoes will not be reduced due to the addition of carbon plates, but also prevent the excessive increase of the stiffness of the shoes, so as to maintain the comfort of wearing.

Therefore, in order to effectively integrate the carbon plate into the midsole, we must use more advanced and thicker cushioning and rebound materials. This material not only needs to have excellent cushioning performance, but also can fully rebound, which is compatible with the carbon plate to provide a good wearing experience. At the same time, the thickness of this material should be moderate. It is necessary to avoid the loss of elasticity and compliance of running shoes due to the addition of carbon plates, and to prevent the increase of shoe weight due to the increase of thickness. Fortunately, the new generation of nylon foam material PEBAX just meets these requirements.