What is a Supercritical Gas?

A supercritical gas is a gas that exceeds the critical temperature, critical pressure, and critical volume state of a substance's gas-liquid. In a narrow sense, it refers to a gas or fluid that exceeds a critical temperature state. The name of supercritical gas can also be called high density gas, high pressure gas, ultra high pressure gas, etc. according to different fields. The properties of a gas in a supercritical state are completely different from its properties at normal temperature and pressure. For example, it has properties similar to liquid, small surface tension (almost close to 0), thermal conductivity higher than normal pressure gas, low viscosity, etc., and can be easily controlled by pressure adjustment.

The properties of a gas in a supercritical state are completely different from its properties at normal temperature and pressure. For example, it has properties similar to liquid, small surface tension (almost close to 0), higher thermal conductivity than normal pressure gas, and low viscosity, and its properties can be easily controlled by pressure adjustment. The following table shows the properties of the gas under supercritical temperature and normal pressure.

Properties of gas under supercritical and normal temperature and pressure

nature	Gas type
	Normal temperature and pressure gas	Supercritical gas
Viscosity coefficient / (uPa · s)	1 × 10 -4 ~ 3 × 10 -4	2 × 10 -5 ~ 1 × 10 -4
Thermal conductivity / [kW · (m / K)]	5 × 10 -3 ~ 3 × 10 -2	3 × 10 -2 ~ 7 × 10 -2
Diffusion coefficient / (cm 2 / s)	5 × 10 -6 ~ 3 × 10 -4	1 × 10 -8 ~ 1 × 10 -6
Density / (g / cm 3 )	6 × 10 -4 ~ 2 × 10 -3	0.2 ~ 0.5

Commonly used supercritical gases are ethane, ethylene, propane, toluene, CO ₂ and the like. Among them, CO _{2 is the} most widely used.

Supercritical gases have a high density of liquid substances,

Supercritical gas leaching, as a new type of separation technology, has aroused great interest and attention, especially it can perform separation operations at lower temperatures, thus saving energy and preparing the human body from various natural materials. There are clear advantages in harmless, healthful products.

In the field of polymer processing, supercritical gas is also widely used in the preparation of microporous polymers, polymer particles, and auxiliary modified polymers.

Preparation of microporous polymers by supercritical gas

Microcellular polymer refers to a foam material with a cell diameter of 0.10 ~ 10.0m, a cell density of 10 ⁹ to 10 ¹⁵ cells / cm ³ , and a material density that can be reduced by 5% to 10% compared to before foaming. Compared with other materials, it has high impact strength, low dielectric constant, and low thermal conductivity.

The use of supercritical gas to prepare microporous polymers is a physical foaming technology first developed by the Massachusetts Institute of Technology with the aid of supercritical carbon dioxide gas. This technology is more environmentally friendly than the traditional polymer chemical foaming technology, because the foaming agent of the microcellular polymer is a gas that does not pollute the environment, such as carbon dioxide or nitrogen.

In addition, compared with traditional polymer chemical foaming technology, microcellular foaming has the following characteristics: (1) microcellular foaming is induced by the thermodynamic instability of a homogeneous supercritical gas / polymer system; (2) ) The nucleation number of microcellular foam is much larger than that of general chemical foaming technology; (3) Because the cell size of microcellular polymer is smaller than that of traditional chemical foaming polymer, this requires cells The growth period should be controlled within 0.01s. Obviously, the microcellular polymer cell growth control technology is more demanding than the general foaming technology.

Preparation of polymer particles by supercritical gas

Polymer particles have been widely used in the chemical industry. It can be used as adsorbents, stationary phases for chromatographic columns, and catalyst carriers. The technology of preparing polymer particles with supercritical gas has attracted people's attention. The main preparation methods include: rapid expansion method, anti-solvent method, and compressed fluid anti-solvent method.

(1) Supercritical gas rapid expansion method (RESS)

The supercritical gas rapid expansion method (RESS) uses the characteristics of the supercritical gas's ability to dissolve as a function of pressure. The process is to inject the polymer-dissolved supercritical gas into the container at a high speed through the nozzle, and the supercritical gas expands into a gaseous state due to the pressure reduction in the container, so that the solubility of the solute in it is rapidly reduced, and polymer particles are precipitated. The polymer particles prepared by this method are widely used due to their small size, simple operation, and low equipment requirements. However, this method is only applicable to polymers dissolved in supercritical gas, so its application in preparing polymer particles that cannot be dissolved in supercritical gas is limited.

(2) Supercritical gas anti-solvent method (SAS)

The supercritical gas anti-solvent method (SAS) first dissolves a polymer insoluble in a supercritical gas in a suitable organic solvent, and then places the organic solvent in a supercritical gas. Since the supercritical gas has a very high solubility in many organic solvents, Large, so that the volume of the organic solvent will expand rapidly after the supercritical gas diffuses into the solvent. A large amount of supercritical gas dissolved in an organic solvent can greatly reduce the ability of the organic solvent to dissolve the polymer, and supersaturation in the solution can precipitate polymer particles with extremely fine particle sizes. The SAS method can control particle size, shape and structure very flexibly. SAS can be regarded as a complement to RESS, because it can overcome the limitations of RESS.

Supercritical gas changes polymer rheology and phase behavior

Many literatures report that supercritical carbon dioxide can reduce the melting point of crystalline polymers, reduce the viscosity of polymer melts, reduce the glass transition temperature of glassy polymers, and reduce the dispersed phase of blends after interacting with polymer melts. Size, increase the free volume of the polymer, and increase the mobility of the molecular chain.

Supercritical gas is also widely used in the processing of blend systems to achieve the purpose of changing the phase behavior of the blend. In the case of supercritical CO ₂ injection, PS / PMMA (polymethyl methacrylate) blends were prepared using a high-pressure vessel and a twin screw. It was found that the addition of carbon dioxide can significantly reduce the size of the dispersed phase PMMA. This phenomenon is mainly Caused by a decrease in the two-phase viscosity ratio. ^[5]