Supercritical fluid refers to a liquid whose temperature and pressure are both above the critical point. Gas under supercritical conditions, also known as supercritical fluid (SF), is a substance that exists in fluid form above the critical temperature (Tc) and critical pressure (Pc). Usually there are carbon dioxide (CO2), nitrogen (N2), etc.
Application of Supercritical Fluid
1、 Supercritical fluid extraction
Supercritical CO2 has the advantages of non toxicity, non corrosiveness, non flammability, high purity, and low price. It has excellent mass transfer properties, high diffusion coefficient, and low viscosity. Compared with other supercritical fluids, it has relatively low critical pressure and sensitive critical temperature, making it suitable for processing thermosensitive biological and natural products.
The principle of supercritical CO2 fluid extraction (SFE) separation process is to utilize the relationship between the solubility of supercritical fluid and its density, that is, to use the influence of pressure and temperature on the solubility of supercritical fluid. In a supercritical state, the supercritical fluid is brought into contact with the substance to be separated, allowing it to selectively extract components of polarity, boiling point, and molecular weight in sequence. Of course, the extract obtained from each pressure range cannot be single, but the optimal ratio of mixed components can be obtained by controlling the conditions. Then, by using pressure reduction and temperature rise methods, the supercritical fluid can be transformed into ordinary gas, and the extracted substance can be separated or basically precipitated, thus achieving the purpose of separation and purification. Therefore, the supercritical CO2 fluid extraction process is composed of a combination of extraction and separation processes.
2、 Supercritical fluid foaming
Material foam is an effective way to reduce its density and expand its functional application. Foam materials have the characteristics of light weight, heat insulation, sound absorption, earthquake resistance, high specific strength and low price, and are widely used in the fields of commodity packaging, sports equipment, medical equipment accessories, automobile industry and construction.
Supercritical fluid foaming technology is an effective method for preparing microporous plastics that has rapidly developed in the past 20 years. The obtained microporous plastic has smaller cell size, higher cell density, and better comprehensive performance. In addition, there are various ways of supercritical fluid foaming molding, which is suitable for foaming high-temperature melting temperature polymers. Therefore, this technology provides a new way for the preparation of special engineering plastic foam.
Compared to other foaming methods, supercritical fluid foaming mainly includes batch method and continuous extrusion method.
Intermittent method
The intermittent method belongs to the static foaming method, which usually involves placing polymer samples into an autoclave for static adsorption, and then selecting the method of primary pressure relief or secondary oil bath heating based on the relationship between saturation temperature and polymer glass transition temperature for polymer foaming. However, whether it is a first pressure relief or a second heating, the temperature required is much lower than its melting temperature. Therefore, the intermittent foaming process is not limited by the melting temperature of the polymer.
In addition, the intermittent foaming device has a relatively simple structure and strong controllability. It is not only an effective method for studying the formation mechanism of microporous polymers and determining process parameters, but also provides guidance for the design and process parameter conditions of injection molding and extrusion molding equipment.
Continuous extrusion method
Supercritical extrusion foaming is a dynamic foaming method based on traditional extrusion or injection molding, and is suitable for industrial production. The main difference between it and traditional extrusion and injection molding is that it requires special equipment and technical means to inject supercritical fluid into the melt plasticizing section of the extruder or injection molding machine, so that the supercritical fluid and polymer melt form a homogeneous system at high temperature and high pressure. Finally, foaming and cooling are carried out through injection molding filling or pressure drop during extrusion.