The preparation methods of micro-foamed polymer materials include batch method and continuous method. The basic principles of the two methods are consistent, that is, the polymer system saturated by supercritical fluid enters the thermodynamic unstable state through rapid pressure discharge or rapid temperature rise, thus inducing a large number of gas nuclei to form simultaneously in the polymer matrix and obtaining micropore structure. My summary of its basic process is shown in figure 1, which can be divided into the following stages:

1. To form polymer/gas saturation system under certain temperature and pressure, non-reactive gases (CO2 or N2) were dissolved in the polymer by appropriate methods to form homogeneous polymer P gas saturation system. The dissolution process of gas is controlled by diffusion, which is related to the saturation concentration of gas, gas pressure and system temperature.

2. The formation of the bubble core makes the polymer P gas saturated system rapidly become oversaturated system and enter the thermodynamic instability state through pressure sudden drop or temperature sudden rise. The phase separation of the polymer and gas induces nucleation in the polymer system. The gas core is larger than a critical size (r) to be stable. According to the classical nucleation theory, r3 by polymer P gas interfacial tension (gamma) and used for the gas pressure of saturated polymer (Δ P) decision;

3. The gas in the bubble growth system diffuses into the bubble core, making the bubble growth and the free energy of the system continue to decrease. Bubble growth is controlled by allowable growth time, system temperature, degree of oversaturation, system stress state and viscoelasticity. The growth of vesicles is often accompanied by the merging of vesicles and the rupture of bubble walls. Vesicle coalescence refers to the phenomenon of large size vesicles merging with surrounding vesicles due to the uneven distribution of bubble sizes. The formation of large size vesicles is conducive to reducing the instability of the microbubble system. However, when the bubble wall cannot withstand the bidirectional tensile effect caused by the growth of adjacent bubble holes, it will break, making the adjacent bubble holes connected together to form larger bubble holes. Bubble wall rupture usually occurs at the later stage of bubble hole growth, at which time the bubble wall becomes very thin due to the increase of bubble hole size. In the literature, the above phenomena are collectively referred to as bubble coalescence without strict differentiation.

4. In the process of growth, a large amount of gas diffuses into the bubble hole or escapes from the polymer foaming system, resulting in continuous decrease of the driving force of bubble growth; At the same time, the rigidity of polymer matrix increases gradually. Both of these effects can inhibit the growth of vesicle and eventually fix the structure of vesicle. Early studies mainly focused on the effects of technological parameters in the preparation process of polymer micro-foam materials, such as saturated temperature, saturated pressure, saturation time, foaming temperature, foaming time, pressure relief rate, etc., on the bubble morphology and mechanical properties of materials. In recent years, with the deepening of experimental research and theoretical understanding of polymer microfoam, more methods have been used to improve the cellular morphology of microfoam polymer materials. Articles come from the Internet

Supercritical foaming equipment