Principles and Characteristics of Two-Stage Supercritical CO₂ Foaming Technology

(1) Core Innovation of Two-Stage Foaming Process

The core innovation of two-stage supercritical CO₂ foaming technology lies in the synergistic effect of pressure difference and surface tension inside and outside foam cells. Research conducted by Lv Xia’s research team at the College of Polymer Science and Engineering, Sichuan University proves that this technology can directly convert rigid engineering plastics into closed-cell reentrant concave foam. The precise control of thermodynamic and kinetic conditions throughout the foaming process is the key to this transformation.

The essential difference between two-stage technology and traditional single-stage foaming lies in the time-space separation of conflicting foaming conditions. Single-stage equipment completes gas permeation and foaming simultaneously inside one reactor. To speed up gas diffusion, the process usually runs at high pressure near the polymer softening point, where the polymer matrix features minimal viscosity and mechanical strength. Instant pressure relief creates an extremely high supersaturation state of dissolved gas, generating powerful driving force for bubble growth. Applying maximum expansion force when the material is weakest keeps the process in an unstable thermodynamic zone, resulting in a narrow process window and high sensitivity to parameter fluctuations.

The two-stage process perfectly solves this contradiction. The first stage performs gas saturation under low temperature and high pressure, with the temperature set below the polymer softening point. The polymer maintains high strength, high modulus and excellent dimensional stability to completely inhibit bubble nucleation and growth. Sufficient high pressure forces supercritical CO₂ to dissolve and penetrate evenly into the polymer to form a homogeneous supersaturated system. In the second stage, the prefabricated material is placed in a secondary foaming environment with moderate backpressure (2–5 MPa). Slow heating gradually reduces the polymer modulus and viscosity to the optimal foaming window, matching matrix rheology with bubble growth kinetics. Trimming scraps generated during production can be crushed and regenerated online with the PS Foam Recycling Machine, which can be directly blended back into extrusion to build a closed-loop low-carbon production line.

(2) Reentrant Concave Cell Structure & Thermally Induced Deformation Mechanism

Exclusive reentrant concave microporous foam can be produced by matching the Two-Stage Supercritical CO₂ Foaming Production Line and PET Foam Sheet Extruder with complete Energy Storage Insulation Board Equipment, which serves as the core foundation for self-temperature-control energy storage insulation materials.

Schematic diagram of the formation mechanism of reentrant concave cell structure

The formation of reentrant concave cells is closely related to two-stage process parameters. By adjusting the key parameters of the first and second stages, R-PCF (reentrant concave Si-PC foam) with varying transformation degrees can be manufactured. During foaming, bubbles formed in the first stage shrink spontaneously under pressure holding instead of merging into large bubbles. Mass new microbubbles nucleate and grow during pressure relief in the second stage, while the original bubbles expand again to form the final large cells, laying the foundation for unique reentrant concave structures.

The thermally induced deformation mechanism is coordinated by multiple physical processes. First, reentrant concave cells expand elastically outward under heat and restore their original shape after cooling, enabling controllable shape change with temperature fluctuation. Second, thermally induced phase transition acts as the thermodynamic basis for deformation. Studies show R-PCF undergoes reversible phase transition at 40–60°C, closely linked to polymer glass transition and CO₂ adsorption-desorption behavior. Third, the pressure difference inside and outside cells provides dynamic driving force for cell deformation as gas pressure changes with temperature.

(3) Precise Regulation of Multi-Grade Pore Structure

Another core advantage of two-stage supercritical CO₂ foaming technology is the accurate regulation of bimodal or multi-modal pore size distribution via two-stage pressure release. In the two-stage process, bubbles generated in the first pressure drop shrink during pressure holding; numerous tiny new bubbles nucleate and grow in the second slow pressure relief, while initial bubbles re-expand to form large pores, creating distinct bimodal foam structures.

The two-stage pressure relief system produces dual-size cells: bubbles from the first stage shrink under constant pressure, while massive microcells form in the second slow pressure drop. The combination of micro and macro pores delivers both ultra-low thermal conductivity and superior energy absorption. The identical two-stage pressure relief procedure adopted in relevant research successfully produces microporous thermosetting polyurethane foam with cell size below 5 μm. The supporting PS Foam Recycling Machine can handle production waste to meet the demands of large-scale low-carbon manufacturing.