Supercritical Fluid Foaming Technology: Autoclave, Injection, Extrusion, and Bead Foaming

Supercritical Fluid (SCF) refers to a fluid where both pressure and temperature exceed the critical point. In the CO₂ phase diagram (Figure 1), point O is the triple point where three phases coexist in equilibrium. Point C is the end of the gas-liquid equilibrium line, known as the critical point, with corresponding critical temperature and pressure.

SCF retains gas-like properties while exhibiting liquid-like characteristics. Its diffusion coefficient is 10-100 times that of liquids, with excellent permeability, flowability, heat transfer, and mass transfer properties. This gives it strong dissolving capacity for many substances, enabling extraction and separation of non-volatile and thermosensitive materials at lower temperatures.

For example, supercritical carbon dioxide (scCO₂) exists when temperature and pressure exceed 31.1℃ and 7.38 MPa, combining gas and liquid properties with enhanced mass transfer performance.

As physical blowing agents, supercritical fluids offer advantages including mild supercritical conditions (31.1℃, 7.37MPa), high mass transfer coefficients, strong solvating ability, and compatibility with most polymers—earning them the title of "ideal physical blowing agents." They were first used in foaming common polymers like polystyrene (PS) and polypropylene (PP).

The formation of cellular structures can be described in three stages: 1.Formation of polymer/gas homogeneous system; 2.Creation of supersaturated system through rapid pressure reduction or temperature increase to induce nucleation; 3.Bubble growth under driving force, followed by solidification as driving force diminishes and polymer matrix temperature decreases.

Supercritical fluid foaming technologies for high-performance polymers include autoclave foaming, injection molding foaming, extrusion foaming, and bead foaming.

I. Autoclave Foaming

Autoclave foaming (solid-state foaming) includes one-step foaming (pressure-induced nucleation) and two-step foaming (temperature-induced nucleation), as shown in the diagram.

In the one-step method, polymer matrix is immersed in high-temperature (above effective Tg0 of homogeneous system) and high-pressure CO₂ environment to form a homogeneous system, followed by pressure release for nucleation, growth, and solidification. In the two-step method, polymer matrix first absorbs gas at low temperature (below effective Tg0) and high pressure, then undergoes rapid pressure release followed by rapid heating to induce bubble growth, typically in high-temperature oil baths or hot presses.

Autoclave foaming was the earliest foaming technology, offering simple equipment and easy control, but its long production cycle limits it to laboratory-scale research such as exploratory experiments and foaming mechanism studies. Currently, most high-performance polymer foaming research uses autoclave technology.

II. Injection Molding Foaming

Supercritical fluid injection foaming combines supercritical fluid injection units with traditional injection molding machines. Representative technologies include Trexcel's Mucell® technology, Sulzer Chemtech's Optifoam technology, and Demag's ErgoCell technology.

The process involves screw back pressure pushing the polymer/foaming agent homogeneous system forward into an atmospheric pressure, high-temperature mold cavity. Maintaining proper airtightness at the nozzle is crucial for supercritical foaming, with gas injection volume controlled by supercritical fluid pumps.

Key influencing factors include injection orifice diameter, screw speed, mold temperature, and SCF injection volume. Advantages include batch production of microcellular polymer foam products, effective weight reduction, material cost reduction, and shorter production cycles.

III. Extrusion Foaming

Extrusion foaming is a continuous process combining SCF technology with extruder's continuous processing capability, enabling mass production of polymer foam materials and widely used in industrial production.

During extrusion foaming, polymers are in molten state, allowing composite formulation at any stage. Foaming is typically achieved through rapid pressure release at the die. Depending on die design, products include rods, tubes, films, and sheets.

In the 1990s, Professor Park's research group at the University of Toronto pioneered SCF extrusion foaming technology, successfully foaming various polymers including PS, PP, and PLA, while systematically studying foaming mechanisms.

IV. Bead Foaming

Bead foaming uses expandable and pre-expanded beads. Expandable beads are mostly amorphous polymers that can encapsulate blowing agents below Tg and require pre-foaming before welding.Expandable beads have higher density (smaller volume) than pre-expanded beads, offering transportation advantages. Recently, combining extrusion foaming with underwater pelletizing for continuous production of expandable/pre-expanded beads has gained attention.

Bead welding is critical in bead foaming. Using pressurized high-temperature steam to weld polymer beads is currently the most important technology for producing large, complex-shaped, high-expansion 3D foam products.

Industry Challenges and Outlook

Despite significant achievements, supercritical foaming technology faces challenges:

1. Late and limited research on special engineering plastics foaming;

2. Inability to mass-produce high-performance polymer foam materials with complex 3D shapes, requiring advanced foaming technologies;

3. Single functionality of high-performance polymer foam materials, needing modification and structural design for properties like flame retardancy.