Introduction to Polypropylene Foaming Technology
With increasing demands for environmental protection, waste recycling, and cost-effectiveness in products, physical foaming methods using agents such as CO2, N2, and isopentane have gained significant attention. Currently, CO2 is the most widely used foaming agent.
The basic method for preparing microporous polymer materials using supercritical fluid involves creating a highly saturated polymer melt/gas mixture and inducing thermodynamic instability during the cooling process. By controlling parameters like pressure and temperature, microcellular structures are formed within the polymer matrix, with the supercritical fluid acting as the nucleating medium. The key steps in this process are as follows:
Polymer/Gas Saturation System
At a certain temperature, an appropriate method is used to dissolve a high-pressure, non-reactive gas (e.g., CO2 or N2) into the polymer, forming a homogeneous polymer/gas saturation system. The gas concentration typically ranges from 5% to 20%. The diffusion of gas within the polymer is slow and can be accelerated by increasing temperature and pressure.
Nucleation
By reducing pressure and/or increasing temperature, the polymer/gas system enters a thermodynamically unstable state, becoming supersaturated. This triggers homogeneous and heterogeneous nucleation, leading to the formation of numerous gas bubbles.
Bubble Growth
The supersaturated gas diffuses into the bubbles, causing them to grow and reducing the system's free energy. Bubble growth is controlled by factors such as time, temperature, supersaturation, stress, and the viscoelastic properties of the system.
Microporous Structure Stabilization
Methods like quenching are used to stabilize the bubble structure.
The uniform, high-concentration polymer/gas system and precise control of nucleation and bubble growth are critical to the process. The microcellular foams produced typically have pore sizes ranging from 5-30 μm. Compared to traditional foamed sheets, these microporous materials exhibit 30%-40% higher tensile and compressive strength for the same density, and they can be produced on existing production lines. The combination of supercritical fluid technology and plastic injection molding has made the direct production of microcellular polypropylene injection-molded products a reality.
High Melt Strength Polypropylene Foaming Technology
In conventional polypropylene foaming, viscosity sharply decreases above the crystallization melting point, making temperature control during extrusion difficult. However, polypropylene must maintain sufficient flowability in the extruder while also having adequate melt strength and elasticity to preserve a regular bubble structure. High melt strength polypropylene is therefore crucial in the foaming process.
For example, Profax F814 resin, produced by a foreign company, has long side chains introduced during the post-polymerization process, giving it 9 times the melt strength of conventional homopolymers with similar flow characteristics. The behavior of bubbles in linear PP and branched PP during foaming differs significantly. Linear PP exhibits high open-cell content, and the bubbles merge quickly, even under rapid cooling. In contrast, branched PP tends to form closed-cell structures with minimal bubble merging, making it suitable for achieving high melt strength.
Cross-Linked Polypropylene Foaming Technology
Some companies have also adopted cross-linking processes to produce polypropylene foams, such as mixing PP with PE and cross-linking the PE. For example, a company has developed a micro-crosslinked polypropylene foam using a two-stage process: first, extruding a 3mm thick solid sheet, then crosslinking it with peroxide or irradiation, and finally placing it in a high-pressure vessel (up to 69 MPa) with N2 to induce foaming. This method results in a foam with 10% closed-cell structure and a density of 0.3g/cm³. This foam is used in applications like automotive parts and sporting goods.
The key to this process is cross-linking the PP resin before foaming, which reduces melt viscosity and minimizes bubble rupture during foaming. Cross-linked PP foams exhibit significantly better heat resistance (30-50°C higher) and thermal creep performance (100 times better) compared to non-crosslinked foams. However, PP's high crystallinity and difficulty in cross-linking present challenges, requiring precise control of reaction conditions to minimize degradation.
Nucleating Agents in Foaming
Producing microcellular PP foams with conventional thermoset or amorphous thermoplastic technologies is difficult due to the low gas solubility in the crystalline region of PP, which limits bubble nucleation and growth. Adding small amounts of sodium benzoate as a nucleating agent can lower the surface tension of the polymer, promoting bubble nucleation. However, talc, which forms a strong bond with PP, is not effective as a nucleating agent and should not be used.