Fabrication of Polyimide Bead Foams via Utilizing Supercritical CO ₂ and Ethanol as the Co‐Foaming Agents

Polyimide (PI) foam exhibits outstanding thermal stability over a wide temperature range together with excellent mechanical strength, which has been widely applied in aerospace, rail‐transport and other advanced‐technology fields. However, PI foam is still predominantly produced by chemical foaming of prepolymers, which is complex, costly and involves volatile organic solvents, and its physical foaming still faces the problem of harsh foaming conditions and limited expansion ratio. Here, we developed a green and effective strategy using supercritical CO 2 (scCO 2 ) and ethanol (EtOH) as co‐foaming agents to fabricate PI bead foams. The addition of EtOH enhanced the solubility of co‐foaming agents in the PI matrix via hydrogen bonding, lowering foaming temperature and pressure while widening the process window. Under optimized conditions of 277°C and 10 MPa, PI foamed beads achieved a high expansion ratio of 11.21, an average cell size of 122.23 μm and a cell density of 5.66 × 10 6 cells cm −3 . The molded PI bead foam parts ( ρ = 0.22 g cm −3 ) exhibited a compressive strength of 4.14 MPa at 10% strain. This work demonstrated a scalable, environment‐friendly scCO 2 /EtOH route for manufacturing high‐performance PI bead foams with uniform cell structure and good bead adhesion.