Fabrication of Lightweight Microcellular Polyimide Foams with Three-Dimensional Shape by CO2 Foaming and Compression Molding

Microcellular structure endows polymeric foams with the improved mechanical properties, but the preparation of lightweight microcellular polyimide (PI) foams with a large size is challenging and inefficient, because of low gas solubility, high stiffness, and an extremely long saturation time. In this study, PI foam was prepared by solid-state microcellular foaming technology with the compressed CO2 as a physical blowing agent and tetrahydrofuran as coblowing agent. The presence of coblowing agent was verified to increase the gas sorption of PI, causing a dramatic increase in the expansion ratio of microcellular PI beads from 2.9 to 15.7. Using a novel compression molding process, the prepared PI foams were molded into the 3-D shaped products. Before the molding, the foamed PI beads were coated by poly(ether imide) (PEI)/chloroform solution. The contact angle tests indicated that PEI/chloroform could infiltrate well PI foams' surface, which facilitated the formation of strong interbead bonding between bead foams. The thickness of the coated PEI layer and the interbead bonding regions were the important parameters to adjust the bending and compression properties of the molded PI foam (MPI) product. The experimental results indicated that the bending strength and compression strength (at 10% strain) of MPI sample with density of 137.7 kg/m(3) were 1.27 and 1.59 MPa, respectively.