Lightweight and High-Performance Polymethacrylimide Foams for Complex-Shaped Applications Based on the Highly Foamable Precursor Beads

Lightweight and high-performance complex-shaped polymeric foams are highly desirable for applications in aerospace, ships, wind power, transportation, etc. This work successfully synthesized the highly foamable P(MAA-co-MAN-co-tBMA) beads via aqueous suspension polymerization, which can be used as foamable precursors for the preparation of polymethacrylimide (PMI) foams by in-mold thermal foaming. For the synthesis of highly foamable P(MAA-co-MAN-co-tBMA) beads used as PMI precursor beads, the oil phase consists of methacrylic acid (MAA) and methacrylonitrile (MAN) as monomers with azodiisobutyronitrile (AIBN) as an initiator and tert-butyl methacrylate (tBMA) as a copolymerizable foaming agent, while the aqueous phase is composed of polyvinyl alcohol (PVA) as the dispersant, NaNO₂ as the aqueous phase inhibitor, and NaCl as a salting-out agent. The chemical structures, foaming behaviors, expansion ratio, and thermal properties of the foamable P(MAA-co-MAN-co-tBMA) beads were investigated in detail. The highly foamable P(MAA-co-MAN-co-tBMA) beads with an average bead diameter of 0.74 mm can be obtained, and their expansion ratio can reach as high as 50 upon free thermal foaming at 230 °C for 30 min. The lightweight and high-performance PMI foams with low mass densities of 60–120 kg/m³, an outstanding thermal stability of 400 °C, an excellent compression strength of 0.69–2.20 MPa, and low compression creep deformations of 1.14–3.39% at the high temperature of 140 °C for 24 h were prepared via in-mold thermal foaming. Moreover, the scalable preparation of complex-shaped PMI foam products is demonstrated based on the highly foamable P(MAA-co-MAN-co-tBMA) beads, which is promising for applications in aerospace, ships, wind power, transportation, etc.