Carbon nanotubes in microwave-assisted foaming and sinter molding of high performance polyetherimide bead foam products

Abstract

The preparation of high-performance polymer foams, especially special engineering thermoplastic foams, with a required geometry shape remains a big challenge worldwide. In this paper, polyetherimide (PEI) bead foam products were innovatively fabricated via the microwave (MW)-assisted foaming and selective sintering. During this process, the pre-expanded PEI beads were coated with carbon nanotubes (CNTs) and were then saturated in compressed carbon dioxide/tetrahydrofuran (CO2/THF) coblowing agent to obtain expandable PEI (EPEI) beads. The presence of CO2/THF was verified to soften and lower the glass transition temperature (Tg) of PEI matrix. Upon MW irradiation, the surface localized CNTs were intensively heated and served as hotspots, which increased the expansion ratio of EPEI beads to 8.2 fold. Taking advantage of the distinctive heating behaviors between CNTs and PEI matrix, the numerous heating sites (CNTs) upon MW irradiation strongly facilitated the melting of the PEI bead surface and caused a chain entanglement across the interfacial region, and tens of EPEI beads were molded into a product in a self-designed Teflon mould. As a model system, PEI/CNTs bead foam sample with 15 mm-thick and a density of 0.28 g/cm3 has been successfully prepared, which featured high surface quality and strong inter-bead bonding strength (up to 7 MPa). The MW selective heating mechanism was studied by using COMSOL based on the first principles. This study provided a novel approach for fabrication of high-performance polymer bead foam parts.