Highly expansive, thermally insulating epoxy/Ag nanosheet composite foam for electromagnetic interference shielding

In this work, epoxy foams with high volume expansion ratio and excellent electromagnetic interference (EMI) are fabricated by the supercritical carbon dioxide as blowing agent. So far, the volume expansion ratio of about 20 for epoxy foam reported in this work is the highest in literature and reported for the first time, related to the strategy of introducing hyperbranched epoxy and solvent-free covalent nanofluid hybrid material of nanocarbon black as modifier and nucleated agent. Meanwhile, incorporating commercial silver nanosheet (AgNS) into above-foamed system achieves two synergistic effects: one is the significant increase of electrical conductivity and the other is the well controllable cell structure to avoid cell coalescence. In turn, the uniform cell distribution of epoxy foams is favorable for the formation of more efficient conductive networks related to the biaxial stretching on rearrangement in the cell wall and inter-connection of AgNS. Multi-properties including electrical conductivity of 89.12 S/m, specific EMI SE (SSE) of 334.59 dB·cm³/g and thermal conductivity of 58.71 mW/m·K are achieved in the epoxy composite foam with 20.0 wt% AgNS. The EMI mechanism is dominated by absorption rather than reflection due to electromagnetic waves attenuated by AgNSs in cell walls after the waves reflecting several times in cells. This work provides SCCO₂ foaming strategy as a feasible path to generate epoxy conductive polymer composite (CPC) foam with lighting weight, low thermal conductivity, outstanding electrical conductivity, excellent EMI SE and accessible mechanical strength.