Effect of vertical aligned carbon nanofiber/carbon nanotube on the mechanical, electrical and electromagnetic interference shielding properties of epoxy foams

In this study, one-dimensional (1D) carbonaceous blends containing multi-walled carbon nanotube (MWCNT) and carbon nanofiber (CNF) were added into epoxy resin (EP) to obtain hybrid powder though ball milling. Limited foam with vertically aligned nanofillers was fabricated through limited-foaming epoxy hybrid in a mold and Two-step foam was prepared by further freely foaming a preformed Limited foam in an oven. The cellular structure, compressive, electrically conductive and EMI shielding properties of these two kinds of epoxy composite foams were carefully investigated. Limited foam showed an isotropic electrical resistivity while the electrical anisotropy started to emerge after the second-step foaming. This result was ascribed to the twisty MWCNT bridged the aligned CNF to build conductive pathways in horizontal direction, thus eliminating the electrical anisotropy of Limited foam. However, these loose interconnections between MWCNT and CNF in horizontal direction were more seriously destroyed by the further growth of bubbles, consequently endowing the Two-step foam with electrical anisotropy. Adding CNF into EP/MWCNT foams not only improved the electromagnetic interference (EMI) shielding effectiveness (SE), which was due to the synergistic effect between hybrid nanofillers in enhancing the electrical conductivity, but also enlarged the anisotropy in EMI SE of both Limited foam and Two-step foam. As compared to anisotropic bubbles, the vertically aligned MWCNT/CNF in composite foams exhibited a weak effect on anisotropy in compressive strength. Moreover, a remarkably anisotropic expansion behavior of preformed foam during the second-step foaming was observed and this behavior can be tuned by the ball milling time and the content of vertically aligned nanofillers.