Broad-band electrical conductivity of carbon nanofibre-reinforced polypropylene foams

Closed-cell carbon nanofibre-reinforced polypropylene (CNF–PP) foams (bulk density 300 kg m −3) were produced and studied regarding their broad-band electrical conduction behaviour, with the main objective of producing lightweight rigid PP-based materials for electrical applications. [Display omitted] ► PP–CNF foams display higher conductivity than the solid composites. ► Foaming causes higher CNF dispersion and reduces interfibre distance. ► Tunnel conduction model fits well to experimental values of electrical conductivity. ► Through- and in-plane conductivities are affected by the foam cellular structure. ► Within certain limits a master curve was constructed for the foamed composites. The influence of foaming a semi-crystalline polymer reinforced with different concentrations of carbon nanofibres (0–20 wt.%) on the formation of an electrically conductive network was studied at room temperature using an impedance analyzer over a wide interval of frequencies (from 10 −2 to 10 6 Hz). Composites were prepared by melt-compounding using a twin-screw extruder, and later chemically foamed. Although composite materials displayed lower conductivities than expected, assuming a percolative behavior, foaming promoted a tunnel-like conduction at lower CNF concentrations than in the solids. At higher CNF concentrations, no great improvements were achieved as tunneling conduction decreased with increasing local crystallinity. Foams showed electrical conduction characteristics typical of a conductive random-distributed fibre-like system, while the behavior of the solids was closer to a system of spherical particles, related to CNF aggregation. The anisotropic cellular structure of the 20 wt.% CNF composite foamed by a physical foaming process disrupted the preferential in-plane CNF orientation attained during solid preparation, with these foams showing higher through-plane conductivity and more isotropic electrical properties than the chemically-foamed ones. It has been demonstrated that foaming PP–CNF composites resulted in the formation of a conductive network at lower CNF concentrations than in the solids, with foams showing the potential for use in conductive high-performance lightweight composite systems.