Electrical properties and electromagnetic interference shielding effectiveness of polypropylene/carbon fiber composite foams

Foamed and solid polypropylene/carbon fiber (PP–CF) composites containing various CF contents (0–10 vol.%) were injection-molded. Foamed composites were achieved using dissolved pressurized nitrogen gas. The effects of foaming on the fibers inter-connectivity and orientation, electrical percolation threshold, through-plane electrical conductivity, longitudinal and transversal in-plane conductivities, dielectric permittivity, and electromagnetic interference (EMI) shielding effectiveness (SE) were investigated. Cell growth increased the fibers inter-connectivity by biaxial stretching of the matrix and also changed the fiber orientation. The introduction of foaming reduced the density of the injection-molded samples by 25%, lowered the volume fraction of the percolation threshold from 8.5 to 7 vol.% CF, enhanced the through-plane conductivity up to a maximum of six orders of magnitude, increased the dielectric permittivity and resulted in the increase of the specific EMI SE up to 65%. Moreover, the uniformity of in-plane and through-plane conductivities as well as EMI SE along the injection-molded samples was greatly improved by foaming. The relationships between the microstructure and electrical properties were also established. The results reveal that lightweight conductive products with lower fiber content and enhanced electrical and EMI shielding properties can be fabricated with the aid of injection foam molding for applications in electronics, aerospace and automotive industries.