Effects of processing methods on the electrical conductivity, electromagnetic parameters, and EMI shielding effectiveness of polypropylene/nickel-coated carbon fiber composites

The effects of composite preparation methods on the electrical conductivities, the electromagnetic parameters and the electromagnetic interference (EMI) shielding effectiveness of polypropylene (PP)/nickel-coated carbon fiber (CF) composites were investigated. The composites were prepared by injection molding machine, internal mixer, and screw extruder. The electrical properties results showed the PP/CF (70/30, wt%) composites prepared by injection molding demonstrated the highest electrical conductivity and EMI shielding effectiveness, which were 1.75x10(1) S/cm and 48.4 dB at the frequency of 10 GHz, respectively. These results seem mainly due to the increased CF length when the PP/CF composite was prepared by injection molding, which was advantageous in forming a conductive network of the composite. The results of the electromagnetic parameters of the PP/CF composites showed that the increased electrical conductivity of the composite prepared by injection molding was mainly due to the increased dielectric constants (epsilon' and epsilon") of the PP/ CF composite. This enhancement in dielectric constants seems related to the percolation at a lower concentration of the CF, which was affected by the increased CF length of the composite prepared by injection molding process. The results of dielectric loss and magnetic loss factors of the PP/CF composite showed that the major electromagnetic absorbing mechanism was dielectric loss, namely dipole polarization and interface polarization between filler and matrix, which resulted in improved EMI absorption values. The total EMI shielding effectiveness (SET) of the PP/CF composite comprised 85.1% EMI shielding effectiveness by absorption (SEA), and 14.9% EMI shielding effectiveness by reflection (SER), which suggests that the EMI shielding was predominantly by the absorbing mechanism of the incident electromagnetic wave.