Impedance analysis and electromagnetic interference shielding effectiveness of conductive carbon black reinforced microcellular EPDM rubber vulcanizates

The effect of processing variables (filler and blowing agent loadings) and operating variables (frequency and sample thickness) on the impedance and electromagnetic interference (EMI) shielding effectiveness of excess conductive carbon black reinforced solid and microcellular EPDM vulcanizates has been investigated. Increasing frequency showed a decrease in real part of complex impedance for both filler and blowing agent loadings, whereas the imaginary part showed a slight peak in the range of 9-10 GHz followed by a subsequent decrease. The measured impedance values were plotted as Nyquist plots (Argand diagrams) and a Resistor-Capacitance model was assumed. Increasing blowing agent loadings showed a marginal increase in the bulk resistivity, whereas increasing filler loadings had a significant decrease. The EMI shielding effectiveness increased monotonically with filler loading and showed a maximum of around 75 dB at 60 phr loadings, thus making them suitable for shielding applications. Increasing blowing agent loadings showed an increase in shielding effectiveness at low loadings (2 phr) whereas further increase showed a rapid decrease. This has been explained on the basis of formation of voids in the composite, which affect the absorbance capacity of the microcellular vulcanizates. However, the microcellular composites showed sufficiently high values in the range of 50-60 Db, thus making Vulcan XC 72 reinforced microcellular EPDM vulcanizates as ideal packaging material for EMI shielding applications.