EMI shielding of intrinsically conductive polymers

Thermoplastic blends which contain a uniformly dispersed conductive filler would be expected to offer significant improvements in performance. Efforts to obtain such a conductive plastic composite with carbon black have been largely unsuccessful, due to the fact that such composites lack sufficient conductivity and mechanical properties to meet the performance requirements. In this report we summarize our progress in the development of highly conductive polymer blends for EMI shielding. These blends are composed of a matrix polymer such as PVC or nylon compounded with an intrinsically conducting polymer (ICP). The authors present the details of the electrical performance which they have obtained from test samples of these blends. It is demonstrated that a very high level of shielding performance can be achieved by the use of intrinsically conductive polymers in thermoplastic blends. The most reliable measure of this performance is obtained from far-field shielding measurements. Nevertheless, near-field measurements provide a useful method for comparing the relative shielding effectiveness of different samples. We have been successful in developing expressions for both near and far-field shielding which are predictive of performance based on a knowledge of dc conductivity and sample geometry. It is clear from the data that the relative conductivities of the samples measured is closely correlated with their measured near-field and far-field shielding effectiveness. The higher the conductivity, the greater the shielding, independent of whether it is measured in the near- or the far-field limit.