Electrical Resistivity of Carbon-Black-Loaded Rubbers

Uses are growing for rubbers with varying levels of resistivity. High electrical resistivity is very much essential in wire and cable insulation applications. Low levels of resistivity are needed for electrostatic discharge in phonograph records and many medical, industrial, and military products and for semiconductive cable compounds. The level of resistivity depends upon the number of contacts or near contacts between conductive particles in the rubber matrix. Loading level is obviously a major determinant in addition to physicochemical characteristics of the black. In the latter regard, the highly conductive grades are characterized by small particle size, high structure, high surface porosity, and low volatile content. One would, therefore, seek the reverse of those factors for high-resistivity rubbers. One of the goals of materials research now is to create new materials with physicomechanical properties tailored to a particular application and to understand the physical processes which determine the end properties. In this review, an attempt has been made to discuss the electrical properties of carbon-black-loaded rubber composites, a class of materials which covers the range from insulators to conductors. The carbon-black-loaded rubbers are formed by dispersing carbon black into the rubber. The compounding is done by adding the carbon black to the rubber, mixing at temperatures above Tg and subjecting the mixtures to high shears until a uniform blend is obtained. The carbon-black particles may be as small as 14 nm in diameter or as large as 300 nm, and they may be individually dispersed or agglomerated in micron-sized clusters. Morphology of the rubber has a profound effect on its electrical properties. High electrically resistive rubbers are becoming increasingly important. Their wide array of applications include antistatic products, shielding materials, insulating membranes, resistors, etc. In the vicinity of the crystalline transition region the rubber shows a dramatic resistivity increase which can be utilized for self-regulation processes. Compounds suitable for such various applications differ appreciably in the nature of their components and composition depending on the specific performance required.