| Carbon black-filled polymer composites have been studied for years due to their ability to conduct electricity when loaded above the percolation threshold. Often times the amount of carbon black required to achieve sufficient conductivity creates difficulties in processing and brittle final composites. In an effort to improve the mechanical behavior of these filled composites without sacrificing electrical conductivity, latex is being used as the matrix phase rather than the standard polymer solution or melt. It has been found that the percolation threshold can be reduced, by more than an order of magnitude (from approximately 15 vol% in a solution to less than 1.5 vol% in a monodisperse latex), when carbon black is added to latex rather than a comparable solution-based polymer. This dramatic drop in the critical carbon black concentration is due to the excluded volume effect produced by the latex and lack of polymer stabilization during film formation. In a polymer solution, carbon black is free to exist anywhere and is often stabilized in the liquid by a thin coating of polymer. In latex, however, the polymer particles exclude a portion of the possible volume that could otherwise be occupied by the carbon black and the solid polymer particles in latex cannot coat the individual filler particle to provide stabilization. Carbon black within the latex-based composite exists as a segregated network, which allows for the lowered percolation threshold. Electrical conductivity of these composites is measured with a four-point probe apparatus and mechanical properties are evaluated using a dynamic mechanical analyzer. These new carbon black-filled latex composites are a significant achievement and are able to provide high electrical conductivity, while retaining polymeric mechanical properties and ease of processing. |
