Dispersion state of carbon black(CB) was studied in polymer blends which are incompatible with each other. It was found that CB distributes unevenly in each component of the polymer blend. There are two types of distribution. (1) One is almost predominantly distributed in one phase of the blend matrix, and in this phase fillers are relatively homogeneously distributed in the same manner as a single polymer composite. (2) In the second, the filler distribution concentrates at interface of two polymers. As long as the viscosities of two polymers are comparable, interfacial energy is the main factor determining uneven distribution of fillers in polymer blend matrices. This heterogeneous dispersion of conductive fillers has much effect on the electrical conductivity of CB filled polymer blends. The electrical conductivity of CB filled polymer blends is determined by two factors. One is concentration of CB in the filler rich phase and the other is phase continuity of this phase. These double percolations affect conductivity of conductive particle filled polymer blends.

Dispersion of fillers and the electrical conductivity of polymer blends filled with carbon black

The objective of this article was to review the use of carbon black (CB) as a conductive filler in polymers and polymer blends. Important properties of CB related to its use in conducting polymers are discussed. The effects of polymer structure, molecular weight, surface tension, and processing conditions on electrical resistivity and physical properties of composites are discussed. Several percolation models applicable to CB/polymer blends are reviewed. Emphasis is placed on recent trends using polymer blends as the matrix to obtain conducting composites at a lower CB loading. A criterion for the distribution of CB in polymer blends is discussed. © 2002 Wiley Periodicals, Inc. Adv Polym Techn 21: 299–313, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.10025

Carbon black filled conducting polymers and polymer blends

The electrical conductivity of mixtures of conductive and insulating materials is reviewed In general, the conductivity of such mixtures increases drastically at a certain concentration of the conductive component, the so-called percolation concentration Among the parameters influencing the percolation concentration, the filler distribution, filler shape, filler/matrix interactions and the processing technique are the most important ones On the basis of these parameters, different models have been proposed aimed at the prediction of the conductivity or the percolation concentration It will be shown here that statistical, geometric or thermodynamic models explain the conductivity behaviour of specific mixtures on the basis of insufficient assumptions However, the conductivity seems to be predictable with the help of structure-oriented models

Models proposed to explain the electrical conductivity of mixtures made of conductive and insulating materials

The electrical conductivity of carbon particle-filled polymers was measured as a function of carbon content to find a break point of the relationships between the carbon content and the conductivity. The conductivity jumps by as much as ten orders of magnitude at the break point. The critical carbon content corresponding to the break point varies depending on the polymer species and tends to increase with the increase in the surface tension of polymer. In order to explain the dependency of the critical carbon content on the polymer species, a simple equation was derived under some assumptions, the most important of which was that when the interfacial excess energy introduced by carbon particles into the polymer matrix reaches a “universal value”, Δg
*, the carbon particles begin to coagulate so as to avoid any further increase of the energy and to form networks which facilitate electrical conduction. The equation well explains the dependency through surface tension, as long as the difference of the surface tensions between the carbon particles and the polymer is not very small.

Electrical conductivity of carbon-polymer composites as a function of carbon content

The modeling of the electrical conductivity of polymer composites reinforced with conductive fibers is investigated. Existing models generally can be divided into percolation theories and non-percolation theories. The basis of the percolation theory is the fact that the conductivity of the composite increases dramatically at a certain fiber concentration called the percolation threshold. This theory can be used to model the behavior of the composite or to predict the percolation threshold itself. Non-percolation theories include terms, which account for microstructural data such as fiber orientation, length, and packing arrangement. A comparison of experimental data with predictions from the various models reveals that only the percolation theory is able to accurately model the conductive behavior of an actual composite. Two alternative new models, which predict the volume resistivity of a composite using microstructural data, are evaluated. The first model relates resistivity to the concentration and orientation of the fibers, while assuming perfect fiber-fiber contact. The relationship between resistivity and fiber concentration predicted by the model is in qualitative agreement with actual data, and predictions of the anisotropy in volume resistivity compare well with experimental results. The second model accounts for the effect of fiber-fiber contact and fiber length on composite resistivity. Predictions are in excellent agreement with experimental data for polypropylene composites reinforced with nickel-coated graphite fibers.

Estimation of the volume resistivity of electrically conductive composites

When conducting particles are added to a nonconducting matrix, conductivity increases abruptly at a critical concentration. The theory of infinite chains is used to predict the variation of conductivity with particle concentration. Factors which determine the critical concentration are discussed.

F. Bueche

Papers

Electrical resistivity of conducting particles in an insulating matrix