Electrical conduction of graphite filled high density polyethylene composites; experiment and theory

IntroductionDisorderedconductor–insulatorcompositesexhibitavarietyof phenomena, some of which have important commercialapplications. Among large number of composite materials,conductive polymer composites have been extensively usedin resistors, self regulating heaters, over current and overtemperature circuit protection devices, antistatic materials,and materials for electromagnetic interference shielding. Inorder to analyze the conducting behavior in such systems onmicroscopic scale, several models have been proposed, suchaspercolationtheory[1],effectivemediatheoryasprojectedby Bruggeman [2], aggregate structure model which over-come the disadvantage of effective media theory [3], andMcLachlan’s general effective medial (GEM) theory whichcombines most of the features of both percolation andeffective mediatheory[4].GEM equationis aninterpolationbetween Bruggeman’s symmetric and asymmetric theories.The format of equation is in the same mathematical form asof the percolation equation of conductivity when the ratio ofconductivities of the two components is infinitely large.Whileintroducingthisequationithasbeen supposedthatthebinary system is microscopically homogeneous, the particlesize distributions are infinitely wide and the particles are incontact with each other with no voids. This equation isapplicable to all volume fractions and not only near theinsulator–conductor transition as in percolation theory.In the past few decades, some works on effective mediatheory on polymer composites filled with carbon blackparticles have been reported [3, 5], but the use of graphite asfiller is almost unavailable in literature. The experimentalstudies in which the critical behavior of polymer compositeshave been investigated using effective media theory are sofar concerned with system of sphere-like conducting parti-cles embedded in an insulating matrix. A reasonableagreement between the experimental results and theoreti-callypredictedcriticalbehaviorofconductivitywasfound[5].This work presents the study of decisive behavior ofpercolation process using general effective media theory ina composite in which the conducting particles localized inan insulating matrix are better described as flakes, thanspheres. Previous experimental studies reported by thisgroup for graphite/polymer composites are concerned withthe qualitative features of conductivity [6] and the theo-retical studies are confined only to the percolation theory[7]. No attempt has been made to explain the conductivitybehavior using GEM theory. In the present work, con-ducting behavior of a series of graphite/HDPE compositeshas been studied experimentally. A comparative analysis ofapplicability of GEM equation for expressing of conduc-tivity behavior of HDPE composites filled with differentfillers has been made. The effect of processing methodsbesides shape and size of filler particles has been discussed.ExperimentalCommercial grade of high density polyethylene used in thiswork was procured from Reliance industry, India. Flakeshaped graphite powder [7] supplied by Graphite India wasselected as filler. The particle size was ranging from 10 to20 lm with a resistivity of 7.5 9 10