Nonlinear Surface Electrical Resistivity of Graphite–Polyurethane Composite Films Ganesh K. Kannarpady, 1 Bharath Mohan, 2 Abhijit Bhattacharyya 1 1 Smart Materials and MEMS Laboratory, Department of Applied Science, University of Arkansas at Little Rock, Little Rock, Arkansas 72204-1099 2 Delbarton School, Morristown, New Jersey 07960 Received 24 October 2005; accepted 12 March 2006 DOI 10.1002/app.24514 Published online 14 June 2007 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: In this article, we study the surface resistivity of graphite–polyurethane composites using voltage–current characteristics. The evolution of the percolation network of graphite in polyurethane is qualitatively studied using optical micrographs. As expected, the surface resistivity decreases as a function of graphite concentration. In particular, the surface resistivity of the 69% graphite–polyurethane composite is about four orders of magnitude lower than the surface resis- tivity of the 27% graphite–polyurethane composite. The elec- trical resistivity of the composite is found to be highly nonlin- ear with respect to an increasing voltage at a low graphite weight fraction. On the other hand, the nonlinearity is signifi- cantly milder at higher weight fractions. The reasons behind the nonlinearity are discussed. Very preliminary studies indi- cate that very low weight fractions of single wall carbon nano- tubes (e.g. 2.5%) are sufficient to generate electrical conductiv- ities comparable to much higher loading fractions ( 60% and higher) of the heavier graphite particles. Ó 2007 Wiley Periodi- cals, Inc. J Appl Polym Sci 106: 293–298, 2007 Key words: conductive filler composite; surface resistivity; graphite; polyurethane; CNT INTRODUCTION Historically, conductive polymers and their compo- sites are of great research interest because of their wide range of applications in batteries, sensors, electronics, electromechanical actuators, drug delivery, etc. 1 Two common types of conductive polymers are inherently conducting polymers 1 and polymeric composites con- taining conductive fillers. 2 In the case of inherently conducting polymers, these are made electrically con- ductive by doping with anions or cations. On the other hand, the electrical conductivity in conductive poly- meric composites is attained by the addition of conduc- tive particles to a polymer matrix. The conductive par- ticles form a percolation network within the polymeric matrix, resulting in a conductive composite. While inherently conducting polymers require a complex chemical process for its preparation, the conductive polymer composites can be easily prepared by a simple polymerization process or by mechanical mixing. Poly- urethane is a commonly used electrically insulating polymer that has excellent properties like high load bearing capacity, superior impact resistance, light weight, and high elasticity. 3 These properties make this material an excellent alternative for metals in applications such as sleeve bearings, wear plates, sprockets, rollers, and various other parts. Significant benefits include weight reduction, noise abatement, and wear improvements. 3 Moreover, polyurethane has an outstanding resistance to oxygen, sunlight, and gen- eral weather conditions. 4 These properties make the polyurethane very useful as protective coatings for materials prone to environmental degradation, e.g., metals and alloys. There is a long and rich history on conductive poly- mer composites. The relation between the dynamic percolation process and the surface state of carbon black was studied by Katada et al. 5 They have studied the effect of annealing temperature and filler concen- trations on percolation time in carbon black-filled poly- methyl-methacrylate (PMMA). The electrical properties of polymers filled with different types of conducting particles were studied by Xiao-Su et al. 6 Wang et al. 7 have studied the electrical conducting properties of car- bon black-filled high density polyethylene (HDPE) composites. The observed conductivity was explained with three models, viz., conduction via nonohmic con- tacting chains, conduction via ohmic contacting chains, and a combination of these mechanisms based on the different loading fractions of carbon black. Also, electri- cal conductivity of carbon black-HDPE was studied by Correspondence to: A. Bhattacharyya (axbhattachar@ualr. edu). Contract grant sponsor: Defense Advanced Research Projects Agency [DARPA; administered and monitored by Army Research Office); contract grant number: #DAAD19- 02-1-0270. Journal of Applied Polymer Science, Vol. 106, 293–298 (2007) V V C 2007 Wiley Periodicals, Inc.