Carbon Black for Electrically Conductive Polymer Applications

19

Michael E. Spahr, Raffaele Gilardi, and Daniele Bonacchi

Contents

Definition........................................................................................ 376

Introduction...................................................................................... 377

Conductive Carbon Black Properties and Manufacture........................................ 378

Material Characteristics Describing Conductive Carbon Black................................ 379

Intrinsic Electrical and Thermal Conductivity of Carbon Black............................... 382

Manufacture of Conductive Carbon Black..................................................... 384

Carbon Black-Polymer Composites............................................................. 386

The Insulator-Conductor Transition......................................................... 386

Carbon Black Properties, Critical Carbon Black Concentration, and Mechanical

Compound Properties........................................................................ 387

Polymer Nature and Properties.............................................................. 389

Compound Processing....................................................................... 392

Conduction Mechanisms in Carbon Black-Polymer Composites.............................. 394

Percolation Models.............................................................................. 396

Cross-References................................................................................ 399

References....................................................................................... 399

Abstract

Carbon black is incorporated into polymers for permanent electrostatic discharge protection, explosion prevention, and polymer applications that require electrical volume resistivities between 1 and 10⁶ ^ cm. Typically, the so-called conductive carbon black is used since grades that belong to this specialty carbon black family impart electrical conductivity to polymers at lower critical volume fractions than conventional carbon black. Hence, conductive carbon black materials influence to a lower degree the mechanical properties of the resulting conducting polymer compound.

M.E. Spahr (*) • R. Gilardi • D. Bonacchi

IMERYS Graphite and Carbon Ltd., Bodio, Ticino, Switzerland

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R. Rothon (ed.), Fillers for Polymer Applications, Polymers and Polymeric Composites: A Reference Series, DOI 10.1007/978-3-319-28117-9_32

Conductive carbon black grades are produced by furnace black processes and by specially designed processes like the ENSACO® process or are obtained as by-products from the gasification of hydrocarbons; these processes are based on the thermal-oxidative decomposition of hydrocarbons. In contrast, acetylene black being another conductive carbon black is formed during the exothermic decomposition of acetylene to carbon black and hydrogen occurring above 800 °C in the absence of oxygen.

Conductive carbon black grades show a large carbon black structure indicated by a high void volume. The void volume can be characterized by the oil absorption number (OAN) being above 170 mL/100 g of carbon for typical conductive carbon black. The oil absorption number at a given compression state (COAN) is attributed to the difference in sensitiveness of the carbon black structure toward compression observed between different carbon black grades. Therefore, the COAN indirectly indicates the resistance of the carbon black structure toward shear stress as well as the ability of carbon black to form a conductive network and maintain it in the polymer compound.

Usually the critical carbon black volume fraction at which the polymer compound becomes electrically conductive is decreasing with increasing COAN. The steplike transition from the insulating to the conducting state, which occurs at the critical carbon black volume fraction when incorporating carbon black into the polymer, can be described by a percolation mechanism. The amount of carbon black required to make a polymer compound conductive is, besides the carbon black type, influenced by the polymer type and polymer properties like crystallinity, viscosity, and surface tension. Due to the occurrence of shear stress during the dispersion of the carbon black in the compounding process as well as during the finishing process to the final polymer article, both compounding and finishing have to be considered as well when determining the amount carbon black required for a conductive polymer compound. Statistical, thermodynamic, and structure-oriented percolation models are the best applicable to describe at a theoretical scientific level the formation of the conductive carbon black network in the polymer matrix and to calculate the percolation from the insulating to the conducting state.

Keywords

Conductive polymer composites • Carbon black • Carbon black structure • Carbon black dispersion • Compound processing • Electrical conductivity • ESD protection • Percolation

Definition

Carbon black is incorporated in polymers for permanent electrostatic charge protection, explosion prevention, and polymer applications that require electrical resistivities below 10⁶ ^ cm. At the critical volume fraction of the carbon black grade, the carbon black-polymer compound percolates from an electrically insulating to a conducting domain. The capability of a carbon black material to impart electrical conductivity to a polymer compound depends on its ability to establish and maintain in the insulating polymer matrix a conductive network in which the electronic charge carriers move mainly by a tunneling mechanism. Conductive carbon black fillers impart electrical conductivity to polymers at lower critical volume fractions than conventional carbon blacks and hence influence to a lower degree the mechanical properties of the resulting conducting polymer compound. The key property of this family of special carbon black grades is a high carbon black structure, i.e., a high void volume. High-structured carbon black materials are preferred fillers to make polymers conductive as they allow high polymer concentrations to be maintained while establishing the conductive network. Besides the carbon black properties, also the polymer properties and the processing of the carbon black-filled polymer compound influence the critical volume fraction and insulator-conductor transition. Statistical, thermodynamic, and structure-oriented percolation models are the best applicable to describe at a theoretical scientific level the formation of the conductive carbon black network in the polymer matrix.

Introduction

Polymers are usually electrically insulating with electrical surface resistivities of above 10¹² Q cm. However, some polymer applications require lower electrical resistivity which can be achieved by dispersing special additives above a threshold concentration into the polymer matrix. Such polymer composites are classified according to their electrical volume or surface resistivity in ranges determining the final application of the polymer compound (Norman 1970; Funt et al. 1993; Markarian 2008). Polymer compounds within a surface resistivity range of 10¹⁰—10¹² Q sq^-1 achieved by the addition of organic antistats provide low static decay rates required in antistatic applications. Antistats are widely used in packaging such as films, thermofused containers, and PET bottles in all of which they reduce the attraction of dust or help to keep surfaces separate. Electrostatic dissipation at faster static decay occurs within a surface resistivity range of 10⁶-10¹⁰ Q sq^-1. The optimum surface resistivity range of polymer compounds used for permanent electrostatic discharge (ESD) protection spans from 10⁶—10⁹ Q sq^-1 and is required, e.g., for packaging of electronics, trays, conveyor belts, casings, containers or hoses for flammable or explosive substances, and antistatic flooring. Conductive polymer materials are classified in the surface resistivity range of 10¹—10⁶ Q sq^-1 which is above the range of 10^-1-10^-5 Q sq^-1 being typical for metals. Conductive polymer composites are used for electromagnetic interference (EMI) shielding applications that require surface resistivities lower than 10 Q sq^-1 and in self-limiting switches and heaters. Electrically conducting polymer compounds with volume resistivities below 10³ Q cm are used in the jacketing, insulator, and conductor shielding of power cables. Used in these shielding layers around the conductor, they prevent partial discharge at the interface between the insulation layers and conductor and eliminate any field stress by homogenizing the electrical field around the conductor. In addition they smooth out any sharp edges at the conductor surface. Specific conductive carbon black-polymer composites are positive temperature coefficient (PTC) materials that, when heated, become resistive at a defined temperature.

Carbon black is electrically conductive; most carbon black grades have an electrical volume resistivity in the range of 10^—1—10² ^ cm. Carbon black therefore imparts conductivity to polymers (Funt et al. 1993). High carbon black concentrations are required with conventional carbon blacks to achieve good electrical properties, and this reduces certain composite mechanical properties and ease of processing while increasing the costs. Generally special carbon black grades, so-called conductive carbon blacks, are used which allow to reach the final resistivity level in filled polymer composites at lower concentration than conventional carbon black grades. Over the past few decades, static dissipative and conductive polymer composites have become part of the polymer world, and conductive carbon black remains the major conductive filler, due to the fact that in most cases conductive carbon black grades offer the best performance as well as the most economical solution. Conductive carbon blacks are specialty blacks that represent only about 1% of the global annual carbon black production estimated at about 11 Mio. tons (calendar year 2012). About 50% of the conductive carbon black volumes are used for power cables, 36% for plastic, and 14% for rubber compounds. Alternative more recent conductive additives used for ESD protection application are inherently dissipative polymer (IDP) materials. Although metal powders are intrinsically more conductive than carbon black, they are less frequently used in conductive polymer composites. Some conductive metal powders or fibers of steel, aluminum, and copper have the tendency to oxidize to form an electrically insulating layer on their surface. Noble metals like gold and silver powders are economically not feasible for most conductive polymer applications. Nevertheless, metal-coated polymers (MCP) or metal powder-filled polymer composites are applied, in particular for EMI shielding.