Carbon Black for Electrically Conductive Polymer Applications

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 Ω 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.

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.