Carbon Black Properties, Critical Carbon Black Concentration, and Mechanical Compound Properties

Carbon blacks are electrically conductive and impart electrical conductivity to the typically insulating polymer. Generally, conductive carbon black grades are preferred as they allow reaching the percolation at lower concentration than conventional carbon black and therefore influence to a lower degree the mechanical properties of the resulting polymer compound. The capability of a carbon black to impart electrical conductivity to a polymer compound depends on its ability to establish and maintain a conductive network in the insulating polymer matrix. Hereby, the carbon black aggregates assemble continuous paths that allow the transport of electrical charges through the polymer matrix. The key property of conductive carbon black is the high carbon black structure, i.e., the high void volume. Conductive carbon black can accept high polymer levels while maintaining the carbon black network. The higher the OAN and COAN of the carbon black grade, i.e., the higher the carbon black structure and the ability to maintain the carbon black structure under the influence of mechanical stress like compression and shear forces occurring during the compounding process, the lower is the carbon black concentration required to make the polymer compound conductive. At the percolation threshold, all carbon black materials show quite a constant total volume calculated from the product of COAN and volume fraction of the carbon black grade. This underlines the fact that all carbon black materials can achieve a low electrical resistivity level, but the selection is based on other criteria as well, for example, mechanical properties and dispersion. In addition, the carbon black surface should be free of any organic residue or chemical groups that could deteriorate the electrical contacts or act as electron traps. The quality and number of carbon black interparticle contacts as well as the carbon black microstructure mainly influence the ultimate resistivity level. Figure 7 illustrates the electrical volume resistivity as a function of the carbon black loading in polypropylene for several examples of commercial conductive carbon black grades. The percolation threshold decreases with increasing

Fig. 7 al volume resistivity of carbon black compounds at different carbon black loadings.

• (a) ENSACO® 350G (E350G, OAN = 320 mL (100 g)^-1), ENSACO® 250G (E250G, OAN = 190 mL (100 g)^-1), Ketjenblack EC300J (EC300J, OAN = 330 mL (100 g)^-1), and ultraconductive carbon black (UCCB, OAN = 560 mL (100 g)^-1) in high-density polyethylene (compounding by twin-screw extruder, compound processing by injection molding);
• (b) ENSACO® 350G (E350G, OAN = 320 mL (100 g)^-1), ENSACO® 250G (E250G, OAN = 190 mL (100 g)^-1), VULCAN® XC72R (N-472, OAN = 175 mL (100 g)^-1), and P-type carbon black (P-type, OAN = 100 mL (100 g)^-1) in low-density polyethylene (compounding by twin- screw extruder, compound processing by compression molding at 2 min/170 °C)

OAN of the carbon black grades indicating the influence of the carbon black structure on carbon black concentration at which the compound becomes conductive.

A proper selection of a conductive carbon black is made in such a way that other performance criteria, especially the mechanical properties, are met while the compound is electrically conductive. With the addition of carbon black into thermoplastics at low loadings, usually an increase of the tensile and flexural modulus, i.e., the stiffness and rigidity of the resulting polymer compound, can be observed. At high carbon black concentration, the tensile strength decreases again. The tensile elongation at break generally decreases, but much variety can be observed depending on the polymer grade and especially the dispersion quality. Also the impact properties typically decrease when incorporating carbon black in the polymer due to the increased brittleness of the resulting polymer composite. The increase of the melt viscosity and stiffness accompanied with the incorporation of carbon black in a polymer are drivers to minimize the carbon black content in a conductive compound.

However, a low critical volume fraction is not always equivalent to best performance. Low-surface-area conductive carbon black ENSACO® 250G achieves in a high-density polyethylene (HDPE) compound at equal electrical resistivity level a higher fluidity and similar mechanical properties compared to extra-conductive carbon black ENSACO^® 350G, as summarized in Table 2. The reason for this behavior could be found in the lower surface area of ENSACO^® 250G together with the higher achievable dispersion degree. For many carbon black grades, the

Table 2 Melt flow index and mechanical properties of ENSACO® 250G and ENSACO® 350G containing compounds of HDPE with adjusted carbon black loadings resulting in the same level of electrical volume resistivity

melt viscosity of corresponding polymer compounds at the same weight percent carbon loading is highly dependent on the BET SSA. In high-surface-area carbon blacks, the DBP absorption or OAN usually follows the surface area mostly created by the particle porosity and is also related to the melt viscosity (Fox 1982). The critical pigment volume concentration (CPVC) of a carbon black grade is a function of its OAN and apparent density, 5 CB:

It can be seen that both electrical resistivity and melt viscosity show a threshold behavior at the CPVC. Both properties change as a function of the carbon loading at the critical volume fraction at which a continuous carbon network is established throughout the sample. The carbon network, once established, makes the polymer compound more rigid, as well as more conducting. The melt viscosity has an influence on the mobility of the carbon black aggregates. As described below in more detail, a high melt viscosity can significantly retard the coagulation (structuring) of the aggregates to the conductive network. In that case a higher carbon black volume fraction is needed to reach the critical concentration of carbon black in the polymer compound.