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Conductive Carbon Black Properties and Manufacture
A general overview about the primary and secondary properties of carbon black, typical related analysis methods, and the principles of the carbon black manufacturing processes are given in the cross-reference Carbon Black as Polymer Filler or in other reviews (Medalia 1982; Hess and Herd 1993; Kuhner and Voll 1993; Probst 1993; Taylor 1997; Wang et al. 2004). The section below focuses on the main material characteristics of conductive carbon black, reviews the intrinsic conductivity and conductivity mechanism of carbon black, and describes the specific manufacturing processes of conductive carbon black grades used at an industrial level.
Material Characteristics Describing Conductive Carbon Black
The microstructure of conductive carbon black can range from paracrystalline primary particles consisting of concentrically arranged continuous layers of hexagonally arranged carbon atoms to primary particles containing small graphitic or turbostratic domains. In Fig. 1 HRTEM images compare the microstructure of an acetylene black indicating small graphitic domains with a planar arrangement of the carbon layers with the microstructure of ENSACO® 250G showing distorted carbon layers that are concentrically arranged within the particle around the growth center or nucleus.
Characteristic feature of the conductive carbon black family is the large carbon black structure formed by more or less spherically shaped primary carbon particles with an average diameter of typically several tens of nanometers. The coalesced primary particle units are coated by continuous carbon layers that in this manner bind them by strong covalent bonds to rigid aggregates as it is indicated for ENSACO® 250G by the transmission electron microscope (TEM) image in Fig. 1a. The major fraction of aggregates in conductive carbon black contains a multitude of primary particles arranged in three-dimensionally branched particle chains, fibrous, or grapelike shapes that can have sizes of up to several hundred of nanometers. The aggregates form agglomerates which are held together physically. The carbon black structure of a typical conductive carbon black, ENSACO® 250G, is illustrated by transmission electron microscope (TEM) and scanning electron microscope (SEM) images in Fig. 2.
The complexity of the arrangements of the carbon black particles, aggregates, and agglomerates results in a relatively high void volume and the possibility to establish
Fig. 1 HRTEM images of typical primary particles for ENSACO® 250G (a) and acetylene black (b)
Fig. 2 Mechanical work required to compact carbon black grades with different levels of carbon black structure in equal pressure increments from 1 to 5 kN cm-2 at the corresponding compression density of the sample. The numbers given in brackets in the graph legend indicate the OAN of the corresponding carbon black grade in mL (100 g carbon)-1
and maintain a carbon network in polymer compounds. The void volume is a measure to compare the structure of carbon blacks. The void volume depends on the size and shape of the aggregates, the aggregate agglomeration, and the porosity of the primary particles. Therefore, the carbon black structure can be considered as the sum of a number of accessible voids by unit weight given by (i) the interaggregate space, (ii) the interstices within the aggregates, and (iii) the porosity of elementary particles. The higher structure level of the carbon black is, the higher is the volume of the voids. Dibutyl phthalate absorption (DBPA) today replaced by oil absorption number (OAN) (ASTM D2414) is employed to measure the void volume and thus the average structure level of carbon black. The OAN is higher the more complex the carbon black structure is. Carbon black grades with an OAN of above 170 mL/100 g are specified as conductive carbon black.
The carbon black structure is very sensitive to the state of compression of the carbon black. Important stages during carbon black production and processing that can cover a broad range of compression states are the (1) carbon black powder after the gas separation step in the manufacturing process, (2) compacted carbon black flakes or pelletized carbon black, and (3) the polymer compound. The compressed oil absorption number (COAN) measured by ASTM D3493 at a given compression state is attributed to the difference in sensitiveness of the carbon black structure toward compression observed for different carbon black grades. Therefore, the COAN can indirectly indicate the resistance of the carbon black structure toward shear stress during the compounding process as well as its ability to form a conductive network and maintain it in the polymer compound. A similar concept takes into account the mechanical resistance of carbon black to compression by measuring the decrease of void volume with increasing compaction pressure at a given weight (ASTM D6086). Probst et al. suggested that the mechanical work being necessary to compact carbon black is ruled by electrostatic surface charges contained in the carbon black aggregates (Grivei and Probst 2003). These electrostatic surface charges are responsible for the aggregate agglomeration which
Fig.3 TEM images indicating the different level of porosity in the primary particles of extra-conductive ENSACO™ 350G (Image a, BET SSA of 770 m2 g-1) and ultra-conductive carbon black (Image b, BET SSA of 1400 m2 g-1)
consequently is reversible and depends on the interaction with the physical environment. Figure 3 shows the mechanical compaction energy calculated from the volume changes achieved by incremental pressure increases at the corresponding compression density of the carbon black samples. Higher structured carbon blacks require significantly higher mechanical work to achieve a given volume density. With a model based on the electrical interpretation of the mechanical properties, it was possible to calculate a superficial charge carrier concentration in the range of 1016 cm-3 increasing with the OAN of the carbon black materials. Intrinsic electrical resistivity values calculated with this model were found to be consistent with values obtained from measurements of the pure carbon black at high pressure (see also following section). These results supported the possible relationship between the resistance of the structure to mechanical stress and the electrical behavior of carbon black governed by electrostatic surface charges ruling the carbon black agglomeration.
The surface area is, besides the OAN, the other secondary carbon black property used to describe conductive carbon black. The specific surface areas calculated from nitrogen isotherms (ASTM D6556) according to the Brunauer, Emmett, and Teller model (BET SSA) of conductive carbon black grades can largely vary from low- surface-area ENSACO® carbon black grades or acetylene black (45-75 m2 g-1) to supra-conductive carbon black grades (ca. 200 m2 g-1), extra-conductive carbon black (ca. 800 m2 g-1), and ultra-conductive carbon black (above 1200 m2 g-1). The porosity can be neglected in the low-surface-area carbon black grades for which the BET SSA increases with the primary particle size. For carbon black above ca. 150 m2 g-1, pores in the primary particles contribute to the BET SSA. A further increasing BET SSA is created mainly by an increasing porosity of the primary particles. The TEM images of the high-surface-area ENSACO® 350G and ultraconductive carbon black in Fig. 4 indicate the existence of a significant amount of pores in the primary particles. For the latter carbon black material, the large pores hollowed the primary particles that almost completely lost the typical spherical character.
Fig. 4 TEM (a) and SEM (b) images indicating the high carbon black structure of ENSACO® 250G consisting of agglomerates of carbon black aggregates with branched shape. An example of one aggregate is indicated by the circle
Table 1 lists a selection of commercial conductive carbon black grades categorized according to their OAN in conductive carbon black, extra-conductive carbon black, and ultra-conductive carbon black and gives their main properties.
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