Carbon black aggregation

A minimum of 10 to 35 parts carbon black to 100 parts of mbber is required to obtain a resistivity in the order of 10 Q-cm. At that loading the carbon black particles, which have an average radius of 10 nm, form grapelike aggregates that provide continuous paths for the electrical current. Special purpose mbbers containing even more carbon black have resistances as low as 1 Q-cm (129). The electrical resistivity of mbber with carbon black is sensitive to strain history, probably because of temporary dismptions of the continuity of the carbon black aggregates. 

Small-angle X-ray scattering (SAXS) data have made it possible to deduce the localisation of organic additives (pigments) in the bulk of isotactic polypropylene (iPP) [344]. This work has confirmed that the additives are located in the amorphous phase, in spite of their crucial influence on the formation of the crystalline phase of iPP. SAXS has also been used to study the 3D structure of different carbon-black aggregates, and silica-filled SBR rubber compounds [345]. 

Fig.36. Variation in electrical conductivity (o) with molecular weight for polyethylene composites filled with 4% by volume carbon black, demonstrating the effects of orientation (I), degradation (II) and flow-induced segregation of carbon black aggregates (III). ( ) injection moulded (O) compression moulded (unoriented) [181]...

The latter can be of two types, either purely mechanical, and be associated with the occlusion of rubber into carbon black aggregates (occluded rubber), are more complex and involve physical and chemical interactions, they will then be related to bound rubber. 

It was shown, on the one hand, that gum-filler interactions are associated with the immobilization of a certain amount of rubber on the surface or inside the carbon black aggregates, and, on the other hand, that the corresponding bound or occluded rubbers play important roles in the reinforcement process due either to a restriction of elastomer chain mobility in the vicinity of the filler or to an increase of the effective volume of the latter. What are now the effects exerted by a filler on the stress-strain behavior and the modulus of cured rubbers ... 

Figure 48. Phase-contrast electron micrograph of a carbon black aggregate...

Figure 10.10 Schematic representation of the physical network structure in a carbon-black-filled elastomer [62]. The symbol - indicates elastomer - carbon black adsorption junctions. The length scales in this figure and the EPDM/carbon black volume ratio are fictional. For simplicity, none of the contacting carbon black aggregates, which form agglomerates, have been included...

The room temperature 129Xe spectra of the three carbon blacks are shown in Figure 12.17 and, surprisingly, the line width of these Xe resonances is much smaller than that of Xe in the composites. Although it is known that magnetic impurities and susceptibility effects excessively broaden the 13C line width of carbon blacks, these effects clearly are not as serious for Xe adsorbed at the outer and inner surfaces of the carbon black aggregates [20]. 

Fractal Analysis of Primary Carbon Black Aggregates... 

The exponent df is denoted mass fractal dimension or simply fractal dimension. It characterizes the mass distribution in three dimensional space and can vary between lfractal analysis of furnace blacks was performed, e.g., by Herd et al. [108] or Gerspacher et al. [109, 110]. The solid volume Vp of primary aggregates is normally determined (ASTM 3849) from the cross-section area A and the perimeter P of the single carbon black aggregates by referring to a simple Euclidean relation [108] ... 

Fig. 18 TEM-micrograph of carbon black aggregates (N339) prepared from ready mixed S-SBR-composites with 60 phr filler (in-rubber state)...

Fig. 19 a Fractal analysis according to Eq. (11) of primary carbon black aggregates (N339) prepared from S-SBR-composites with 60 phr filler (in-rubber state). Vp is evaluated from Eq. (12). b Fractal analysis according to Eq. (11) of the same set of primary carbon black aggregates (N339) as shown in Fig. 19a. Np is evaluated by using Eq. (13)...

This behavior can be understood if a superimposed kinetic aggregation process of primary carbon black aggregates in the rubber matrix is considered that alters the local structure of the percolation network. A corresponding model for the percolation behavior of carbon black filled rubbers that includes kinetic aggregation effects is developed in [22], where the filler concentrations and c are replaced by effective concentrations. In a simplified approach, not considering dispersion effects, the effective filler concentration is given by ... 

In the case of carbon black as a filler, the ultimate tensile strength of a plastic compound results from interfacial effects between the carbon black and the resin. These interfacial effects are enhanced by the presence of a higher concentration of carbon black aggregates (higher loading) and by decreased aggregate size. 

A similar approach has also been developed by Susteric [108], who compares the behavior during low-amplitude deformation of rubbers, loaded with aggregated carbon black, with the visco-elastic behavior of macromolecules undergoing high-frequency deformation. The specific features of the breaking of carbon black aggregates defined by the deformation amplitude of loaded rubbers are described by the above author by a mathematical model developed for the description of the dynamic, visco-elastic behavior of polymer molecules. This approach revealed... 

---