Volume carbon black

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]...

Vulcani2ed mbber is an insulator (volume resistivity is 10 Q-cm), and the static generated by mbber tires created serious problems in vehicles until the introduction of electrically conductive carbon black as a reinforcing pigment. An excellent correlation was found between the potential generated and the resistivity of the tires (127,128) (see Rubber natural). 

Stmcture is usually measured by a void volume test such as the absorption of dibutyl phthalate (DBPA) (15), or by bulk density measurements of the carbon black under compression. In order to eliminate the effects of pelletizing conditions the DBPA test has been modified to use a sample that has been precompressed at a pressure of 165 MPa (24,000 psi) and then broken up four successive times (24M4) (16). This procedure causes some aggregate breakdown and is claimed to more closely approximate the actual breakdown that occurs duting mbber mixing. 

Perhaps the first practical application of carbonaceous materials in batteries was demonstrated in 1868 by Georges Le-clanche in cells that bear his name [20]. Coarsely ground MnO, was mixed with an equal volume of retort carbon to form the positive electrode. Carbonaceous powdered materials such as acetylene black and graphite are commonly used to enhance the conductivity of electrodes in alkaline batteries. The particle morphology plays a significant role, particularly when carbon blacks are used in batteries as an electrode additive to enhance the electronic conductivity. One of the most common carbon blacks which is used as an additive to enhance the electronic conductivity of electrodes that contain metal oxides is acetylene black. A detailed discussion on the desirable properties of acetylene black in Leclanche cells is provided by Bregazzi [21], A suitable carbon for this application should have characteristics that include (i) low resistivity in the presence of the electrolyte and active electrode material, (ii) absorption and retention of a significant... 

Fig. 12. Flow curves of poly(isobutylene), containing different concentrations of active filler (acetylene carbon black). Concentration (in volume percent) is indicated near the curves. A is the region of flow for stresses exceeding the yield stress B is the region directly adjacent to the yield stress...

In pressing, the threshold concentration of the filler amounts to about 0.5% of volume. The resulting distribution of the filler corresponds, apparently, to the model of mixing of spherical particles of the polymer (with radius Rp) and filler (with radius Rm) for Rp > Rm as the size of carbon black particles is usually about 1000 A [19]. During this mixing, the filler, because of electrostatical interaction, is distributed mainly on the surface of polymer particles which facilitates the forming of conducting chains and entails low values of the percolation threshold. 

For the second method the threshold concentration of the filler in a composite material amounts to about 5 volume %, i.e. below the percolation threshold for statistical mixtures. It is bound up with the fact that carbon black particles are capable (in terms of energy) of being used to form conducting chain structures, because of the availability of functional groups on their surfaces. This relatively sparing method of composite material manufacture like film moulding by solvent evaporation facilitates the forming of chain structures. 

The important yet unexpected result is that in NR-s-SBR (solution) blends, carbon black preferably locates in the interphase, especially when the rubber-filler interaction is similar for both polymers. In this case, the carbon black volume fraction is 0.6 for the interphase, 0.24 for s-SBR phase, and only 0.09 in the NR phase. The higher amount in SBR phase could be due to the presence of aromatic structure both in the black and the rubber. Further, carbon black is less compatible with NR-cE-1,4 BR blend than NR-s-SBR blend because of the crystallization tendency of the former blend. There is a preferential partition of carbon black in favor of cis-1,4 BR, a significant lower partition coefficient compared to NR-s-SBR. Further, it was observed that the partition coefficient decreases with increased filler loading. In the EPDM-BR blend, the partition coefficient is as large as 3 in favor of BR. 

Figure 18.1 is the typical stress-strain curves of the filled rubber (SBR filled with fine carbon black, HAF),

The thickness is 2 nm for the GH layer and 3-8 nm for the SH layer, therefore the total thickness of the GH and SH layers is 5-10 nm. Generally, the thickness of the SH layer is smaller in fine carbon-filled mbber than in the coarse one. In the case of the fine carbon black like HAF, its diameter being about 30 nm, the thickness is 2 nm for the GH layer and 4—5 nm for the SH layer, for example. When the volume fraction of HAF carbon is 20% in SBR, the total thickness of both layers (6-7 nm) corresponds to the volume fraction of 30%-35% to the total mbber. The 2 nm thickness of the GH layer is a little less than 10% of the diameter of a fine carbon black (20-30 nm), but it is only 1% of that of a coarse carbon black (100-200 nm). 

When carbon black (fine carbon black) is dispersed homogeneously and its volume fraction

less than nearly 0.2, the SH layers of adjacent carbon particles are still separated from each other in matrix mbber. In this case, the molecules in the SH layer shde, orient, and extend along the extension direction and finally produce strands of oriented molecules under large extension, as shown in Figure 18.8. In this situation, as the molecules inside the SH layer are extended much more... 

As a next step, we can roughly estimate the modulus of the gel (without carbon black) from Figure 18.13. Although the modulus of carbon gel is 9-10 times larger than that of the fiUed NR vulcanizate at 20°C, we must consider the difference of the content of carbon black in both materials. Although the volume fraction of carbon black is 0.20 (corresponding to 50 phr) in the filled NR vulcanizate, since the gel (without carbon black) is about 45% of the total NR, the volume fraction of carbon black in the carbon gel is 0.36. On the other hand, if we compare the modulus of... 

Now, we show the relation between the ratio of 8 to Tq, 8/ro and the volume fraction of carbon black (p in Table 18.1, when the diameter of the hard particle (including carbon black, the GH layer and a little more contribution from the cross-links at the surface of particle) is tq and the distance between the hard particles is 8. In the carbon black-filled rubber (ip g 0.23-0.25), the fact that the stress of the filled system is 10-15 times larger than that of the unfilled rubber as shown in Figure 18.1 indicates that more than 90% of the stress of the system is supported by the supernetwork and the remainder of the stress results from the matrix rubber. In the present calculation, however, we can ignore the contribution from the matrix mbber. 

Now, the dream has become a reality. Figure 19.5 shows 3D-TEM images of three NR vulcanizates, cured by sulfur/N-cyclohexyl-2-benzothiazolyl-sulfenamide (CBS) curing system, CB-10, CB-40, and CB-80 containing 10, 40, and 80 phr high-abrasion furnace (HAF) carbon black. In the black and white images the contrast was reversed, and the white is identified as the carbon black. Association to aggregates is observed even in CB-10 (volume fraction is 0.0498). 

FIGURE 19.6 Dependency on carbon black loading of perimeter distance between the nearest aggregates obtained from three-dimensional-transmission electron microscopic (3D-TEM) images (dp) and volume resistivity at room temperature (pv). (From Kohjiya, S., Kato, A., Suda, T., Shimanuki, J., and Ikeda, Y., Polymer, 47, 3298, 2006. With permission.)... 

As physical properties of the carbon-black-loaded vulcanizates, their volume resistivity (/>v) were measured and plotted in Figure 19.6. Interestingly, the behavior is very similar with that of dp. 

By relating the endpoint of crushed DBF absorption to the void space within and between equivalent spheres of aggregates, and assuming the spheres to be packed at random, Wang et al. obtained the following equation for the effective volume fraction of carbon black ... 

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