Carbon black networking

FIGURE 22.1 Transmission electron microscopy (TEM) micrograph of a carbon black network obtained from an ultrathin cut of a filled rubber sample. 

Carbon Black Networking on Mesoscopic Length Scales... 

Fig. 30a behaves similarly to that of the NBR/N220-samples shown in Fig. 29, i.e., above a critical frequency it increases according to a power law with an exponent n significantly smaller than one. In particular, just below the percolation threshold for 0=0.15 the slope of the regression line in Fig. 30a equals 0.98, while above the percolation threshold for 0=0.2 it yields n= 0.65. This transition of the scaling behavior of the a.c.-conductivity at the percolation threshold results from the formation of a conducting carbon black network with a self-similar structure on mesoscopic length scales. 

Little information has been published on the question of how filler network structure actually affects the energy dissipation process during dynamic strain cycles. The NJ-model focuses on modeling of carbon black network structure and examination of the energy dissipation process in junction points between filler aggregates. This model was further developed to describe the strain amplification phenomenon to provide a filler network interpretation for modulus increase with increasing filler content. 

Extensive studies on different rubber compounds (see, for example, Table 1 in [105]) yield Ec 0.05 to 0.15 eV per filler-filler bond [105,106], i.e., typical values for physical (van der Waals like) bonds. Similar values were obtained within an approach which assumes a hypothetical analogy between the structure of a statistical carbon black network and that of a Gaussian elastomeric (unfilled) polymer network [107]. As in the Kraus approach, the carbon black network scission process is assumed to be thermally activated. 

Payne, A.R. (1964). The elasticity of carbon black networks. Journal of Colloid Science. 19 744-754. 

Gerspacher, M., O Farrel, C.P., and Yang, H.H., Carbon black network responds to dynamic strains methods and results, Elastomerics, 123, 35, 1901. 

FIGURE 36.3. TEM micrograph of a carbon black network obtained from an ultrathin cut of a filled rubber sample. Reproduced from M. Kluppel and G. Heinrich, Kautschuk, Gummi, Kunstsoffe 58, 217-224 (2005) with permission from Huthig. 

Even a modest increase in strain amplitude can greatly reduce the dynamic modulus of a carbon-black-filled rubber [58,80,88-90]. Because the effect on the modulus of unfilled rubbers is very small by comparison, the effect has largely been attributed to the carbon black aggregate-aggregate network [58,90]. The difference between a modulus measured at low strain and that masured at very high strain (or a value extrapolated to infinite strain) has been used as a measure of carbon black network per se [58]. The loss of dynamic shear modulus of filled rubber which occurs with increases in strain amplitude is greater if the rubber is not vulcanized [80]. 

The low-strain storage modulus drops as a result of the brief application and release of a large strain, and then its recovery with time. We feel that these effects are due to a loss or breaking up of a carbon black-carbon black network, followed by a reformation (or partial reformation) thereof. The phenomenon is largely due to the carbon black because it is almost nonexistent in the case of unfilled, uncured natural rubber. 

Meier JG, Kluppel M (2008) Carbon Black Networking in Elastomers Monitored by Dynamic Mechanical and Dielectric Spectroscopy. Macromol Mater Eng 293(1) 12-38... 

Meier J G, Mani J W and KlUppel M (2007) Analysis of carbon black networking in elastomers by dielectric spectroscopy, Phys Rev B 75 054202-1-054202-10. 

Yielding is observed in elongation of compounds. This occurs at large deformations. The decrease of modulus with increase of the strain amplitude occurs at very small deformations (Payne effect) [9]. This may also be regarded as a yielding of the carbon black network. When the steady state flow measurement is carried to very low shear... 

In detail, the fast recovery is clearly related to the surface area and the structure of carbon black, the larger the surface areas by nitrogen absorption (see Chapter 9) and the lower the structure by dibutyl phthalate (DBF) absorption (see Chapter 9) the larger is the swell. The slow recovery was not clearly related to the carbon black properties. What causes the fast and slow recovery is the subject of future study. The fast recovery may primarily be related to the deformation of carbon black network and the slow recovery to the deformation of the rubber matrix. 

For a higher loading where the modulus decreases [18] with the increasing strain amplitude, Medalia [16] took the G values at 10% strain amplitude. At this point the modulus has decreased significantly, presumably as a result of breaking the carbon black network. The calculated G were in good agreement with the observed ones within 10% for the same 12 carbon blacks. 

J.G. Meier, M. Kliippel. Carbon black networking in elastomers monitored by dynamic mechanical and dielectric spectroscopy. Macromol. Mater. Eng., 293, 12-38,2008. 

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