Black-filled systems

He subsequently assumed, that the whole body of the carbon black filled system can be represented by one single RC circuit. He was able, using this model, to calculate accurately the AC properties as a function of the frequency for a carbon black filled PVC system. This model failed, however, in the calculation of the AC properties of the above decribed, conductive epoxy resin systems. 

The polymers are elastomeric polyethers, sold under the Stat-Rite S-Series in alloys with base polymers such as acetal, ABS, PP, PETG, and others. The conductive polymer reportedly has minimal effect on the mechanical properties of the base resin. Prices are higher than carbon-black filled systems, but lower than carbon-fiber materials. 

Silica-filled tread compounds for passenger car tires have become state-of-the-art and have taken a significant share from conventional carbon black-filled systems. In addition to the well-known benefits of fillers such as reinforcement (Fig. 11) and increased cut resistance, silica-based compounds are mainly successful because they provide improved rolling resistance (leading to lower fuel consumption), better wet and snow track resistance, and thermal stability. 

CARBON BLACK-FILLED BLOCK COPOLYMER THERMOPLASTIC ELASTOMER SYSTEMS, 714 Fundamentals for Black-Filled Systems, 714 Experimental, 715 Rheology, 716... 

Unfortunately, for block copolymer thermoplastic elastomers, any trials to accommodate the viscosity changes by carbon blacks into theoretical considerations, or to fit the equations cited onto the rheological behavior of black-filled systems have not been conducted up to now. Of course, such trials will be relatively complex and time-consuming work because thermoplastic elastomer shows intricate properties due to its unique microstructure. 

It is important that the filler be bonded to the polymer for this theory to apply. A test case is where polymer and filler are not similar chemically and thus have poor adhesion. Ethylene-propylene terpolymer and finely divided silica make such a system. Under the same conditions of mixing and cross-linking, a gum (unfilled) compound has a tensile strength of less than 1 MPa. A loading of 100 g of silica for 100 g of polymer raises this to 3.6 MPa. However, when the system is stressed, the polymer pulls away from the filler surface, forming vacuoles. Such vacuoles have been observed in carbon black-filled systems under the electron microscope. An indirect effect that they have is to cause opacity, a whitening at the stressed point. 

Examples of Cure Systems in NR, SBR, and Nitrile Rubber. Table 6 offers examples of recipes for conventional, semi-EV, and EV cure systems ia a simple, carbon black-filled natural mbber compound cured to optimum (t90) cure. The distribution of cross-links obtained is found ia Figure 9 (24). 

Fig. 9. Distribution of sulfur cross-links ia a carbon black-filled NR, where A is the conventional, B the semi-EV, and C the EV system.

Table 3. Properties of Carbon Black-Filled Natural Rubber Vulcanizates With Various Cure Systems ...

The model in Figure 3.20a applies to ACM-silica, while Figure 3.20b suits the ENR-silica system. Kraus constant, C, determined from the slope of the plots in Figure 3.19, quantifies the mbber-silica interaction in these systems. CACM/siUca is 1-85 and CENR/siika is 2.30 and these values are significantly higher than the reinforcing black-filled mbber composites [65]. Hydrogen-bonded interaction between the SiOH and the vicinal diols in ENR is responsible for this, whereas dipolar interaction between ester and SiOH in ACM-silica only results in weaker adsorption of the mbber over the filler surfaces. 

Although many interface models have been given so far, they are too qualitative and we can hardly connect them to the mechanics and mechanism of carbon black reinforcement of rubbers. On the other hand, many kinds of theories have also been proposed to explain the phenomena, but most of them deal only with a part of the phenomena and they could not totally answer the above four questions. The author has proposed a new interface model and theory to understand the mechanics and mechanism of carbon black reinforcement of rubbers based on the finite element method (FEM) stress analysis of the filled system, in journals and a book. In the new model and theory, the importance of carbon gel (bound rubber) in carbon black reinforcement of rubbers is emphasized repeatedly. Actually, it is not too much to say that the existence of bound rubber and its changeable and deformable characters depending on the magnitude of extension are the essence of carbon black reinforcement of rubbers. 

The new interface model and the concept for the carbon black reinforcement proposed by the author fundamentally combine the structure of the carbon gel (bound mbber) with the mechanical behavior of the filled system, based on the stress analysis (FEM). As shown in Figure 18.6, the new model has a double-layer stmcture of bound rubber, consisting of the inner polymer layer of the glassy state (glassy hard or GH layer) and the outer polymer layer (sticky hard or SH layer). Molecular motion is strictly constrained in the GH layer and considerably constrained in the SH layer compared with unfilled rubber vulcanizate. Figure 18.7 is the more detailed representation to show molecular packing in both layers according to their molecular mobility estimated from the pulsed-NMR measurement. 

Nevertheless, it is obviously shown in Figure 18.1 that the stress-strain curve of the filled mbber gives the clear stress upturn, thus its tensile strength becomes 30 MPa. Therefore, the fundamental question is what happens or what stmcture is produced in the carbon black-filled mbber under large extension, which newly generates the stress upturn. In the case of the fine carbon black-fiUed system, when carbon blacks are dispersed ideally, the carbon gel makes the continuous phase at the... 

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. 

With the modern ABS braking system contact temperatures are kept low, particularly on wet surfaces so that the log a v is relatively high and the silica-filled compound is superior to the standard black-filled one. With locked wheel braking the contact temperamres are high and log a-iv values are low, and the ranking of the braking performance of these two compounds reverses. 

On the other hand, Schaefer ( ) has shown from selective saturation experiments of amorphous cis polyisoprene, crystalline trans polyisoprene, as well as carbon black filled cis polyisoprene, that the resonant lines are homogeneous. The linewidths in these cases are thus not caused by inhomogeneous broadening resulting from equivalent nuclei being subject to differing local magnetic fields. The results for these systems are thus contrary in part to what has been found here. 

Solid-state 13C NMR spectra of carbon black filled, uncured and sulfur-vulcanised HR were recorded at 22.6 MHz. The line broadening of the filled polymer relative to the unfilled polymer is attributed to incomplete motional narrowing of the NMR lines [53, 54] Incorporation of filler also results in a decrease in the signal-to-noise ratios in the spectra, but fundamentally it does not obscure the qualitative and quantitative nature of the spectra for the moderately cured elastomer systems. 

If the estimated fitting parameters are compared to the predicted values of percolation theory, one finds that all three exponents are much larger than expected. The value of the conductivity exponent ji=7A is in line with the data obtained in Sect. 3.3.2, confirming the non-universal percolation behavior of the conductivity of carbon black filled rubber composites. However, the values of the critical exponents q=m= 10.1 also seem to be influenced by the same mechanism, i.e., the superimposed kinetic aggregation process considered above (Eq. 16). This is not surprising, since both characteristic time scales of the system depend on the diffusion of the charge carriers characterized by the conductivity. 

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