Carbon-black compounds

The rheological properties of gum and carbon black compounds of an ethylene-propylene terpolymer elastomer have been investigated at very low shear stresses and shear rates, using a sandwich rheometer [50]. Emphasis was given to measurements of creep and strain recovery at low stresses, at carbon black flller contents ranging between 20 and 50% by volume. The EPDM-carbon black compounds did not exhibit a zero shear rate viscosity, which tended towards in-Anity at zero shear stress or at a finite shear stress (Fig. 13). This was explained... 

Fig. 3.65 Left TM-AFM images left height, right phase) showing the filler microdispersion in the unvulcanized compounds, (a) unvulcanized EPDM filled with modified silica (Compound 1) (z-scale height, 310 nm, phase, 30°) (b) unvulcanized EPDM filled with carbon black (Compound 2) (z-scale height, 365 nm, phase, 35°). Right (c), (d) filler distributions as determined from the analysis of the phase images (a) and (b) Reproduced with permission from reference [141]. Copyright 1999. American Chemical Society...

This has led to a large number of carbon-black compounds on the basis of—by now—almost all thermoplastic polymers [30], Rubbers have also been antistatically treated. 

One of the properties of carbon-black-filled conductive compounds that is of paramount importance in the processing of carbon-black compounds is their enormously increased melt viscosity compared with the... 

The rheology of conductive carbon-black compounds (and in general of solvent-free dispersions in polymers)... 

Painstaking investigation of the production and processing of carbon-black compounds, and above all the study of molecular and supramolecular structures,... 

It was the proof of dispersion [4b], the conductivity jump based on it due to flocculation [17a] and the optimisation thereof [17b,d], that first forced us to develop the new non-equilibrium thermodynamics theory of heterogeneous systems [19,17c,37] that applies to carbon-black compounds and ICP blends. The presumed necessity to achieve good dispersions compelled us to polymerise ICPs of increasingly high purity and to develop, to this end, the analytical techniques that yielded contributions to the understanding of structural aspects (see Sections 3,7.1 and 3.6). 

Our first attempts in this direction led to completely new findings in the field of carbon-black compounds and also directly to new developments capable of practical exploitation. In conjunction with ICP research we have developed new ideas about interactions and structure formation in solvent-free polymer dispersions. This materials-oriented approach has also—with processing research as an integrated element—made contributions to a better fundamental understanding of... 

This is where unexploited potential is waiting to be tapped. Rewarding subjects for materials research into carbon-black compounds and ICPs deserve greater attention than they are receiving. 

The most outstanding property of carbon black compounds is of course their electrical conductivity, which today may be as much as 30 S/cm (laboratory produced) [71]. Much more interesting than considering conductivity values in isolation, however, is an examination of conductivity per se. For example, given a steady increase in the concentration of carbon black, the increase in the conductivity of the compound is not linear instead, a moderate increase is followed at a certain carbon black concentration by a sudden jump, which is again followed by a moderate increase . This sequence of events was formerly called percolation. The carbon black concentration necessary for the conductivity leap is known as the critical volume concentration mechanical properties deteriorate drastically as the carbon black concentra-... 

Percolation theory is a statistical/geometrical approach to explaining the shape of the conductivity curve in carbon-black compounds. The theory does not explicitly embody aspects of thermodynamics, though these are implicit in it. 

Looking at the curve of the density of carbon-black compounds against increasing carbon-black concentration, we find that the density initially shows a linear increase with the carbon-black concentration, until at c (determined beforehand) it stagnates or even decreases slightly, after which the expected increase... 

In studies of CO2 adsorption on carbon-black compounds with different concentrations of carbon-black, a sudden increase in adsorption at < >, was observed (studied by Tanioka et al, described in [72]), an observation that cannot be explained with the aid of percolation theory. 

Figure 11.13. SEM-picture of a carbon-black compound with a carbon-black concentration of 5% [17d].

Scanning electron microscopy (SEM) data for carbon-black compounds and conductive polymer blends [72c], supported by recent transmission electron microscopy (TEM) evaluations [79,80] (shown in Figure 11.39) were made, they also contradict the assumption of a statistical distribution. We find complete dispersion below the critical volume concentration (I) and a sudden stiucture formation ( branched flocculate chains ) at the critical volume concentration. This structural feature remains at higher concentrations. 

However, In aoss-linked PE and carbon black compounds, lower-molecular-weight, sulfur-bridged phenolic AOs provide comparable or better performance than high-weight AOs... 

Many proposals have been made in which OM (mainly PAni) blends could be used. Some of them are visionary and creative, like roofs coated with photovoltaic cells, wallpaper with electrical heating capability, heated textiles, dust filters, and many more [62]. Often the expectation that such blends would have properties superior to those of carbon black-fiUed blends, in conductivity or in mechanical or colour aspects, guided the vision. Whereas PAni blends can actually deliver somewhat higher conductivity values (up to 50 S/cm, the best value for laboratory samples, see Ref [22b] and Ref [23a], 5 S/cm for technical scale [65]) compared to those of carbon black compounds (best values around 0.5 S/cm), the other presumed advantages are not there. Nor are mechanical or processing properties, electrochemical stability under applied voltage and current (like for heating devices), or the color aspects of PAni blend any better than with carbon black compounds. 

There is also often a misunderstanding in the scientific community that carbon black compounds are a relatively bad compromise. This is not the case, as many high performing compounds have been developed and have been in commercial use for many years (cf Ref [63]). 

Both the systems, the PAni blends and the carbon black compounds, however, are based on the same structural principle the conductive phase is the dispersed phase in nanoscale, which suddenly self-organizes into complex networks above the critical concentration [58]. This is the reason for aU properties, including mechanical or rheological, and for abrasion. But, most of the poorer products have been replaced by products based on subtle and successful developmental work (cf Ref [63]). As a consequence, the market does not ask for replacements of carbon black compounds, which provide more or less comparable properties and this at a higher price. (Neste Oy offered PP and PE blends with PAni as a replacement for carbon black compounds in 1995, and stopped the program in 1996.)... 

There was little discussion of this perspective during the next 25 years, and only in the 1960s was there renewed attention. In 1962, Zakharenko et al. [Zl] in Moscow reported shear flow measurements of rubber-carbon black compounds. In 1972, Vinogradov et al. [V8], also in Moscow, reported similar results for other rubber-carbon black compounds and indicated the occurence of yield values. At the same time similar behavior was reported for talc-polypropylene compounds by Chapman and Lee [C8] of Shell and for titanium dioxide-polyethylene compounds by Minagawa and White [M29]. 

From about 1980, there have been extensive investigations of the shear viscosity of rubber-carbon black compounds and related filled polymer melts. Yield values in polystyrene-carbon black compounds in shear flow were found by Lobe and vhiite [L15] in 1979 and by Tanaka and White [Tl] in 1980 for polystyrene with calcium carbonate and titanium dioxide as well as carbon black. From 1982, White and coworkers found yield values in compounds containing butadiene-styrene copolymer [Ml, M37, S12, S18, T7, W29], polyiso-prene [M33, M37, S12, S18], polychloroprene [S18], and ethylene-propylene terpolymer [OlO, S18]. Typical shear viscosity-shear stress data for rubber-carbon black compounds are shown in Figs. 5(a) and (b). White et al. [S12, S18, W28] fit these data with both Eq. (56) and die expression... 

Recently, Osanaiye et al. [OlO] have made extensive measurements of creep in rubber-carbon black compounds at very low stresses. It was found that there were stresses below which there was no flow. The yield values determined by these authors were somewhat lower than those reported earlier. 

Studies of transient behavior of rubber-carbon black compounds were first reported by Mullins and Whorlow [M51, M52] in 1950. They found that there were strong time-dependent thixotropic effects in rubber-carbon black... 

Lobe and White [L15] studied stress relaxation following imposed strains in polystyrene-carbon black compounds and found that the stresses did not decay to zero, but to a finite value of stress roughly equal to the yield value of Eqs. (56) and (57). Montes et al. [M37] have found similar effects in rubber-carbon black compounds. This is shown in Fig. 7. Montes et al. found similar effects in stress relaxation following shear flow. 

There have been many studies of the dynamic viscosity ri o)) and complex viscosity of rubber-carbon black compounds and other filled systems. 

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