Carbon-black filled polymers

The Emerman model described in the previous section is hardly applicable to the carbon black-filled CCM as the black particles have sizes of hundreds angstrom and such a composite, compared with the molding channel size, may be considered as a homogeneous viscous fluid. Therefore, the polymer structure, crystallinity and orientation play an important role for such small particles. The above-given example of manufacture of the CCM demonstrates the importance of these factors being considered during processing of a composite material to and article with the desired electrical properties. 

FIGURE 18.1 Stress-strain relation of HAF carbon black-filled styrene-butadiene mbber (SBR). (From Chikaraishi, T., Koubunshi Gakkkai (Japan), No 25, Polymer Free Discussion, 1987.)... 

FIGURE 18.18 Temperature dependence of the stress-strain relation in carbon black-filled natural mbber (NR). (From Fukahori, Y. and Seki, W Polymer, 33, 1058, 1992.)... 

J.L. Leblanc and B. Stragliati, An extraction kinetics method to study the morphology of carbon black filled mbber compounds, J. Appl. Polym. Sci., 63, 959-970, 1997. 

Physical properties of carbon black-filled EPR and EPDM elastomers have been found to be comparable with the suUur-cured analogues [372]. Aromatic oils increase the optimum dose requirement for these compounds due to the reaction of the transient intermediates formed during radiolysis of the polymer with the oil as well as energy transfer which is particularly effective when the oil contains aromatic groups. The performance and oxidative stability of unfilled EPDM as well as its blend with PE [373], and the thermal stabdity and radiation-initiated oxidation of EPR compounds are reported by a number of workers [374,375]. 

Chemicals like polyorthoaminophenol, diphenylamine in small amounts have been found to decrease the yield of cross-linking [388]. The tensile strength of the carbon black-filled polychloroprene compounds has been found to be comparable to the conventional thermally cured one. The physical properties [389] have been observed to improve on adding cross-linking promoters like A,A -hexamethylene-bis-methacrylamide into the polymer matrix. 

Flandin L, Hiltner A, Baer E. Interrelationships between electrical and mechanical properties of a carbon black-filled ethylene-octene elastomer. Polymer. 2001 Jan 42(2) 827-38. 

In contrast to the spin-lattice relaxation parameters, which remain invariant, a sijbstantial broadening of the resonant lines occurs upon crystallization. The effect is relatively modest for cis polyisoprene at 0°C and 57.9 MHz, where comparison can be made at the same temperature. Here there is about a 50% increase in the linewidths upon the development of 30% crystallinity. Schaefer (13) reports approximately 3- to 5-fold broader lines (but they are still relatively narrow) for the crystalline trans polyisoprene relative to the completely amorphous cis polyisoprene at 40°C and 22.5 MHz. It is interesting to note in this connection that for carbon black filled cis polyisoprene the line-widths are greater by factors of 5-10 relative to the unfilled polymer. 

Table VI. Effect of Crosslinking on Brittleness of Carbon Black-Filled Polymer"...

Electrodes in a capacitively coupled conductivity detector were made by injection molding carbon-filled polymer into a preformed PS chip. The polymer consisted of three conducting formulations 8% carbon black filled PS, 40% C fiber filled nylon-6,6, and 40% C fiber filled high-impact PS [774]. In another report, a movable contactless conductivity detector was also developed to allow the distance of the electrode to be adjustable [775],... 

Elastomeric components and compositions in BR/SBR and NR/BR/SBR blends have been studied by 13C solid-state NMR. The MAS spectra are of sufficient quality for polymer identification of the carbon black filled vulcanisates in most cases [51]. 

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. 

The presence of free radicals deriving from carbon black could also complicate the interpretation of NMR data in the case of filled rubbers, because radicals may cause a substantial decrease in T2. Two types of radicals have been detected in carbon-black-filled rubbers localised spins attributable to the carbon black and mobile spins deriving from rubbery chains [86]. Mobile spins are formed because of the mechanical breakdown of polymer chains when a rubber is mixed with carbon black. The concentration of mobile spins increases linearly with carbon black loading [79, 87]. 

Elastomer-filler interactions were the subject of many intensive investigations. Kaufmann and co-workers [17] investigated carbon-black-filled EPDM by nuclear spin relaxation time measurements and found three distinct regions in the material. These regions are characterised by different mobility of the elastomer chains a mobile region in which the polymer chains have no interaction with the filler particles, loosely bound rubber in an outer shell around the carbon black particles and an inner shell of tightly bound elastomer chain with limited mobility. 

This exchange model is supported by experiments on samples with more carbon black (Figure 12.19). In samples with more carbon black the Xe chemical shift shifts in the direction of pure carbon black, just as expected for simple exchange effects. More work is needed to clarify, for example, the decrease of the line width with decreasing temperature. The results show, however, that 129Xe NMR has the potential to characterise the interface between the polymer and filler particles in carbon black filled materials, for which it is known that they are difficult to characterise by other spectroscopic techniques. 

Carbon C (electrographite, Acheson graphite) Carbon black-filled polymers, graphite-filled plastics, graphite-felt, glassy carbon (anode only), porous carbon, poly-p-phenylene (synthetic metal)... 

Among conventional electrode materials are Pb and Hg as cathodes (they have a high H overpotential and hence are likely to prefer to give electrons to compounds that will accept them at less cathodic potentials than that needed for H2 solutions). Pb02, Fe203, and SiC are examples of useful anode materials. Since about 1980, several newer electrode materials have found a place. For cathodes, Ni has become important. Cr203, TiC, and carbon-black-filled polymers have been used as anodes. 

The Payne effect of carbon black reinforced rubbers has also been investigated intensively by a number of different researchers [36-39]. In most cases, standard diene rubbers widely used in the tire industry, bke SBR, NR, and BR, have been appbed, but also carbon black filled bromobutyl rubbers [40-42] or functional rubbers containing tin end-modified polymers [43] were used. The Payne effect was described in the framework of various experimental procedures, including pre-conditioning-, recovery- and dynamic stress-softening studies [44]. The typically almost reversible, non-linear response found for carbon black composites has also been observed for silica filled rubbers [44-46]. 

An explanation of the observed relaxation transition of the permittivity in carbon black filled composites above the percolation threshold is again provided by percolation theory. Two different polarization mechanisms can be considered (i) polarization of the filler clusters that are assumed to be located in a non polar medium, and (ii) polarization of the polymer matrix between conducting filler clusters. Both concepts predict a critical behavior of the characteristic frequency R similar to Eq. (18). In case (i) it holds that R= , since both transitions are related to the diffusion behavior of the charge carriers on fractal clusters and are controlled by the correlation length of the clusters. Hence, R corresponds to the anomalous diffusion transition, i.e., the cross-over frequency of the conductivity as observed in Fig. 30a. In case (ii), also referred to as random resistor-capacitor model, the polarization transition is affected by the polarization behavior of the polymer matrix and it holds that [128, 136,137]... 

A much more general mechanism for this kind of application is schematically represented in Fig. 3(b). A third phase (3) is inserted as an active material between the contact plane (1), made of metal or any inert metal-free material such as glassy carbon or carbon-black-filled polymers with a good electronic conductivity, and the... 

Figure 5.21. Excess storage modulus of carbon black filled polybutadiene vs. annealing time. [Adapted, by permission, from Boehm G G A, Nguyen M N, J Appl. Polym. Sci, 55, No.7, 1995, 1041-50.]...

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