Carbon black filled polymers/rubber

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]. 

P. Annadurai, A.K. Mallick, D.K. Tripathy, Studies on microwave shielding materials based on ferrite- and carbon black-filled EPDM rubber in the X-band fiequency. J. Appl. Polym. Sci. 83, 145-150 (2002)... 

Ding, T. Wang, L. Wang, P., Changes in Electrical Resistance of Carbon-Black-Filled Silicone Rubber Composite During Compressions. J. Polym. Sci., Part B Polym. Phys. 2007,45,2700-2706. 

Owing to the different mechanical properties, filler particles can be in most cases differentiated from the polymeric matrix in SFM. Examples include carbon black or silica-filled rubbers, carbon-black-filled polymer blends or salt-loaded block copol5nner micelles. For the latter case, Spatz and co-workers demonstrated... 

Laraba-Abbes F, lenny P, Piques R (2003) A new Tailor-made methodology for the mechanical behaviour analysis of rubber-like materials n. Application to the hyperelastic behaviour characterization of a carbon-black filled natural rubber vulcanizate. Polymer 44 821-840... 

Heinrich G and Vilgis T A (1993) Contribution of Entanglements to the Mechanical Properties of Carbon Black Filled Polymer Networks, Macromolectdes 26 1109-1119. Witten T A, Rubinstein M and Colby R H (1993) Reinforcement of Rubber by Fractal Aggregates, J Phys II Prance 3 367-383. 

A.K. Bagchi, K.K. Sirkar. Extrusion die swell of carbon black filled natural rubber. /. Appl. Polym. Sci., 23,1653-1670,1979. 

At the other extreme, in the formation of composite materials, especially filled polymers, fine particles must be dispersed into a highly viscous Newtonian or non-Newtonian liquid. The incorporation of carbon black powder into rubber is one such operation. Because of the large surface areas involved, surface phenomena play an important role in such applications. 

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. 

When filler concentration is low, g 1. Each filler is bound only once. Carbon black filled rubber does not form gel if only small amounts of carbon black are used. The molecular weight of polymer in the matrix affects the fraction of bound polymer according to the equation ... 

The molecules able to be efficient H-atom donors to alkyl radicals are not restricted to hindered phenols. It has long been noted in the rubber industry that secondary aromatic amines are effective radical scavengers, and study of these materials has shown that they produce a range of coupling products that function in a way similar to the phenols. The problem of the intense colour of these by-products is limited when they are used in carbon-black-filled rubber but a major limitation in other polymers (Zweifel, 1998). The exceptions are the polyamides, in which aromatic amines have a greater stabilization efficiency than do hindered phenols. This may be related to the general observation in polyamides (e.g. nylon-6) that the amine-terminated polymer is more stable than the carboxylic acid-or methyl- terminated polymer. 

Of particular importance for detection of chemical or physical change in polymer materials are mobility filters, which are sensitive to differences in the numbers of molecules within a given window of correlation times. Within reasonable approximation such filters are relaxation filters. Here, Tj filters are sensitive to differences in the fast motion regime while T2 and Tip filters are sensitive to the slow motion regime. Which time window is of importance can be seen from Fig. 5.7 [101]. It shows a double-logarithmic plot of the mechanical relaxation strengths Hi(t) for two carbon-black filled styrene-butadiene rubber (SBR) samples as a function of the mechanical relaxation time T. They have been measured by dynamic mechanical relaxation spectroscopy. In terms of NMR, the curves correspond to spectral densities of motion. But the spectral densities relevant to NMR are mainly those referring... 

In the present paper, which is intended as a review of more recent progress only, emphasis is placed on the physical approach, but not to complete exclusion of the nature of the polymer-filler bond. Because of the overwhelming importance of carbon black as a reinforcing filler, and because most of the pertinent literature on reinforcement concerns carbon black filled rubbers, much of the discussion will be directed to carbon black reinforcement. However, the principles involved are general and apply qualitatively also to other fillers. 

Proton spin resonance measurements on carbon black filled rubbers confirm the relatively small effect of the black on local segmental mobility. Waldrop and Kraus (107) were unable to find evidence for two spin-lattice relaxation times (one for surface rubber and one for bulk rubber) and found very little effect of carbon blacks on the position of the minimum in the spin-lattice relaxation time (7j) vs. temperature curve. The shape of the curve was also substantially unaffected (107). Extraction of free rubber from an uncross-linked SBR-HAF black mix did not accentuate the effect of the carbon black. More recently Kaufmann, Slichter and Davis (108) reported the observation of two spin-spin relaxation times (T2) in the bound rubber phases of polybutadiene and ethylene-propylene rubber, each reinforced with 50 phr of an SAF black (155 m2/g surface area). The amount of fully immobilized polymer was only 4% of the total, but the remainder of the bound rubber displayed... 

The response of unvulcanized black-filled polymers (in the rubbery zone) to oscillating shear strains (151) is characterized by a strong dependence of the dynamic storage modulus, G, on the strain amplitude or the strain work (product of stress and strain amplitudes). The same behavior is observed in cross-linked rubbers and will be discussed in more detail in connection with the dynamic response of filled networks. It is clearly established that the manyfold drop of G, which occurs between double strain amplitudes of ca. 0.001 and 0.5, is due to the breakdown of secondary (Van der Waals) filler aggregation. In fact, as Payne (102) has shown, in the limit of low strain amplitudes a storage modulus of the order of 10 dynes/cm2 is obtained with concentrated (30 parts by volume and higher) carbon black dispersions made up from low molecular liquids or polymers alike. Carbon black pastes from low molecular liquids also show a very similar functional relationship between G and the strain amplitude. At lower black concentrations the contribution due to secondary aggregation becomes much smaller and, in polymers, it is always sensitive to the state of filler dispersion. 

It has been remarked that time (frequency) - temperature reduced data on carbon black filled rubbers exhibit increased scatter compared to similar data on unfilled polymers. Payne (102) ascribes this to the effects of secondary aggregation. Possibly related to this are the recent observations of Adicoff and Lepie (174) who show that the WLF shift factors of filled rubbers giving the best fit are slightly different for the storage and loss moduli and that they are dependent on strain. Use of different shift factors for the various viscoelastic functions is not justified theoretically and choice of a single, mean ar-funetion is preferred as an approximation. The result, of course, is increased scatter of the experimental points of the master curve. This effect is small for carbon black... 

Carbon black is the most widely used conducting filler in composite industry. Carbon black filled immiscible blends based on polar/polar (65), polar/nonpolar (63,66), nonpolar/nonpolar thermoplastics (67,68), plastic/rubber and rubber/mbber blends (69,70) have already been reported in the literature. The properties of carbon black filled immiscible PP/epoxy were reported recently by Li et al. (60). The blend system was interesting because one of the components is semicrystalline and the other is an amorphous polar material with different percolation thresholds. The volume resistivity of carbon black filled individual polymers is shown in Fig. 21.23. 

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