Rubber-carbon black interaction

Some General Comments on Carbon Black/Rubber Interactions. .. 24... 

Leblanc JL (2013) What large amplitude oscillating shear characterization and modeling reveal about carbon-black/rubber interactions. Rubb Chem Technol 86 261-285... 

For small strains the stress-relaxation rate of vulcanized rubbers at long times is proportional to tan 8 (178). This will also be true at large strains if strain-time factorization applies. The implication of this for the results of Cotten and Boonstra (150) is that tan 8 in unswollen vulcanizates is only little affected by carbon black-polymer interactions at strain levels between 75 and 250% elongation (and at very low frequencies) and suggests that the substantial increases in tan 8 observed in filled rubbers at small strains are due primarily to the effects of secondary filler aggregation. 

In the 1960s, carbon black-elastomer interaction was considered as the result of a chemical bonding (Bueche, 1961, 1960) between acidic surface functions and natural rubber alkaline moieties (Donnet and Heinrich, 1960 Le Bras and Papirer, 1983). So many studies have been conducted to increase carbon black activity by surface oxidation (Le Bras and Papirer, 1983) oxygen at high temperatures, H2O2, ozone, nitric acid. The type of oxidation used determines the number and the type of functions obtained it is interesting to underline that such chemical modifications are used at industrial scale for specialty carbon blacks (inks, pigments). 

In attempting to predict the direction that future research in carbon black technology will follow, a review of the literature suggests that carbon black-elastomer interactions will provide the most potential to enhance compound performance. Le Bras demonstrated that carboxyl, phenolic, quinone, and other functional groups on the carbon black surface react with the polymer and provided evidence that chemical crosslinks exist between these materials in vul-canizates (LeBras and Papirer, 1979). Ayala et al. (1990, 1990) determined a rubber-filler interaction parameter directly from vulcanizatemeasurements. The authors identified the ratio a jn, where a = slope of the stress-strain curve that relates to the black-polymer interaction, and n = the ratio of dynamic modulus E at 1 and 25% strain amplitude and is a measure of filler-filler interaction. This interaction parameter emphasizes the contribution of carbon black-polymer interactions and reduces the influence of physical phenomena associated with networking. Use of this defined parameter enabled a number of conclusions to be made ... 

As an example of the use of equation (4.5) to determine the viscosity of suspensions, one can refer to e works of Mullins [94] and Feldman and Boiesan [95] on rubbers containing fillers which are chemically inactive like wood flour or chemically active like carbon black. When the filler introduced is chemically inactive (with any ) or chemically active (with < 0.10), the quadratic form of equation (4.5) with aj = 2.5 and X2 = 14.1 could be used to give a good estimate of the viscosity of the suspension. For higher concentrations of the chemically active filler (carbon black), particle interaction begins and the viscosity of the suspension increases markedly and equation (4.5) as such cannot then be used for an estimate. However, if particle interaction leads to agglomeration, then Mullins [94] and Feldman and Boiesan [95] recommend the use of aj = 0.67 , and 02 = 1.62 in equation (4.5), where a, is the index of asymmetry of the elastomer macromolecules. 

Droste D H and DiBenedetto A T (1969) The glass tranation temperature of filled polymers and its effect on their physical properties, J Appl Polym Sci 13 2149-2168. Waldrop M A and Kraus G (1969) Nuclear magnetic resonance study of the interaction of SBR with carbon black. Rubber Chem Tech 42 1155-1166. 

Having the interaction site well identified in a filled polymer system, in terms of chemical activity and surface, and a clear picture of the nature of the polymer-filler interaction allow quite convincing theoretical models to be developed. Such a favorable situation is however restricted to a few cases, namely silica/polysiloxane systems. With other systems, either the nature of the polymer-filler interaction is badly known or the size of the interaction site cannot be clearly quantified, or both. In such case however, the successful silica/PDMS case provides some interesting guidelines when assuming that, whatever are the respective chemical natures of the filler and the polymer, at least the physics is the same. As we have seen the author has successfully adapted this model to the case of carbon black/rubber systems, with however the additional difficulty that the surface area of the interaction site Aq cannot be known a priori (see Chapter 5, Section 5.1.5). 

N. Nakajima and R. A. Miller, "Processing Ease and Rubber Carbon Black Interaction," paper presented atMCL meetings Montreal, 1987. 

The important yet unexpected result is that in NR-s-SBR (solution) blends, carbon black preferably locates in the interphase, especially when the rubber-filler interaction is similar for both polymers. In this case, the carbon black volume fraction is 0.6 for the interphase, 0.24 for s-SBR phase, and only 0.09 in the NR phase. The higher amount in SBR phase could be due to the presence of aromatic structure both in the black and the rubber. Further, carbon black is less compatible with NR-cE-1,4 BR blend than NR-s-SBR blend because of the crystallization tendency of the former blend. There is a preferential partition of carbon black in favor of cis-1,4 BR, a significant lower partition coefficient compared to NR-s-SBR. Further, it was observed that the partition coefficient decreases with increased filler loading. In the EPDM-BR blend, the partition coefficient is as large as 3 in favor of BR. 

D-TEM gave 3D images of nano-filler dispersion in NR, which clearly indicated aggregates and agglomerates of carbon black leading to a kind of network structure in NR vulcanizates. That is, filled rubbers may have double networks, one of rubber by covalent bonding and the other of nanofiller by physical interaction. The revealed 3D network structure was in conformity with many physical properties, e.g., percolation behavior of electron conductivity. 

Incorporation - the carbon black is distributed into the rubber matrix but not into the desired state for complete reinforcement. At this stage of mixing the rubber penetrates the voids in the large agglomerates of carbon black. It is also at this stage that strong interaction between the rubber and black surface occurs in the case of small particle sized blacks with low structure, which makes the next step of dispersion difficult to achieve. 

The reaction with free radicals plays an important role in the interaction of carbon black with rubber (103) and with styrene (58, 102). 

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