Carbon-black-filled rubbers polymer-filler interactions

Next 129Xe experiments on an EPDM terpolymer, which is present as the elastomer component in a composite material with carbon black will be discussed. The question investigated for these materials is whether the existence of any polymer-filler interaction can be detected by 129Xe NMR. This interaction influences the mobility of the elastomer chains in a relatively large shell around the filler particles. This fraction is called the bound rubber fraction. It is generally believed that the bound rubber fraction influences the mechanical and frictional properties of the filled elastomer [17, 18]. 

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. 

Two major fillers used in the rubber industry are silica and carbon black. Carbon black is black because it absorbs/scatters all radiation, including infrared, impinging on it. Hence, simple transmission spectroscopy of carbon black filled specimens is not straightforward and is usually not possible unless the samples are very thin. Carbon black filled samples have not been readily examinable using micro-spectroscopic methods. Silica filled systems are more amenable to microscopic techniques [76] and can be examined to determine silica-polymer(rubber) interactions. The presence of inorganic materials, e.g., transition metal complexes in... 

As shown by Figs. 15 and 16 (carbon black) filled compounds exhibit a more complex dependency on strain and temperature essentially because strong interactions develop between the filler and a part of the rubber matrix. An approach of the complex modulus function G (y, T, [where stands for filler volume fraction] of carbon black filled mbber compounds was recently proposed by the author [28] but its detailed discussion is outside the scope of the present chapter. It will only be noted here that an appropriate equation for G (y, T, combines a member that represents the response of the polymer matrix to increasing strain (the so-called polymer component) and a member that describes the response of a rubber-filler network embedded in the matrix (the so-called filler component). 

Incorporating carbon black and carbon nanotubes into the polymer matrix leads to conductive materials. A lot of work has been devoted to investigations of the electrical properties of these filled materials. The electrical conduction process depends on several parameters such as processing techniques used to mix fillers with rubber, fiUer content and filler characteristics (particle size and structure) as well as polymer-filler interactions. 

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). 

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. 

Micron-sized fillers, such as glass fibers, carbonfibers, carbon black, talc, and micronsized silica particles have been considered as conventional fillers. Polymer composites filled with conventional fillers have been widely investigated by both academic and industrial researchers. A wide spectrum of archival reports is available on how these fillers impact the properties. As expected, various fundamental issues of interest to nanocomposites research, such as the state of filler dispersion, filler-matrix interactions, and processing methods, have already been widely analyzed and documented in the context of conventional composites, especially those of carbon black and silica-filled rubber compounds [16], It is worth mentioning that carbon black (CB) could not be considered as a nanofiller. There appears to be a general tendency in contemporary literature to designate CB as a nanofiller - apparently derived from... 

In view of the long history of research efforts on filled rubbers, it is not surprising that the initial works on filled polymer blends appeared in publications authored by rubber compounders and carbon black vendors [17, 18]. For instance, Walters and Keyte [17] observed that the compound ingredients, such as CB and zinc oxide, were not homogenously dispersed in rubber blends. Hess et al. [18] also reported a series of fundamental observations. First, they observed that filler particles tend to remain in the lower viscosity phase, in the absence of significant filler-matrix interactions. However, in the presence of strong polar-polar interactions between the filler particles and one of the phases, the particles were found to be selectively dispersed in the more polar phase and the viscosity became less important. More recently. Portal et al. [19] also presented similar observations about selective localization of CB particles in the natural rubber (NR) phase in NR/ polybutadiene blends. 

In the rubber industry the distribution of particle size is considered to be important as it affects the mechanical properties and performance. Aggregate size also varies with particle size. Aggregates can have any shape or morphology. The fundamental property of the filler used in a filled elastomer is the particle size. This affects the reinforcement of elastomer most strongly. One of the sources of reinforcement between the carbon black surface and the rubber matrix is the van der Waals force attraction. Also, rubber chains are grafted onto the carbon black surface by covalent bonds. The interaction is caused by a reaction between the functional group at the carbon black particle surface and free radicals on polymer chains. Hence, filler-rubber interface is made up of complex physical-chemical interaction. The adhesion at the rubber-filler interface also affects the reinforcement of rubber. When the polymer composites are filled with spherical filler (aspect ratio of the particle is equal to unity), the modulus of the composite depends on the modulus, density, size, shape, volume ratio, and number of the incorporated particles. 

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