Rubber occluded

The latter can be of two types, either purely mechanical, and be associated with the occlusion of rubber into carbon black aggregates (occluded rubber), are more complex and involve physical and chemical interactions, they will then be related to bound rubber. 

It was shown, on the one hand, that gum-filler interactions are associated with the immobilization of a certain amount of rubber on the surface or inside the carbon black aggregates, and, on the other hand, that the corresponding bound or occluded rubbers play important roles in the reinforcement process due either to a restriction of elastomer chain mobility in the vicinity of the filler or to an increase of the effective volume of the latter. What are now the effects exerted by a filler on the stress-strain behavior and the modulus of cured rubbers ... 

It follows that this strain amplification effect will be more important if a high structure carbon black is used. In this case, indeed, the real volume concentration of the filler will be significantly increased by the amount of occluded rubber trapped in the aggregates. At strains high enough, the occluded rubber, though anchored or... 

The (pc value represents the actual size of filler aggregates in the mix it includes, naturally, the filler object itself plus a significant volume of polymer that is shielded from deformation by aggregate tortuosity. This part of the polymer that will not be deformed is usually called occluded rubber (Kraus, 1970 Medalia, 1970 1974). 

Nevertheless, occluded rabber must not be confused with the polymer part whose molecular mobility is changed by adsorption. Occluded rubber, which is mainly trapped in aggregate fractal sites, only represents a part of the volume of elastomer whose molecular motion is slowed down. 

Occluded rubber and viscosity increases with filler structure and loading on return, specific surface area of the filler has an influence on green mix viscosity. 

As in the green state, the strain amplification, due to the limited volume of the actually deformable phase, remains the first-order result of filler incorporation. For a given macroscopic deformation, the actual deformation of the polymeric matrix will always be much higher, obviously depending on the filler volume and its structure, which defines occluded rubber volume. 

The presence of reinforcing fiUers also increases the non-Newtonian behavior of elastomers. This effect is mainly due to the fact that the incorporation of fillers in elastomers decreases the volume of the deformable phase. As discussed in the following text, this decrease is not limited to the actual volume of the filler, but must also include the existence of occluded rubber. So, when filled mixes are submitted to shear forces, because of the lower deformable volume, the actual deformation and speed of deformation are much higher than in unfilled mixes [1,134]. This phenomenon is usually called strain amplification effect, obviously strain amplification is not specific to reinforced systems but to any filled polymer. 

The viscosity of composites with silica content from 10 to 70 phr, prepared with the solution procedure with an amine as catalyst, was found to increase with the silica content, being however lower than the viscosity of composites containing nanostructured silica. This difference, attributed to the lower amount of silanols on the surface of in situ generated silica, could be also attributed to the lower structure of in situ silica, that means to the absence of occluded rubber. The /shape factor of the Guth equation (Equation (2.1)) was calculated to be 2.53. 

The mechanism by which fillers enhance the hardness and modulus is reasonably well understood, at least in qualitative terms. The stiffening is in part attributed to the absence of deformation within the rigid filler particles, and in part with the immobilization of the rubber at the interface between the rubber matrix and the filler particles, plus the hydrodynamic effect. " For blackfilled vulcanized rubber, this immobilization is attributed to the strong physi-sorption of the rubber in the filler structure and is often called occluded rubber. ... 

Non-linear mechanical properties were observed for rubber eomposites and referred to as the Payne effect. The Payne effeet was interpreted as due to filler agglomeration where the filler clusters formed eontained adsorbed rubber. The occluded rubber molecules within filler elusters eould not eontribute to overall elastic properties. The composites behaved similarly to rubber composites with higher filler loading. Uniform and stable filler dispersion is required for rubber composites to exhibit linear viscoelastic behaviour. Payne performed dielectric measurements on SBR vulcanizates containing silica or carbon black. The dielectric data were used to construct time-temperature superposition master curves. The reference temperature increased with crosslinking but not significantly with filler. Comparison of dynamic mechanical and dielectric results for the SBR blended with NR was made and interpreted. ... 

The effect of filler structure on the rubber properties of filled rubber has been explained by the occlusion of rubber by filler aggregates (45). When stmctin-ed carbon blacks are dispersed in rubber, the polymer portion filling the internal void of the carbon black aggregates, or the polymer portion located within the irregular contours of the aggregates, is imable to participate fully in the macrodeformation. The partial immobilization in the form of occluded rubber causes this portion of rubber to behave like the filler rather than like the polymer matrix. As a result of this phenomenon, the effective volume of the filler, with regard to the stress-strain behavior and viscoelastic properties of the filled rubber, is increased considerably. 

The effect of carbon black on hysteresis depends primarily on the particle size of the filler and is related to breakdown and reformation of the agglomerations and the network, to slippage of polymer chains around the periphery of the filler clusters and the presence of occluded rubber. Figure 13 shows the difference of the temperature profiles of carbon black and silica filled rubber compoimds. 

As mentioned above, the incorporation of conventional fillers into elastomeric systems usually causes an increase in the modulus ascribed to the inclusion of rigid particles and to the creation of additional cross-links by polymer-filler interactions. The anisometry of particle aggregates or agglomerates is also expected to increase the modulus as well as the occluded rubber, considered as a typical mechanical interaction and consisting of elastomer chains trapped inside the filler aggregates. This occluded rubber, which is assumed to be partially shielded from deformation, increases the effective filler concentration. 

Considering a highly irregular shape and a presence of occluded rubber in cavities of the carbon black aggregates, Medalia [16] introduced effective volume to represent filler concentration. His effective volume fraction, V, replaces the real volume fraction, v, in Equation 8.8. The effective volume fraction, V, is not an adjustable parameter, but calculated from the DBP absorption [17] (see also Chapter 9). The shear storage modulus, G, was measured at 25 °C and 0.25 Hz, with 20 phr of carbon black loading where G was practically independent of the strain amplitude. With 12 carbon blacks of varying particle size and structure, the calculated, G, from the equation. 

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