Filled rubbers rubber-filler interaction

Figure 15.33 shows benzene uptake by natural rubber samples. Filled samples absorb less solvent (lower swelling). The carbon black containing sample had a lower benzene uptake than the silica filled sample. The lower swelling of the carbon black containing sample is due to high bound rubber content, the crosslink density of the black filled vulcanizate, and a strong rubber-filler interaction. 

Leblanc, J. L. 2002. Rubber-filler interactions and rheological properties in filled compounds. Progress in Polymer Science 27 627-687. 

In order to determine the influence of rubber matrix polarity on filler dispersion and rubber-filler interaction, two kinds of rubber NBR and SBR, were chosen. SEM pictures of the rubber vulcanizates, filled with reference and 48 min plasma treated wollastonite, are presented in Fig. 12.4. Morphology of SBR/wollastonite samples does not reveal any changes, explaining strengthening of the material (see Section 12.3.3). 

Any changes to filler particles SFE and its components effect on mechanical properties of rubber vulcanizates filled with the modified filler. Improvement of mechanical properties of the materials originates increased rubber-filler interaction and better dispersion of filler particles in rubber matrix. 

In order to understand the growth of the bound rubber, we need to start from the incorporation of the filler, when the rubber-filler interaction commences. Boonstra and Medalia [11] interpreted the occurrence of the torque maximum (the second peak) as the result of filling the void of the agglomerate with rubber. This view was supported by the density measurements. The dispersion did not start, because no brown tint appeared in the rubber phase. Thus, the increase of the torque was interpreted as the result of the effective increase of the filler concentration, when the increasing amount of rubber becomes... 

Providing the filler content is above or near a critical level of 12-13%, BdR measurement yields a swollen, coherent sample at the end of the extraction process. This suggests that there are rubber-filler interactions, strong enough to resist the solvatation process, which consequently lead to a tri-dimen-sional morphology in uncured filled rubber compounds. Microphotography evidences of such a 3D structure were published in the early 1970s and... 

Rubber-filler interactions sites are like "knots" whose effect superimposes to chemical reticulation. With reference to the theory for rubbery elasticity, in a filled network, the d5mamic modulus is proportional to the overall networking density N and the temperature T, according to (see details in Appendix 5.6) ... 

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. 

It has been well established that wear resistance of filled rubber is essentially determined by filler loading, filler morphology, and polymer-filler interaction. For fillers having similar morphologies, an increase in polymer-filler interaction, either through enhancement of physical adsorption of polymer chains on the filler surface, or via creation of chemical linkages between filler and polymer, is crucial to the enhancement of wear resistance. In addition, filler dispersion is also essential as it is directly related to the contact area of polymer with filler, hence polymer-filler interaction. 

The lowering of die swell values has a direct consequence on the improvement of processability. It is apparent that the processability improves with the incorporation of the unmodified and the modified nanofillers. Figure lOa-c show the SEM micrographs of the surface of the extrudates at a particular shear rate of 61.2 s 1 for the unfilled and the nanoclay-filled 23SBR systems. The surface smoothness increases on addition of the unmodified filler, and further improves with the incorporation of the modified filler. This has been again attributed to the improved rubber-clay interaction in the exfoliated nanocomposites. 

The interaction between two fillers particles can be investigated by measuring the Payne effect of a filled rubber compounds. In this measurement, dynamic properties are measured with strain sweep from a very small deformation to a high deformation. With the increased strain, the filler-filler network breaks and results in a lower storage modulus. This behavior is commonly known as the Payne effect... 

An important feature of filled elastomers is the stress softening whereby an elastomer exhibits lower tensile properties at extensions less than those previously applied. As a result of this effect, a hysteresis loop on the stress-strain curve is observed. This effect is irreversible it is not connected with relaxation processes but the internal structure changes during stress softening. The reinforcement results from the polymer-filler interaction which include both physical and chemical bonds. Thus, deforma-tional properties and strength of filled rubbers are closely connected with the polymer-particle interactions and the ability of these bonds to become reformed under stress. 

A low-resolution proton NMR method is one of the few techniques that have so far proved to be suitable for studying elastomer-filler interactions in carbon-black-filled conventional rubbers and silica-filled silicon rubbers [20, 62, 79]. It was pointed out by McBrierty and Kenny that Many of the basic characteristics of filled elastomers are revealed by low resolution spectra while the more sophisticated techniques and site specific information refine interpretations and clarify motional dynamics [79]. 

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