Filler-filler network

In addition to increases in high-strain loss modulus, reductions in low-strain loss modulus are also observed. This may be attributed to the improvements in polymer-filler interactions which may reduce the amount of filler networking occurring in the compound. The low-strain losses are dominated by disruptions in the filler-filler network, the Payne effect. 

The surface of silica is covered by a layer of acidic silanol and siloxane groups. This highly polar and hydrophilic character of the filler surface results in a low compatibihty with the rather apolar polymer. Besides, highly attractive forces between silica particles result in strong agglomeration forces. The formation of a hydrophobic shell around the silica particle by the sUica-sUane reaction prevents the formation of a filler-filler network by reduction of the specific surface energy [3]. 

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... 

A series of six stress-strain cycles with a crosshead rate of 600 mm/min was applied to specimens having a parallel length of 25 mm and a cross-section of 1 x 4 mm2 on a tensile testing machine. The samples were continuously stretched in six hysteresis cycles up to 60% of their elongation at break values, as shown in Fig. 47. This procedure is an established one and widely practiced for elastomeric composites reinforced with fillers such as carbon black and silica, which tend to build a strong filler-filler network [83]. 

T filler-filler networking parameter calculated from ratio of storage modulus at high and low strains... 

Obviously, if any coupling agent is used, polymer adsorption will naturally occur (Bomo, 1989 Killian etal., 1987) in addition, because of the high polarity of silica, some direct interaction between silica aggregates will also take place and constitute an additional filler-filler network. These effects will not happen in silica-reinforced systems when an appropriate amount of coupling agent is used. 

Figure 8.20 shows the XRD patterns for CB- and clay-filled NR composites. A broad peak is apparent for OMt-filled NR B composites. This peak disappears completely in the case of EOMt-filled NR-CB composites. Although the NR composites are filled with significant amounts of CB the absence of any peak indicates a complete exfoliation of the clay minerals when they are mixed in expanded form. Strain-induced filler-filler network breakdowns by dynamic tension mode at constant frequency and temperature was carried out and the... 

Figure 8.21 Filler-filler network (Payne effect) in hybrid filler filled NR composites.

In 2013, cellulose nanocrystals (CNs) were fabricated from sulfuric acid hydrolysis of cottonseed finter. The crystals were then utifized to prepare nitrile rubber (NBR)/CNs nanocomposites by mixing a water suspension of CNs and the NBR latex directly. CNs formed a strong filler-filler network in the NBR matrix which resulted in an obvious Payne effect [110]. 

Filler modification affects the viscoelasticity since the variation in storage modulus with strain changes with rate of dispersion. This is illustrated in Fig. 6. The unmodified fillers in mbber cause agglomeration and thus result in high Payne effect due to strong inter-aggregate interaction of filler. With modification, the Payne effect of the filled compounds changes as the filler-filler networks is... 

The authors also describe the effect of temperature on the Payne effect. With increasing temperature the amplitude of the Payne effect decreases significantly (Fig. 12). Very surprisingly, enhanced Payne-like behavior was observed for rubber vulcanizates at room temperature where filler-filler and filler-polymer interaction are not observed in comparison to the typically filled vulcanizates. The authors concluded that in addition to the contribution from the filler-filler network, there are many other factors that affect the nonlinear viscoelastic behavior. Nevertheless, the Payne effect is assumed to arise from the elementary mechanism consisting of adsorption-desorption of polymer chains from the surface of the particles [50]. Besides the experimental investigation, the authors have applied the Maier... 

Besides the interaction between the polymer and the filler, an interaction between filler particles occurs, predominantly above a critical concentration threshold, the percolation threshold. The properties of the material change drastically, because a filler-filler network is established. Eor example an over proportional increase of electrical conductivity of a carbon black filled compound. But even at... 

Three zones indicated in G, can also be detected in the graph of loss modulus (G") versus strain. In low strain, the first linear viscoelastic zone, G" of reclaimed is lower than NR due to its filler content. In higher strains, G" of NR compounds decreases due to disentanglement of polymer chains whereas, reclaimed rubber has different behavior. In medium strains, loss modulus of reclaimed rubber increases due to energy dissipation for mbber-filler and filler-filler networks breakdown and then decreases due to mbber chains disentanglement. 

The filler-filler network can be reformed again after a certain time interval. Payne revealed that the value of E is largely recoverable upon return to smaller amplitudes in the linear regime. So, flexible mbber chains allow the filler particles to rearrange again to form a three-dimensional filler network in the rubber matrix [99]. In order to investigate the ability to recover the strain sweep experiments were also carried out in the reverse direction from higher to lower strain amplitudes for the samples with unmodified CNT dispersed by ethanoUc suspension. 

It is observed that the values do not reach its initial position within the relaxation time of the experiment, but a recovery of the E values have been attained (Fig. 18). This behaviour of a rubber can be explained by the stress softening effect during the dynamic strain. Nevertheless, a high extent of recovery in the reverse amplitude sweep indicates that a good filler-filler network has been re-established at a low loading of tubes in the S-SBR-BR matrix. So, at least it can be said that rather than damage or permanent break of the tubes, the amplitude sweep disrupted the filler-filler network in the rubber matrix. It is noted that the absolute values of E at small amplitudes are somewhat differed from each other as compared with the value obtained from the phr CNT-filled compound. The difference may be developed from ageing of the samples. 

The nonlinear viscoelastic behavior of filled vulcanizates is somewhat different from that of filled compounds, since the chemical crosslink network of the rubber matrix is formed and, the physical rubber-filler networks and filler-filler networks are enhanced during curing at a relatively high temperature [7]. Speaking from a broad sense, filled vulcanizates can be viewed as a double network structure in which the nanoparticles supplement the inherent viscoelasticity of crosslink rubbers with additional physical network junctions. 

Chen and Xu [58] studied the G of uncured NR/ZDMA compounds during consecutive strain sweeps (Fig. 8). After the first sweep, the rupture of the weak filler-filler network formed by high load of ZDMA leads to an apparent softening behavior. However, the polarity of ZDMA and the softened rubber molecules by first sweep are beneficial to accelerate the aggregation of ZDMA, resulting in a subsequent interesting behavior that the G of third and forth sweep showing an increase [58]. 

Generally speaking, G shows a rapid decrease at a high strain about 100-200 %, which is mainly contributed by the rubber matrix. In this high strain amplitudes region, the filler-filler networks have been destroyed completely and rubber is stretched to deform. In Chen s study, the G curves of 10 phr and 20 phr are very close, whereas the 40 phr and pure NR are very close. This reflects that the higher density of cross-links formed in the samples with 10 phr and 20 phr ZDMA than in neat NR and sample with 40 phr ZDMA. 

The nonlinear viscoelastic behavior of the composites of natural rubber filled with surface-modified nanosilica was studied with reference to silica loading [191]. The effect of temperature on the nonlinear viscoelastic behavior has been investigated. It was observed that Payne effect becomes more pronounced at higher silica loading. The filler characteristics such as particle size, specific surface area, and the surface structural features were found to be the key parameters influencing the Payne effect. A nonlinear decrease in storage modulus with increasing strain was observed for unfilled compounds also. The results reveal that the mechanism includes the breakdown of different networks namely the filler — filler network, the... 

The dynamic viscoelastic properties of nanosilica-filled natural rubber composites was investigated. The objective of the present study was to look at the nonlinear viscoelastic behavior of natural rubber filled with commercially used nanosilica. The Payne effect is assumed to arise from the elementary mechanism consisting of adsorption-desorption of macromolecular chains from the filler surface. It was found that because of the small particle size and high specific surface area, nanosilica forms stronger and more developed filler-filler network and the breakdown of these networks results in larger Payne effect. Also, the amount and morphology of the fillers played a major role on the Payne effect. 

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