Fillers aggregation

In a polymeric matrix the filler, even if present in minimum quantities) is always more or less agglomerated [212]. Microscopic studies have confirmed the existence of two types of primary structures in composites [213], i.e., filler aggregates with particles bound together firmly enough, and agglomerates — systems of weakly interrelated aggregates. 

FIGURE 29.4 Filler aggregation and dispersion. (From Li, Y. et al.. Rubber Chem. TechnoL, 67, 693, 1994.)... 

Medalia, A. 1. Filler aggregates and their effect on reinforcement . In Le renforcement des elastomeres. Paris CNRS 1975, pp. 63-79... 

The objective of making good anode carbon, which has the property values already specified, involves four important operations which are applied differently for Soderberg and prebake anode fabrication. These four operations are (1) filler aggregate sizing (2) paste pitching level determination, (3) paste compaction, and (4) compacted composite baking. 

The most important objective for anode filler aggregate sizing is to obtain a high vibrated aggregate bulk density (23). For prebake anodes, maximum aggregate bulk density is required to achieve maximum baked carbon density. For Soderberg anodes, this requirement is somewhat modified by paste rheological requirements... 

Little information has been published on the question of how filler network structure actually affects the energy dissipation process during dynamic strain cycles. The NJ-model focuses on modeling of carbon black network structure and examination of the energy dissipation process in junction points between filler aggregates. This model was further developed to describe the strain amplification phenomenon to provide a filler network interpretation for modulus increase with increasing filler content. 

Several performance characteristics of rubber such as abrasion resistance, pendulum rebound, Mooney viscosity, modulus, Taber die swell, and rheological properties can be modeled by Eq 7.34. " A complex mathematical model, called links-nodes-blobs was also developed and experimentally tested to express the properties of a filled rubber network system. Blobs are the filler aggregates, nodes are crosslinks and links are interconnecting chains. The model not only allows for... 

Figure 10.6 gives a summary of data on different fillers in UHMWPE. The total degree of crystallinity, as determined by the enthalpy of crystallization, increases with filler concentrations up to 40-50% and then gradually decreases. This decrease is caused by filler aggregation which decreases its nucleation ability. 

Secondary Filler Aggregation Effects and Viscoelastic Behavior. . 197... 

Of the experimental obstacles, attainment of a reliable equilibrium stress (the statistical theory is an equilibrium theory) is perhaps the most serious one. Pre-straining to a higher extension is sometimes resorted to as a means for hastening the attainment of a level stress value, but stress-softening is an undesired complication. At low extensions secondary filler aggregation makes a large contribution to the modulus which is... 

The response of unvulcanized black-filled polymers (in the rubbery zone) to oscillating shear strains (151) is characterized by a strong dependence of the dynamic storage modulus, G, on the strain amplitude or the strain work (product of stress and strain amplitudes). The same behavior is observed in cross-linked rubbers and will be discussed in more detail in connection with the dynamic response of filled networks. It is clearly established that the manyfold drop of G, which occurs between double strain amplitudes of ca. 0.001 and 0.5, is due to the breakdown of secondary (Van der Waals) filler aggregation. In fact, as Payne (102) has shown, in the limit of low strain amplitudes a storage modulus of the order of 10 dynes/cm2 is obtained with concentrated (30 parts by volume and higher) carbon black dispersions made up from low molecular liquids or polymers alike. Carbon black pastes from low molecular liquids also show a very similar functional relationship between G and the strain amplitude. At lower black concentrations the contribution due to secondary aggregation becomes much smaller and, in polymers, it is always sensitive to the state of filler dispersion. 

In spite of the often large contribution of secondary filler aggregation effects, measurements of the time-temperature dependence of the linear viscoelastic functions of carbon filled rubbers can be treated by conventional methods applying to unfilled amorphous polymers. Thus time or frequency vs. temperature reductions based on the Williams-Landel-Ferry (WLF) equation (162) are generally successful, although usually some additional scatter in the data is observed with filled rubbers. The constants C and C2 in the WLF equation... 

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 discussion of stress-softening phenomena it has become accepted to exclude the considerable softening that occurs at small strains due to disruption of the secondary filler aggregation (Section VII-1) and to reserve the terms stress-softening to the processes which occur at large strains, where secondary particle aggregation effects are small or absent. 

EN XXX Tests for (bituminous bound fillers) filler aggregate (in preparation). Part 1 Delta ring and ball test . 

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