Dispersion, Aggregate Sizes, and Distances

The quality of particulate filler dispersion in the elastomer matrix is of primary importance for compound mechanical and use properties (Gotten, 1983 Funt, 1986 Gerspacher and O Farrell, 1993 Bomo andMorawski, 1983 Richmond etal., 1993). 

In very recent years, tiller dispersion characterization has been brought again into light because of the difficulties encountered to disperse silica in rubber (Bomal etal., 1998,1993). 

As discussed earlier, dispersibility and so dispersion is mainly controlled by the strength of interaction between agglomerates and/or aggregates, which is a direct consequence of their surface energy. The influence of surface energy on dispersion has been clearly demonstrated by the use of different matrices when matrix surface energy increases and becomes closer to filler surface energy, dispersion is facilitated. 

In addition to filler s surface energy, which is of major importance, dispersion can also be influenced by filler s morphological properties. 

Dispersion is highly influenced by filler surface area the higher the surface area, the lower the dispersion (Sone et al., 1992). This result is probably due to the fact that high surface area usually has smaller aggregates, which will develop more interactions with their neighbors in the dry state. 

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This value can be relatively low, nc - 2-4 in the initial aggregates.3 An increase in nc in the concentrated suspensions, and the shortening of distances between particles with increasing Csi02, affect the dispersion structure with time. Therefore, gelation of the concentrated dispersion (at 293 K) occurs faster, e.g. at Cs o2 = 16.7 wt.% in 2 days, than at Cs o2 <8 wt.%. Sonication leads to a lower turbidity of the suspensions compared to suspensions made from ball-milled silicas because of a different size of residual aggregates... 

A dispersion Is a system made of discrete objects separated by a homogeneous medium In colloidal dispersions the objects are very small In at least one dimension. Colloidal sizes range from 1 to 100 nm however these limits are somewhat arbitrary, and It Is more useful to define colloids as dispersions where surface forces are large compared to bulk forces. Here we are concerned with systems where the dispersion medium Is a liquid examples are droplets In emulsions or mlcroemulslons (oll/water or water/oll), aggregates of amphiphilic molecules (surfactant micelles), foams, and all the dispersions of solid particles which are used as Intermediates In the manufacture of ceramics. At this stage we are not too concerned with the nature of the constituents, but rather with the structures which they form this Is a geometrical problem, where the system Is characterized by Its surface area A, by the shapes of Its Interfaces (curvatures - b ), and by the distances between opposing surfaces (d — concentration parameter). 

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