Aggregation and agglomeration

Up to this point only the surface chemistry and size of the primary filler particles have been examined. However, important evidence exists that these particles are not randomly distributed throughout the elastomeric matrix. Indeed, much evidence shows that a complex state of aggregation is important for reinforcement in both carbon black and silica fillers. Two levels of structure have been identified in reinforcing fillers beyond the 

The quality of crystalline products can be strongly inflnenced by processes such as aggregation (absence of supersatrrration) artd agglomeration in supersatrrrated solu-tiorrs (Judat and Kind 2004 Schwarzer arrd Peukert 2004). There is a great number 

At first the simple case will be discussed that the difierent forces are neutralizing and the resulting interaction energy is zero. It is practical to introduce a volume-based population balance v according to 

This is reasonable because the volume of all particles lemains constant in the absence of nucleation and crystal growth. The volume-based population balance is given by 

5dis(M) - dis( ) formation rates of birth and death events of particles 

Here p (in m /s) is the rate corrstant of aggregation or the aggregation kernel which depends on the particle volrrmina u and (y-u). 

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Figure 6.2 Particle formation via aggregation and agglomeration adapted from Low and White, 1975)...

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. 

Note Alternative definitions of aggregate and agglomerate are used in catalysis [2]. The distinction offered by these definitions is in conflict with the distinction understood in the wider context and with the concepts of aggregation and agglomeration. To avoid confusion the definitions proposed here are recommended. 

Data presented in Table 1 show that depending on the kind of oxide matrices (polymer type (with polar or nonpolar groups), and deposition technique, we obtain carbon-silica materials of different porous structure and carbon content. 

From known relationships between surface area and dissolution, it is reasonable to predict that ultrafine particles may increase the dissolution rate of relatively insoluble compounds. If these particles are then stabilized to avoid aggregation and agglomeration, and yet retain fluidity, then a useful drug product could be obtained. 

In the case of fumed powders, the results of particle size analysis depend veiy strongly on the characterization method. Each method measures a different particle property, from which sphere equivalent diameters are calculated. The underlying models assume homogeneous, spherical particles, which does not apply to the porous aggregates and agglomerates of these materials. 

It is apparent from the data that particles of a few nanometers in size can only be made on industrial scale by synthetic methods. On the other hand, these particles are either intentionally or unintentionally aggregated and agglomerated in their powder forms. Thus, for the dispersion of fillers, agglomerate and aggregate size is usually as relevant as the primaiy particle size. Fillers, which are obtained by various milling and classification processes, can also be obtained in the form of small particles, but usually not below 100 nm. 

For the formulation of suspensions, the hydrophobic or hydrophilic solid is dispersed in a aqueous or nonaqueous medium to produce a system that covers a wide particle size range, typically 0.1-5 pm. This process requires the presence of a surfactant (dispersant) that satisfies four criteria (i) wetting of the powder by the Uquid (if) the dispersion of aggregates and agglomerates into single units (iii) comminution of the large particles into smaller units and (iv) stabilisation... 

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