Dispersion of agglomerates

Dispersion of agglomerates has been applied in the polymer processing industry for at least 50 years. It is concerned with the incorporation and deagglomeration of additives in the polymer matrix with the ultimate goal being the reduction of the price or the improvement of the properties of the final product. Of course, if the additive exists in the form 

The size of the particles and their ability to interact with each other characterize the type of cluster as cohesionless or cohesive. A cohesionless cluster is formed from noninteracting particles or from large particles ( 1 mm), and its dispersion is determined only by the total deformation of the primary phase. In this case, the dispersion is achieved by peeling off particles from the surface of the cluster by tangential velocity components close to the particles. On the other hand, a cohesive cluster includes interacting particles or very small particles or particles dispersed in a medium other than the polymer matrix, and its dispersion depends on the applied stresses (or equivalently on the deformation rates). 

Usually, additive particles are of maximum size of about 100 qm and the cohesive forces cannot be neglected. The significance of the cohesive forces is in the disintegration of the cluster, which requires that the hydrodynamic forces exceed the cohesive forces. Assuming the aggregates are formed by nontouching spherical particles of like material, the Bradley-Hamaker theory (Elmendorp, 1991) allows one to calculate the attractive van der Waals force between two particles as 

C = R R2Ad. Ri, R2, and d are the radii of the spheres and their distance, respectively. When the two spheres are 

The hydrodynamic forces acting to rupture the cluster can be calculated by assuming simple geometrical shapes. Tadmor and Gogos (1979) assumed that the agglomerate consisted of a dumbbell (two spheres connected with a hypothetical rod to transmit the forces between them) suspended in shear and elongational flow fields. The maximum hydro-dynamic force acting on the rod to rupture the dumbbell (of equal-size spheres) was shown to be 

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The dispersion of agglomerates is strongly dependent on mixing time, rotor speed, temperature, and rotor blade geometry [3], Figure 3.28 [6,4] shows the fraction of undispersed carbon black as a function of time in a Banbury mixer at 77 rpm and 100°C. The broken line in the figure represents the fraction of particles smaller than 500 nm. 

Fig. 5.13 Schematic representation of the influence of wicking and swelling on the dispersion of agglomerates in liquids.

Dispersion of agglomerating substances, as most micronised substances are, requires much attention, see Sect. 29.3. In small batches of cutaneous preparations it is common practice to use a minimal amount of base for the first phase of dispersion. 

Agglomeration might be the cause of poor flowability, for instance of a powder mixture for filling of capsules or tablet dies. The addition of anhydrous colloidal silica (Aerosil 200V) may be effective for the dispersion of agglomerates. Silica is added to 0.5-1 % (w/w) of the total weight of active substance and excipients. 

For dispersion of agglomerates amid another substance usually a (non-rough) pestle and mortar, an three roll mill (ointment miU, see Sect. 28.6.6) or a rotor-stator mixer (see Sect. 28.6.2) are used as a tool. Sometimes a beaker mixer/ blender (Sect. 28.6.5) or the Stephan mixer (Sect. 28.6.1) is used). Trituration of an active substance with eye ointment base is done in a stone mortar with a stone pestle because a lot of force is needed. Sometimes it is possible to disperse agglomerates on an ointment tile with a spatula, but a drawback is that no great force can be exerted. For the dispersion of a sohd substance amid other solids, a plastic or metal mortar can be used. 

In the next sections two types of mixing of solids are discussed random mixing and ordered mixing. In practice, the mixing process mostly consists of a combination of the two. Dispersion of agglomerates may precede the mixing of solids or it may take place simultaneously [26]. 

Dispersing a soUd into a Uquid may be done by trituration in a mortar (see Sect. 29.3). The first step in the preparation is usually the dispersion of agglomerates and wetting of the particles. Then the active substance should be homogeneously distributed into the entire preparation. Dispersion can also be done by vigorous mixing with a rotor-stator mixer. This method is mainly used if the soUd is added at once or in parts, to all of the liquid (or aU of the semisoUd). 

The particle size is determined in the raw material. In the design phase the effective dispersion of agglomerates (see Sect. 29.3) and possible particle growth during storage (see Sect. 18.4.2) have to be validated. 

The dispersion of agglomerates requires that they experience a sufficiently high mechanical stress Tdisp that eventually exceeds the agglomerate strength agg- The ratio of the two quantities, the fragmentation number Fa, is thus a measure on the effectiveness of the dispersion process (Rwei et al. 1990 Baldyga et al. 2008) ... 

Colloid Mills These mills impose intense fiictional work and they are used for dispersion of agglomerated particles. 

Both the dispersed and the continuous phases are fed into the blending or compounding equipment in the form of pellets. The deformation and the dispersion starts after heating both components to temperatures above their melting point. Similarly to the dispersion of agglomerates, the hydrodynamic force is the deforming and disruptive force and the interfacial tension force is the cohesive force of the dispersed phase. The ratio of these two forces or stresses is called the Capillary (or Weber) Number, Ca ... 

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