Nucleation of particles

Agglomerate formation and growth by the nucleation mechanism proceed when there is sufficient free surface liquid to establish pendular bondings between the particles. It results in small, loose agglomerates which may grow further by coalescence with free particles. Insofar as the loose agglomerates survive the agitation they are likely to consolidate because of agitation and effects of the liquid bondings and, thereby, gain further strength.

Nucleation of particles is influenced by the cohesive forces expressed by equation 5 and probably also by the viscous forces of the moving pendular bridge. According to equation 5, reduction of the particle size gives rise to increased bonding strength. This is the reason why fine powders agglomerate more easily than coarse powders.

It is a general experience that for efficient agglomeration the binder liquid must have good wetting properties on the solid particles, but there are few data on the effect of the contact angle upon agglomerate growth. Ritala etal. (1988) found that granulation of fine particulate sulfur was difficult using a binder liquid with a contact angle of 56°. When the contact angle was reduced to 37°, by addition of a wetting agent, considerably less liquid was required but the granulation was still unsatisfactory. The effect of the contact angle is difficult to interpret, because the addition of a wetting agent changes both the surface tension and the contact angle. The literature on wet granulation gives the impression that a contact angle lower than about 25° in most experiments results in acceptable agglomeration.

The literature on binders for granule and tablet formulations shows that significant determinants for optimum granulation are the wetting of the solid by the binder and the binder adhesion and cohesion (Krycer eta!., 1983). The relative influence of these factors was assessed by Rowe (1988, 1989a-c, 1990) in studies on the thermodynamic energy of cohesion and adhesion. Although these studies relate to the properties of granules and tablets, they demonstrate that wetting and spreading of the binder influence the granule morphology. It is probable that the binder-substrate interaction influences the formation and growth of agglomerates. Parker etal. (1990, 1991) showed that binder-substrate interactions influence the rheological behaviour of microcrystalline cellulose wetted with different binder solutions. In addition to the effect of binder concentration and, hence, liquid viscosity, the spreading coefficient also affected the maximum torque recorded during wet massing. Different molecular weight grades of the polymers showed different torque readings at equivalent viscosities, indicating that the interactions between binder and substrate are dependent on the grade of the polymeric binder. In an earlier study, Jäger and Bauer (1984) demonstrated the benefit of using blends of low and high molecular weight grades of povidone as granule binder. Parker et al. (1990) found that increasing binder concentration and, hence, viscosity produced a greater maximum torque and also a reduced liquid requirement at the maximum. This agrees well with the 'lubricating' effect of adding a binder to the granulation, as shown in Fig. 4. The binder reduces the particle interactions and, therefore, improves the deformability of the moist agglomerates.

Figure 10 shows the final granule size obtained by wet granulation of lactose with various binder liquids in a fluidized bed granulator, i.e. a process characterized by low shear effects. As pointed out by Ennis etal. (1991), the resulting granule size is almost the same as the mean size of the droplets of the atomized binder liquid. As the binder droplet is deposited onto the bed it immediately absorbs surrounding particles. Because the particles have insufficient energy to rebound or break away, the drop structure is maintained in the formation of the agglomerate. Granule growth by fluidized bed granulation proceeds primarily by the nucleation mechanism insofar as there are primary particles present.

The effect of the droplet size shown in Fig. 10 can explain the many reports in the literature on effects of the binder liquid viscosity upon the final granule size in fluidized bed granulation (Kristensen and Schaefer, 1987). A high liquid viscosity gives rise to relatively large drops by atomiza-tion and, hence, a relatively large granule size.

Rg. 10 Effect of mean droplet size on the final granule size in granulation of lactose with aqueous binder solutions in a fluidized bed granulator (Glatt WSC 15). Binder solutions: gelatine 4% (O); Povidone K25 10% (A); methylcellulose 2% (A). Reproduced from Schaefer and Woerts (1978a) with permission from the authors.

Rg. 10 Effect of mean droplet size on the final granule size in granulation of lactose with aqueous binder solutions in a fluidized bed granulator (Glatt WSC 15). Binder solutions: gelatine 4% (O); Povidone K25 10% (A); methylcellulose 2% (A). Reproduced from Schaefer and Woerts (1978a) with permission from the authors.

In high shear mixers, growth by coalescence is significant in addition to the nucleation mechanism. The drop size of the atomized binder liquid does not affect the granule size (Holm etal., 1983), as the particles absorbed by the drops break away because of the intensive agitation.

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