Melt Granulation Process and equipment

Melt granulation - also called thermoplastic granulation - is an agglomeration process based upon the use of a binder material which is solid at room temperature and softens or melts at slightly elevated temperatures, usually above 50-60°C. When melted, the action of the liquid binder is similar to the action of a binder solution in a wet granulation process. The binders normally used for melt granulation are polyethylene glycols (Rubinstein, 1976; Ford and Rubinstein, 1980; Pataki etal., 1983; Kinget and Kernel, 1985; Schaefer et al., 1990b). The use of hydrophobic binders such as waxes and stearic acid have been investigated for the purpose of preparing sustained release products (McTaggert etal., 1984; Flanders etal., 1987). A range of hydrophobic, meltable substances for the preparation of matrix pellets with prolonged release properties have been investigated by Thomsen etal. (1993b).

When using melt granulation the stages of liquid addition and drying of a wet granulation process are eliminated. It is, perhaps, of greater importance that granulation of water-sensitive materials is possible. A disadvantage for heat-sensitive materials is the elevated product temperature required to ensure melting and distribution of the binder material. In some processes, the final product temperature may be increased to more than 100°C and, thus, cause liberation of water of crystallization.

The binder material is added to the starting materials either as a powder or in molten form to the preheated starting materials. The product must be heated to a temperature above the melting point of the binder. This can be achieved by a heating jacket (Kinget and Kernel, 1985) or by heat of friction caused by intensive agitation (Schaefer etal. 1990b). At a production scale, it is advantageous to use a high shear mixer with a power input sufficient to generate the required product temperature within an acceptable time. Figure 21 compares the correlation between relative swept volume and impeller rotation speed in different laboratory-scale high shear mixers. The Baker Perkins 10 mixer is clearly capable of producing a high energy input. Flanders etal. (1987) examined three scales of Baker Perkins high shear mixers (10, 60 and 6001) and found that melt granulation by heat produced solely by friction is possible in all of them. The authors mentioned that the Fielder and Diosna high shear mixers are unsuitable for melt granulation

fig. 21 Correlation between relative swept volume and impeller rotation speed in laboratory scale high shear mixers. Reproduced from Schaefer etal. (1992a) with permission from the authors.

fig. 21 Correlation between relative swept volume and impeller rotation speed in laboratory scale high shear mixers. Reproduced from Schaefer etal. (1992a) with permission from the authors.

purposes because of the long process time required to achieve the necessary product temperature. The Pellmix 10 high shear mixer, which has a bowl volume of 501, is shown to possess the necessary energy input to ensure melting by frictional heating (Schaefer etal., 1992a). In the work by Kinget and Kernel (1985), a Gral 10-1 mixer equipped with a heating jacket was used. The molten binder was added to the preheated powder.

When melt granulation is performed in a high shear mixer that provides a high energy input to the product, the physical conditions required to produce rounded granules with a narrow size distribution, i.e. pellets, are met. Melt granulation in high shear mixers has, therefore, potential as a simple and fast method for pelletization (see later).

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