The process of granulation aims to transform powders into solid aggregates or agglomerates called granules. Melt granulation or thermoplastic pelletizing are two granulation processes by which granules are obtained through the addition of either a molten binder or a solid binder which melts during the process. In the latter case the plastic properties of the binder are used. After the granulation the binder crystallizes at room temperature.
Whatever the process used, agglomerates obtained are either granules with an irregular shape and a broad particle size distribution or pellets of spherical shape, particle size distribution ranging from 0.5 to 2 mm and a matrix structure (1). The granules or pellets can be used either directly into sachets, capsules or compressed into tablets.
1. Principle of melt granulation
Mechanisms of agglomeration of both melt granulation and thermoplastic pelletizing are similar to those of wet granulation. As a matter of fact, even if the meltable binder is classically introduced in the solid-state, it melts during the process and acts as a classical wetting agent.
1.1. Nucleation step
During the nucleation step the binder comes into contact with the powder bed and some liquid bridges are formed, leading to the formation of small agglomerates. The size, the viscosity of the binder and the speed of the impeller influence the mechanism of nucleation.
Fine or atomized binders with low viscosity and high impeller speed favor a homogenous distribution of the binder at the surface of the powder whereas large particles of binder with a high viscosity and low impeller speed preferably induce the immersion of the powder into the binder (2). The mechanism of nucleation depends on the binder viscosity at high impeller speed and the binder size at low speed (3).
1.2. Densification step
The mechanism of densification depends on the liquid saturation of granules and their ability to deform themselves during collisions. These collisions can lead to the coalescence of two granules or the layering of fine particles at the surface of pre-existing granules. Two main hypotheses are postulate to explain the densification step (4):
- The steady-state densification happens with easily deformable granules. It is favored by large particles and low-viscosity binders.
- Densification through induction happens by coalescence of solid-granules. The presence of molten binder at the surface of the granule is necessary to permit the coalescence of granules after collision. The liquid saturation of the granules should be from 80 to 100% in order to insure the deformability of the surface. It is favored with fine particles and viscous binders.
1.3. Attrition-breakage step
In fact there is an equilibrium between densification and breakage of granules. Granules that cannot withstand the shear of the mixers will break and their fragments will participate to the densification by layering (5;6).
In a high-shear mixer, that phenomenon of breakage insures the control of the granule size distribution.
High-shear mixers are the best equipments for melt granulation or thermoplastic pelletizing. As a matter of fact they allow a sufficient increase of the product temperature inside the equipment to melt the binder and to shorten the process time. They are also equipped with a double-heated jacket in order to quickly obtain the melting temperature of the binder inside the bowl. The shear exerted by the impeller allows the mixing and densification of particles and the chopper controls the granule size distribution. The choice of the high-shear mixer can influence the melt granulation process.
2.1. Volume and height of the bowl
The volume of the bowl influences the granule size distribution (7). Granules produced with high-countenance bowl are generally more spherical and their surface is smoother. The big quantity of product inside the bowl favors its movement and hence its pelletization. The temperature increase is also greater with these equipments thanks to lower heat losses. The height of the bowl determines the vertical movement of the product and the contact between the agglomerates and the impeller (8-10). A small height of bowl favors the contacts of the granules with the impeller and their densification.
2.2. Design and position of the impeller
The impeller is defined by the number of blades (2 or 3), their geometry (plane or Z-shape) and the angle of the impeller with the bottom of the bowl (ranging from 30 to 60°).