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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.
2. Equipments
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°). |
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