8 Basic Granulation Properties

There are many formulation and process variables involved in the granulation step, and all of these can affect the characteristics of the granulation produced. These are:

a) Particle size and shape

b) Surface area

c) Strength and Friability

d) Flow Properties

e) Compressibility

f) Moisture Content

g) Granules porosity

a) Particle size and shape: The particle size of a granulation is known to affect the average tablet weight, tablet weight variation, disintegration time, granule friability, granulation flowability and the drying rate kinetics of wet granulation.

b) Surface area: Granulation properties mainly dependent on the size and surface area of particles and granules. It is fairly common practice to determine the surface area of finely drug powder. Particle size or availability surface area of the drug can have a significant effect on the dissolution rate.

c) Density: Granule density may influence compressibility, tablet porosity, dissolution, and other properties. Dense particles are generally less cohesive than less dense particles of the same size and shape.

d) Strength and Friability: The strength of a wet granule is due mainly to the surface tension of liquid and capillary forces. These forces are responsible for initial agglomeration of the wet powder.

e) Flow Properties: There are many types of forces which can affect the flow properties of a solid such as Frictional forces, Surface tension forces, Machanical forces, Electrostatic forces & Cohesive forces or van der waals forces.

f) Compressibility: The percentage compressibility is commonly known as Carr's index. This index is interpreted in the following way: The higher the compressibility, the poorer the flowability.

Scale of flowability:

Compressibility index (%)	Flow character	Hausner ratio
1-10	Excellent	1.00-1.11
11—15	Good	1.12—1.18
16—20	Fair	1.19—1.25
21—25	Passable	1.26—1.34
26—31	Poor	1.35—1.45
32—37	Very Poor	1.46—1.59
>38	Very Very Poor	>1.60

Methods for measuring Compressibility:

Specific volume is determined by pouring a know mass of blend into a graduated cylinder. The volume is read off the cylinder and the specific volume is calculated by dividing the volume by the mass of the blend and get untapped density. Then the granulated cylinder is vibrated on a shaker of a time period. This vibration reduces the volume that the blend occupies in the graduated cylinder. The percentage compressibility is calculated by the following equation:

% Compresdibility = {100 × (P-A)}/P

where,

P = Packed density

A = Untapped density

g) Moisture Content: Control of moisture content in granulation is very important and it could affect the physucal and chemical performance of final dosage forms. Moisture could affect the flow of granules, tablet compression, tablet DT, crystal habit, chemical stability and so on.

h) Granules porosity: Increase in intragranular porosity,  increases the propensity of the granules to fragment leading to formation of stronger tablets. For granulations, which are less prone to fragmentation, increased intragranular porosity increased the degree of deformation, resulting in formation of a closer intragranular pore structure during compression and stronger tablets.