granulation Flashcards
what is granulation?
- granulation is the process of size enlargement of powders
- powder particles adhere to form larger particles
- used for production of tablets/capsules, or may be used as a dosage form themselves
what are some of the reasons for granulation?
to prevent segregation in the powder mix improve flow properties improve compressibility densification of powder reduction of dust hazard increase wettability of hydrophobic drugs
why do you prevent segregation in the powder mix?
- increases uniformity of active ingredient and excipients
- powders segregate due to differences in size & density
- ideally, a granule will contain all the constituents of the mix
- don’t want segregation of the granules themselves –control size distribution
why do you want to improbe a powders flow properties?
- enhance flow rate and uniformity•many powders don’t flow very well e.g. fine particles are cohesive/adhesive
- poor flow could lead to poor capsule/tablet weight uniformity i.e. non-uniform metering of volumetric dispensing
- granules are larger and often more uniform in shape
- slightly hygroscopic materials may absorb moisture, however the granule flow properties may be unaffected due to size (powders may adhere with moisture and form cake)
why do you want to improve compressibility?
- some powders difficult to compress
* granules often easier to deform/fragment & produce stronger tablets
why do you want to densify powder?
- granules are denser than parent powder
* occupy less volume per unit weight
why is it beneficial to reduce hazard due to grnaulation?
fine powders are dust hazard – toxic and explosive
why do you want to increase wettability of hphobic drugs?
• mixing drug with hydrophilic excipients
what are the requirements for granules used in tab manufacture and capsule filing?
similar shape & colour
• narrow particle size distribution
• not more than 10 %w/w particle fines (i.e. below 100 μm)
• good flow
• appropriate mechanical strength
• moisture content 3-5 %w/w
• disintegration into initial powder particles
what are the two types of granulation?
wet and dry granulation
what is wet granulation?
- mixture of powder and binder liquid
- formation of granules
- drying
- size classification (sieving)
what is dry gransulation?
- formation of slugs (on heavy duty tablet machine) or compacts (roller compaction) from dry powder
- coarse grinding
- size classification (sieving)
what is the mechanism of granule formation?-dry granulation
or drygranulation, adhesion of particles due to applied pressure. Short range van der Waals forces will dominate. Large granules then milled/sieved
what is the mechanism of granule formation for wet granulation?
in wetgranulation, liquid is mixed into dry powder. Granulation starts with:
•nucleation– small number of particles combine to form pendularstate, which builds into capillary state with more liquid
•then transition growth, where either single particles are added to the nuclei by pendular bridges or nuclei combine. This represents a suitable end point for granules for tablets/capsules – not too large
•if continued, get ball growth, giving large spherical granules increasing size with time. Too large
what are the possible mechanisms of ball growth?
- coalescence. Two or more granules join to form a larger granule
- breakage. Granules break into fragments which adhere to other granules, forming a layer of material over the surviving granule
- layering. A powder mix is added to a bed of granules, the powder forms a layer increasing the granule size
- Abrasion Transfer. Agitation of the granule bed leads to the attrition of material from granules. This abraded material adheres to other granules
what are mechanisms of wet granule formation?
- Hardening binders.When adhesive is included in the granulating solvent it forms liquid bridges. Adhesive hardens or crystallizes on drying to form solid bridges to bind the particles.
- Crystallization of dissolved substances.The solvent may partially dissolve one of the powdered ingredients. When the granules are dried, crystallization of this material acts as a hardening binder.
give an example of a binder liquid and resultant granules/ tabs?
starach/ pregel starch/ gelatine
the type of binder affects the granules properties
what are methods of wet granulation?
wet massing –low shear granulation
• powder mixed with binder solution in planetary mixer
what occurs in wet massing- low shear granulation?
moist mass then transferred to a granulator e.g. oscillating granulator
•mass forced through a sieve screen. Mesh size determines granule size
•moist granules collected on tray and transferred to tray drying oven
what are the disadvantages of tray dryer?
- long drying time
- solute migration may occur due to convective drying at surface
- aggregation of granules due to solid bridge formation
what is a high shear mixer granulation?
•stainless steel mixing bowl with three-bladed impeller revolving in horizontal plane. Also, a chopper is included that revolves in horizontal plane (sometimes vertical plane) in upper regions of granulating mass
what are the properties of a high shear mixer granulation?
- dry powder mixed by impeller• granulating fluid added and mixed in by impeller
- chopper switched on when moist mass formed –produces bed of fine, granular material
- granules discharged through a sieve into a bowl for fluid bed drying
what are the advantages of a high shear mixer granulation?
• advantage – mixing, massing & granulation all done rapidly in same equipment.
what are fluidized bed granulators? and how do they work?
- similar principle to fluidized bed dryer
- air blown through powder bed to fluidize and mix
- filter prevents escape of fine particles
- binder solution pumped through spray nozzle over particles
- temperature of air then increased
