Pharmaceutics: Pellets and Extrusion Flashcards
What is the difference between a pellet and granule?
granules are agglomerates of powder particles whilst pellets are agglomerates of granules.
Similarities between pellets and granules?
- can both contain multiple drugs and excipients
- both coated individually for modified release
- commonly filled into capsules or pressed into tablets
- smaller than tablets and capsules so can pass through stomach quicker
- don’t disintegrate
- better able to pass through pylorus intact
- can manufacture incompatible ingredients separately then incorporate into a single dosage form.
advantages of using pellets over granules?
pellets have
- a greater density
- better flow properties
- lower friability
- narrower particle size distribution
- more uniform packing
- smoother surface for more uniform coating
Why do pellets have many advantages over granules?
- they have a smoother, more spherical morphology
2. because of compression and densification during pellet formation
How are pellets formed?
by extrusion-spheronisation
What are the steps in extrusion-spheronisation?
- agglomeration
- extrusion
- pelletisation
- speronisation
sometimes 3 is spheronisation and 4 is drying.
what is extrusion?
forcing semi-solid materials through apertures in a screen shaping them into cylindrical/rod-shaped products (extrudate) of uniform diameter.
What are the raw materials used in extrusion?
- wet mass
- polymer melt
How can you vary the shape an size of the extrudate?
change the shape and size of the apperture in the screen which the raw material passes through.
Types of extruders?
- screw
- sieve
- basket
- roll
- ram
How does the screw extruder work?
moves material through a cylinder
How does axial screw extrusion work in the screw extruder?
the raw material is passed through the cylinder and extruded at the end of the cylinder where the screen is.
How does radial screw extrusion work in the screw extruder?
raw material passes through cylinder and is extruded through the screen that is in the cylinder walls
How does a sieve and basket extruder work?
the material is pushed through screen by rotating/oscillating device e.g. a blade
what is the difference between sieve extruder and basket extruder?
- for sieve extruder, the screen is at the bottom of the chamber
- for basket extruder, the screen is in the chamber wall.
How does a roll extruder work?
the materials pass through the screen using rollers and extrudate collected within the rollers or outwith (outside) the rollers
How does a ram extruder work?
material is compressed and forced through the screen by a moving pistion.
What are the extrusion parameters to consider?
- the starting material properties
- the extruder design
- extruder speed
- extruder temperature.
What do you look at when considering the properties of the starting material?
the moisture content of the starting material and its cohesiveness
what parameters do you look at when considering the extruder design?
the screen geometries and dimensions.
Why should the extruder speed not be too high?
high speeds can result in shark-skinning (rough edges on the extrudate)-see slide 17 for image.
why must you control the extruder speed?
coz temp can affect the flow behaviour and stability of thermoliable formulations.
What materials are required for spheronisation?
granules and crude pellets
what equipment is used during spheronisation?
a spheroniser
explain the process of spheronisation!
- granules and crude pellets are placed on a rotating friction plate
- they’re broken down into smaller particles as the friction plate rotates
- then they’re made spherical through collision with the friction plate, spheroniser wall and each other
What is the product at the end of the spheronisation process?
spherical pellets
what causes the rope movement of the spheroniser?
- particles are thrown outwards by centrifugal force
2. particles rebound inwards upon impact with spheroniser wall
three mechanisms of spheronisation?
- plastic deformation
- dumb-bell break-up
- attrition-agglomeration
pellet properties that are important for performance are…?
- particle size
- particle shape
- surface morphology
- friability
- tensile strength
- density
- porosity
- disintegration time
- dissolution time
What are two types of friction plate patterns used in a spheroniser and which is preferred and why?
- cross-hatch and radial friction plate patters.
- the cross-hatch pattern is preferred although the radial pattern is gentler.
- This is because the radial pattern is unsuitable for large disc diameters.
Parameters to consider when using a spheroniser.
- friction plate rotational speed
- charge volume or mass
- retention time
Why do we need to consider the friction plate rotational speed of a spheroniser?
as smaller friction plates would require a higher speed and the speed at the outer diameter is very important.
Why do we need to consider the Charge volume or mass of a spheroniser?
- overloading the plate may affect particle interactions with plate and wall.
- underloading may affect particle-particle interactions.
Why do we need to consider the retention time of a spheroniser?
normal retention time is between 3-8 minutes.
- a longer retention time may increase sphericity and produce a narrower particle size distribution but overall larger particle sizes.
