Lecture 4: powders and granules 1 Flashcards
What are the overall dimensions of powders?
Less than 1000 µm
What are the overall dimensions of granules?
More than 1000 µm
What are the different usage of granules and their respective sizes?
- Final dosage form (in capsules) - 1-4mm
- Granules as intermediates (make into powder and then into tablets) - <1mm
What is granulation?
The process by which dry powder particles are process to make them adhere to form larger multiparticulate entities called granules.
What are the types of granulation?
Wet & dry granulation
What are the properteies we consider when formulating with powders?
- Particle size distribution
- Particle shape
- Density
- Flowability
- Compaction properties
What is the particle size range for most APIs?
0-50μm
What is the range of particle sizes in the direct compression zone, which is needed to make a tablet?
80-160 μm
How do you get the bulk volume of a powder?
Pour the powder very gently, so that it naturally flows in, and you dont try to push the powder down, so it will be spaced apart
This is the bulk volume - no external consolidation
How do you get the tapped volume of a powder?
Pour gently and tap out the air bubbles so the powder is compact as much as possible (until no further reduction in volume)
This is the tapped volume
How do you calculate the bulk density of a powder?
Bulk density = weight / V0
V0 is the bulk volume
How do you calculate the tapped density of a powder?
Tapped density = weight / Vf
Vf is the tapped volume
Which is higher - tapped or bulk density for the same powder?
Tapped density because its volume is lower
What is the true density of a powder?
The density of a powder compact, which has 0% pore inside
This is theoretical because even if you compress the pwder into a tablet, you will have air pockets inside
What is the machine used to measure the true density of a powder?
Helium pycnometer
What is powder flow?
It represents the movement of solid particles due to force imbalance
(gravity is sometimes the most impt factor to consider for powder flow)
What are some ways that combat gravity to reduce powder flow?
- Adhesion thru molecular interactions
- Friction thru mechanical interlocking
- Electrostatic attractions
How does the force of gravity on powder particles change with sphere diameter?
As sphere diameter increases, so does the force of gravity.
How does the force of smooth adhesion on powder particles change with sphere diameter?
As sphere diameter increases, so does the force of smooth adhesion.
How does the force of rough adhesion on powder particles change with sphere diameter?
As sphere diameter increases, rough adhesion initially decreases because the size change allows for rearrangement of surface morphology and smooth out.
After that it begins to increase with increasing particle size.
What is the particle size at which you begin to see powder flow?
Beyond 10^-3 m
Because gravity > adhesion beyond this size
What are the different types of flow behaviours, and are they good or bad?
- Rathole: bad
- Arch: bad, worst one
- Funnel flow: not bad
- Mass flow: good
What is funnel flow?
First in last out flow sequence
What is mass flow?
First in first out flow sequence
What are some factors that influence powder flow?
- crystal structure
- particle size (gravity &adhesion)
- particle shape (mechanical interlocking)
- Moisture (adhesion, capillary force, electrostatic charge) structure
- Density (true & bulk) (gravity)
- Stress state (e.g., tapping a hopper)
How can crystal stucture affect powder flow?
Crystalline strtures have more ordered packing which could improve flow
Amorphous structures may lead to irregular packing and poorer flowH
How can particle size affect powder flow?
particle size <10^-3 m: adhesion > gravity, which reduces flow because they stick to each other
particle size >10^-3 m:
gravity> adhesion, which enhances flow because gravity overcomes adhesion
How can particle shape affect powder flow?
Irregular or angular particles can interlock, increasing cohesion and reducing flowability.
Spherical particles tend to flow better due to less mechanical interlocking.
How can moisture affect powder flow?
Moisture can increase adhesion through capillary forces, leading to clumping and reduced flow.
It can also induce electrostatic charges (ionisation?), further affecting particle behavior.
How can density affect powder flow?
UNSURE
Higher true density means that particles are heavier and can pack more closely together. This can help improve flow as it allows for more effective use of gravity.
A higher bulk density means that there is more material in a given volume, which can help overcome the forces that make particles stick together (adhesion). It helps the powder flow better by providing more weight that helps pull the particles apart.
How can stress state affect powder flow?
Applying stress (like tapping) can compact particles, reducing voids and improving flowability.
However, if stress leads to particle deformation or agglomeration, it may negatively impact flow.
Powder flow depends on both ____ and _______.
fill in the blanks
Powder properties
equipement design
How do you calculate the compressibility (carr) index?
carr index = 100 x ((v0-vf)/v0))
How do you calculate the Hausner Ratio?
Hausner ratio = v0/vf
How do you interpret compressibility (carr) index values?
carr index = 100 x ((v0-vf)/v0))
The higher v0 is than vf, the poorer it flows because its harder to get to the tapped volume
The lower the Carr index the better the flow.
How do you interpret the Hausner Ratio values?
Hausner ratio = v0/vf
The higher v0 is than vf, the poorer it flows because its harder to get to the tapped volume
The lower the Hausner ratio the better the flow.
What is the angle of repose and how is it measured?
Allow the powder to flow naturally onto a surface and measure the angle of the cone pile it forms –> angle of repose
How do you interpret angle of respose values?
The lower the angle of repose the better the flow of the powder because it spreads more easily onto the surface.
Why is angle of repose not the best method to measure powder flow?
- If you rpowder flows too poorly, its not going to move out of the hopper onto the surface for you to measure
- If the powder flows too well the angle of repose measured may not be accurate because the effct of gravity is significant –> the pile it forms is almost flat
- Hence angle of respose is only good for powders with moderate flow.
How does the flow through an orifice method work to measure powder flow?
- A container is filled with powder up to a regulated height
- An orifice at the bottom is opened, allowing the powder to flow out
- the orifice must be less than half of the diameter of the container - The rate of powder flow (g/s or mL/s) is recorded by the balance
- A better flowing powder will have a faster rate
- If you have a powder flow where the powder at the sides gets stuck and doesn’t flow, you may anticipate long term storage and degradation problems.
What are the disadvantages of using the flow through an orifice method to measure powder flow?
- If the powder has very poor flowability, it may not even flow out of the orifice, so you cant quantify the flow rate
- The flow rate depends on material properties like humidity, particle size and shape which have to be controlled
What is the minimum value of the Hausner ratio?
Hausner ratio = v0/vf
minimum value is 1
v0 cant be less than vf
How is unconfined yield strength (fc) measured, and how do you interpret it to understand powder flow?
- The powder is first confided in a box and a pre shear stress is applied on top to standardise the amount of initial stress on the powder
- The sides are removed and the powder remains consolidated
- The force on top is then increased until the powder breaks and starts to flow –> fc
- The greater the force required, the worse the powder flow because it takes more strength to get the powder to flow
How is major principle stress measured, and how do you interpret it to understand powder flow?
- The powder is first confided in a box and a pre shear stress is applied on top to standardise the amount of initial stress on the powder
- The force on top is then increased until the powder breaks and starts to flow
- This doesnt neccessarily say much about the flow
How is the flow function measured, and how do you interpret it to understand powder flow?
ffc = major principle stress / unconfined yeild strength
Since a higher unconfined yield strength indicates worse flow, a higher ffc indicates better flow.
In a graph of unconfined yield strength vs major principle stress, how do we find the ffc?
It is the gradient
the steeper the gradient the worse the flow.
How does particle size impact flow?
Usually, for larger particle size we see better flow because the surface area is minimized so the adhesion and cohesion forces are smaller
There are exceptions so you have to see the plot to understand.
How can we use crystal engineering to improve particle flow?
Use this to change particle size, surface morphology to make it smoother
How can we use particle engineering to improve particle flow?
coat the surface to make it smoother with eg: nanosilica
