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Pharmaceutics > Powder Flow > Flashcards

Flashcards in Powder Flow Deck (35)
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1

Classification of powders is based on...

particle size e.g. coarse, fine, very fine

2

Powders are..

a complex fluid comprising a mixture of solid particles and air.

3

What is the flowability of a powder?

How well a powder flows.

4

How do powders flow?

A tug of war between driving forces that promote flow and resistive that inhibit in.

Driving force needs to exceed resistive force.

5

Examples of driving forces (2)

Gravity

mechanical agitation

6

Examples of resistive forces (2)

cohesion/adhesion

friction

7

What is the difference between adhesion and cohesion?

Both are attractive forces between two forces.

Adhesion is when 2 chemically different surfaces

Cohesion is when 2 chemically similar surfaces

8

Gravitational force equation

W = m x g


m - mass of particle
p - particle acceleration

9

Van der Waals forces equation, FVdw =

A x R1 x R2 / 6d2 (R1+R2)

A = Hamaker constant
d = surface-to-surface distance between particles
R1 = radius of one particle
R2 = radius of other

10

Electrostatic forces equation: Fe =

Fe = k Q1 x Q2 / d2

k - constant
Q1 - charge on one particle
Q2 - charge on other particle
d2 = distance between particles

11

What does mechanical interlocking do?

physically obstructs movement of other particle.

12

How does friction inhibit flow?

Acts in opposite direction of the movement, inhibiting particle movement.

13

Friction equation =

Ff = uFr

u = coefficient of friction
Fr = reaction force

14

Condition that favours powder flow:

large
spherical
small surface area
high density

15

Specific surface area equation

total surface area / mass

16

The larger the specific surface area...

more bonding
more cohesive
greater resistive forces

POORER FLOWABILITY

17

Do spherical particles flow well?

yes because has the smallest specific surface area than any other shape.

18

Packing geometry is...

the position of solid particles relative to one another.

19

Porosity is..

volume fraction of air

1 - k

20

Packing fraction is..

the volume fraction of solid particles (k)

21

What is bulk density?
Equation

Density of solid particles when the powder is allowed to settle naturally

mass / bulk volume

22

What is the tapped density?
Equation

density of solid particles with minimal amount of entrapped air

mass / tapped volume

23

True density equation

Bulk density / packing fraction

24

How does density (true density) affect flowability?

greater mass at same volume
greater gravitational force

lower specific surface area
smaller resistive forces
less cohesive

better flowability

25

How does moisture content affect flowability?

too much will cause them to stick

moderate provides balance between lubrication and cohesion.

26

What's mass flow vs funnel flow?

The way the powder exists a hopper. Mass flow is an example of powder with good flowability while a funnel flow is not.

27

Flow rate equation

Amount of powder discharged / discharge duration

28

2 methods of determining flow rate

Method 1
measure amount of powder discharged over a fixed duration

Method 2
measure time taken to discharge a fixed amount of powder

29

Flow rate influenced by: (3)

- hopper shape
- office shape
-orfice diameter

30

Another way of measuring flowability: (3)

measure bulk volume in a graduated cylinder

consolidate powder on a tapped volume tester to a standardised set of parameters

measure tapped volume in the graduated cylinder

compressibility index and harsher ratio