Compression Physics

Question 1

The process of compression entails?

Answer 1

The process of compression entails a reduction in the bulk volume hence increase in the density of a material as a result of displacement of the gaseous phase due to an application of pressure.

Question 2

What does compression wrt to tableting involve?

Answer 2

It involves the filling of an appropriate volume of drug particles/granules into a die cavity so that it is compressed between the upper & lower punch into a consolidated single solid matrix and its subsequent ejection from the die cavity as an intact tablet.

Question 3

Tablet compression is characterized by?

Answer 3

 Transmission of forces during compression
 Distribution of forces in tablets
 The effect of applied pressure on the relative volume of the powder
 Inter-particulate adhesion and cohesion forces
 Tablet compression energies
 Mechanical strength of tablets

Question 4

What are the objectives of compression?

Answer 4

Formation of tablets that do not stick to the punch surfaces and the die wall hence prevention of capping.
Formation of tablets with acceptable mechanical properties in terms of hardness and friability.
Formation of tablets that comply with the pharmacopoeia limits for tablet weight variation and content uniformity.
Formation of tablets with appropriate disintegration times and dissolution rates.

Question 5

What is compression pressure?

Answer 5

Compression pressure is an external axial force which occurs when a physical force is pressed inward on the solid surface of an object, deforming it either permanently or temporarily, depending on the material and the
magnitude of the force.

Question 6

The extent, degree and the direction of the deformation from compression pressure depends on?

Answer 6

The extent, degree and the direction of the deformation depend on such factors as

1. The properties of the
   APIs and excipients
2. The type and concentration of ingredients in the formulation
3. Particle size
4. Particle shape
5. Molecular weight
6. Solubility
7. Formulation processes 8. Wetting properties
8. Polymer properties
9. Type of binding force that exist during consolidation, etc.

Question 7

Why is the determination of compression pressure is often empirical?

Answer 7

The United States Pharmacopeia and the European Pharmacopeia did not specify a limit of compression pressure required to produce a cohesive compact, and therefore its determination is often empirical depending on the critical quality attributes of the material to be compressed.

Question 8

The application of Compaction pressure on the drug particles has what effect?

Answer 8

Application of Compaction pressure on the drug particles rearrange the particles into closer packing which leads to initial volume reduction.

Question 9

What is consolidation?

Answer 9

Consolidation is an increase of a material density following particle-particle interaction.

Question 10

The physics of compaction may be simply stated as?

Answer 10

Thus the physics of compaction may be simply stated as the compression & consolidation of two phase (particulate solid & gas) systems due to the applied force.

Question 11

Compression cycle is divided into following 4 phases

Answer 11

Pre-compression
Main compression
Decompression
Ejection

Question 12

What is involved in precompression?

Answer 12

 It is the stage where small force is applied to the powder bed in order to form partial compacts before undergoing the main compression.

 It is characterized by transitional packing of the particles/granules (bulk density due to particle re-
arrangement).

 The pre-compression force is usually kept smaller than the main compression force.

 However, the force required is dependent on the material type as powders which are prone to brittle fracture would require higher pre-compression force than main compression, this leads to higher tablet hardness.

 However, powders with elastic property require gradual application of force to reduce elastic recovery and allow stress relaxation.

 Optimal tablet formation could be achieved when the pre-compression and compression forces are similar.

Question 13

What is involved in main compression?

Answer 13

During main compression, the applied compression force to the granule/powder bed results in particle arrangement that no further filling of the void exits.

It results in the formation of inter-particulate bonds.

 A further increase of compression force causes deformation at the point of contact such that particle fragmentation and/or deformation occur.

Fragmentation cause further densification with the infiltration of the smaller fragments into
the void space.

With some materials fragmentation doesn’t occur because the stresses are released by plastic
deformation.

Plastic deformation is related to a change in particle shape which occurs as an
attempt to release the stress (visco-elastic flow).

These deformation produce new clean surfaces that are potential areas for further bonding.

Question 14

What is deformation?

Answer 14

Deformation is the change of geometry of a solid when it is subjected to opposing forces.

Question 15

The amount of
deformation is called?

Answer 15

Strain

Question 16

Particle deformation occurs in many ways including?

Answer 16

 Elastic deformation – Elastic substances exhibit reversible process, i.e., particles when deformed resume their original
shape instantaneously.

 Plastic deformation – Plastic substances exhibit irreversible process, i.e., deformation of particles results in a permanent
change of the particles shape. The force required to initiate a plastic deformation is known as yield stress.

 Viscoelastic deformation – A viscoelastic material will return to its original shape after any deforming force has been removed. However, it will take time to do so.

 Brittle fracture – Brittle particles generally increase more contact points for bonding because of the fragmentation of the original particles into smaller units.

Question 17

The type of deformation depends on?

Answer 17

The physical properties of the material, as well as, on the rate and magnitude of the applied force and the duration of locally induced stress.

Question 18

What is involved in decompression?

Answer 18

Decompression stage occurs when the applied compression force is removed.

This leads a new set of stresses within the tablets as a result of elastic recovery which is
increased by the forces (stresses) necessary to eject the tablet from the die.

If these stresses are not sustained by the tablet, it can lead to structure failure (tablet capping or lamination) when the tablets show high rate and degree of elastic recovery.

Question 19

When does fracturing of surfaces occur?

Answer 19

Fracturing of surfaces occur when the tablet undergoes brittle fracture during decompression.

Question 20

Plastic deformation is another way to relieve stress and is time independent. T or F?

Answer 20

False

Plastic deformation is another way to relieve stress and is time dependent.

Question 21

The rate of
decompression also affects the tablet structure failure. T or F

Answer 21

True

Question 22

The use of a plastically deforming agent such as ____, ____ is recommended.

Answer 22

PVP, MCC

Question 23

What is involved in ejection?

Answer 23

Ejection is the last stage of compression cycle which involves breaking of the adhesions between die wall and compact surface.

It occurs when the lower punch rises & pushes the tablet upward due to the continued residual die wall friction.

When the tablet is ejected from the die wall, it causes friction due to shear between the tablet and the die wall further resulting in heat generation.

Absorption of this heat further leads to bond formation.

As the tablet is removed from the die the lateral pressure is relived & the tablet undergoes elastic recovery with an increased (2-10%) in the volume of that portion of the tablet removed from the die.

Question 24

The success or failure to produce an intact tablet depends on?

Answer 24

1. The stresses induced by elastic rebound.
2. The associated deformation processes during decompression.
3. Ejection.

Question 25

Why must any dimensional change during decompression must occur in the axial direction?

Answer 25

As the upper punch is withdrawn from the die cavity, the tablet is confined in the die by a radial pressure. Consequently, any dimensional change during decompression must occur in the axial direction

Question 26

What is the importance of lubrication during ejection?

Answer 26

Lubrication generally reduces ejection forces thereby reducing the possibility of capping or laminating.

Question 27

Powders of smaller particle size require lower ejection forces to remove the tablets. T or F?

Answer 27

False. Powders of smaller particle size require higher ejection forces to remove the tablets.

Question 28

Explain the compaction process

Answer 28

 Compaction is the process of alteration of powders into a coherent specimen or compacting powder or granules.  During compaction, powder particles are closely packed because of particle rearrangement, which leads to initial volume reduction.  The particle rearrangement becomes difficult with increasing compaction force and causes particle deformation, and fragmentation.  Bonds are established between the particles as they move into closer proximity to each other during the volume reduction process.  These particle-particle interactions increase the mechanical strength of the powder bed and help in sustaining the rising compression forces leading to consolidation.  Deformation and fracture/plastic flow occur which results into fusion of the drug particles hence tablet formation.

Question 29

List the events that occur in the process of compression

Answer 29

Transitional repacking. Deformation at point of contact. Fragmentation &/or deformation. Bonding. Decompression. Ejection.

Question 30

Particle rearrangement ceases after a certain point of increasing compression force. The volume reduction at this stage is governed by?

Answer 30

Plastic and elastic deformation and/or fragmentation of the particles.

Question 31

What powder characteristics affect the possible inter- or intra-particulate bonds among the particles?

Answer 31

Particle size, particle size distribution, density, surface properties, and inter-particulate voids

Question 32

What process variables affect the possible inter- or intra-particulate bonds among the particles?

Answer 32

Moisture content, flow rate, particle diameter, and die-cavity diameter

Question 33

Why is compression pressure an important parameter in tablet formulation system?

Answer 33

Compression pressure is an important parameter in tablet formulation system as it influences tablet properties such as density, porosity, surface area, hardness, friability, disintegration and dissolution, etc.

Question 34

Density decreases with the increase in compression force, while porosity increases. T or F?

Answer 34

False Density increases with the increase in compression force, while porosity decreases.

Question 35

Hardness increases with the increase in compression force, while friability decreases.

Answer 35

True

Question 36

A linear relationship exists between disintegration time and compression pressure. T or F?

Answer 36

False. An exponential relationship exists between disintegration time and compression pressure.

Question 37

The disintegration time increases with the increase in pressure. T or F?

Answer 37

True

Question 38

The effect on dissolution is dependent on the tablet type which could be?

Answer 38

Immediate release and controlled release.

Question 39

However, the effect of compression pressure on tablets is complemented by ____ and ____ , which in turn may affect ____ and ____.

Answer 39

the material properties and the formulation factors, mechanical strength and release properties

Question 40

Why is the selection of the optimal compression conditions for a given tablet formulation system?

Answer 40

The selection of the optimal compression conditions for a given tablet formulation system can avoid the potential tableting problems thus saving significant time and resources.