Video Summary - Fundamentals of Pharmaceutical Processes Compaction

Main Ideas and Concepts (What the Lecture Teaches About Compaction)

Compaction / Compression: Definition

Compaction is the change in a material’s particle arrangement when pressure/stress is applied.
During compaction, the interparticle (void) space decreases or changes.
The resulting behavior depends strongly on the material type and nature.

Where Compaction Occurs in Pharmaceutics

Compaction is used during:

Tableting (forming tablets)
Granule formation
Direct compression
Milling / particle processing
Capsule filling
- Involves bulk powder packing
- Includes volume reduction via repacking and deformation

Material-Dependent Porosity Behavior

Rigid materials (e.g., sodium carbonate)
- Compaction can increase porosity (more void space than would occur with tapping).
- Such powders are dilatant: they expand/fluff rather than contract when stressed.
Soft / spongy materials (e.g., kaolin)
- Compression causes deformation that reduces porosity.
- Final porosity is less than what would happen from tapping.

How Compaction Changes Tablet/Granule Surface Area

As compaction force increases:

Specific surface area increases to a maximum
- Noted up to around 0.9 at a force near 3
Then specific surface area decreases at higher forces

Explanation

Initial increase: new surfaces are created as particles fragment.
Later decrease: cold bonding increases interparticle bonding, reducing void space and thus reducing surface area.

Interfacial Phenomena Driving Compaction Outcomes

At the air–solid interface, forces differ from the bulk and affect particle interaction:

Free surface energy
- Associated with small-distance attractive forces
Adhesive forces
- Between unlike particles
Cohesive forces
- Between like particles

Adsorption (gas/liquid adherence to surfaces)

Physisorption: via van der Waals forces
Chemisorption: via covalent/ionic bonding

Powder Volume, Porosity, and Density

Types of Void Spaces and Volumes

Powders contain different void spaces, creating different volumes:

Open intra-particulate voids
- Within a particle; open to the outside
Closed intra-particulate voids
- Within a particle; closed to the outside
Interparticle voids
- Between particles; can trap air/moisture

Volume definitions

Bulk volume (Vb): total powder volume including all voids (open/closed intra- + interparticle)
True volume (Vt): volume of solid material only (excluding void spaces larger than molecular dimensions)
Granular volume (Vg): solid volume including intra-particulate voids, excluding interparticle voids

Porosity

Porosity is the ratio of void space volume to bulk volume
Void space volume = Vb − Vt
Porosity can be expressed as a ratio or percentage porosity

Densities

Bulk density = mass / bulk volume (includes inter- and intra-voids)
Granular density = mass / granular volume (includes intra-voids)
True density = mass / true volume (excludes all void spaces)
Tap density = mass / volume after tapping (used to estimate packing/tightness)

How Compaction Links to Flow Properties

Powder flow depends on interparticle/interfacial forces and material attributes:

Frictional forces (particle rubbing)
Surface tension forces (between unlike materials)
Mechanical interlocking (from particle shape)
Electrostatic forces (minor charges; can cause clumping/aggregation)
Cohesive / van der Waals forces

Additional factors include:

Particle size and especially distribution
Shape
- Globular → smoother flow
- Irregular → interlocking → slower flow
Charge interactions
- e.g., anionic vs cationic can aggregate
Adsorbed moisture
- makes powders sticky → worse flow

Particle Size Trend for Cohesiveness / Flow

> 100 μm
- “Non-cohesive”, permeable, fluidizable
- Low compressibility, lower shear strength
< 10 μm (“fines”)
- More cohesive, more compressible
- Less fluidizable

Flow Characterization: Carr’s Index and Hausner Ratio

Carr’s Index (Compressibility Index)

[ \text{Carr’s index} = \left(\frac{\text{tap density} - \text{bulk density}}{\text{tap density}}\right)\times 100 ]

Hausner Ratio

[ \text{Hausner ratio} = \frac{\text{tap density}}{\text{bulk density}} ]

Carr’s Index Flowability Categories

5–15: excellent flow
12–16: good flow (overlaps noted in provided ranges)
18–21: fair / passable
23–35: poor
33–38: very poor
>40: very very poor

If Flow Is Poor (Interventions)

Drying
Add non-hygroscopic / flow-improving materials
- Examples: talc, stearates
Use spray drying or freeze drying
- to make fine, globular particles
After processing, recheck flow properties to confirm improvement

Angle of Repose and Flow Relationship

Meaning of Angle of Repose (θ)

Angle of repose (θ) represents resistance to particle movement.

Formula

[ \tan(\theta) = \frac{2h}{d} ]

Measurement

Allow powder to flow from a hopper/flask until a heap forms
Measure:
- heap height = h
- heap diameter = d
Compute θ and infer flow quality

Noted Correlation

θ > 40°: very poor flow
θ up to 25°: excellent flow
For good flow, Carr’s index ~ 10
Therefore, θ and Carr’s index correlate, reflecting compaction/compressibility tendencies

Deformation and Strain During Compaction

Deformation: finite change in geometry from opposing forces
Strain: relative amount of deformation (dimensionless)

Deformation / compression outcomes

Elastic deformation: reversible
Plastic deformation: irreversible (remains deformed)
Brittle fracture: breaks particles into fines, which can fill void spaces

Compaction Differences: Tableting vs Capsule Filling

Hard Gelatin Capsule Filling

Typically uses lower force
Involves more repacking and deformation
Less brittle fracture compared to tablets

Tablet Processes (e.g., Roll Compaction / Extrusion)

Typically involve higher compression force
Include:
- repacking
- elastic/plastic deformation
- brittle fracture

Bonding Mechanisms: Cold Bonding vs Fusion Bonding

Cold Bonding

Occurs when particle surfaces approach within < ~50 nanometers
Free surface energy enables direct bonding

Fusion Bonding

If processing causes local temperature rise
Contact regions may melt, producing stronger bonds

Effects

Cold/fusion bonding can increase mechanical strength and hardness
As bonding strengthens the structure, voids decrease
Result: tablets become harder to break

Formulation Performance Implications (Especially Immediate-Release)

For immediate-release (and dispersible) tablets, manufacturers generally avoid:

Brittle fracture (and excessive hardening)
Fusion bonding

Reason

These can make tablets too hard, preventing adequate disintegration.

Downstream examples

Immediate release is expected to disintegrate within ~5 minutes
- overly hard tablets fail
Dispersible tablets expected to disperse within 1–2 minutes
- overly hard tablets fail

Overall Conclusion

Compaction is a stress-responsive phenomenon occurring during processing stages such as:

hopper flow
compression
transportation, etc.

Transportation can also contribute via:

friction / mechanical stress
sometimes fusion bonding if temperature is not controlled

Unifying theme: the interplay of interparticle forces determines:

packing
deformation/bonding
hardness
flow
final product quality

Methodologies / Instruction-Like Procedures

A) Measure Powder Flow Using Angle of Repose

Add powder to a hopper/flask and allow it to flow to form a heap.
Measure:
- heap height = h
- heap diameter = d
Compute:
- (\tan(\theta) = 2h/d)
Interpret:
- θ > 40° → very poor flow
- θ up to 25° → excellent flow
Use correlation to infer likely Carr’s index behavior from θ.

B) Calculate Flowability Using Carr’s Index and Hausner Ratio

Determine:
- tap density
- bulk density
Compute Carr’s index:
- (\text{Carr} = ((\text{tap density} - \text{bulk density})/\text{tap density})\times 100)
Compute Hausner ratio:
- (\text{Hausner ratio} = \text{tap density}/\text{bulk density})
Interpret Carr’s index categories:
- 5–15 excellent
- 12–16 good
- 18–21 fair/passable
- 23–35 poor
- 33–38 very poor
- >40 very very poor

C) Improve Poor Powder Flow

If Carr’s index / Hausner / θ indicate poor flow, apply one or more:

Drying
Add non-hygroscopic excipients (e.g., talc, stearates)
Use spray drying or freeze drying to create fine globular particles
After processing, recheck flow properties and verify improvement toward target ranges

D) Manage Compaction Effects to Meet Tablet Performance Goals

To avoid excessive bonding/hardening (especially for immediate-release/dispersible tablets), consider:

particle size and shape
applied stress/force level and rate
interparticle force behavior (adhesive/cohesive/adsorption/electrostatic/frictional)
likelihood of brittle fracture or fusion bonding
re-evaluate downstream properties (e.g., disintegration/dispersibility targets):
- immediate release ~5 min
- dispersible ~1–2 min

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Fundamentals of Pharmaceutical Processes Compaction

Key takeaways