Relaxation tests for the time dependent behavior of pharmaceutical tablets: A revised interpretation

Area of Science:

• Pharmaceutical Science
• Materials Science
• Rheology

Background:

• Relaxation tests are crucial for evaluating pharmaceutical powder and tablet strain rate sensitivity.
• Interpreting these tests is challenging due to the simultaneous occurrence of viscoelasticity and viscoplasticity.
• Axial pressure alone cannot differentiate between these two phenomena as it decreases in both cases.

Purpose of the Study:

• To demonstrate how monitoring die-wall pressure during relaxation tests can differentiate viscoelastic and viscoplastic behaviors.
• To investigate the influence of compaction pressure on the dominance of these phenomena.

Main Methods:

• Theoretical analysis of pressure evolution during relaxation.
• Experimental validation using specially designed compaction cycles.
• Testing on four different pharmaceutical excipients.

Main Results:

• Die-wall pressure decreases during viscoplasticity and increases during viscoelastic relaxation.
• Viscoplasticity predominates at low compaction pressures.
• Viscoelasticity becomes more prominent at high compaction pressures.

Conclusions:

• Monitoring die-wall pressure is a viable method to separate viscoelastic and viscoplastic effects in pharmaceutical relaxation tests.
• Low pressures are suitable for assessing viscoplastic properties.
• High pressures should be avoided to prevent misinterpretation due to significant viscoelastic contributions.