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Purpose: Determine the strain rate sensitivity
affect due to increased loading rates on the resulting physical
properties of tablets.
Method: Experiments have been conducted on a
10-station rotary tablet press that show certain excipients change
their deformation characteristic from plastic to brittle as a
function of the loading rate. Punch tangential velocities were
varied from 100 to 800 mm/sec in ten increments while maintaining
the applied force constant. Tablets collected from each of the
ten loading rates were measured and broken to determine the tensile
strength of each compact. Based on the initial results additional
tests will be performed on six MCC based excipients as well as
two model API formulations. Future tests will be performed on
a tablet press that will extend the punch tangential velocities
up to 1400 mm/sec to determine if the trend from plastic to brittle
behavior will continue at the higher loading rates. Oscilloscope
traces were obtained at each loading rate and the areas before
and after peak were compared. The area ratios supported the tensile
strength results.
Results: Classic brittle deforming materials
exhibit no reduction in tablet strength as the loading rate is
increased. This was confirmed in the above experiments. By contrast,
plastically deforming materials suffer when loading rates are
increased. Preliminary results with at least one such excipient
show an initial loss in tablet strength with increased loading
rates, followed by a leveling off and a subsequent increase in
strength as the loading rates were increased still further.
Conclusion: Pharmaceutical materials display
clearly different properties as loading rates increase. At least
one material showed a "bathtub curve" as the loading
rates increased. The implementations are that there could be a
turret speed range for a particular tablet press that will not
produce tablets with sufficient hardness, while turret speed higher
or lower will.
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