Is "Yield Stress" As Important As "Viscosity?"

By Robert McGregor, Global Marketing and High End Product Manager, Brookfield Engineering Laboratories, Inc.

“Yield Stress” could be a term that applies to our daily lives. When under pressure, if the stress is too great, you start to “change” in a figurative sense. In the world of science and engineering, fluids and semi-solid materials that are subjected to a strong-enough force will start to move. Basically they “change” from their original shape and move in response to the applied force, remaining in motion as long as the force is great enough.

Materials in motion can be measured for resistance to flow. “Viscosity” is the parameter of interest. Quality Control departments throughout the pharmaceutical industry use viscometers to evaluate test samples and verify that the measured viscosity falls between established limits. This measured value can actually relate to how the material is processed in manufacturing or to what the end user may sense when the material is used.

The pertinent question is how “Viscosity” relates to “Yield Stress”? Should R&D, Manufacturing, and Quality Control all be concerned about Yield Stress? R&D’s job is to characterize the flow properties of new formulations. This includes measuring physical properties like viscosity and yield stress. QC test methods handed down from R&D most often include a viscosity test, but seldom one for yield stress. This is not so much an oversight as a pragmatic decision to limit the amount of work required by QC to certify product acceptability.