(479c) Measurement of Unconfined Yield Strength In Small Size Cells



Pharmaceutical companies often create very small quantities of viable drug mixture on a laboratory scale.  Extensive analysis is conducted to tease out as much data as possible about the characteristics of the material.   Particle size, particle shape, surface area, and other particle scale properties are often the extent of the characterization.  While these preliminary properties can help with qualitative product design, they are not generally the primary material behavior measurements the formulator needs to effectively design the product.  This information is especially lacking when attempting to infer cohesive flow properties from particle scale flow properties.  For example, particle size and surface area are often measured to infer dissolution rates (a bulk scale property); it is much better to be able to measure the property that controls the behavior directly. It would be highly beneficial to have a means of measuring cohesive flow properties directly with very limited material.  This paper presents test results from a single axis strength measurement device capable of measuring unconfined yield strengths using a representative sample of 0.1 cc volume of material.  This new test technique compacts the material using centrifugal forces and induces failure stresses within the material through controlled application of centrifugal forces.  Direct measurement of unconfined yield strength has been accomplished at consolidation stress levels of 0.04 KPa, which is almost one order of magnitude smaller than in typical direct shear cell measurements.  This new test technique is compared with standard direct shear measurements using the Schulze, Johanson uni-axial, and Jenike methods.  The technique shows good agreement with existing measurement methods.  Because of the small volume and lower stress level measurements, this new technique can provide formulators direct measurement of bulk unconfined yield strength before typical process development times, thus shortening the time to market for viable drug mixtures.  Consequently, process design can be initiated in conjunction with formulation development.