A larger SI indicates a greater likelihood that sticking will occur.
Defining SI for Assessing Adhesion of Powder to the Punch
One cause of sticking is that when a powder is being compacted, the adhesive force between powder particles of the tablet and the punch surface exceeds the adhesive forces of powder particles within the tablet. Φp represents the frictional force acting between particles in the powder bed, and Φw represents the frictional force between the powder and the punch surface. The larger these values, the higher the friction and adhesion of the powder. We defined SI, which represents the degree of adhesion of a powder to the punch surface, as the value obtained by dividing Φw by Φp according to the following formula.
Sticking is a failure of pharmaceutical production that occurs when a powder containing a large amount of adhesive is being tableted. This is most frequently observed when long-term tableting is carried out, making it extremely difficult to predict its occurrence during the tablet formula design stage. The efficiency of the pharmaceutical production process could be improved if it were possible to predict whether a particular formulation was likely to stick during tableting. To address this issue, in the present study we prepared tablets composed of blended ibuprofen (Ibu), a highly adhesive drug, and measured the degree of adherence of powder particles to the surface of the tablet punch. We also measured the shear stress of the powder to determine the practical angle of internal friction (Φp) of the powder bed as well as the angle of wall friction (Φw) relative to the punch surface. These values were used to define a sticking index (SI), which showed a high correlation with the amount of Ibu that adhered to the punch during tableting; sticking occurred at SI >0.3. When the amount of lubricant added to the formulation was changed to yield tablets exhibiting different SI values without changing the compounding ratio, sticking did not occur at SI ≤0.3. These results suggest that determining the SI of a pharmaceutical powder before tableting allows prediction of the likelihood of sticking during tableting.
Predicting the Occurrence of Sticking during Tablet Production by Shear Testing of a Pharmaceutical Powder
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