Effect of Compressibility and Powder Flow Properties on Tablet Weight Variation

Abstract

A powder flowmeter has been designed to provide both quantitative and qualitative data relating to powder flowability. Three directly compressible powders, Emdex, Emcompress and magnesium oxide as well as a three component powder mixture was assessed for flowability, angle of repose and particle size. Compressibility indices were determined for all the above materials as well as for the fractions of each which consisted of a particle size below 315 μm. Sieve analysis was performed on the above powders in order to establish groups consisting of cohesive, mildly cohesive and non-cohesive fractions and their respective flow-time profiles were subsequently determined. Scanning electron microscopic analysis was performed to obtain information on the particle size, shape and size distribution. The interrelationships between flow rate, angle of repose, compressibility index and coefficient of tablet weight variation were established using both a single punch and a high-speed rotary tabletting machine.

A three-dimensional plot was constructed to illustrate the influences of flow rate, angle of repose and compressibility index on the coefficient of tablet weight variation. Whilst it was established that particle size has a significant effect on uniformity of flow, the data also indicated that when the compressibility index exceeded a value of about 20% a significant increase in tablet weight variation resulted irrespective of the powder flow rate.