A New Method of Estimating Volume During Powder Compaction and the Work of Compaction on a Rotary Tablet Press from Measurements of Applied Vertical Force

Abstract

The volume per unit mass of a powder bed, V’, during compaction on a rotary tablet press has been expressed as a function of pressure, P using a modification of Kawakita’s equation: V = (V’o –V’)P’/(P+P’)+V’, where V’o, V and P’ constitute a set of unique values for a given powder or powder mix under specified tableting conditions. The volume, V, is determined from the machine deformation constant which is the relationship between applied vertical force and the deformation of the tablet press and the punches. An iterative method is described which allows the determination of V’o, V’ and P’ from the slope and intercept of V vs 1/(P+P’) where all values are evaluated at peak pressure. By substituting these values into the equation, the volume of a given powder bed during compaction up to peak pressure can be accurately predicted from the pressure vs time curve. This method of estimating volume and hence punch displacement, is much simpler than an earlier analytical method which was derived from direct measurements of punch displacement under running conditions. Since volume is an explicit function of pressure, the work of compaction is also a function of pressure. Estimates of the work of compaction are in good agreement with values calculated using our previous method. Values of V’o, V’ and P’ are reported for 35 pharmaceutical materials and could be incorporated into a database library of drugs and tableting excipients. This database could then be used for the quality control of incoming raw materials (batch to batch assessment) and for the comparison of materials from alternative sources. The experimental methodology and method of calculation should, in principle, be applicable to any rotary tablet press and together with other tableting parameters (such as compression time, peak offset time, decompression time, elastic recovery and work of compaction) would provide a simple, inexpensive method for the in process validation of tablet compression.