Friction coefficients of tablet masses

Radial die-wall pressure as a reliable tool for studying the effect of powder water activity on high speed tableting

The effect of moisture as a function of water activity (Aw) on the compaction process is important to understand particle/water interaction and deformation. Studying powder/moisture interaction under pressure with radial die-wall pressure (RDWP) tool was never done. The aim of our study was to use this tool to study this interaction at high compression pressure and speed. Moreover, the effect of changing ejection cam angle (EA) of the machine on ejection force (EF) was investigated. Also, a new tool for prediction of tablet sticking was proposed. Materials with different deformation behaviors stored at low and high moisture conditions were used. Compaction simulation guided by modeling was applied. High Aw resulted in a low residual die-wall pressure (RDP) for all materials, and a high maximum die-wall pressure (MDP) for plastic materials, p < 0.05. This was due to the lubricating and plasticizing effects of water, respectively. However, microcrystalline cellulose showed capping at high Aw and compaction pressure. By increasing compression pressure at high Aw for all materials, effective fall time (EFT) was increased, p < 0.05, showing tendency for sticking. Increasing EA caused an increase of friction and EF for powders, p < 0.05. RDWP was a useful tool to understand particle/moisture interaction under pressure.

Benefits of die-wall instrumentation for research and development in tabletting

Instrumented presses used in tabletting research and development are normally equipped to measure punch force and displacement. Die-wall monitoring is rare, probably because instrumentation and calibration are quite difficult. The authors critically examine the tenets of radial pressure measurement in compression physics. The theoretical background concerning axial to radial stress transmission during the different phases of the compression cycle is presented. The literature reporting on the use of radial stress measurement to assess the self-lubricating properties of materials or the effect of lubricants is reviewed. Examples of interpretation of radial pressure cycles to define the basic material behaviour are given. The influence of particle size and shape as well as that of process and formulation variables on die-wall response are also discussed. Substantial inconsistencies can be seen in the literature with respect to the interpretation of experimental data, often because of the poor reliability of results and mostly because powders are essentially not solid, isotropic bodies. There is also a distinct lack of complementary tabletting parameters that would help understanding their comparative benefits. For this reason, original data on 13 model compounds are presented together with a classification of the materials encountered in pharmaceutical tabletting, based on selected parameters. In conclusion, none of the determined parameters, including those derived from radial pressure measurement, is able, alone, to predict the material behaviour under compression. Although die-wall instrumentation contributes little to the development of improved tablet formulations, it is valuable for characterising the mechanical properties of the materials. This is particularly advantageous given that the mechanical properties account for variations in tabletting performance to a much greater extent than the magnitude of the interparticulate attractions. Nevertheless, because of the peculiar nature of powders compared to solid, isotropic bodies, there is a need to develop new models for analysing their behaviour and to put more emphasis on examination of time-dependent deformation in the later stage of the compression cycle.

Effect of lubricant level and applied compressional pressure on surface friction of tablets

A method for measuring friction coefficients of compacted surfaces (tablet surfaces) is described. Frictional coefficient measurements were carried out at various tableting compression forces. The effect of magnesium stearate concentration was found to be a log-linear approach to an asymptote (which would represent a minimum frictional coefficient).