Thermodynamic analysis of compact formation; compaction, unloading, and ejection. II. Mechanical energy (work) and thermal energy (heat) determinations of compact unloading and ejection

Evaluation of In-Die Compression Data for a Deeper Understanding of Altered Excipient Properties upon Temperature Rise

The thermodynamic analysis of tablet formation includes the thermal and mechanical analysis during compression. The aim of this study was to evaluate alterations of force-displacement data upon temperature rise as an indicator for changed excipient properties. The tablet press was equipped with a thermally controlled die to imitate the heat evolution from tableting on an industrial scale. Six predominantly ductile polymers with a comparably low glass transition temperature were tableted at temperatures ranging from 22-70°C. Lactose served as a brittle reference with a high melting point. The energy analysis included the net and recovery work during compression, from which the plasticity factor was calculated. The respective results were compared to the changes in compressibility obtained via Heckel analysis. Elevated temperatures reduced the necessary work for plastic deformation for the ductile polymers, which was reflected in decreasing values for the net work of compaction and the plasticity factor. The recovery work slightly increased for the maximum tableting temperature. Lactose showed no response to temperature variations. Changes in the net work of compaction showed a linear correlation to the changes in yield pressure, which could be correlated to the glass transition temperature of a material. It is therefore possible to detect material alterations directly from the compression data, if the glass transition temperature of a material is sufficiently low.

Impact of die wall material on the mechanical properties of paracetamol tablets

Tablet quality can be affected by material, configuration and design of the tooling which comprise punches and dies. Much research attention had centred around punches, with very little reported on the dies. Dies with modified bore lining materials or inserts are available for special applications. However, the influence of such die types on tablet properties and the compaction process has not been well studied. Often, the reason for selecting dies with harder lining material is for improved wear resistance. In this study, flat-faced and convex-faced tablets were produced from paracetamol granules using dies with different bore inserts. Tablet properties and response parameters of the compaction process were evaluated to understand the impact of die mechanical and surface properties on the compacts formed. Compaction pressure was found to have the greatest impact on tablet elastic recovery (r ≥ 0.96, p < 0.01 in all bivariate correlations) and thus affecting the tensile strength. Choice of die inserts could impact the mechanical properties of convex-faced paracetamol tablets, particularly at high compaction pressures.

Temperature evolution during compaction of pharmaceutical powders

A numerical approach to the prediction of temperature evolution in tablet compaction is presented here. It is based on a coupled thermomechanical finite element analysis and a calibrated Drucker-Prager Cap model. This approach is capable of predicting transient temperatures during compaction, which cannot be assessed by experimental techniques due to inherent test limitations. Model predictions are validated with infrared (IR) temperature measurements of the top tablet surface after ejection and match well with experiments. The dependence of temperature fields on speed and degree of compaction are naturally captured. The estimated transient temperatures are maximum at the end of compaction at the center of the tablet and close to the die wall next to the powder/die interface.