Tablet ejection: A systematic comparison between force, static friction, and kinetic friction.

The magnitude of the frictional forces during the ejection of porous pharmaceutical tablets plays an important role in determining the occurrence of tabletting defects. Here, we perform a systematic comparison between the maximum ejection force, static friction coefficient, and kinetic friction coefficient. All of these metrics have different physical meanings, corresponding to different stages of ejection. However, experimental limitations have previously complicated comparisons, as static and kinetic friction could not be measured simultaneously. This study presents a method for simultaneously measuring the maximum ejection force, static friction coefficient, and kinetic friction coefficient in situ during tablet ejection in routine compaction simulator experiments. Using this method, we performed a systematic comparison, including variations of (1) ejection speed, (2) compaction pressure, (3) material, and (4) lubrication method. The relative importance of each variable is discussed in detail, including how ejection speed alone can be a decisive factor in tablet chipping. The reliability of the newly developed method is supported by excellent agreement with previous studies and finite element method (FEM) simulations. Finally, we discuss the suitability of friction coefficients derived from Janssen-Walker theory and explanations for the phenomenon of die-wall static friction coefficients with apparent values far above unity.

Prediction of the impact of lubrication on tablet compactibility.

The tableting of most pharmaceutical formulations requires the addition of lubricants to reduce ejection forces, prevent tooling damage and tablet defects. The internal addition of lubricants is known to reduce tablet tensile strength, especially of mainly plastically deforming materials. To date, available models show only limited quantitative predictive accuracy for the influence of lubricant concentration on the mechanical strength of tablets. This study aims to fill this gap and present a model based on the Ryshkewitch-Duckworth equation that can estimate the compactibility profiles of lubricated formulations. Binary mixtures of different diluents (microcrystalline cellulose and lactose) were prepared with common lubricants (magnesium stearate and sodium stearyl fumarate) and subsequently tableted. The resulting compactibility profiles were fitted using the Ryshkewitch-Duckworth equation and the derived fit parameters (kb and σ0) were correlated with the lubricant concentration. Subsequently, an empirical model was established which requires a minimum of experimental data and is able to predict the tensile strength of lubricated diluent tablets. Consequently, the developed empirical model is an interesting and valuable addition to the existing multi-component compacting models available and offers the opportunity to accelerate experimentation in the development of new tablet formulations.

Assessment of surface caking of powders using the ball indentation method.

Powder caking is a ubiquitous problem, which could significantly decrease product quality and lead to economic losses. Hence it is important to know the conditions under which it occurs. The caking behaviour of three powder materials (PVP, HPC and CaHPO4) has been investigated by the ball indentation method (BIM) as affected by relative humidity (RH), temperature and time. The resistance to powder flow, as indicated by the hardness is measured by a ball indenting the powder bed surface. The surface hardness increases with increasing RH and temperature, indicating caking of the powder bed. Moreover, the temperature and RH show a coupled effect on powder caking. Irreversible caking is formed in PVP and HPC at 75% RH; the particles coalesce and the volume of powder bed is significantly reduced with time. However, the caking of CaHPO4 is reversible. To examine the caking mechanism of PVP and HPC, the critical glass transition RH is determined at 25°C and 45°C. The values are 63% and 53% RH for PVP and 61% and 50% RH for HPC, respectively. The glass transition moisture content in the ball indentation experiments is comparable with that determined by the dynamic vapor sorption measurement. BIM could be a fast and effective method for the assessment of powder surface caking.