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.

Highlights

• Method to measure friction coefficients in standard compaction simulator experiment.
• Experiments conducted with different materials, speed, density, and lubrication type.
• Ejection speed and excipient material are the dominant factors affecting friction.
• Kinetic friction coefficients depend only weakly on the material.
• Oscillatory stick–slip identified as potential cause for high friction coefficients.

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 one.

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2.1. Materials

The following three powders were used: micro-crystalline cellulose (MCC, Avicel PH200®, 𝐷v50 of 199 μm), dibasic calcium phosphate dihydrate (DCPD, Emcompress premium®, 𝐷v50 of 212 μm), and partially pregelatinized starch (Starch, SEPISTAB^TM ST 200, 𝐷v50 of 192 μm). The effective true densities of these powders have been determined previously using the method by Sun (2003) and van der Haven et al., 2022, van der Haven et al., 2024. These powders were blended with 1 wt% of magnesium stearate (MgSt) for the purpose of internal lubrication. Blending was performed using a 3D shaker TURBULA®T 2 F for 5 min at 25 rounds per minute.

The materials in this study have been chosen to represent a range of mechanical properties that is representative of powders encountered within the pharmaceutical industry. MCC and starch are extremely common excipients, respectively with predominantly plastic and (visco-)elastic behaviour. DCPD, on the other hand, is a common excipient with hard and brittle behaviour, more closely resembling the mechanical properties of a typical API. The selected materials therefore provide a decent sample of different mechanical properties whilst also providing a point of reference by having been extensively studied in the past.

2.2. Compaction and ejection

All tabletting experiments were performed on a compaction simulator; the STYL’One Evolution press (MEDELPHARM, Beynost, France), equipped with a 80 kN load cell, instrumented die, and flat-faced punches with a 11.28 mm diameter. The sampling frequency was set to 5 kHz. Compression (loading) and decompression (unloading) were the same for all experiments, using a V-shaped double-ended compression (DEC) profile with punch velocities of 2 mm s⁻¹, giving a total compression speed of 4 mm s⁻¹. The compression force was adjusted such that the final height of all the tablets was 4.0 mm. Punch positions were corrected for punch deformation using the Analis software (MEDELPHARM, Beynost, France). During decompression, the top punch retracts to fully detach from the tablet. The lower punch does not necessarily fully detach from the tablet because there is some freedom of movement between the lower punch and the actuator moving this punch. However, decompression is symmetrical and the force on the lower punch always reaches 0±5 N at the end of decompression. Punch positions were also adjusted such that the radial pressure sensor in the die wall was always positioned at the centre of the band of the tablet.

Dingeman L.H. van der Haven, René Jensen, Maria Mikoroni, Umair Zafar, James A. Elliott, Ioannis S. Fragkopoulos,
Tablet ejection: A systematic comparison between force, static friction, and kinetic friction, International Journal of Pharmaceutics, 2024, 124369, ISSN 0378-5173, https://doi.org/10.1016/j.ijpharm.2024.124369.

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