Hard capsules for dry powder inhalation (DPI) are treated with an external lubricant in order to improve their flow properties and to prevent them from sticking together during storage. There are different external lubricants in commercial use, which can lead to differences in flow and processability, such as tribocharging, , widely known as an issue that can occur during capsule filling due to the interaction of capsule surface and capsule filling machine. The tribo-electric effect of insulator-metal contacts is a complex phenomenon that still lacks sufficient scientific understanding Therefore, this study aimed to examine the charging behaviour of two different capsule materials, which are lubricated with three different substances, during capsule filling at two different relative humidities (RH) and filling speeds. It turned out that the charge after storage, directly out of the transport aluminium bag, was very low, evidencing in-process charging. Results proved that lubrication has an impact on the magnitude and the polarity of charge dependent on the capsule material. Tribo-charging of the capsules was concluded to be a result of the combination of capsule material and lubricant. Generally, capsules showed in-process charging dependent on the lubricant and capsule material in combination with filling speed and filling humidity when interacting with stainless steel parts of the capsule filling machine.
The charging behaviour of capsules during processing depends on capsule material, external lubricant and fill weight. The same lubricants do not necessarily result in comparable charging results on different capsule materials. The lubricant in combination with the capsule material seems to affect charging more than filling speed and filling humidity.