(492c) Influence of an External Lubricant on the Tribo-Charging Behaviour of Capsules

During the manufacture of empty two-piece hard capsules, an external lubricant is applied to the outer surface of the capsules to prevent sticking to each other after packaging and to improve transfer of the empty capsule during filling operations. This lubricant also helps the capsule to minimize friction in a capsule filling machine and therefore is needed for a smooth processing [1, 2]. For the external lubricant, the same substances are used for capsules comprised of either gelatin or hydroxypropyl methylcellulose (HPMC). These lubricants, for example, are magnesium stearate (MgSt), sodium lauryl sulphate (SLS) and carnauba wax (CW). The structural differences among lubricant can lead to a different work function of the material (the minimum energy needed to withdraw an electron of the surface of a solid), which places them at different positions of the tribo-electric series. When materials with different work functions brought into contact and are separated, electrons are transferred and tribo-electric charges are created. Tribo-charges are often responsible for many manufacturing issues and can influence the quality, safety and efficiency of pharmaceutical products [3] or for example the aerodynamic performance of a dry powder inhaler (DPI). Tribo-charges can also cause problems in terms of powder mixing homogeneity and dosing accuracy [4], and can lead to particle adherence on surfaces and agglomeration of particles during manufacturing or powder release from the capsule [5].

This study examines the impact of the charging behaviour of capsules that are externally lubricated with different materials, before and after transport over stainless steel and PVC. The transport over steel was intended represent the transfer from the bulk container to a filling machine hopper-feeder, whereas the transport over PVC could give an idea of how the capsules behave/or will be charged within a packaging material.

Material and Methods

Different types of size 3 gelatin capsules (Quali-G^®) and chemically gelled HPMC capsules (Quali-V^®-I) were received from Qualicaps (Qualicaps Europe, S.A.U., Spain) with various variations of colorant and external lubricant. Gelatin capsules were supplied with the external standard lubricant SLS, unlubricated, MgSt, and CW. Batches with 1g and 2g of MgSt on 10kg of capsules were received, to study the influence of the amount of external lubricant. Gelatin capsules with MgSt or CW as external lubricant were further divided into two groups, in which different colorants were used. HPMC capsules were also used in an unlubricated state, as well as with CW, which is the standard lubricant for HPMC capsules, SLS, and MgSt.

Charge measurements were performed in triplicate using 2.0g ±0.15g capsules on the GranuCharge™ (Granutools, Belgium) instrument. The room was conditioned to a relative humidity (RH) 50% ±3% at 23.5°C ± 0.6°C, whereas the conditions inside the device remained constant at 50% ±2% at 23.5°C ± 0.5°C. Charge measurements started with the measurement of the initial charge to mass ratio (q0) by pouring the capsules directly out of the bag into the inbuilt Faraday cup of the device. Afterwards, the capsules were transferred to the grounded vibrational feeder of the GranuCharge and fed into a V-shaped steel or PVC tube to drop into the Faraday cup again after transport. Then, the charge to mass ratio was calculated for a second time (q1). By subtracting the initial charge to mass ratio from that obtained after sliding through the V-tube yielded the charge to mass ratio (Δq = q1-q0) that describes the charge gained.

To differentiate the surface energy (SE) of the capsules with different external lubricants, contact angle measurements were made using the pendant drop method (Easydrop system, Krüss GmbH, Germany) at 22°C ± 0.3°C. For this purpose, the contact angle of water and diiodomethan was measured on the outer surfaces of the capsules (as received). Surface energy was calculated via Owens-Wendt-Rabel-Kaelble method (OWRK) [6].

Results and Discussion

Overall, the charge to mass ratio (Δq) of capsules after the transport over steel and PVC was positive, although a higher charge to mass ratio was observed when capsules were in contact with PVC tube walls. Gelatin capsules on steel charged in a range of 0.63 (± 0.05) to 2.87 (± 0.07) nC/g whereas HPMC charged from 0.43 (± 0.06) to 1.45 (± 0.06) nC/g. Charges of gelatin and HPMC capsules after passing over PVC surface were from 0.72 (± 0.27) to 2.62 (± 0.71) nC/g and 1.61 (± 0.05) to 3.05 (± 0.13) nC/g, respectively. Capsules with MgSt showed an exception because they were charged to the same amount on both materials. As Staniforth and Rees [7] stated, MgSt is an electropositive and therefore able to lower the charging effect of the capsule material and wall interactions. Within one type (gelatin or HPMC) of capsules, the un-lubricated capsules showed the highest charging and those lubricated with SLS showed the lowest charging. It can be considered, that the electron transfer of the HPMC and gelatin is hindered due to the work function of the contacting surface of the capsule material or the external lubricant. The batches with two different amounts of MgSt showed that a lower amount of lubricant gave higher charging. This seems to fit to the theory that lubricants sometimes work as antistatic agents as it is also reported for coated particles [8]. A part of the lower charging tendency of HPMC capsules compared to gelatin capsules can be explained by the antistatic properties of HPMC itself [9] although other ingredients of the capsule formulation like the gelling agent or different colorants also play a role . The effect of a lower charging tendency due to lubricants is supported by the fact that a lower amount of external lubricant lead to an increase in charge. In order to assess the batch to batch variability, two batches of gelatin capsules without lubricant were measured. The two batches gave the same results in charging, although the surface energy was slightly different. Overall, SE showed results between 3.18 to 8.08 mN/m for HPMC capsules and 26.6 to 38.59 mN/m for gelatin capsules, respectively. The HPMC capsules with the highest SE showed the lowest charging tendency within the group of HPMC capsules, whereas in gelatin the highest SE also gave the highest charging tendency. It can be assumed that the lubricants cover the main part of the capsule surface but do not fill all cavities of the underlying material and therefore the differences between HPMC and gelatin occur.

Conclusion

In summary both, the external lubricant and the capsule materials themselves have an influence on the tribo-charging behaviour of the capsules. The low amount of lubricant on a single capsule (only 2% of the average capsule weight), seems to smoothen the surface of the capsule. Since it is not known how uniform the distribution of the external lubricant is, it is possible that some regions of the capsule surface are not covered enough to protect the capsule material from directly interacting with the environment. Hence, the results for charging can be a mixture of the interactions of the capsule material and the external lubricant. The work functions of HPMC and gelatin on one hand and the work functions of the lubricants on the other hand do not explain the differences on their own. Calculated work function values depend on well-defined molecular structures, therefore real work function values are hard to define for HPMC and gelatin as impurities in the material and the orientation of functional groups on the surface also have an impact. Overall, tribo-charging effects of capsules seem to be an interplay of capsule materials, parameters like surface roughness of the capsule and the interacting machine, water content and lubricant attributes as well as the lubrication level. It could be shown, that lubrication of capsules plays a role in tribo-electrification.