(98d) Impact of the Micro/Nano- Structural Properties of Materials on Tribo-Charging: A Case Study on Salbutamol Sulphate Particles

Powder tribo-charging occurs due to the charge exchange between two solid surfaces that are brought into contact and then separated. During powder manufacturing, particles can acquire electrostatic charges from their frequent collisions within the powder and the surfaces of the processing equipment. The attained electrostatic charges can alter powder behaviours and affect its processability, i.e., reducing flow, inducing particle agglomeration and/or segregation, causing adhesion to surfaces, etc. Besides the type of processing operation (e.g., sieving, blending, conveying, etc.) and environmental conditions (i.e., temperature and relative humidity) used during production, the charging propensity of powders also depends on their multiple material attributes¹, particularly, their solid-state characteristics (e.g., crystallinity), presence of impurities, etc. For instance, crystalline salbutamol sulphate (SS) has been reported to charge distinctly, compared to its amorphous counterpart². The presence of surface and bulk impurities can also contribute to powder tribo-electrification³. In this context, the present work aims at evaluating the impact of solid-state, including different amorphous contents and impurity contents, on the tribo-charging tendency of SS powders.

MATERIALS AND METHODS

Materials

Salbutamol sulphate (SS) was purchased from Fagron GmbH Co.KG (Barsbüttel, Germany) and engineered to obtain particles with different solid-state properties.

Methods

Powder engineering via ball-milling

Particle engineering was performed via ball-milling on a PM 100 planetary mill (Retsch, Germany), equipped with a 250 ml agate jar. Milling was performed at 400 rpm for different milling times (i.e., 10, 20, 40, 120 min) at room temperature.

Solid-state characterization of the salbutamol sulphate powders

The crystallinity of SS particles was analysed via wide angle X-ray scattering (WAXS) and attenuated total reflectance-Fourier transformed infrared (ATR-FTIR) spectroscopy. Information on the structural heterogeneity of the samples at nano-scale was obtained by the analysis of the small angle X-ray scattering (SAXS) signal. Karl Fischer (KF) titration was performed to determine the water content of the samples, whereas particle size and specific surface area were characterized by laser diffraction and gas adsorption, respectively. The impurities arising from the milling process were quantified via high performance liquid chromatography (HPLC). Results of the powder characterization of the engineered samples were compared to the starting material (i.e., crystalline powder) and to spray-dried SS (i.e., amorphous powder). For spray-drying process, the same parameters described elsewhere were used⁴.

Charge evaluation

The electrostatic charge density of the milled powders and raw material was measured using the GranuCharge™ (GranuTools, Belgium) apparatus at 22 ± 2 °C and 38 ± 3% RH. Approx. 2 g of powder were used for the analysis by measuring the powder charge before and after flow in contact with the stainless-steel tubes of the GranuCharge™ device and the charge density of the samples expressed as difference between the two charge values. Charge density data was normalized by the specific surface area of the powders to allow a proper comparison between the samples.

Statistical analysis

Principal component analysis (PCA) was selected as multivariate data analysis method with the primary aim of identifying material attributes contributing to powder tribo-electrification. Analysis was performed using Simca 16.0 software (Umetrics, Sweden).

RESULTS AND DISCUSSION

The room temperature ball-milling process induced a progressive amorphization of the SS powders with increasing milling time as confirmed from the changes noticed in both FTIR and WAXS patterns. A completely amorphous sample was obtained after 120 min of milling, as shown by the absence of Bragg peaks in X-ray pattern of the material. Similarly, a broad halo was found for the spray-dried SS, further confirming the complete amorphicity of the 120 min sample. In addition to powder amorphization, the milling energy applied on the material also led to the drug degradation in solid-state and resulted in the appearance of impurities, particularly at longer milling times. All the engineered powders showed similar particle size distribution (i.e., x₅₀~7.2 µm; span~3.7), whereas a narrower distribution was obtained for the starting material (i.e., x₅₀=8.3 µm; span=2.7) and the spray-dried sample (i.e., x₅₀=7.0 µm; span=2.2). Since tribo-charging is particularly sensitive to the presence of water, the water content of the samples was quantified and found to be in the range between 3.6-4.7 wt% for all the produced samples. These amounts were considerably higher, when compared to the starting material (i.e., 0.2 wt%) and the spray-dried sample (i.e., 1.7 wt%). Regarding charge evaluation, all the milled powders and the starting material were found to charge positively before and after contact with the stainless-steel surface of the GranuCharge tubes. However, distinct charging trends were obtained for the different milling times. Charge density was found to initially decrease with milling time, reach a minimum at 20 min and thereafter, progressively increase up to 120 min (i.e., fully amorphous sample). The tribo-charging behaviour of fully crystalline and amorphous SS has also been explored in earlier studies^2,5, where, likewise to our current results, amorphous SS was found to charge to a higher extent (compared to its crystalline form). However, the effect of different degree of amorphicity and impurities on the SS tribo-electrification have not been investigated before. Both factors could potentially impact tribo-charging by inducing variations to surface chemical composition of the materials, thus altering their effective work function (i.e., driving force of the tribo-charging phenomenon)^2,3,5,6. Statistical analysis of the results presented in this study revealed underlying correlations between the various material attributes (Figure 1). From the arrangement of the powder properties in the loading plot (i.e., plot displaying the variables in the principal components planes), the main source of variability among the samples was attributed to the particle size of the materials and their amorphous and water content. Those variables were in fact located along the first principal component (PC), explaining 53.3% of variability in the data, whereas the surface charge density (Δq), impurities and nano-heterogeneity were contributing to the second principal component of the PCA model. Surprisingly, powder tribo-charging was found to be not only correlated to solid-state properties, such as the amorphous content and impurity content, but a negative correlation was also revealed in respect to the structural heterogeneity of the samples at the nano-scale level. This observation suggests that the presence of heterogeneous domains at the nano-scale might disrupt charge transfer within the material, resulting in charge mitigation.

CONCLUSION

The present study showed that changes in solid-state properties and impurity level of SS powders can alter their tribo-charging behaviour. Statistical analysis revealed a correlation between powder tribo-electrification and the amorphous and impurity contents. Also, a relationship between the structural heterogeneity of the particles at the nano-scale and tribo-charging was found. This suggests an inter-dependence between amorphous and impurity contents and the nano-heterogeneity of materials and its consequent effect on tribo-charging.

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