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Effect of grain size distribution on powder properties
This article will explain the influence of grain size distribution on different powder properties.
Powder granulometry
Powders are polydisperse granular materials by nature. A narrow grain size distribution is generally difficult to obtain. Moreover, in industrial processes, powders are mixtures of particles having different chemical natures and different sizes. As an example, pharmaceutical powders are generally made of an active ingredient, a binder and other excipients. Although the chemical properties are of huge importance, the grain size distribution of a powder has an important impact on its processability and on the properties of the final product. Indeed, the grain size distribution influences different properties such as the bulk density, the flowability or the tribocharging, that determine the behaviour of a powder. Thereby, many problems encountered with powders can be solved by an adequate choice of the granulometry.
Bulk Density
The bulk density of a powder depends on its grain size distribution. Mixing different particle sizes generally leads to a higher bulk density of the whole material, compared to a monodisperse granular material. Concrete is a direct example of particle size distribution design to increase the density. Indeed, smallest particles like sand or fumed silica are used to fill the remaining voids between granulates particles. The bulk density of the initial mixture has direct consequence on the mechanical properties of the final product. The mechanical resistance of pharmaceutical tablets is also dependent on the bulk density of the initial powder used in the tableting process.
While extensively studied with model particles such as spherical glass beads, a good understanding of the effect of particle size distribution is necessary for powder engineering improvement. The optimization of the bulk density represents a big challenge, especially when the range of particle sizes is large. The number of possibilities, varying the relative proportion of constituents, can then rapidly increase which requires an efficient method of investigation. The GranuPack is designed to limit the operator intervention by a strict and automated protocol that allows a precise, repeatable, and systematic investigation of the bulk density of powder mixtures [1,4].
Powder flow
Powder flowability is one of the major characteristics on which industrials focus. Many problems encountered in powder manufacturing processes comes from a lack of flowability. The powder flowability is known to be influenced by the presence of small particles inserted between larger ones. This is one of the main roles of excipients added in many powders. Recent studies highlighted the influence of a small quantity of small particles on the rheology of the powder [2,3].
Based on the rotating drum method, the rheology of a mixture of two particle sizes is seen to change with the relative proportion of fine and coarse particles. A rheological index can be defined to highlight this behaviour. The GranuDrum is then an adequate instrument for investigating the rheology evolution of powder mixtures.
IMPACT OF POWDER PROPERTIES ON THE RHEOLOGICAL BEHAVIOR OF EXCIPIENTS
The tap density method performed with the GranuPack was used to characterize the powder flowability, regarding the typical relaxation time of the bulk density curve. The packing dynamics highly depends on the proportion of small particles in the mixture and on the size ration between small and large particles [3].
FROM JAMMING TO FAST COMPACTION DYNAMICS IN GRANULAR BINARY MIXTURES
While good mixing is required in processes involving powders of various chemical natures, difference in size inexorably leads to granular segregation. This phenomenon can lead to serious problems, especially in pharmaceutical industries. In a tableting process, when the die is filled with powder, segregation during the flow can lead to large dosage variations of the active ingredient of the final tablets.
Moreover, segregation is known to decrease the bulk density of a granular mixture. As a result, the mechanical properties of the final product can be altered. Fundamental research was conducted with the GranuPack to highlight the effect of segregation on the bulk density of granular mixtures [4,5].
FORCED SEGREGATION IN BINARY GRANULAR MIXTURES
Electrostatic charge
Powders have generally the ability to build up charge during flow. While known since a long time, especially by industrial which are regularly confronted to this phenomenon, the tribocharging of powders remains poorly understood from a fundamental point of view. However, a powder that gain charges can lead to a change of rheological properties, and then to a change of flowability as the Coulomb forces can attract or repulse grain between them. This often leads to technical problems in many industrial manufacturing processes.
Few is known about the effect of the granulometry on the ability of a powder to build up charge. Recent works [6,7] studied the effect of size dispersion on the tribocharging, using two particle sizes. The GranuCharge instrument measures automatically and precisely the quantity of electrostatic charges created inside a powder during a flow in contact with a selected material. From a practical point of view, the tribocharging of granular mixtures can be investigated with the GranuCharge varying the different proportions and size of the particles and help industrials to have a better understanding of the impact of the granulometry on charging of their powders.
LEARN MORE ABOUT THE GRANUCHARGE
References
[1] Pillitteri, S. (2021). Mélanges et compaction d'assemblages granulaires (Doctoral dissertation, Université de Liège, Liège, Belgique).
[2] Janssen, P. H., Depaifve, S., Neveu, A., Francqui, F., & Dickhoff, B. H. (2021). Impact of Powder Properties on the Rheological Behavior of Excipients. Pharmaceutics, 13(8), 1198.
[3] Pillitteri, S., Lumay, G., Opsomer, E., & Vandewalle, N. (2019). From jamming to fast compaction dynamics in granular binary mixtures. Scientific Reports, 9(1), 1-7.
[4] Pillitteri, S., Opsomer, E., Lumay, G., & Vandewalle, N. (2020). How size ratio and segregation affect the packing of binary granular mixtures. Soft Matter, 16(39), 9094-9100.
[5] Pillitteri, S., Lumay, G., Opsomer, É. and Vandewalle, N., 2021. Forced segregation in binary granular mixtures. In EPJ Web of Conferences (Vol. 249, p. 02001). EDP Sciences.
[6] Lee, V., Waitukaitis, S. R., Miskin, M. Z., & Jaeger, H. M. (2015). Direct observation of particle interactions and clustering in charged granular streams. Nature Physics, 11(9), 733-737.
[7] Wang, H., Fotovat, F., Bi, X. T., & Grace, J. R. (2019). Tribo-charging of binary mixtures composed of coarse and fine particles in gas–solid pipe flow. Particuology, 43, 101-109.