Behaviour of magnesium stearate at particle–particle interfaces: Microdynamic flowability to monitor distribution in powders

Oral solid dosage forms (OSD) manufacturing requires a series of unit operations, such as blending, roller compaction, granulation, capsule filling and tablet manufacture. In these processes, frictional forces between powder and processing equipment often generate mechanical stress on the instruments and may compromise the quality of the final product [1]. The addition of lubricant agents to the formulation is necessary for obtaining high quality products with efficient and trouble-free processing. Despite the large variety of lubricants, magnesium stearate (MgSt) remains the most widely used as a cost-effective, non-toxic compound with excellent anti-adherent properties. During blending, free MgSt agglomerates are at first adhered on the surfaces of excipient and drug particles, and thereupon the shear forces induce mechanical deagglomeration and delamination within the powder mass [8]. This modifies the surface properties and functionality of the formulation constituents [5,6]. Excessive lubricant amount and/or shear contribute to delamination on the surfaces and the entire blend is overlubricated [18]. The physical properties of the tablet are then adversely affected, such as reduced tensile strength (TS), delayed tablet wettability and drug dissolution rate [5,7].

Although overlubrication and related problems are well known in OSD manufacturing, current technology does not allow early detection of this phenomenon in powders [[12], [13], [14], [15], [16],30]. Most studies assumed a uniform distribution of the lubricant, despite high cohesion of the small aggregates, short blending times and low recommended amounts. It has been recognized that MgSt coverage is often discontinuous [[19], [20], [21]]. For instance, near-infrared (NIR) and X-ray spectroscopy (EDX) largely succeeded in regular particle morphologies such as granule blends, but poor reliability has been reported in powders because it was difficult to distinguish between uniform distribution and excessive blending [17,30]. There is a clear gap in research on this topic over the past few decades. End-product testing and/or tablet physical properties remain the first choice for determining the endpoint of lubricant blending in powders, with a limited understanding of the process and considerable time and resources consumption for risk assessment of overlubrication.

Powder flowability is also affected because MgSt modifies the area of contact between the sliding interfaces, the type and strength of the attractive forces between the contacting surfaces, and the shearing and rupture of the materials at the contact points and the surrounding area during sliding [2,22]. The effect of MgSt addition has been evaluated using state-of-the-art flow measuring techniques such as angle of repose, avalanche time, powder rheometry and shear cell, that is, both the static and dynamic measurements and different shear conditions [3,5,[8], [9], [10],23,24]. The consensus is that MgSt improves flowability by diminishing particle roughness, and only decreases for certain materials by significantly increasing the lubricant amount and/or blending time. This glidant effect was more pronounced as powder cohesion increased and did not have a significant impact for free-flowing powders. In addition, dynamic flow measurements were reported to be more sensitive and provided more accurate and reproducible results than static measurements. Considering the above, the decrease in flowability at long blending times, even for small amounts of MgSt suggested to us that, when heavily delaminated, surface coverage increases the shearing and rupture of the materials at the contact points cohesion. Moreover, the flowability differences between testing conditions may be related to the magnitude of physical contacts between particles during measurement.

In this study, it was hypothesized that an accurate description of the frictional properties in the powder mass could correlate with the distribution of MgSt on the particle surface. This phenomenon may be too weak for early detection in a dynamic flow regime with available flow testing technologies, as many interaction mechanisms are involved. Our previous research introduced microdynamic powder flowability to quantify cohesive phenomena arising from surface modification by colloidal silica [27]. This time, we attempt to determine the nonuniform distribution of MgSt in powders, based on the modified flow behaviour at particle–particle interfaces, towards a better understanding of lubricant adhesion and the impact on the properties of blends and tablets. The TS of tablets was investigated as secondary response.