An investigation of magnesium stearate mixing in a V-blender through gamma-ray detection

Magnesium stearate (MgSt) is a widely used powder lubricant in the pharmaceutical industry whose mixing dynamics remain poorly understood. In this work, MgSt was used as a radioactive tracer to investigate its mixing behaviour in a V-blender. MgSt was mixed with a microcrystalline cellulose and spray-dried lactose pre-blend, and a number of samples were then extracted from the powder bed with a core sampler. The amount of lubricant measured in each sample allowed the investigation of the effect of the blender fill volume and rotational speed, the pre-blend composition and the lubricant loading profile on mixing performance. Results indicate that, of these 4 parameters, the rotational speed and the pre-blend composition had the most significant effect on mixing behaviour. Contrary to our expectations, the MgSt loading profile had little effect on mixing performance. Both results indicate the overriding importance of the shear force applied on the powder bed on mixing performance. In a final experiment, the mixing performance of MgSt was compared to that of Sodium Lauryl Sulfate (SLS) at identical fill volumes and rotational speeds. Results show that, in spite of its larger particle size, SLS is more difficult to mix to the other excipients than MgSt.