The Effect of Magnesium Stearate Admixing in Different Types of Laboratory and Industrial Mixers on Tablet Crushing Strength

It is generally known that hydrophobic lubricants such as magnesium stearate can have a strong negative effect on the binding properties of directly compressible filler-binders. It was found that the decrease in binding forces is not only dependent on the tablet ingredients and the lubricant concentration used, but especially on the mixing time and mixing procedure. Most studies were performed, however with small laboratory scale mixers. In order to evaluate the effect of magnesium stearate admixing in different types of laboratory-scale and industrial mixers, the decrease in crushing strength was measured for a test formulation during mixing with the lubricant in different mixers. The formula used consisted of 90% a-lactose monohydrate 100 mesh, 9.5% microcrystalline cellulose and 0.5% magnesium stearate. The mixers used were two laboratory scale mixers: a 2 litre Turbula mixer and a 13 litre cubic mixer and five production scale mixers: a 45 litre drum mixer, 90 litre, 200 litre and 900 litre planetary mixers and a 1.000 litre V-shaped mixer, respectively. For the test formulation used, it was found that the effect of lubricant admixing on tablet crushing strength was strongly dependent on type, size and rotation speed of the mixer used. When operated at the same rotation speed, the decrease in crushing strength was much faster for the large industrial mixers than for the small laboratory mixers. These differences were explained by differences in shear forces during the mixing process and the efficiency of the mixing procedure. For the industrial mixers the decrease of the tablet crushing strength as an effect of lubricant admixing was mainly determined by the rotation speed and only to a small extent by the type and size of the apparatus. Moreover no effect of load could be observed between the mutual industrial mixers used. For a prediction of the effect of lubricant admixing on tablet crushing strength in large mixers, efficient laboratory mixers, operating at high rotation speeds can be used. For this purpose a 2 litre Turbula mixer is a valuable tool in preformulation wor