Stress relaxation studies of granules as a function of different lubricants.

Viscoelastic properties of granules may be studied using stress relaxation. The effect of viscoelastic properties of different lubricants, namely magnesium stearate (Mgst), talc and precirol, on granule compaction properties was examined using texture analyzer TA-XT2i at low pressure. Normalized compact curves of stress relaxation have been discussed in relation to some parameters (flowability, porosity, viscoelasticity as well as particle size). The literature shows that viscoelasticity is always present and it produces an accompanying plastic deformation.This study revealed that bonding in compacted granules lubricated with Mgst was higher than those in compacts lubricated with the other two lubricants being studied. When studying the partial porosity of granule beds, we see that this is the result of stored energy, like the tablet case and the problem of its capping. The small stress relaxation due to talc or precirol suggested that these materials deformed principally by energy storage. However, a qualitative characterization of Mgst as tablet lubricant would be that it avoids the accumulation of stress in the compact that causes the problem of capping due to the entrapped air and therefore facilitates the optimization of pharmaceutical manufacturing. It has been possible to normalize stress relaxation using the Wischert model, represented by the sum of several exponentials, according to the nature of the lubricant. The use of texture analyzer TA-XT2i was considered to be a good technique for the evaluation of the stress relaxation of solid particles in the compression process at low pressure. It permitted the observation that viscoelasticity is influenced by the lubricant used. The brittle fracture index, like Carr's index values, has been correlated with the viscoelastic characteristics of granules.