Kinetics of elastic recovery in roll compaction

Elastic recovery ($\mathrm{ER}$) has been investigated and discussed extensively in the field of tableting. However, until now only limited data is available regarding $\mathrm{ER}$ in roll compaction. Therefore, a previously established in-line measurement technique was rolled out to further investigate the kinetics of $\mathrm{ER}$ in roll compaction and the effects of specific compaction force ($\mathrm{SCF}$) and roll speed ($\mathrm{RS}$). In-line laser triangulation measurements at different positions within a roll rotation as well as measurement over time after the process has been stopped were utilized. Pure microcrystalline cellulose ($\mathrm{MCC}$) and two placebo powder blend formulations were analysed. Successful fit of the contained $\mathrm{ER}$ profiles emphasized that the $\mathrm{ER}$ on the roll surface is build out of two exponential kinetics. Starting with a dominating fast $\mathrm{ER}$ (${\mathrm{ER}}_A$), characterized by a high increase of the ribbon thickness after passing the gap width, followed by a slower $\mathrm{ER}$ (${\mathrm{ER}}_B$). Sigma minus plot analysis showed that increasing $\mathrm{RS}$ led to an accelerated ${\mathrm{ER}}_A$ and ${\mathrm{ER}}_B$ which was related to the viscoelastic behaviour of $\mathrm{MCC}$. The $\mathrm{SCF}$ only had an effect on the kinetics of $\mathrm{ER}$ if a brittle filler was added to the mixture. The conducted study established the first approach in literature to characterize the kinetics of $\mathrm{ER}$ in roll compaction. It supports the understanding and characterization of relaxation times and the effect of the $\mathrm{RS}$ and $\mathrm{SCF}$ in roll compaction.