Use this URL to cite or link to this record in EThOS:
Title: Self-Lubrication Phenomena in Pharmaceutical Tabletting
Author: Toson, Severine
ISNI:       0000 0004 2681 5726
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2009
Availability of Full Text:
Access from EThOS:
Access from Institution:
This thesis presents an experimental study designed for the characterization of the tablettingbehaviors of Starch 1500?, Starcap 1500?, LHPC LH-11 and LHPC LH-B1, chosen for their displayof self-lubrication properties, in the form of efficient stress transmission, low ejection forcesand anti-capping propensities, with two specific aims:Firstly, the mechanisms underlying the tabletting performance of the materials were investigated. Secondly, in order to develop a robust experimental characterization method, the value ofthe experimental indicators used in this work was assessed. The time dependence of plastic flow was evidenced through a strain rate sensitivity study: thevariations of a set of experimental parameters with the compaction rate were observed. On theone hand, the axial (AER) and diametral (DER) tablet relaxations provided an indirect measureof the elastic deformation of the powder bed. On the other hand, the Heckel Yield Stress (?yH),Kawakita parameters a and b, and the maximum stress transmission coefficient (STCM), allowedan evaluation of the extent of plastic flow. Secondly, the initial (SRR0), final (SRRF) and average (SRRAv) stress relaxation rates, as wellas the proportion of stress lost during relaxation (?PM) of the four powders were quantified fromstress relaxation experiments. Also, a new approach based on linear combinations of exponentialdecay functions, was proposed for the analysis of stress relaxation curves obtained for powderbeds. Finally, the maximum ejection force and profiles, used in conjunction with the unejected compactsurface roughness and the final tablet shape served to elucidate the different relaxation phenomenataking place during the unloading and ejection phases, as well as investigate die wallfriction. The mechanical strength of the final tablets allowed an estimation of the final particlecohesion achieved. The major finding of this work is that the tabletting behaviors of the powders result from thespecific balance between their degrees of plastic and elastic deformations. More precisely, plasticflow governs the compact consolidation through interparticulate bonding, but also the strengthof the adhesive junctions formed between the tablet and the die wall, responsible for friction. Axial elastic recovery of the tablet during unloading causes the weakening of interparticulatebonds through their stretching, but also the cleavage of the compact-die wall plastic junctionsand thus a reduction in the friction force between them. The fine-tuning of these two phenomenacould lead to the formation of a tablet of satisfactory mechanical strength with minimal die wallfriction.
Supervisor: Briscoe, Brian Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral