This thesis is 'concerned with aspects of dynamic compaction of pharmaceutical powders. The research is mainly aimed at the influence of compression rates on the residual resilience and strength of the tablet. A wide range of compression strain rates (103 - 10 per sec. ) is covered for di-pac-sugar, sodium chloride, potassium bromide, paracetamol, copper sulphate, avicel, lactose and calcium phosphate. The influence of compression rate on the form of the pressure-density relationship including the IHeckel and 1: awakita diagrams is investigated. Theoretical and experimental work are conducted on the relationship between radial and axial pressures during uniaxial straining. The general tendency for all the powders tested except for the paracetamol d. c., is to exhibit increased compaction pressure with strain rate up to 105 per sec. Due to morphological and compositional factors paracetamol d. c. softens with the rate of straining up to about 102 per sec. At higher rates it behaves like other powders. Generally, at high strain rates, the brittle behaviour dominates the process and tablet capping becomes more evident. The scope, origin and the main objectives of the present work are presented in Chapter 1. A brief historical account of the development of uniaxial compression techniques is given in'Chapter 2. Past work in the field of uniaxial compaction is reviewed and. classified. Various dynamic processes are classified according to the rates of compaction. Discussion of the various mechanisms responsible for, and governing, the way in which a powder consolidates when subjected to uniaxial compaction is presented together with a brief discussion of the capping phenomenon. Chapter 3 describes the construction and operation of a specially designed die incorporating a pressure pin for radial pressure measurement. The effect of compression rate on the form of the characteristic compaction curves, for all powders, is investigated using the above die on a servohydratlic variable speed machine (ICI simulator) at constant compression rate conditions (10-110 per sec). Investigations, using this apparatus indicate that all materials tested exhibit strain rate sensitivity to varying degrees (i. e. increased resistance to compaction as the speed is increased). In Chapter 4a brief review and discussion of viscoelastic aspects and theoretical modelling of the compaction process are presented. By plotting axial versus radial pressures during successive compression and decompression cycles at various speeds and by observing the decay of both pressures under constant volume conditions, it is possible toi draw certain conclusions about the relative elastic-plastic behaviour of each material. Investigations ' indicate that the use of linear viscoelastic theory could be useful in predicting the general trend of the process but insufficient to model the response of the various powders to the different loading conditions. Accurate modelling would require more experimental work and analysis of a high level. Also, the elastic recovery of all the compacts within the die is found to range from 8-120. Chapter 5 describes various aspects of forming and testing of tablets on an Instron machine at constant speed. Crack initiation, propagation and modes of failure during the Brazilian test are investigated by employing -high speed photography. An analysis of the effect of die and punch rigidity on the Kawakita relationship is attempted. An account of 'the variation of the. ejection pressure and the residual tensile strength with the maximum applied pressure is given. in this-chapter, the effect of ageing time and the effect of the ejection speed, for both straight and tapered dies, are also examined. Investigations indicate that, during the Brazilian test, in all compacts for all materials, the crack starts at the centre of the tablet and propogate up and down towards the plattens. Three main types (modes) of failure are observed: The 'traditional' mode of failure, in compacts of elastic-plastic deforming materials , (such like di-pac-sugar, paracetamol); The (random) mode of failure in plastically deforming compacts (such like sodium chloride potassium bromide), End capping which is observed in almost all materials compacted at high speeds. The values of the constant "b" (in the Kawakita Equation) is significantly influenced by the die and punch materials while the constant "a" remains unchanged. Also, the ejection pressure is increased with the increased axial maximum pressure and decreases with increasing speeds of compaction. All materials tested, show slight increase in the compact tensile strength as the. ageing time is increased - Also all compacts formed in the tapered die show higher tensile strength than those formed in the straight die over the range of ejection speed tested. Chapter 6 describes the construction and operation of an apparatus used for quasistatic"and medium rate (10 2_ 103 per sec) uniaxial compaction of powders. This apparatus allows the powder volume, 'axial and radial compaction pressures to be measured simultaneously. The investigations indicate that the properties of the resulting compacts are influenced by the compaction rate employed. Also, the compact tensile strength at this range is found to decrease together with the capping pressures. Chapter 7 describes the construction and operation of especially designed high pressure air projectile launcher compaction apparatus suitable for high rate uniaxial compaction of powders. Both, the axial pressure and powder volume can be measured on this apparatus. Problems of stress wave reflections in the system are overcome by using long load cells which enable accurate measurements to be obtained. Almost in all compacts, formed on this apparatus, ' end capping is observed. Chapter 8 gives a detailed description of the design, construction and operation of a high pressure triaxial cell working at pressures of up to 207 MPa, together with the associated equipment. Also, the sample preparation procedure is established. Preliminary, tests carried out on di-pac-sugar, lactose and paracetamol d. c. indicate that the powder characteristics can be designed by controlling the axial-and radial stresses. Chapter 9 describes the theory, equipment and the experimental results of a new technique based on the Split Hopkinson Bar principle to determine 'tension and compression Young's moduli of'a. tablet. The investigation indicates that the modulus in compression is higher than that in tension (in some cases by almost 20t). Finally, chapter 10 outlines general conclusions and suggestions for further work.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:385033 |
| Date | January 1985 |
| Creators | Es-Saheb, Mahir Hamdi Hamed |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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