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Compression Mechanics of Powders and Granular Materials Probed by Force Distributions and a Micromechanically Based Compaction Equation
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Pharmacy.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The internal dynamics of powder systems under compression are as of yet not fully understood, and thus there is a necessity for approaches that can help in further clarifying and enhancing the level of understanding on this subject. To this end, the internal dynamics of powder systems under compression were probed by means of force distributions and a novel compaction equation.

The determination of force distributions hinged on the use of carbon paper as a force sensor, where the imprints transferred from it onto white paper where converted through calibration into forces. Through analysis of these imprints, it was found that the absence of friction and bonding capacity between the particles composing the powder bed had no effect on how the applied load was transferred through the system. Additionally, it was found that pellet strength had a role to play in the homogeneity of force distributions, where, upon the occurrence of fracture, force distributions became less homogenous.

A novel compaction equation was derived and tested on a series of systems composed of pellets with differing mechanical properties. The main value of the equation lay in its ability to predict compression behavior from single particle properties, and the agreement was especially good when a compact of zero porosity was formed.

The utility of the equation was tested in two further studies, using a series of pharmaceutically relevant powder materials. It was established that the A parameter of the equation was a measure of the deformability of the powder material, much like the Heckel 1/K parameter, and can be used as a means to rank powders according to deformability, i.e. to establish plasticity scale. The equation also provided insights into the dominating compression mechanisms through an invariance that could be exploited to determine the point, at which the powder system became constrained, i.e. the end of rearrangement. Additionally, the robustness of the equation was demonstrated through fruitful analysis of a set of diverse materials.

In summary, this thesis has provided insights and tools that can be translated into more efficient development and manufacturing of medicines in the form of tablets.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. , 56 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Pharmacy, ISSN 1651-6192 ; 159
Keyword [en]
compression, powder, mechanical properties, pellet, tablet, compaction equation, force distribution
National Category
Medical and Health Sciences
Identifiers
URN: urn:nbn:se:uu:diva-171874ISBN: 978-91-554-8319-7 (print)OAI: oai:DiVA.org:uu-171874DiVA: diva2:512577
Public defence
2012-05-16, B21, BMC, Husargatan 3, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2012-04-24 Created: 2012-03-28 Last updated: 2012-08-01Bibliographically approved
List of papers
1. An effective-medium analysis of confined compression of granular materials
Open this publication in new window or tab >>An effective-medium analysis of confined compression of granular materials
2009 (English)In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 194, no 3, 228-232 p.Article in journal (Refereed) Published
Abstract [en]

A simplified model for confined compression of granular materials is considered, which idealizes the collection of particles as a (central) force network. Applying an effective-medium procedure, an equation with micromechanically well-defined parameters is derived, which relates the applied pressure to the engineering strain of the powder during uniaxial compression. Despite the simplicity of the model, comparison with experimental data for mm-sized spherical granules indicates that this equation is able to satisfactorily predict the overall compression profile from single-particle data.

Keyword
Granular materials, Compression, Effective-medium theory, Pharmaceuticals, Mathematical modelling, Powder technology
National Category
Pharmaceutical Sciences
Identifiers
urn:nbn:se:uu:diva-123115 (URN)10.1016/j.powtec.2009.04.012 (DOI)000267842700009 ()
Available from: 2010-04-23 Created: 2010-04-23 Last updated: 2016-05-13Bibliographically approved
2. Effect of lubrication on the distribution of force between spherical agglomerates during compression
Open this publication in new window or tab >>Effect of lubrication on the distribution of force between spherical agglomerates during compression
2010 (English)In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 198, no 1, 69-74 p.Article in journal (Refereed) Published
Abstract [en]

We employ the carbon paper technique to aid the understanding of in die force and spatial distributions, upon compression of approximately I mm sized spherical agglomerates (pellets) of microcrystalline cellulose (MCC). The aim in this study was to test for the effect of lubricant film on force and spatial distributions. Pellets of MCC were formed via granulation and extrusion/spheronisation. Investigation of pellet bed compression was performed on a materials tester. Prior to compression studies the pellets were characterised for bulk density. size and deformability. Two pellet types were investigated; MCC and MCC lubricated with magnesium stearate. The carbon paper technique relies upon carbon paper as the medium for transferring imprints from compressed pellets onto photo quality paper. The digitised images of these imprints form the basis of analysis through the use of image processing software. Using the carbon paper technique within the range of 10-30 MPa indicates that lubrication does not have a significant effect on the distribution of forces between spherical agglomerates during uniaxial compression. Spatial analysis of the imprints revealed that the lubricated pellets exhibited a higher packing order than the unlubricated ones at low applied pressures (10 and 20 MPa), a difference that could not be observed at 30 MPa. Hence interparticle friction and/or cohesion appear to influence the initial particle rearrangement, whereas confinement is suggested to dominate at higher pressures.

Keyword
Granular materials, Compression, Force distributions, Carbon paper technique, Lubrication
National Category
Pharmaceutical Sciences
Identifiers
urn:nbn:se:uu:diva-137888 (URN)10.1016/j.powtec.2009.10.016 (DOI)000274281400010 ()
Available from: 2010-12-16 Created: 2010-12-16 Last updated: 2013-10-31Bibliographically approved
3. Effect of spherical-agglomerate strength on the distribution of force during uniaxial compression
Open this publication in new window or tab >>Effect of spherical-agglomerate strength on the distribution of force during uniaxial compression
2011 (English)In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 206, no 3, 283-290 p.Article in journal (Refereed) Published
Abstract [en]

We employ the carbon paper technique with the aim of investigating the effect of spherical-agglomerate (pellet) strength on force distributions, through confined compression of approximately 1 mm sized pellets formed from microcrystalline cellulose and polyethylene glycol. The carbon paper technique relies on the transference of imprints from compressed pellets onto white photo quality paper, which are digitised and processed via image processing software. The investigated pellets can both deform plastically and develop localised cracks in response to an applied stress, while remaining largely intact during confined compression. Our results indicate that such crack formation - henceforth referred to as fracture - has a decisive influence on force distributions. Previous work on non-fracturing systems has found that the distribution of normalized forces tends to narrow with increasing particle deformation. No narrowing is observed after the point of fracture in this study and the width of the distributions - as quantified by the standard deviation of non-normalized forces - is found to increase with the difference between non-normalized mean force and fracture force. Additional corroborative results show that spatial force-force correlations typically exhibit a marked change once the fracture force is exceeded.

Keyword
Granular materials, Compression, Force distributions, Carbon paper technique, Particle failure
National Category
Pharmaceutical Sciences
Identifiers
urn:nbn:se:uu:diva-147764 (URN)10.1016/j.powtec.2010.09.031 (DOI)000286299200011 ()
Available from: 2011-03-01 Created: 2011-02-28 Last updated: 2013-11-06Bibliographically approved
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5. On the practical utility of an effective medium based compaction equation: application to a diverse set of pharmaceutically relevant materials
Open this publication in new window or tab >>On the practical utility of an effective medium based compaction equation: application to a diverse set of pharmaceutically relevant materials
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(English)Manuscript (preprint) (Other academic)
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-171873 (URN)
Available from: 2012-03-28 Created: 2012-03-28 Last updated: 2012-08-01

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