A small-angle X-ray scattering study of powder compaction

This work demonstrated a novel and potentially important application of two-dimensional small-angle X-ray scattering (2D-SAXS) to investigate powder compaction. SAXS from powder compacts of three materials commonly used for pharmaceutical tabletting exhibited azimuthal variations, with stronger intensity in the direction of the applied compaction force, relative to the transverse direction. This implied that compaction of a (macroscopic) powder could also produce changes on the molecular (nanometre) scale, which can be probed by 2D-SAXS. Two possible explanations for this effect were suggested. A combination of anisometric (i.e. elongated or flattened) granules with anisotropic morphologies could result in azimuthal variation in X-ray scattering due to granule orientation. It is expected that this mechanism would require relatively low packing density, so may operate during die filling. Granule re-orientation appeared less likely at higher packing densities and compaction pressures, however. Under these conditions, the changes in the 2D-SAXS patterns would be consistent with the powder granules becoming relatively flattened in the compression direction, with corresponding changes in their nano-scale morphology. The magnitude of this effect was found to vary between the materials used and increased with compaction pressure. This suggested that 2D-SAXS studies could provide useful information on force-transmission within a compressed powder. Further analysis of the data also suggested differences in the compaction mechanisms (i.e. granule re-orientation, deformation or fragmentation) between the materials studied.     

Wall friction in the compaction of pharmaceutical powders: measurement and effect on the density distribution

In this paper, the axial density profile of tablets of microcrystalline cellulose (MCC) powder compacted in nonlubricated die is investigated by finite element modelling (FEM). The Drucker-Prager/Cap model was adopted for the compaction behavior of powder. The material parameters of the model, including the die wall friction coefficient, were estimated from experimental data of die compaction where the initial density of powder is taken uniform. Changes of Young's modulus with density was measured with a four-point beam bending test. The results of the simulation of the compression and the decompression steps were used to calculate the axial density distribution. Comparison with the measured data presented in [A. Michrafy, M.S. Kadiri, J.A.D. Dodds, Wall friction and its effects on the density distri-bution in the compaction of pharmaceutical excipients, Chem. Eng. Research and Design, Vol. 81, Part A, September (2003)] is discussed.     

Localized Densification during the Compaction of Alumina Granules: The Stage I—II Transition

Tungsten marker layers and X-ray computed tomography were used to monitor the compaction of spray-dried alumina. Local density changes and compaction curves show that the well-known Stage I—II transition indicates an alteration in the direction of transmitted pressure through the uppermost layer. Stage I pressure is dispersed in local agglomerate contacts until "pockets" of low density are eliminated. In Stage II, these deformed/fragmented agglomerates behave as a continuous body and interact with wall friction to transmit pressure nonuniformly into the bulk. Discrete element simulations reproduced the compaction curves and provided clear explanation of the density distribution development.     

Localized Densification during the Stage II–III Transition - Compaction Efficiency at High Pressures

Localized densification and compaction efficiency at high pressures was studied using X-ray computed tomography. Stage III begins with the initiation of a region of uniform average density within the overall high-density zone. No further densification occurs in this region; additional pressure is transmitted into adjacent, less-dense zones and the die wall. This localized increase in wall friction continuously decreases compaction efficiency. Discrete element modeling was used to visualize force transfer. The transition was governed by events that ranged from the microscale to the macroscale. Terapascal levels of pressure were required to produce a uniform compact.     

Uniaxial compaction behaviour and elasticity of cohesive powders

The compression and compaction behaviour of bentonite, limestone and microcrystalline cellulose (MCC) — three cohesive powders widely used in industry were studied. Uniaxial compression was performed in a cylindrical die, 40 mm in diameter and 70 mm high, for three selected cohesive powder samples. The initial density, instantaneous density and tablet density were determined. The influence of maximum pressure and deformation rate was examined. The secant modulus of elasticity E_sec was calculated as a function of deformation rate v, maximum pressure p and powder sample. After compaction experiments in hydraulic press at three pressures - p = 30, 45 and 60 MPa - and two different deformation rates, the strength of the produced tablets was examined in a material strength testing machine.From uniaxial compression tests performed on the universal testing machine for loading and unloading, the modulus of elasticity E was calculated on the basis of the first linear phase of unloading. The total elastic recovery of tablets was also obtained.     

The effect of processing parameters on pharmaceutical tablet properties

The preferred drug delivery system today is represented by tablets, which are manufactured using high speed rotary presses where the powder material is compressed in a die between rigid punches. Compression represents one of the most important unit operations because the shape, strength and other important properties of the tablets are determined at this time. These properties are dictated not only by the characteristics of the powder constituents (which are determined by the properties of the constituents, mixing and granulation), but also by the selection of process parameters imposed by production machinery. This paper focuses on the die fill and the compaction parameters.Die fill on high speed rotary tablet production presses is a complex phenomenon. On most presses the powder is deposited into the die under the effect of the gravity. Die fill is facilitated by the paddle wheels operating in the feed frame and the suction effect, whereby the lower punch is withdrawn while the die opening is exposed to powder in the feed frame. An experimental shoe-die system was developed to examine the effect of the contributing factors. High speed video observations enabled a detailed examination of the die fill process. The flowability of powders was quantified using the concept of critical velocity. It was illustrated that a detailed understanding of die fill could contribute to the design of feed frames as well as optimisation of press parameters in order to ensure consistent and efficient die fill, thus maximising the productivity of the presses.The compaction parameters are discussed with reference to tablet strength. Results generated using a compaction simulator as well as a number of rotary tablet presses are presented for a range of pharmaceutical excipients and placebo formulations.As a result of combined interactions between the material behaviour during compaction, powder-die wall friction and process parameters during die fill and compaction, the resulting tablets are in general non-homogeneous. X-ray computed tomography is employed to characterise the internal density distribution in tablets. The effect of tablet structure on friability, erosion and disintegration behaviour is examined.     

A 2-dimensional small-angle X-ray scattering study of the microphase-separated morphology exhibited by thermoplastic polyurethanes and its response to deformation

The morphological behaviour of poly(ether-urethane)s undergoing deformation was studied using two-dimensional small-angle X-ray scattering (2D-SAXS). The data was analysed using the Zernike-Prins and Percus-Yevick models, which fitted the data well and indicated morphologies composed of discrete, elongated hard segment (HS) microdomains. Although the formulations contained relatively large HS weight fractions, relatively low volume fractions of scattering bodies were indicated, which implied limited segmental de-mixing. Possible explanations for this were discussed.Curve-fitting the 2D-SAXS data for deformation experiments using the Zernike-Prins model, indicated that HS microdomains initially became aligned with the applied strain and subsequently fragmented. This suggested a morphological basis for the observed mechanical properties of these materials, which will be explored in more detail, in subsequent publications.     

A validation procedure for numerical models of ceramic powder pressing

This paper describes an experimental procedure to validate numerical models used to simulate powder pressing. It consists mainly of two steps: closed die uniaxial pressing followed by isostatic pressing. The uniaxial pressing causes a non-homogeneous density distribution in the pressing direction as a consequence of friction between die walls and powder. In the isostatic pressing, less compacted regions have a larger volumetric strain, resulting in a non-trivial shape of the re-compacted part, which computes indirectly the previous density distribution. Experimental data from both steps are compared to the results from finite element models. The Drucker-Prager/Cap constitutive model was used to represent the compaction of alumina powder. Several simulations covering a range of parameters obtained from the literature were performed to calibrate the model, through an inverse analysis. The developed procedure sheds a light in the methods to calibrate and/or validate constitutive models used for powder pressing.     

Preparation of La0.8Sr0.2CoO3-δ sputtering targets using a deformable compaction die

Fabrication of sputtering targets deviates from the customary practices in ceramic processing as their production volumes are often quite low. The use of hot-press, in this context, greatly facilities the fabrication of sputter targets since both the density and the dimensions of the target are controlled during the pressing. In the absence of hot press, however, the fabrication requires extensive preliminary work, but difficult to justify due to limited volume production. In this study, in place of customary rigid die, we propose the use of a deformable die which greatly simplifies the fabrication procedure. In this approach, polytetrafluoroethylene (PTFE) rings are used as compaction die filled with powders, tapped to uniform density. The die is then deformed between parallel platens whereby compacting the powders. The method relies on the fact that the pressing leads to almost no change in the internal diameter of the ring. This approach was illustrated with the fabrication of 2 in. La_0.8Sr_0.2CoO_3-δ (LSC-113) target where the deformable die was dimensioned by preliminary experiments on PTFE rings of small diameter. Sputter targets of sintered density greater than 0.95 and dimensions within the tolerances of the sputter gun were successfully fabricated. It is proposed that the approach may also be applicable to flat products of irregular shape, as high friction in tapped particulate media makes the lateral flow difficult, confining the compaction mainly to axial direction.     

Evaluation of Force-Displacement Measurements during one-sided powder compaction in a Die. The quantitative evaluation of a model of die-wall friction from experiments with various diameters,with a critical examination of the influence of various other machine variables

With a microgranulation of lactose, experiments at various diameters of punches and die, namely from 7 to 17 mm, have been performed in order to determine the influence of the diameter on the pressure required to achieve a given state of density during compaction.These measurements have been performed under strictly standardised conditions, but small alterations in the experimental conditions are unavoidable as, for example, the depth o f penetration of the upper punch in the die, the state of die-wall lubrication, the precision with which punches fit in the die, the amount of powder in the die, etc- The influence of a number of 'machine variables' of this kind as well as the influence of the diameter on the pressures required for compaction have been investigated and their magnitude was established. Finally, it could be concluded that the upper punch pressure required to effect equal states of density decreased with an increase of the diameter, whereas the lower punch pressure proved to be independent o f the diameter- The experimentally obtained results could be accounted for quantitatively with an earlier published model o f die-wall friction.     

Evaluation of force—displacement measurements during one-sided powder compaction in a die — the influence of friction with the die wall and of the diameter of punches and die on upper and lower punch pressure

On an instrumented single-punch press, working at an overall rate of 30 strokes per minute, the influence of diameter of punches and die on upper and lower punch pressure, measured during the densification of a powder with an initial height of 8.0 mm, has been investigated.The upper punch pressure necessary to effect a certain state of density, proved to decrease with increasing diameter, whereas the lower punch pressure proved to be independent of the diameter. The upper punch pressure differences, found for diameters ranging from 11 to 17 mm, could not be accounted for by differences in the actual speed of compaction between the individual force displacement measurements and it could be concluded that these pressure differences were exclusively due to an influence of the diameter on the process of compaction.By using a number of hyportheses concerning friction with the die wall, it was possible to derive a mathematical model for the pressure distribution on the upper punch which qualitatively agreed with the experimental results.     

Some studies in the displacements of powder material during compaction

On the basis of the results of tribological investigations during the compaction of iron powder type NC.100.24 and of the distribution of porosity in sintered compacts, the character of powder material displacements was described for the last stage of compaction, i.e. after establishment of a preset compacting pressure. In those investigations, the model used was of the compaction of a powder material in a rotary die. The choice of such a model results from the necessity of programming an experiment with a more complex state of stresses within the mixed powder being compacted than that in standard static compaction.In the paper decisive conditions are determined for depth of penetration of shearing stresses in the compacted material, as well as conditions of change of the state of friction of the compact against the stationary mandrel surface from static to kinetic friction.     

The use of hardness in the study of compaction behaviour and die loading

An experimental relationship between density, hardness and compacting pressure is obtained for the isostatic compaction of Alcoa grade 1202 atomised aluminum powder. These results are used to evaluate the use of hardness measurement in the determination of density contours within compacts and pressure distributions at the compact—die interface. Density contours and pressure distributions are presented for closed die compacts; the results are in general agreement with those reported in the literature for more complex techniques. The technique is shown to be suitable for use in many situations.     

An integrated study of die powder fill, transfer and compaction process using digital image correlation method

A major advantage of the powder metallurgical (P/M) manufacturing process is its ability to shape powder directly into a final component with a primary goal of a high quality, homogeneity of density and mechanical properties and productivity. In this research, powder die filling, powder transfer and powder compaction process have been studied in succession using a novel experimental set-up that utilizes a high strength transparent wall section to observe and record the particle movement and powder compaction during the entire sequence leading up to the formation of a green part. The natural powder pattern itself, as observed from the transparent wall section, is utilized for obtaining full-field displacement and strain measurement. The test set-up and the strain measurement technique offer a means of quickly obtaining density distribution data in select cases. In addition to the above, several powder flow characteristics during die filling, powder transfer and powder compaction under a range of test conditions have been noted through a series of high-speed photographic recordings.The observations reveal increased porosity in the die wall region due to friction and formation of shear bridges during powder transfer stages during suction filling. Spatial density data from optical strain measurements in the top, middle and bottom regions of the die are consistent with similar bulk density measurements from mass and volume of the 3 regions.     

Roll compaction of maize powder

This paper presents a study of a roll compaction process as a dry granulation method for typical food materials such as maize powder. This process is widely applied in industry as it can continuously produce large quantities of granular product at comparatively low cost. The objectives of this work were to predict the roll compaction performance from a simple measurement involving uniaxial die compaction using the classical Johanson model. This involved determination of the optimum operating conditions for the production of granules as evaluated by apparent density.In the current work, a smooth counter-rotating rolling mill with a roller diameter of 0.08 m and a roller width of 0.20 m was used. The operating conditions for the rolling mill are shown to be influenced by parameters such as the roll gap, the roll speed, the feed powder amount, and the friction ratio. Material properties such as the compressibility factor and the angle of wall friction were investigated using uniaxial die compaction. The angle of wall friction was analysed using both contact mechanical and continuum mechanical approaches.The results indicated that this simplified approach can be used to provide a quantitative prediction of the extent of the roll compaction performance, and can be used to design optimal roller geometries and operating conditions.     

The determination of the coefficient of dynamic friction of organic powder compacts on steel

A practical experimental model system has been successfully used to study the frictional response of organic powder compacts sliding across a polished steel plate, representative of the die bore of a production compaction system. This system offers a controlled approach to the study of frictional phenomena occurring during compaction and facilitates a more detailed investigation into the fundamental mechanisms of friction than a simple resolution of forces within a punch and die apparatus.For acetylsalicyclic acid sliding on steel, the dynamic friction coefficient was found to be dependent on the displacement and, to a lesser extent, the initial normal load, whereas for PTFE on steel, the dynamic friction coefficient was independent of displacement and load and estimated at 0.09. Thus, soft organic materials exhibit very different frictional characteristics to those of brittle materials. These differences reflect the differing importance and magnitude of the three frictional components, adhesion, shear and ploughing.     

Wall friction measurement and compaction characteristics of bentonite powders

This paper presents a new method (referred to as direct method) for measuring wall friction during powder compaction and ejection. The accuracy for wall friction force measurement by both new and conventional methods (referred to as indirect method) were first studied according to the theory of error propagation. The error sources for compact density measurement in both methods were also examined. Based on the accuracy in the measurement of wall friction force and compact density, the direct method is compared favorably to the indirect method. Two bentonites, Black Hills bentonite and Zhi-Hsing bentonite which were considered as the candidate buffer materials for the geological disposal of high-level radioactive wastes were adopted to conduct a series of compaction and ejection tests. The compaction characteristics of bentonite blocks were expressed in terms of compressibility curve, wall friction ratio and friction index. The Gurnham's equation was used to describe the compressibility curve. The effects of the aspect ratio of block on the friction ratio and friction index are discussed from both experimental and theoretical point of views. Ejection profiles of the compacted bentonite blocks during ejection phase were presented. The proposed measuring method is validated by comparing the readings of ejection force recorded by both upper load cell and ring-type load cell.     

The importance of the amount/thickness of die wall lubricant for UO2 pellets pressing

External lubrication is often used to complete compaction process of powder materials. The main goal of this method is generally to reduce the amount of admixed internal lubricant (Zinc stearate powder) within the raw material. The application of external lubricants enhances the density uniformity and the mechanical strength of the resulting compacts. This study investigates the effects of the external lubricant amount for UO₂ powder compaction and the properties of the corresponding green pellets (corresponding to the compacts before sintering) without any admixed lubricant in the raw powder in order to evaluate the feasibility of this route in the case of nuclear powder. Results show that there is a quantity or number of layers from which the external lubricant applied on the die wall becomes detrimental to the friction index and the ejection force measured during the pressing cycle. The quality (surface defects, mechanical strength) of the green pellets can also be affected by the amount of lubricant. Thus the quantity and the thickness of the die wall lubricant must be optimized in order to assure an efficient mixed lubrication mode corresponding to the better lubrication mode in our study case.