The Interpretation of Powder Compaction Data - a Critical Review

Multivariate data analysis to evaluate commonly used compression descriptors

Numerous studies elucidated material behavior based on compression analyses. Especially compressibility, compactibility and tabletability were in the focus of these investigations. In the present study, a comprehensive multivariate data analysis was performed using principal component analysis method. Twelve pharmaceutically used excipients were selected for direct compression tableting and subsequent evaluation of several compression analyses. Material properties, tablet properties, tableting parameters and parameters from compression analyses were used as input variables. The materials could successfully be grouped using principal component analysis. Of the tableting parameters, the compression pressure showed the greatest influence on the results. The tabletability was found to be the most important compression analysis in the material characterization. Compressibility and compactibility only played a minor role in the evaluation. Some important insights have been gained for a deeper understanding of the tableting process using the multivariate approach to evaluate the variety of compression data.

Mean yield pressure from the in-die Heckel analysis is a reliable plasticity parameter

Despite the ability to characterize the plasticity of powders in a material-sparing and expedited manner, the in-die Heckel analysis has been widely criticized for its sensitivity to several factors, such as particle elastic deformation, tooling size, lubrication, and speed. Using materials exhibiting a wide range of mechanical properties, we show that the in-die P_y correlates strongly with three established plasticity parameters obtained from the out-of-die Heckel analysis, Kuentz-Leuenberger analysis, and macroindentation. Thus, the in-die P_y is a reliable parameter for quantifying powder plasticity in a material-sparing and expedited manner.

How can single particle compression and nanoindentation contribute to the understanding of pharmaceutical powder compression?

The deformation behaviour of a powder and, thus, of the individual particles is a crucial parameter in powder compaction and affects powder compressibility and compactibility. The classical approach for the characterization of the deformation behaviour is the performance of powder compression experiments combined with the application of mathematical models, such as the Heckel-Model, for the derivation of characteristic compression parameters. However, the correlation of these parameters with the deformation behaviour is physically often not well understood. Single particle compression and nanoindentation enables the in-depth investigation of the deformation behaviour of particulate materials. In this study, single particle compression experiments were performed for the characterization of the deformation behaviour of common pharmaceutical excipients and active pharmaceutical ingredients (APIs) with various, irregular particle morphologies of industrial relevance and the findings are compared with the results from powder compression. The technique was found useful for the characterization and clarification of the qualitative deformation behaviour. However, the derivation of a quantitative functional relationship between single particle deformation behavior and powder compression is limited. Nanoindentation was performed as complementary technique for the characterization of the micromechanical behavior of the APIs. A linear relationship between median indentation hardness and material densification strength as characteristic parameter derived by in-die powder compression analysis is found.

Investigating elastic relaxation effects on the optical properties of functionalised calcium carbonate compacts using optics-based Heckel analysis

Heckel analysis is a widely used method for the characterisation of the compression behaviour of pharmaceutical samples during the preparation of solid dosage formulations. The present study introduces an optical version of the Heckel equation that is based on a combination of the conventional Heckel equation together with the linear relationship defined between the effective terahertz (THz) refractive index and the porosity of pharmaceutical tablets. The proposed optical Heckel equation allows us to, firstly, calculate the zero-porosity refractive index, and, secondly, predict the in-die development of the effective refractive index as a function of the compressive pressure during tablet compression. This was demonstrated for five batches of highly porous functionalised calcium carbonate (FCC) excipient compacts. The close match observed between the estimated in-die effective refractive index and the measured/out-of-die effective THz refractive index supports the validity of the proposed form of the equation. By comparing the measured and estimated in-die tablet properties, a clear change in the porosity and hence, the effective refractive index, due to post-compression elastic relaxation of the FCC compacts, has been observed. We have, therefore, proposed a THz-based compaction setup that will permit in-line monitoring of processes during tablet compression. We envisage that this new approach in tracking powder properties introduced in this preliminary study will lead to the onset of further extensive and detailed future studies.

Micromechanical and physical stability analysis of an irradiated poly (lactic-co-glycolic acid) donut-shaped minitablet device for intraocular implantation

This study pragmatically characterized the micromechanical and physical stability of a poly(lactic-co-glycolic acid) (PLGA)-based ganciclovir (GCV)-loaded donut-shaped minitablet (DSMT) device for intraocular implantation. Thermal and spectroscopic analysis was performed on various drug-polymer permutations. Porositometric profiles were quantitatively analyzed coupled with qualitatively SEM imaging. The tensile strength (TS) and fracture energy (FE) of the device was also determined pre- and post-γ-sterilization. Inimitably, chemometric and molecular modeling provided a supportive confirmatory tool for establishing fundamental correlative suppositions between the transitioned surface morphology and the micromechanical stability after γ-irradiation. Isotherm plot volumes ranged between -0.028 ± 0.022 and 0.110 ± 0.005 m(2)/g for pre- and post-sterilized devices, respectively, revealing a microporous alteration in porosity. Pre-sterilized devices had larger pores (BJHa=286.22 vs. 192.49 Å) and lower FE (151.301 ± 6.089 N/m) and TS (26.396 ± 1.062 N) values while sterilized devices had crystalline matrices that facilitated the superiorly controlled drug release kinetcs obtained. DSC thermograms displayed the characteristic disordered crystallization of GCV and hydration exotherms resulting from ionization during γ-irradiation. FTIR spectrograms showed fingerprint molecular imprints of GCV and axial stretching of hybridized carbons of PLGA with no subversive drug-polymer interactions after γ-irradiation. Integration of the results inveterately revealed that compression and subsequent γ-irradiation of the device affected desirable micromechanical and solid-state stability behavior.

Benefits of die-wall instrumentation for research and development in tabletting

Instrumented presses used in tabletting research and development are normally equipped to measure punch force and displacement. Die-wall monitoring is rare, probably because instrumentation and calibration are quite difficult. The authors critically examine the tenets of radial pressure measurement in compression physics. The theoretical background concerning axial to radial stress transmission during the different phases of the compression cycle is presented. The literature reporting on the use of radial stress measurement to assess the self-lubricating properties of materials or the effect of lubricants is reviewed. Examples of interpretation of radial pressure cycles to define the basic material behaviour are given. The influence of particle size and shape as well as that of process and formulation variables on die-wall response are also discussed. Substantial inconsistencies can be seen in the literature with respect to the interpretation of experimental data, often because of the poor reliability of results and mostly because powders are essentially not solid, isotropic bodies. There is also a distinct lack of complementary tabletting parameters that would help understanding their comparative benefits. For this reason, original data on 13 model compounds are presented together with a classification of the materials encountered in pharmaceutical tabletting, based on selected parameters. In conclusion, none of the determined parameters, including those derived from radial pressure measurement, is able, alone, to predict the material behaviour under compression. Although die-wall instrumentation contributes little to the development of improved tablet formulations, it is valuable for characterising the mechanical properties of the materials. This is particularly advantageous given that the mechanical properties account for variations in tabletting performance to a much greater extent than the magnitude of the interparticulate attractions. Nevertheless, because of the peculiar nature of powders compared to solid, isotropic bodies, there is a need to develop new models for analysing their behaviour and to put more emphasis on examination of time-dependent deformation in the later stage of the compression cycle.

Tabletting of pellet-matrix systems: ability of parameters from dynamic and kinetic models to elucidate the densification of matrix formers and of pellets

Two different types of pellets, i.e. drug-free sugar spheres, and pellets, spray-layered with crystalline theophylline and coated with Eudragit RS/RL, were tabletted each in combination with matrix-forming powder mixtures of Avicel PH200 and PEG 4000. The die fills from pellets and powder mixtures were regarded as two-compartment systems with a volume fraction of the pellets being limited to 0.52 corresponding to a cubic lattice, and the maximum degrees of densifications were adjusted related to the matrix. To data measured during single compression cycles on an instrumented eccentric tabletting machine and transformed appropriately, the Kawakita equation, the Heckel function, and a modified Weibull function were fitted, and the total work of compression was calculated. The Kawakita model fitted well systems with both types of pellets. Its parameters reflected the additional densification of the theophylline pellets separately from that of the matrix formers. The Heckel function could only be applied to systems containing non-porous sugar spheres, since the theophylline pellets underwent considerable densification and deformation. Only, when the Heckel porosity function was related to the volume fraction of the matrix, excluding the sugar spheres, the approximately linear regions for mixtures with increasing volume proportions of sugar spheres occured in comparable regions of densification. Parameters of the modified Weibull function demonstrated an increasing resistance against densification with increasing amounts of pellets. The total work of compression increased steeply with increasing volume fractions for pellets from 0.42 to 0.46 indicating, that the resistance against densification already rose when the pellets were still isolated. In conclusion, the combination of dynamic and kinetic models provides a comprehensible insight into the process of tabletting powder mixtures with pellets. Particularly, the Kawakita model was a suitable tool to differentiate the actual changes in porosity during compression from the compressibility of such complex systems.

Influence of granulating method on physical and mechanical properties, compression behavior, and compactibility of lactose and microcrystalline cellulose granules

The physical and mechanical properties of lactose (LC) and microcrystalline cellulose (MCC) granules prepared by various granulating methods were determined, and their effects on the compression and strength of the tablets were examined. From the force-displacement curve obtained in a crushing test on a single granule, all LC granules appeared brittle, and MCC granules were somewhat plastically deformable. Inter-granular porosity epsilon inter clearly decreased with greater spherical granule shape for both materials. Decrease in intragranular porosity epsilon intra enhanced the crushing force of a single granule Fg. Agitating granulation brought about the most compactness and hardness of granules. In granule compression tests, the initial slope of Heckel plots K1 appeared closely related to ease of filling voids in a granule bed by the slippage or rolling of granules. The reciprocal of the slope in the succeeding step 1/K2 in compression of MCC granules indicated positive correlation to Fg, while in LC granules, no such obvious relation was evident. 1/K2 differed only slightly among granulating methods. Tensile strength of tablets Tt obtained by compression of various LC granules was low as a whole and was little influenced by granulating method. For MCC granules, which are plastically deformable, tablet strength greatly depended on granulation. Granules prepared by extruding or dry granulation gave strong tablets. Tablets prepared from granules made by the agitating method showed particularly low Tt. From stereomicroscopic observation, the contact area between granule particles in a tablet appeared smaller; this would explain the decrease in inter-granular bond formation.

Thermodynamic analysis of compact formation; compaction, unloading, and ejection. I. Design and development of a compaction calorimeter and mechanical and thermal energy determinations of powder compaction

The aim of this investigation was to determine and evaluate the thermodynamic properties, i.e. heat, work, and internal energy change, of the compaction process by developing a 'Compaction Calorimeter'. Compaction of common excipients and acetaminophen was performed by a double-ended, constant-strain tableting waveform utilizing an instrumented 'Compaction Simulator.' A constant-strain waveform provides a specific quantity of applied compaction work. A calorimeter, built around the dies, used a metal oxide thermistor to measure the temperature of the system. A resolution of 0.0001 degrees C with a sampling time of 5 s was used to monitor the temperature. An aluminum die within a plastic insulating die, in conjunction with fiberglass punches, comprised the calorimeter. Mechanical (work) and thermal (heat) calibrations of the elastic punch deformation were performed. An energy correction method was outlined to account for system heat effects and mechanical work of the punches. Compaction simulator transducers measured upper and lower punch forces and displacements. Measurements of the effective heat capacity of the samples were performed utilizing an electrical resistance heater. Specific heat capacities of the samples were determined by differential scanning calorimetry. The calibration techniques were utilized to determine heat, work, and the change in internal energies of powder compaction. Future publications will address the thermodynamic evaluation of the tablet sub-processes of unloading and ejection.

Effects of true density, compacted mass, compression speed, and punch deformation on the mean yield pressure

Compressibility properties of pharmaceutical materials are widely characterized by measuring the volume reduction of a powder column under pressure. Experimental data are commonly analyzed using the Heckel model from which powder deformation mechanisms are determined using mean yield pressure (Py). Several studies from the literature have shown the effects of operating conditions on the determination of Py and have pointed out the limitations of this model. The Heckel model requires true density and compacted mass values to determine Py from force-displacement data. It is likely that experimental errors will be introduced when measuring the true density and compacted mass. This study investigates the effects of true density and compacted mass on Py. Materials having different particle deformation mechanisms are studied. Punch displacement and applied pressure are measured for each material at two compression speeds. For each material, three different true density and compacted mass values are utilized to evaluate their effect on Py. The calculated variation of Py reaches 20%. This study demonstrates that the errors in measuring true density and compacted mass have a greater effect on Py than the errors incurred from not correcting the displacement measurements due to punch elasticity.

Examination of the compaction properties of a 1:1 acetaminophen:microcrystalline cellulose mixture using precompression and main compression

The compaction properties of a 1:1 acetaminophen and microcrystalline cellulose (MCC) mixture have been studied using a compaction simulator to make tablets by single compression and by a combination of precompression and main compression. The tensile strengths of the tablets and the energies involved in the compressions were determined. The tensile strengths of the tablets increased with increases in single compression pressure from 80 to 400 MPa and as the total applied pressure increased from 80 MPa up to around 400 MPa when combinations of precompression and main compression pressures were used. The tablet porosity decreased with increase in main compression pressure while the tablet tensile strengths increased. At minimum tablet porosity, further increase in main compression pressure could no longer result in increase in tablet strengths. Tablets compressed with combinations of precompression and main compression were stronger (2.15 +/- 0.02 to 3.99 +/- 0.1 MPa) than those produced with single compression (0.73 +/- 0.01 to 3.09 +/- 0.05 MPa). The total gross energies of compression increased with an increase in pressure of both the precompression and main compression. The elastic energies during main compression increased with an increase in precompression pressure as the tablet exhibited greater elastic deformation and reduced plasticity on second compression. The increase in elastic energies during main compression may also be because elastic energy is recoverable and is independent of precompression. As the precompression pressure increased, the minimum tablet porosity was attained; hence, the plastic energy during main compression became smaller while the elastic energy increased. Thus, a combination of low precompression and main compression pressures of 160/80 MPa or 80/160 MPa are more advantageous in the tableting of the 1:1 acetaminophen:MCC than a high single compression pressure of 320 or 400 MPa.

Tableting characteristics of micro-aggregated egg albumin particles containing paracetamol

The tableting characteristics of micro-aggregated egg albumin particles containing paracetamol were evaluated and compared with non-micro-encapsulated paracetamol and coagulated egg albumin particles. Mean yield pressure values of micro-aggregated egg albumin particles containing paracetamol and coagulated egg albumin particles were 30.5 and 49.3 MPa, respectively, which were lower than the mean yield pressure obtained for paracetamol (97.5 MPa). Paracetamol tablets obtained with micro-aggregated egg albumin particles did not show the capping characteristic of conventional paracetamol tablets. Crushing strength of paracetamol tablets obtained with egg micro-aggegated particles was similar to that obtained using paracetamol granulated with povidone and gelatin as binders at 3 and 6% (w/w) concentrations. Drug release from the paracetamol tablets depends on the choice of excipients. Crospovidone showed good protective characteristics for the tableting of micro-aggregated particles. Crushing strength of paracetamol tablets formed from egg albumin-coated particles could be increased using crospovidone or microcrystalline cellulose as fillers and was decreased by the use of magnesium stearate. Nevertheless, magnesium stearate was useful to decrease the ejection force.

Comparison of calculated and experimentally determined punch displacement on a rotary tablet press using both Manesty and IPT punches

An indirect method of calculating punch displacement on a rotary tablet press from measurements of the change in punch force with the turret position was in good agreement with direct measurements of punch displacement made using a linear variable displacement transducer (LVDT)-slip ring system. The direct measurements were made during the compaction of three direct compression agents using Manesty punches. However, the agreement between calculated and experimentally determined punch displacements was unsatisfactory when IPT punches were used. The IPT punches have a much flatter punch head profile than the Manesty punches. Due to this difference, the analytic equation does not accurately describe the dynamics of the press under normal operating conditions. Terms in the analytic equation, determined originally under static conditions, were re-evaluated under dynamic conditions for both sets of tooling using the LVDT-slip ring system. Excellent agreement for both IPT and Manesty punches was found between punch displacement calculated using the revised analytic equation and direct experimental measurements. Punch displacements determined from punch head profile and machine geometry only, without taking machine deformations into account, were shown to differ widely from the calculated and experimental values.

Calculation of punch displacement and work of powder compaction on a rotary tablet press

To calculate the work of compaction during tableting it is necessary to have accurate values of force and punch displacement. The direct measurement of punch displacement on a rotary press is both costly and complicated but calculated displacements will be in considerable error unless deflections in the press during compression, are taken into account. By analysing the physical restraints imposed on the punches during tablet compression, an expression for punch displacement was derived. From preliminary measurements made on the table press of machine deflections and punch displacement under static conditions, the terms of this expression were evaluated for dynamic conditions. This analytic solution was then used to determine the true punch displacement and work of compaction from direct measurements of vertical force and turret position.

Work of friction and net work during compaction

Series of tablets were compressed in a reciprocating tablet machine with a gradually increasing die wall friction. The force needed on the upper punch to maintain the tablet dimensions constant increased with the die wall friction while the lower punch force decreased. The change in punch forces due to differences in die wall friction had no effect on the tablet strength. The net work of compaction should be constant under these circumstances. The net work calculated by subtracting the work of friction and the expansion work from the gross work input was constant when the frictional work was calculated according to one of the two equations proposed in the literature (Järvinen & Juslin 1974) while the other appears to give an overestimation of the work of friction.