Estimation of Elastic Recovery, Work of Decompression and Young's Modulus Using a Rotary Tablet Press

A modified Kushner-Moore approach to characterising small-scale blender performance impact on tablet compaction

Continuous Direct Compaction (CDC) has emerged as a promising route towards producing solid dosage forms while reducing material, development time and energy consumption. Understanding the response of powder processing unit operations, especially blenders, is crucial. There is a substantial body of work around how lubrication via batch blender operation affects tablet critical quality attributes such as hardness and tensile strength. But, aside from being batch operations, the design of these blenders is such that they operate with low-shear, low-intensity mixing at Froude number values significantly below 0.4 (Froude number Fr being the dimensionless ratio of inertial to gravitational forces). The present work explores the performance of a mini-blender which has a fundamentally different mode of operation (static vessel with rotating blades around a mixing shaft as opposed to rotating vessel with no mixing shaft). This difference allows a substantially wider operating range in terms of speed and shear (and Fr values). The present work evaluates how its performance compares to other blenders studied in the literature. Tablet compaction data from blends produced at various intensities and regimes of mixing in the mini-blender follow a common trajectory. Model equations from literature are suitably modified by inclusion of the Froude number Fr, but only for situations where the Froude number was sufficiently high (1 < Fr). The results suggest that although a similar lubrication extent plateau is eventually reached it is the intensity of mixing (i.e. captured using the Froude number as a surrogate) which is important for the lubrication dynamics in the mini-blender, next to the number of revolutions. The degree of fill or headspace, on the other hand, is only crucial to the performance of common batch blenders. Testing using alternative formulations shows the same common trend across mixing intensities, suggesting the validity of the approach to capture lubrication dynamics for this system.

Understanding the implication of Kawakita model parameters using in-die force-displacement curve analysis for compacted and non-compacted API powders

The aim of this study was to investigate powder mechanics upon compression using data obtained from force-displacement (F-D) curves. The Kawakita model of powder compression analysis was adopted in order to compare the pressure-volume reduction relationship of the drug powders in relation to the F-D curves. Experiments were carried out on six model drugs (metronidazole, metformin, secnidazole, ciprofloxacin, norfloxacin, and mebeverine). The drugs were compressed at different pressures in the non-processed or processed (using a roller compactor) forms. Results indicate the similarity between the F-D curves and a rearranged form of the Kawakita model. The foregoing enables the calculation of two important powder parameters, “a” (maximum powder volume reduction) and “Pk” (pressure required to achieve half of the maximum volume reduction) from the F-D curves without the need, as in the case of the conventional Kawakita model, to compress powders into tablets at different compression forces.

Graphical abstract

Tablet capping predictions of model materials using multivariate approach

The present study demonstrated the prediction of predominant root causes of capping behavior as a function of the powder rheological and the mechanical behavior of Acetaminophen (APAP) and Ibuprofen (IBU). The authors analyzed powder rheological properties for powder blend permeability, pressure drop, and cohesion. The measured deformation properties were compact porosity, internal air pressure, Brinell hardness, and tensile strength. The data were evaluated qualitatively and quantitatively using multivariate techniques, such as principal component analysis (PCA) and principal component regression (PCR) models, respectively, to identify the effect of powder air entrapment efficiency and mechanical behavior on the tablet capping score. The PCA model indicated that pressure drop, cohesion, API amount, and compression pressure correlated positively, whereas permeability, porosity, internal air pressure, Brinell hardness, and tensile strength correlated negatively to the capping potential. APAP and IBU also showed two independent mechanisms as a function of their amount on the capping score at all compression pressures. APAP and IBU followed an exponential and linear relationship, respectively. Furthermore, the dominant powder rheological and deformation behavior affecting the capping score of each material was identified and quantified using two separate PCR models. These models showed that APAP capping was predominantly dependent on its powder properties, while that of IBU was predominantly based on its deformation properties. In conclusion, APAP and IBU compacts capping had respective air induced and deformation induced capping behavior. The proposed approach can aid in understanding the underlying mechanisms of capping and developing an effective, optimized strategy to ensure tablet quality.

Powder Compression Properties of Paracetamol, Paracetamol Hydrochloride, and Paracetamol Cocrystals and Coformers

The objective was to study the relationship between crystal structure, particle deformation properties, and tablet-forming ability for the monoclinic form of paracetamol (PRA), 2 cocrystals and a salt crystal of PRA in addition to 2 coformers (oxalic acid and 4,4'-bipyridine). Thus, the structure-property-performance relationship was investigated. Analytical powder compression was used for determination of effective plasticity, as inferred from the Heckel yield pressure and the Frenning parameter, and the elastic deformation was determined from in-die tablet elastic recovery. The plasticity could not be linked to the crystal lattice structure as crystals containing zig-zag layers displayed similar plasticity as crystals containing slip planes. In addition, crystals containing slip planes displayed both high and low plasticity. The mechanical properties could not be linked to the tablet-forming ability as the tablet tensile strength, unexpectedly, displayed a tendency to reduce with increased plasticity. Furthermore, the elastic deformation could not explain the tablet-forming ability. It was concluded that no relationship between structure-property-performance for PRA and its cocrystals and salt could be established. Thus, it was indicated that to establish such a relationship, an improved knowledge of crystallographic structure and interparticle bonding during compaction is needed.

Direct compaction: An update of materials, trouble-shooting, and application

Direct compaction (DC) is the preferred choice for tablet manufacturing; however, only less than 20% of active pharmaceutical ingredients could be compacted via DC as its high requirement for functional properties of materials. Materials with improper functionalities could lead to serious troubles during DC manufacturing, such as content non-uniformity, sticking, and capping, all of which profoundly affect the properties of final products and, thus, severely restrict the practical application of DC. With undoubted importance, these seem to be unexpectedly ignored by reviewers but not researchers in terms of many original research articles published recently. Therefore, as an informative supplement and update, this review mainly focused on trouble-shooting and application situation of DC, together with several newly reported materials.

The reality of in-line tablet coating

The possibility of continuous processing in pharmaceutical tablet manufacturing is hampered by the viscoelastic recovery of tablets post-compaction. Compacted tablets are typically aged before coating to allow complete viscoelastic recovery so as to avoid subsequent coating defects. There has been little attempt to overcome tablet recovery in order to enable continuous processing and improve manufacturing efficiency. However, with the introduction of improved or newly developed types of tablet-coating equipment, there is renewed interest in the coating of tablets in-line. In-line tablet coating is defined as the coating of tablets immediately after compaction. It is a one-step highly integrated system that circumvents the delay in processing time typically given to allow viscoelastic recovery of tablets. This review aims to summarize the requirements of an in-line tablet-coating system. The possibility of carrying out in-line tablet coating in the near future will also be discussed.

The effect of compression and decompression speed on the mechanical strength of compacts

The purpose of this work was to investigate the effect of punch speed on the compaction properties of pharmaceutical powders; in particular, to separate out differences between the effect of the compression and decompression events. Tablets were prepared using an integrated compaction research system. Various "sawtooth" punch profiles were followed to compare the effects of different punch speeds on the crushing strength of the resulting tablets. The loading and unloading speeds were varied independently of one another. In general, when the compression speed was equal to the decompression speed, the tablet crushing strength was observed to decrease as the punch velocity increased. When the compression speed was greater than or less than the decompression speed, the results varied, depending on the material undergoing compaction. Reduction of the unloading speed from 300 to 10 mm/sec for pregelatinized starch and microcrystalline cellulose produced a significant increase in crushing strength, whereas no significant increase in crushing strength was observed until the loading speed was reduced to 10 mm/sec. Reduction of the unloading speed had a similar effect on the direct compression (DC) ibuprofen, however, even greater improvement in the crushing strength was observed when the loading speed was reduced. No improvement in the DC acetaminophen tablets was observed when the unloading speed was reduced, however, a significant increase in crushing strength was produced when the rate of loading was reduced. This work showed that the strength of tablets can be improved and some tableting problems such as capping can be minimized or prevented by modifying the rates of loading/unloading.

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.

Characterisation of particle properties and compaction behaviour of hydroxypropyl methylcellulose with different degrees of methoxy/hydroxypropyl substitution

The particle characteristics and compaction behaviour of hydroxypropyl methylcellulose (HPMC) powders from two different suppliers were studied regarding effects of methoxy/hydroxypropyl substitution. Samples included Methocel K4M (low substitution ratio), E4M (medium) and F4M (high) and the corresponding substitution ratios from Metolose: 90 SH 4000, 60 SH 4000, and 65 SH 4000. Characterisation of the particle properties and compaction behaviour of the pure polymers suggested that reported differences in drug release behaviour of Methocel E4M compared with the other two powders may be related to the lower powder surface area, differing particle morphology and lower fragmentation propensity during compaction. In addition, compacts of Methocel E4M were weaker when tested in both axial and radial directions and had different porosity and elastic recovery properties. There were no differences between the polymers in degree of disorder, as evaluated by solid-state nuclear magnetic resonance spectroscopy. The different behaviour of Methocel E4M could, however, be related to the overall higher total degree of substitution of this polymer and in particular the high content of methoxy groups compared to the other polymers. The methoxy substituent is hydrophobic and may, when present in sufficiently high concentrations, change the particulate and mechanical properties of the powder, thus potentially affecting the compactability. The high content of methoxy groups might also decrease the development of inter- and intraparticulate hydrogen bonds during compaction, and suppress the actions of the hydrophilic hydroxypropyl groups, both of which could affect drug release.

The compression of spheres coated with an aqueous ethylcellulose dispersion

Tablets were compressed from commercial samples of Sugar Spheres NF, Sucrose NF, Corn Starch NF, as well as ground spheres and a physical mixture of ground sucrose plus cornstarch. Additional tablets were compressed from spheres that had been coated with a water-soluble cellulosic polymer solution followed by an aqueous ethylcellulose dispersion. Tableting parameters measured "in-die" included work of compression, peak offset time, tablet density, and Young's modulus. Following ejection, tensile strength was determined under diametrical loading. Dissolution of a marker contained in the water-soluble layer was determined for both compressed and uncompressed spheres. Porosities at peak pressure and peak offset times or tensile strength as functions of peak pressure did not differ between tablets compressed from pristine spheres or from ground spheres. Tablets compressed from spheres had higher values for porosity, tensile strength, and peak offset time than those compressed from sucrose or the sucrose: starch mixture. Values for work of compression were higher for tablets compressed from pristine spheres or from starch. This was attributed to the work required for particle deformation and for breaking of the spheres. The greatest elastic recovery during decompression was observed for tablets compressed from pristine spheres or starch. More brittle behavior was observed for tablets compressed from sucrose or the sucrose: starch mixture. Tablets compressed from ground spheres were more brittle than those compressed from the pristine spheres, indicating an effect due to grinding. Most mechanical properties of tablets compressed from the coated spheres were comparable to those of tablets compressed from uncoated spheres. An exception was diametric strain for tablets compressed from spheres coated with the aqueous ethylcellulose dispersion. These values increased since the plasticized ethylcellulose allowed greater distortion of the tablet before failure occurred. The dye marker was released more rapidly from tablets compressed from spheres coated with the aqueous ethylcellulose dispersion than from comparable uncompressed spheres. At both the 5% and 10% coating levels, spheres coated with the aqueous ethylcellulose dispersion fused into nondisintegrating matrices during compression. Little difference in release rates was seen between the two tablets.

The use of energy indices in estimating powder compaction functionality of mixtures in pharmaceutical tableting

A series of binary powder blends comprising of microcrystalline cellulose (Avicel PH101), alpha-lactose monohydrate or theophylline anhydrous were prepared in order to investigate the densification of binary pharmaceutical powder mixes under compaction pressure. It is postulated that the use of derived energy parameters, as well as various evolved indices, calculated from the work expended during the fabrication and/or rupture of a compact can be employed to quantitatively predict the compaction properties of pharmaceutical powder mixes comprised of the same constituents. The relationship between the net work of compression normalized to powder volume and the resulting compact strength for mix constituents can be used to define a pharmaceutical formulation space in which compact mechanical properties can be estimated for other 'virtual mixes' of the same constituents in different proportions. The approach is successfully applied to the prediction of the mechanical properties of a ternary mix of these constituents.

Mechanical properties of powders for compaction and tableting: an overview

This review provides an insight into mechanical properties that are critical to understanding powder processing for tableting. Various parameters that reflect these basic fundamental properties of powder and their evaluation by different techniques are described. Some recent examples in which these techniques are used in drug substance selection, formulation optimization or scale-up are also provided.

Force-time curves of a rotary tablet press. Interpretation of the compressibility of a modified starch containing various amounts of moisture

On a rotary tablet press, the force-time curves are segmented into three phases: the compression phase, the dwell phase during which both stress and strain are variable for plastically deforming materials and the decompression phase. The following seven parameters were investigated: the compression area (A1) and the compression slope (Slc) describing the initial phase, the area ratio (AR) and the peak offset time (t(off)) characterizing the dwell time, the decompression area (A4) and the decompression slope (Sld) describing the terminal phase and the total area under the force-time curve (Atot). AR, t(off), Slc and A1 (the last with limitations) are used for phase-specific allocation of the occurrence of plastic flow, which is found to be a function of compression force and moisture content. Tablet strength, tablet porosity and in-die bulk porosity provide additional information for comprehensive interpretation. The values of A4 for the four starch batches are not significantly different. Sld provides somewhat better information about the elastic compact recovery. In general, however, the short decompression phase seems to be inappropriate for characterization by force-time curve parameters, because it is difficult to separate machine recovery from that of the tablet. Porosity above the porosity limit of the material was found to be a prerequisite for plastic flow within the compact. When the porosity limit is reached, further densification remains elastic and leads to a reduced compact strength during expansion. The area ratio, as a robust in-process control parameter for plastically flowing formulations, is suggested as a means of preventing this effect.

A new method of estimating volume during powder compaction and the work of compaction on a rotary tablet press from measurements of applied vertical force

The volume per unit mass of a powder bed, V', during compaction on a rotary tablet press has been expressed as a function of pressure, P using a modification of Kawakita's equation: V = (V'o-V')P'/(P+P') + V', where V'o, V' and P' constitute a set of unique values for a given powder or powder mix under specified tableting conditions. The volume, V, is determined from the machine deformation constant which is the relationship between applied vertical force and the deformation of the tablet press and the punches. An iterative method is described which allows the determination of V'o, V' and P' from the slope and intercept of V vs 1/(P+P') where all values are evaluated at peak pressure. By substituting these values into the equation, the volume of a given powder bed during compaction up to peak pressure can be accurately predicted from the pressure vs time curve. This method of estimating volume and hence punch displacement, is much simpler than an earlier analytical method which was derived from direct measurements of punch displacement under running conditions. Since volume is an explicit function of pressure, the work of compaction is also a function of pressure. Estimates of the work of compaction are in good agreement with values calculated using our previous method. Values of V'o, V' and P' are reported for 35 pharmaceutical materials and could be incorporated into a database library of drugs and tableting excipients. This database could then be used for the quality control of incoming raw materials (batch to batch assessment) and for the comparison of materials from alternative sources.(ABSTRACT TRUNCATED AT 250 WORDS)