The pharmaceutical industry relies heavily on the high-quality manufacture of solid dose tablets. Monitoring a production batch’s bulk density thus becomes vital, since inaccuracy can result in production losses.
The Haussner Ratio and Carr Index are two of the most common gages to determine flowability in the pharmaceutical sector. Flowability values are therefore computed using fill and tapped bulk density results.
A commonly computed measure is tapped bulk density, which is obtained by tapping a cylinder of material several times and determining the change in volume. However, since this process requires human intervention, it can be prone to errors in bulk density reading.
For instance, when either the Carr Index or Hausner Ratio are used to gage powder flow, an incorrect computation of bulk density will produce erroneous values. While there isn’t a single precise direct test for these powder flow parameters, it is evident that the accuracy of the initial measurements can be improved.
Below, a precise technique for defining fill and final bulk density is described, as well as a more precise way to compute a Hausner Ratio and Carr index.
Importance of Bulk Density
One of the key tests used by the bulk solids industry is bulk density, which also finds application in tablet manufacturing. Bulk density is used to determine fill bulk densities and tapped bulk density (in other words, final compaction). The test can also be used to gage a product’s flowability. The latter is done by computing the Hausner Ratio or Carr Index using this single value as a correlation.
A Haussner Ratio above 1.25 indicates poor flowability, while a Carr Index above 25 clearly indicates poor flowability. However, a Carr Index below 15 is an indication of good flowability.
Thus, it is critical to calculate accurate bulk density because:
- To enable the exact calibration of tablet making equipment in order to ensure the right amount of force exerted on a powder after it has flowed into a dye
- To account for changes in the material’s bulk density owing to environmental conditions such as high humidity or temperature, which may have created an inferior quality product
- To avoid the product to cap or split in cases where the bulk density increases and the tablet making equipment calibrated for a particular force may be unable to compress the powder for tablet production
- To avoid tablet crushing when the bulk density is low and the force exerted during production is higher than necessary
- To prevent significant losses in revenue at high production volumes
Due to the subjectivity in operator judgment and lack of uniformity in tap force during the test, the tapped bulk density test can be inaccurate. Thus, there is an urgent need for a scientific bulk density test method that delivers repeatable results without direct operator involvement. The solution? Shear cell technology that uses an annular shear cell and a wall friction lid.
Shear Cell Technology: Bulk Density, Compressibility Index and Compressibility Ratio
Frequently used in the bulk solids industry, shear cells have the advantage of offering a repeatable and accurate test method to assess flowability. ASTM D6128 explains the basic principle for testing powder using shear cell methodology.
To a sample of powder contained in a trough, an annular shear cell applies several compressive loads. This is done to simulate the consolidation effect of the powder’s self-weight in a bin. With time, the powder in the bin settles and the particles move closer to each other.
Thus, the shear cell can determine friction between particles using a vane lid that shears the particles against each other. This resulting measurement provides the powder’s flowability value, represented as a function of consolidating stress.
Alternatively, the powder sample in the trough can be compressed using an annular shear cell with a flat-surface lid. This is referred to as a wall friction lid, since its surface is identical to that of construction material in the hopper wall for the bin.
A density test on the powder sample in the trough can also be done by using the wall friction lid. This is done by determining the volume reduction of the powder sample with each increase in consolidating stress.
The shear cell supplies complete data – from the initial fill condition to the final bulk density at the maximum consolidating stress – on bulk density. An annular shear cell is depicted in Figure 1 while Figure 2 shows the wall friction lid.
Figure 1. Brookfield Powder Flow Tester with Shear Cell
Figure 2. Brookfield Wall Friction Lid
Further, a wall friction lid performing a standalone bulk density test can be used for users that require data that is comparable to the tap test for calculating Hausner Ratio and Carr Index. Lasting for 100s, the test supplies accurate information for bulk density – including final bulk density and fill density.
The next step involves using the density data to determine a Compressibility Index (which is a measurement similar to a Carr Index) and Compressibility Ratio (similar to Hausner’s ratio). As per Figure 3, sample calculated values for Compressibility Index and Compressibility Ratio are obtained using software that works with the instrument in Figure 1.
Figure 3. Graph Shows compressibility index and ratio
Once a material’s bulk density is characterized, QA/QC tests need to be run in order to compare new material batches as they are being produced.
For example, if the control sample’s initial fill bulk density is 520 kg/m3 with a final bulk density of 784 kg/m3 at the maximum consolidating stress, there needs to be a comparison among these values and subsequent density checks on future batches of material.
Figure 4 portrays the bulk density graphical curves of two powders. The first is the control sample while the other is a new sample under study.
Figure 4. Graph shows bulk density comparison
Pass or fail criteria can be applied to new samples with the help of limits based on the control sample. For instance, the bulk density of the new batch of material comes with an initial fill density of 695 kg/m3 and a final bulk density of 1018 kg/m3.
If these values do not fall between the established pass/fail criteria for the reference powder, the tablet making equipment will fail at compressing the product sufficiently. As a consequence, the product will cap.
What’s more, if such a situation arises before manufacturing, the product can be successfully reformulated or the tablet making press can be recalibrated. Indeed, the compressibility index and compressibility ratio are much more accurate measures than Carr Index and Hausner Ratio.
This is because they depend on definitive consolidating stresses applied to the sample using a flat, wall friction lid. While the compressibility index and compressibility ratio may not match historical Carr Index and Hausner Ratio data, when applied to the same powder samples they tend to exhibit similar trends.
As per Figure 5, the density data curve from a test run can be seen with the annular shear cell in Figure 1. The Compressibility Index and Compressibility Ratio are determined using formulae that are similar to Carr Index and Hausner Ratio. Parameter inputs are used in the form of the initial fill bulk density and final bulk density values. Moreover, the tapped bulk density, which was used to calculate Carr Index and Hausner Ratio, is most likely to be a value in the highlighted circle, as seen in Figure 5.
Figure 5. Initial Fill and Final Bulk Density as compared to Tapped Bulk Density
As demonstrated, bulk density is a significant measurement in the manufacture of solid dose tablets. Importantly, a change in bulk density can cause adverse effects on productivity by causing rework of material or loss of product altogether, thus resulting in lost revenue.
The advantages of shear cells include lack of operator involvement, a quick and accurate bulk density test and a more complete and accurate picture of changes in bulk density. This can be achieved through the use of a shear cell with wall friction lid.
While the pharmaceutical industry has used the Hausner Ratio and Carr Index for decades to gage powder flowability, today, there exists a much more precise method to compute these values automatically. With these results, the comparable Compressibility Index and Compressibility Ratio can be effectively applied to the manufacturing process.
About AMETEK Brookfield
Brookfield, a business unit of AMETEK Inc, is the world leader in viscosity measurement and control of liquids and semi-solids for 80+ years!
We manufacture and distribute globally viscometers the AST 100 for advance sensor technology for simple, direct in-line viscosity measurement, DV2T Touch Screen with temperature measurement, rheometers DV3T for measuring yield stress and viscosity, RST Controlled Stress for challenging rheological measurements, CT3 Texture Analyzers which features compression and tension mode for measuring firmness, springiness and chewiness and PFT Powder Flow Testers which measure yield stress and viscosity flow index and arching dimension used in R&D, QC and inline applications for rheological fluid analysis, tension and compression and powder flow analysis.
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