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Traditional tablet presses do not measure tablets' tensile strength, yet this characteristic strongly influences tablet quality. The author describes a compression technique that accounts for tensile strength and produces tablets with consistent weight and disintegration time.
Controlling tablets' individual weight is critical to tablet pressing. Two factors that are difficult to control, yet are fundamental to a tablet's bioavailability and clinical effect, are tensile strength and disintegration rate. This article will examine a recent development in tablet compression that enables consistent tablet production. The article will explain how the development's flexibility can take into account the critical factors in tablet production and improve tablet quality.
Traditional tablet presses measure a tablet's weight either by measuring its variation in tablet height at precompression or the force at main compression. These techniques, however, overlook one fundamental characteristic: tensile strength. A tablet's tensile strength influences its bioavailability and clinical effect, which are crucial characteristics for any dosage form.
Compression to equal thickness
All rotary tablet presses on the market use the same compression principle. They compress a volume of powder, captured in a die, between two rollers using an upper and lower punch.
By changing the distance between the rollers, one adjusts the force used to compress tablets. Once the distance between the rollers is set, the compression force will stay the same, thus ensuring that tablets are compressed to an equal thickness (ET).
Several factors affect the precise amount of powder that is captured within each die. For example, granule size, size distribution, and variation in punch length can have an effect. In addition, an excessively high rotational speed allows the powder insufficient time to fall into the die. Variations in the amount of powder in each die result in tablets of different weights and densities, and these tablets require different forces for compression to ET.
Figure 1 shows a simple force graph for different tablet weights. Tablet weight variations cause different forces to be applied to the tablet because of the fixed distance between the rollers. Note that although the weight tolerance is symmetrical, the forces are not. The force limits are clearly visible in the figure.
Khan and Rhodes showed that the magnitude of the compression force affects tablet hardness and disintegration time (1). An increase in compression force produces an increase or decrease in disintegration time, depending on the formulation. These results show that an inconsistency in the maximum compression force can result in inconsistent tablet properties, thus affecting the efficacy of the dosage form.
Compression to equal force
Compression to equal force (EF) is a new concept that allows tablets to be compressed at the same peak compression force, independent of tablet weight. This method relies on an air compensator. (see Figures 2 and 3).
The air compensator is installed at the precompression and main compression stations. The assembly is attached to a compression roller mounted on a piston. The piston moves vertically in a cylinder filled with compressed air. The air pressure in the cylinder is preset and kept constant by a pressure-regulating valve and an expansion chamber. Because the surface of the cylinder and the air pressure are constant, the force is also constant.
If the air pressure in the cylinder is such that this maximum compression force is higher than the actual compression force, the system compresses tablets to ET. On the other hand, the press can be set up so that the distance between the two rollers and the air pressure in the air compensator make the compression roller move up at each compression.
In this arrangement, all tablets are compressed at the same peak force, which is determined by air pressure and the distance between the two rollers (see Figure 4). The thickness of individual tablets might vary as their weight varies. Variance is measured and kept within limits, however, to ensure that no problems arise during packaging. The tablet press is equipped with a linear variable displacement transducer sensor that measures compression-roller movement accurately.
A heavy tablet causes the compression roller to move more, a light tablet causes less movement. When compressed to EF, variations in tablet weight do not affect tensile strength. Therefore, tablets maintain a consistent clinical efficacy.
The compression roller's upward movement also has another important advantage: it increases the total compression time (i.e., the dwell time). The more the compression roller moves, the longer the dwell time.
The effect of equal porosity on tablet properties
The author performed an experiment using an ibuprofen 85 DC formulation with 0.5% magnesium stearate. The tabletting was done on the "MODUL P" tablet press (GEA Courtoy, Halle, Belgium). This press is equipped with the air compensator at precompression and main compression.
The advantage of equal porosity tableting is the reduction of variability in density, tensile strength, and disintegration.
Compression to EF. For the above formulation, a series of tablets was compressed at a constant force of 650 daN with a nominal tablet weight of 233 mg.
Tablets were compressed within ± 4% of their nominal weight. All tablets were compressed to the same peak compression force.
Compression to ET. The same series of tablets was compressed to ET. The distance between the two rollers remained constant throughout compression.
Figures 5–8 show the analysis. The weight variation demonstrates an increase of force when the machine uses the ET mode and tablet weights increase.
The influence of tablet weight on density for ET and EF. Density increases as tablet weight increases. Density decreases when tablet weight decreases. The variability of the density during compression to EF is 1.1%. The variability of the density during compression to ET is 8%. Both tests used the same series of tablets within the same weight range.
Testing the tablets for tensile strength. The variability in tensile strength was more significant than that of density. Tablets compressed to EF showed as little as 1.2% variation. Tablets compressed to ET showed as much as 29.6% variation.
Testing the tablets for disintegration time. Disintegration time varied widely according to the compression technique. Tablets compressed to EF showed only 14.5% variability in disintegration time. Tablets compressed to ET, however, showed more than 105% variability in disintegration time.
The data were gathered under controlled conditions for ibuprofen 85 DC, and tablets were compressed within the US Pharmacopoeia's weight-variation limits.
Tablets compressed under equal force (EF) showed a much more consistent density, tensile strength, and disintegration rate than tablets compressed to equal thickness.
Different results might be expected from other drug formulations, but these data clearly indicate the advantage of a rotary press with EF capability.
Machines with both capabilities, therefore, give formulation and production departments great scope for improving tablet quality.
Johan Van Evelghem is a product and compression-technology manager at Courtoy, 9165 Rumsey Rd., Columbia, MD 21045, tel., 410.997.6692, firstname.lastname@example.org
1. K.A. Khan and C.T. Rhodes, "Effect of Variation in Compaction Force on Properties of Six Direct Compression Tablet Formulations," J. Pharm. Sci. 65 (12), 1835–1837 (1976).