Measuring Tablet Hardness: A Primer

Solid tablets are perhaps the most commonly used dosage form for pharmaceuticals. Tablet hardness serves both as a criterion to guide product development and as a quality-control specification. Tablets should not be too hard or too soft. An extremely hard tablet could indicate excessive bonding potential between active ingredients and excipients, which can prevent proper dissolution of the tablet needed for an accurate dosage. By the same token, a softer tablet could be a result of weak bonding and may lead to premature disintegration when ingested by the patient. A soft tablet could also chip or break during processing stages in manufacturing, such as coating and packaging.

Figure 1: Crush test on tablet (CT3 Analyzer, Brookfield Engineering). (ALL FIGURES COURTESY OF THE AUTHOR)

Knowing the mechanical properties of a solid-dose tablet can provide valuable information for optimizing material constituents and the manufacturing process. The types of binders used, the nature of the active ingredient(s), and the composition of the ingredient(s) in the tablet will affect the hardness of the tablet; the tablet press speed, granulation flow, and air in the powder can also potentially affect tablet hardness (1). These factors must be controlled during production and verified after manufacture. As the production-to-market timeline of pharmaceutical products becomes tighter, it is essential to efficiently and effectively quantify the critical properties that will affect product development and performance.

Figure 2: Cylinder probe fractures tablet in crush test (CT3 Analyzer, Brookfield Engineering).

Methods for measuring the mechanical strength of a tablet include crushing, fracturing, and bending tests (2).

Figure 3: Graph of tablet-crush test.

Crush test

The crush test is usually performed on a round tablet standing on its rim, or, for a capsule-like tablet, parallel to the longest axis. This test is sometimes known as a diametrical compression test. The test sample is placed on a base table and compressed against a flat surface cylindrical probe as shown in Figure 1 (CT3 Analyzer, Brookfield Engineering). The cylindrical probe surface, which is larger than the test sample, is moved down to crush the tablet at a constant speed, and the force applied to crush the tablet is measured (see Figure 2). The load-force values obtained will depend on the construction and size of the tablet. Figure 3 shows a typical plot of force load (g) vs. time (s) as the test progresses (TexturePro CT software program, Brookfield Engineering). The highest point on the graph, peak load, is the load required by the analyzer to break the tablet. This point also indicates the tablet's maximum strength before breaking. Subsequent smaller peaks suggest that the test tablet was not fully broken down at the maximum load. The smaller peaks represent continuous fracturing of the tablet until full disintegration.

Figure 4: Fracture test on tablet (CT3 Analyzer, Brookfield Engineering).

Fracture test

The fracture test is accomplished by driving a smaller hemispherical ball probe into the flat surface of a solid tablet (see Figure 4). A suitable probe deformation distance must be chosen to avoid base effect, which is the external influence of the substrate surface on which the tablet is placed. Base effect is caused by compression of a thin sample against the test bed of the analyzer instrument, which inadvertently renders incorrect results. A deformation distance of not more than 60% of the sample height is usually enough for the ball probe to fracture the tablet without causing base effect. The maximum strength of a tablet before breaking is the peak load on the graph plot (see Figure 5). The recorded force applied to fracture the tablet is useful for determining mechanical properties, such as Young's modulus and tensile strength. Young's modulus, which is the ratio of stress over strain deformation (E = σ/ε), describes the stiffness and toughness of a material.

Figure 5: Graph of tablet-fracture test.

Bending test

Another common fracture test on tablets is the snap or bending test. This test is common for an oval tablet shaped like a capsule (i.e., caplet) as well as on a fairly large round tablet. The test is performed with a three-point bend fixture; the tablet is supported at either end and deformed in its center with a knife-like probe, causing it to fracture and break at its weakest point (see Figure 6). To ensure comparability of results, the tablet's orientation in the fixture must be standardized, preferably in a manner that is most readily and easily reproduced by operators (e.g., align the score line of the tablet with the probe blade). In the plot in Figure 7, the peak load (Y-axis) indicates the tablet hardness; harder tablets will give a higher peak load. The distance to peak load (X-axis) is an indication of the elasticity of the test sample. Brittle samples (such as solid-dose tablets) will have a shorter distance or time at failure hardness compared to elastic samples.

Figure 6: A blade splits the tablet in a bending test (CT3 Analyzer, Brookfield Engineering).

Conclusion

These tablet hardness tests provide a meaningful picture as to the amount of force required to fracture the solid-dose tablet. This knowledge will be useful in gauging the tablet's resistance to damage that might occur during production handling, packaging, and storage. Based on this testing, guidelines for acceptable hardness values can be established. The test is also useful for quantifying the internal bonding strength of powder, which will help to achieve compatibility of formulation with performance specifications. Tests can also be used to enhance the production evaluation of tablet consistency between different batches, shifts, and facilities.

Figure 7: Graph of tablet-bending test.

Eric Chiang is product manager for texture analyzers at Brookfield Engineering Laboratories, Middleboro, MA, tel: 1.800.628.8139, e_chiang@brookfieldengineering.com.

References

1. P. N. Davis and J.M. Newton, "Mechanical strength" in Pharmaceutical Powder Compaction Technology, G. Alderborn and C. Nystrom, Eds. (Marcel Dekker, New York, 1996), pp. 165-191.

2. J.M. Newton, G. Alderborn, and C. Nystrom, Pow. Technol. 72 (1) 97-99 (1992).