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An alternative test for measuring the tensile strength of tablets has been proposed by Nystrom et al (1977). Tablet specimens are fixed between a pair of adapters by means of a resin adhesive and subjected to a direct tensile force. Unlike the diametral compression and flexural tests this procedure is presumed to generate a completely uniform stress distribution and pure tensile stress in the whole of the specimen. In addition, the test measures the strength of the specimen in the weakest plane normal to the axis of the tablet unlike that of the diametral compression test which imposes a stress in a known plane. However, it is possible that an additional bending stress (due to the mass of the adapters) and a torsion stress (due to loading) could be introduced during the test if small misalignments are not avoided thus destroying the meaning (Jarosz and Parrott, 1982). Also the adhesive could have a deleterious effect on the test by affecting the specimen’s strength or the stress state. Unlike the diametral compression test the axial tensile strength test is slower to perform because of the curing of the adhesive and specimen preparation necessary (at least 36 hours, Jarosz and Parrott, 1982).

When compared with diametral compression test results this procedure does provide valuable information on the capping or lamination tendency of a formulation. The radial tensile strength (diametral compression test) was found to continually increase with upper punch pressure whereas axial strength rose and then fell with pressure due to capping problems thus indicating that capping is not a random process (Nystrom et al, 1977). This

(52 was studied further where the effect of increasing binder concentration was shown to improve the axial strength (by reducing the axial elastic recovery and hence capping tendency) whilst radial strength remained relatively unchanged (Nystrom et al, 1978). The strength isotropy ratio (axial/radial strength) was also shown to be a valuable parameter for characterising the homogeneity of tablets with ideal, isotropic materials having a value of unity. Low values approaching zero indicated incipient capping or lamination (Alderbom and Nystrom, 1984). However, this ratio has been shown to vary with compaction pressure and should therefore be examined over a range. Duberg and Nystrom (1982) have shown that this ratio in conjunction with scanning electron microscopy provides a rapid method for evaluating the fragmentation propensity of tableting materials, with predominantly brittle fragmenting materials having ratios of near unity illustrating their homogeneous structure.

The homogeneity of compacted powders has also been demonstrated using an indirect tensile strength technique (Newton et al, 1992). Cubical specimens were subjected to both axial and indirect tensile determinations where the latter was performed by compressing them using steel rods across opposite faces. Specimen strengths were measured in the same plane as the direction of compaction and at right angles to this plane. In agreement with earlier work by Duberg and Nystrom (1982), materials undergoing fragmentation (such as lactose) were shown to be more homogeneous than those deforming by plastic and elastic deformation {e.g. Avicel PHlOl) using either technique.

A more direct approach might be possible using a technique described by Cocolas and Lordi (1993). They developed a novel instrumented die using four piezo-electric force transducers enabling axial to radial pressure transmission to be measured for tablet excipients over the compaction profile. Compaction profile results were found to be similar to those reported in the literature with the extent of die wall lubrication influencing the shape of the profile.

1.2.5.5 Compression Tests

The Young’s modulus of elasticity for brittle materials, including pharmaceutical powder compacts, can differ when measured in tension and compression whereas ductile materials have similar values. This can be explained by the pores tending to increase in volume when stressed in a tensile manner whilst decreasing when compressed (Kerridge,

1988). Consequently values determined by beam bending can be influenced by the compressive-tensile stress state as described above and variations in porosity caused by uneven filling of the die along the length of the beam. Compressive determinations of Young’s modulus represent an alternative, with the advantage that specimens are easy to prepare and represent a tablet shape. Measurements are also taken over the whole of the specimen (where the stress state is simpler) and can be easily performed. Such determinations obviously allow the examination of the behaviour of a material under a compressive stress. A procedure for measuring the compressive Young’s modulus of cylindrical specimens has been described by Paddon and Wilson (1976) who measured the slope of a stress-strain curve for a cylinder compressed between the platens of a testing machine. They were able to link the mechanical behaviour of dental cements to their molecular structure and chemical setting reactions.

Kerridge and Newton (1986) and Kerridge (1988) used the technique to examine the compressive Young’s moduli for a selection of pharmaceutical materials. Using the equation derived by Spriggs (1966) values of the modulus at zero porosity were calculated to represent properties similar to the material rather than the specimen. Due to the effects of fiiction at the specimen/platen interface it was suggested that the test could only be used to measure comparative values rather than absolute values.

The compressive strength of pharmaceutical compacts is a poorly researched area. Although most materials are weaker in tension than compression, so that they may well fail in tension under a compressive stress, compression tests still represent an important alternative form of loading. Newton et al (1993) have studied the compressive strength of equi-dimensional lactose and microcrystalline cellulose compacts by compressing them between the platens o f a testing machine and monitoring the force-time curves. Preliminary results were not influenced by the presence of Teflon sheets which were employed to reduce fiictional eflfects. Tests were therefore conducted without the sheets. Lactose specimens showed clear failure by crumbling and a drop in the maximum force whereas for Avicel PH I01 internal failure was not evident and the maximum force reached a plateau which was taken as the failure force. Specimens of the same dimensions were used for diametral compression tests allowing the compressive to tensile strength ratio for the materials to be calculated. The ratio for lactose was greater indicating its brittle nature.