A change in the normal mode of tensile failure following diametral compression has been demonstrated for specimens of constant weight but varying thickness at low porosities (Section 3.2.1.2). Thus the preliminary findings point to either specimen porosity or geometry, or a combination of both as the causative factors.
The second experiment therefore attempted to isolate these two variables by producing specimens of constant thickness. Thus for a particular thickness/diameter ratio the geometry of the specimen was maintained constant whilst the porosity was varied.
The results of Section 3.2.1.4 (Table 3.1) show that by changing the specimen geometry the edge deformation and collapse mechanism of failure becomes apparent at diflfering levels of porosity. However, the results also illustrate the important influence that porosity has on the mechanism of failure. At high porosities all specimens failed in the normal tensile manner irrespective of their geometry and thus both geometry and porosity seem to be causative factors.
The results strongly suggest a specimen-platen interaction for which a possible explanation may be provided. At high compaction pressures (and hence low porosities) the physico-mechanical structure of Avicel PH 102 may be changed. Microcrystalline cellulose consists of microfibrils packed closely together which hydrogen bond neighbouring cellulose chains through the hydroxyl groups (Khan, et al 1988). The application of a large compaction pressure over a prolonged period may increase the proximity of these microfibrils and promote stronger hydrogen bonding leading to a specimen with a stronger physical structure.
The time-dependent nature of microcrystalline cellulose and other excipients is well documented in the literature (David and Augsberger, 1977; Rees and Rue, 1978; Roberts and Rowe, 1985; 1986b; Armstrong, 1989). During prolonged contact between the punch faces and die contents such materials undergo extensive plastic flow leading to stronger particle-particle bonding and therefore a greater tensile strength.
The compaction operation takes approximately five minutes with an Instron Physical Testing Machine when preparing specimens at a compaction rate of 5mm/min, thus the punch faces are in contact with the die contents for a considerable length of time allowing extensive time-dependent flow and particle bonding to occur.
The above factors could result in the formation of a very dense plastic core in the centre of the specimen which is resistant to fracture. In particular, the low porosity specimens appeared to be very plastic and after testing could not be broken into two halves manually following edge deformation and collapse. Deformation and failure of such specimens would thus only occur at the surface and edges of the specimen - the weakest points of the structure.
In addition, the results of Table 3.1 implicate specimen geometry as a causative factor. The determination of tensile strength using the diametral compression test depends upon the correct stress distribution being achieved within the specimen. Variations in the type of failure encountered during diametral compression tests due to changes in the stress state have been documented by Rudnick et al (1963) for ceramic materials and by Fell and Newton (1970b) for pharmaceutical tablets. Rudnick et al (1963) demonstrated that the mechanical properties of both the specimen and load platens determine the stress distribution within the specimen. Thus if the Young’s modulus of both the specimen and platens are high then conditions approaching ideal line loading will be achieved. This results in minimum values for compressive and shear stresses at the centre of the load diameter and infinitely high values immediately at the contact points with the platens. In this case failure is not initiated in tension but may initiate by shear or compression. To alleviate this problem an appropriate padding material should be selected which allows the load to be distributed over a reasonable area ensuring that these high stresses near to the platens are reduced.
Pharmaceutical tablets are normally much softer than the steel platens used in diametral compression tests thus the consequences of line loading conditions are usually avoided. However, it is possible that the low porosity specimens used in this study had a much higher Young’s modulus which was similar in magnitude to that of the platens. Under these conditions line or point loading would be achieved and shear or compressive failure could occur (Fell and Newton, 1970b). The change in the stress distribution within the specimen also seems to have been enhanced by the specimen geometry where the thinner specimens seem to be predisposed to line loading conditions.
Although this could account for the appearance of the edges of the specimens the specimens did not break into numerous irregularly shaped pieces as is usual for failure under shear and compressive stresses. Examination of Figure 3.1(b) also reveals a
reasonable flattening of the specimen at the point of contact with the platens which makes it unlikely that conditions of line loading would have initiated this type of failure.
Finally the proposed causes for this phenomenon do not account for the fact that both Figures 3.2 and 3.3 indicate that a valid value for the tensile strength of the specimen is still being measured even though the appearance of the specimen in Figure 3.1(b) shows no evidence of a diametral fracture. This will be discussed further in Section 3.4.
3.3 Force-Transmission Study for Avicel PH102 Specimens of Constant