COMPARISON WITH POWDER COMPACTS

Seven hundred milligram o f the 60%w/w GMS content pellets and powder mix o f the same composition were compacted to tablets o f the same thickness (2.2.4.1 A). Tablets were also made form the same bulk volume o f the pellets or powder mixture (2.2.4.1C). The compaction mechanism and properties o f the tablets (2.2.4.2.1-2) were analyzed.

The pressure needed to compress pellets o f the same mass to the same tablet thickness was greater than that o f powder mixture by about 5MPa. This indicates that a greater force was needed to deform the structure o f the pellets to obtain tablets o f the same dimensions. As previously noted, the porosity o f the pellet compact was very low, indicating a nearly complete elimination o f the void which was at least 26% when the “spheres” were arranged in the most packed rhombohedral manner. The smooth surfece o f the tablets, with no sign o f the pellet boundaries, could be taken as a proof to this assumption (Plate-3, la, see p-178). The out o f die porosity o f the tablets formed from pellets and the powder mixture did not have a significant difference. Their difference was only by less than 0.5%. This shows the formation o f a similar overall structure after compaction.

The mechanism o f compaction plotted as a pressure/displacement curve (Fig-3.1.12) illustrated that the crucial pressure needed to compact the pellets or powder mixture was exerted on the last 1 mm thickness reduction o f the compacts. The platen was displaced for about 4 mm without any difference in the pressure in both formulations. Neither needed a pressure more than 1 OMPa. This indicated the ease o f deformability o f the GMS-rich powder mixture and pellets. Due to its higher packing ability (i.e, relatively lower compressibility) the pellet-bed was the first to reach to the final tablet thickness, while it took a longer time to rearrange the powder particles, hence to build the pressure needed to compress them to a tablet o f the same thickness.

The work o f compaction (area under the curve), did not seem to have a large variation in the compaction o f the powder or pellet-bed. That was because the difference in the curve was

only at the part where insignificant pressures were registered. This indicates, the low strength of the GMS-rich pellets and their significant degree of deformability, which enabled them to be comparable with a similar powder mixture in terms o f compaction mechanism.

300 250 200 150 100 pellets Ffcwder Dsplacement (mr)

Fig-3.1.12:- Comparison o f the compaction mechanism o f tablets produced from 700 mg pellets o f 1.0-1.18mm size fraction and 700 mg o f powder having the same composition

(60%w/w GMS in MCC/GMS mixture).

These compacts had different strengths as measured by diametral compression. The values of the strength of the tablets prepared fi-om the powder mixtures were about three times higher than those of pellet compacts in both of the tableting machines (Fig-3.1.13a&b). This means, the tablets fi-om the same mass to the same tablet dimensions, and the tablets fi-om the same original bulk volume had a similar relation in terms o f the strength of the powder and the pellet compacts.

p o w d e r

Fig-3.1.13:- Comparison on the tensile strength o f (a) eccentric tableting machine (b) Instron tablets produced from 700 mg pellets o f 1.0-1.18mm size fraction and 700 mg o f powder having the same composition (60%w/w GMS in MCC/GMS mixture).

The main reason for such discrepancy in the strength of the tablets seems the difference in the nature o f the constituents. The powder, had to be mixed, wetted, extruded, spheronized and

dried to be changed to pellets. These all processes may have afifected their rigid tablets forming ability. Also, the variation in particle size and size distribution, consequently the difference in surface area could be another reason. In the eccentric tableting machine the mass o f the tablets was determined by the die volume. As a result the low bulk density o f the powder mixture, produced lighter (by 15%) and slightly more porous (by 6%) tablets. However, these did not affect the relative strength o f the tablets considerably (Fig-3.1.13).

The dimensions o f the tablets produced form the same mass in an Instron were measured immediately after their ejection and again after 72 hours o f their storage at ambient temperature and humidity. From the difference o f these dimensions, the volumetric elastic recovery was calculated and was found to be approximately the same. The insignificant difference in the elastic recovery again proved the total change in the structure o f the pellets. After compaction, the pellets and the powder-mix were plastically deformed in the same way. The loss o f plasticity o f the GMS during pelletization process could then be assumed to be trivial.

From this work, it is possible to conclude that, GMS-rich powder mixture and pellets exhibit a similar mechanism o f compaction, volumetric elastic recovery and final tablet porosity. This was attributed to the inherent and unchanged deformability o f GMS. The difference in the strength was, however, presumably due to the effect o f the pelletization process on the surface nature o f the pellets mainly on the MCC part o f the mixture. This was proved by the failure o f th pellet compacts through the boundaries o f the pellets during diametrical compression (Plate 3.1b).