Interparticulate bonding

Lubricant interfering with interparticulate bonding during compression results in production of tablets with unacceptable strength. 

Bandyopadhyay, R. Grant, D.J. Influence of crystal habit on the surface energy and interparticulate bonding of L-lysine monohydrochloride dihydrate. Pharm. Dev. Technol. 2000, 5 (1), 27-37. 

The consequences of such a force distribution on tablet strength can be profound. Particle deformation, whether elastic or plastic, will be proportional to the force applied, and as has been discussed, this deformation is an essential preliminary to the formation of the interparticulate bonds on which tablet integrity depends. Thus, the porosity of the tablet, and hence its strength, will vary within the tablet. The weakest points in the tablet structure will be those that receive the lowest force i.e., on the face of the tablet adjacent to the stationary punch and on the central axis near to the moving punch. Thus, because of its non-uniform density, some parts of a tablet are stronger than others. 

The effect of the removal of the compressing force must now be considered. Elastic recovery will occur to a greater or lesser extent, which will result in a reduction in the strength of interparticulate bonds and an overall weakening of the tablet. It therefore follows that if a tablet is to be disrupted by elastic recovery, this is most likely to occur at its weakest point. This is just below the top surface, and is the phenomenon often encountered in tablet manufacture known as lamination or capping. With this explanation in mind, some effects associated with capping, and some causes and pragmatic solutions to the problem can now be explained. 

Any applied stress that exceeds the breaking strength of the tablet will also cause the tablet to break at its weakest point. A number of stresses occur when the tablet is removed from the die after compression. The die may become worn at the point in the die where the tablet is compressed, i.e., the die is fractionally wider at this point than elsewhere. Thus, when the tablet is ejected, it is forced through an aperture, the diameter of which is slightly less than that of the tablet itself. This will obviously stress the tablet, and the interparticulate bonds may be overcome at their weakest point. Also as the tablet is extruded from the die, elastic expansion will occur not just in an axial but in a radial direction. The latter occurs progressively, i.e., one segment of tablet is free to expand while the one below is still constrained by the die. Bond disruption will be an inevitable consequence. 

The physical strength of a tablet is dependent on the extent and strength of interparticulate bonds and these in turn are related to the compressive force which is applied. Therefore, the relationship between the applied force and some parameter related to tablet strength is a good indication of the ease with which a given substance will form satisfactory tablets, and may also give an insight into the compaction mechanism of the solid and its mechanical properties. 

Precompression is often used to tamp or apply a small compression force prior to the main compact compression cycle. Rotary tablet presses are often equipped with a separate precompression station, which is positioned between the die-filling feed frame and the main compression station. Typically, precompression is used to improve the quality of tableted products, where it increases the strength of the compact and/or decreases the incidences of capping and lamination. The compact strength is enhanced through the increase in the effective contact time in which the powder particles are in contact under an applied force. During this extended contact time, stronger interparticulate bonds form and stress relaxation occurs. 

Eriksson M, Alderbom G. The effect of particle fragmentation and deformation on the interparticulate bond formation process during powder compaction. Pharm Res 1995 12 1031-1039. 

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