Tablet consolidation mechanisms

The compression cycle profiles may be used to characterize the consolidation mechanisms of powders as they help to characterize the extent of pressure distribution within the powder bed as well as the formed tablet. The compression behavior... 

The final tablet properties are also affected by the consolidation (i.e., bonding) mechanisms of the powder which is influenced by its chemical nature, the surface area of the contact points, contamination (including film coatings such as magnesium stearate), and interparticle distance. The predominant consolidation mechanisms are listed below ... 

Starch is a very widely used tablet excipient, but in its natural state, it does not possess the fluidity and binding characteristics needed as a tablet diluent. The major consolidation mechanism of starch is by deformation with a high elastic component.In addition, starch shows a high degree of lubricant sensitivity. 

A further study was conducted to study the effect of compression temperature on the consolidation mechanism of the polymorphs of chlorpropamide [51]. The effect of environmental temperature on the compression mechanism of the A and C forms of chlorpropamide was investigated with an eccentric type tableting machine with two load cells and a noncontact displacement transducer. The temperature of the sample powders was controlled at 0 and 45°C, and these were compressed at almost 230 MPa. The tableting dynamics were evaluated by Cooper and modified Heckel analyses. The results suggest that particle... 

Stress-strain type equations have been developed for the compaction process, which help provide an understanding of the mechanisms involved in forming a tablet, as well as allowing for the prediction of compaction results. This predictive power of the compaction process is the basis for many scale-up approaches. However, there are compression and consolidation process aspects which are dependent on manufacturing scale, e.g., speed-sensitive materials, and this results in many problems encountered in transferring a technology to production scale. Unfortunately, these scale-sensitive processes have not been as extensively studied, and are less understood. 

Basically, the process of tablet compression starts with the rearrangement of particles within the die cavity and initial elimination of voids. As tablet formulation is a multicomponent system, its ability to form a good compact is dictated by the compressibility and compactibility characteristics of each component. Compressibility of a powder is defined as its ability to decrease in volume under pressure, and compactibility is the ability of the powdered material to be compressed into a tablet of specific tensile strength [1,2], One emerging approach to understand the mechanism of powder consolidation and compression is known as percolation theory. In a simple way, the process of compaction can be considered a combination of site and bond percolation phenomena [5]. Percolation theory is based on the formation of clusters and the existence of a site or bond percolation phenomenon. It is possible to apply percolation theory if a system can be sufficiently well described by a lattice in which the spaces are occupied at random or all sites are already occupied and bonds between neighboring sites are formed at random. 

A fundamental property of a solid is the mechanism by which it consolidates under the influence of a compressing force. There are two principal mechanisms— fragmentation and deformation—though most solids will show a mixture of the two with one mechanism predominating. The mechanism can have a major influence on tablet properties. 

The basic powder compression operation of a rotary tablet press is rather simple and is essentially the same from machine to machine (Fig. 14). The powders are compressed by passing the upper and lower punch between two rollers (or cams), thus mechanically driving punches towards one another to a specified target tip separation. The distance between the rollers can be adjusted to control the maximum degree of powder consolidation within the die (i.e., minimum tip separation), which has a direct influence on final tablet properties, such as thickness and crushing strength. 

Pharmaceutical compacts are complex structures that present difficult challenges when measuring their mechanical properties. Hiestand was a pioneer who quantified the compaction properties of pharmaceutical powders and (105-109) the result of his work are indices known as the Hiestand Tableting Indices. These indices are dimensionless numbers used to describe the mechanical properties and consolidation behavior of materials under compression and decompression. The three main Hiestand Tableting Indices are the bonding index, brittle fracture index BFl), and strain index. 

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