Tablet bonding mechanisms

As indicated by eqn. (4) (Section 2.1), the main variables affecting the strength of an agglomerate, other than the bonding mechanism, are the void ratio and the size of the particles from which it is composed. A number of empirical relationships have been used to correlate the effects of these variables. For example, eqn. (18) has been used [39] to represent the crushing strength of compressed tablets of constant size. 

Adolfsson, A., Gustafsson, C., and Nystrom, C. (1999), Use of tablet tensile strength adjusted to surface area and mean interparticulate distance to evaluate dominating bonding mechanism, Drug Dev. Ind. Pharm., 25,753-764. 

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 ... 

The intermolecular forces theory and the liquid-surface film theory are believed to be the major bonding mechanisms in tablet compression. Many pharmaceutical formulations require a certain level of residual moisture to produce high quality tablets. The role of moisture in the tableting process is supported by the liquid-surface film theory. 

Tribasic calcium phosphate is widely used as a capsule diluent and tablet filler/binder in either direct-compression or wet-granulation processes. The primary bonding mechanism in compaction is plastic deformation. As with dibasic calcium phosphate, a lubricant and a disintegrant should usually be incorporated in capsule or tablet formulations that include tribasic calcium phosphate. In some cases tribasic calcium phosphate has been used as a disintegrant. It is most widely used in vitamin and mineral preparations as a filler and as a binder. It is a source of both calcium and phosphorus, the two main osteogenic minerals for bone health. The bioavailability of the calcium is well known to be improved by the presence of cholecalciferol. Recent research reports that combinations of tribasic calcium phosphate and vitamin D3 are a cost-effective advance in bone fracture prevention. ... 

During tablet formation the following types of bond mechanisms are hypothesized to occur ... 

ITMs provide a valuable service to all phases of tablet manufacture, from research to production and quality control [109 111]. As a research tool, ITMs allow in-depth study of the mechanism of tablet compaction by measuring the forces that develop during formation, ejection, and detachment of tablets. ITMs can also provide clues about how materials bond,... 

Binder A material with a high bonding ability can be used as a binder to increase the mechanical strength of the tablet. A binder is usually a ductile material prone to undergo plastic (irreversible) deformation. Typically, binders are polymeric materials, often with disordered solid-state structures. Of special importance is the deformability of the peripheral parts (asperities and protrusions) of the binder particles [5],... 

Nevertheless, in a previous study dealing with inert matrices of naltrexone-HCl [74], two different excipient percolation thresholds pc2 were found for the matrixforming excipient Eudragit RS-PM the site percolation threshold related to a change in the release kinetics and the site-bond percolation threshold derived from the mechanical properties of the tablet, where the excipient failed to maintain tablet integrity after the release assay. 

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. 

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. 

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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