Tablet mechanical theory

Leuenberger et al. introduced percolation theory in the pharmaceutical field in 1987 to explain the mechanical properties of compacts and the mechanisms of the formation of a tablet [36,37]. Knowledge of the percolation thresholds of a system results in a clear improvement of the design of controlled-release dosage forms such as inert or hydrophilic matrices. 

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 demonstration of the validity of the continuum-based modelling approach to tablet compaction requires familiarity with fundamental concepts of applied mechanics. Under the theory of such a mechanism, powder compaction can be viewed as a forming event during which large irrecoverable deformation takes place as the state of the material changes from loose packing to near full density. Moreover, it is important to define the three components of the elastoplastic constitutive models which arose from the growing theory of plasticity, that is the deformation of materials such as powder within a die ... 

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. 

I. Caraballo, M. Pernandez-Arevalo, M.-A. Holgado, A.-M. Rabasco and H. Leuenberger, Study of the release mechanism of carteolol inert matrix tablets on the basis of percolation theory, Int. J. Pharm., 109 (3) 229-236,1994. 

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