Tablet elasticity

FIGURE 2 Tablet elastic recovery versus residual die wall pressure for acetaminophen tablets containing 4% w/w level of different binders. A high-.shear mixer was used during wet granulation. Source. Adapted from Ref, 77. 

Hiestand Tableting Indices Likelihood of failure during decompression depends on the abihty of the material to relieve elastic-stress by plastic deformation without undergoing brittle fracture, and this is time dependent. Those which relieve stress rapidly are less... 

Higuchi et al. [98] postulated that an additional increase in surface area occurs after this point and that this effect may cause lamination of the tablet due to extensive rebound at decompression. In other words, at the tablet punch powder interface, there may be zones of high density during compression, but upon decompression these zones have elastic rebound and are pulled apart from the rest of the tablet that did not contain this high density. 

When one plots force vs. displacement, the area under the curve thus represents work. In practice, the compression/decompression data take the form shown in Fig. 18. The area under the upward line represents the work done on the tableting mass during compaction, while that under the downward line arise from the fact work is done on the punch by the tablet as a result of the latter s elastic recovery on decompression. 

Fig. 18 A typical force-displacement curve. WF= work done overcoming die wall friction WD, work of elastic recovery Wp/, net work involved in tablet compact formation.

One should note that BC represents a highly elastic material as little plastic deformation or brittle fracture has occurred. Also, sharp differences between the slope CD and DE are indicative of weak, or failed, tablet structures. The RDWF estimated from these plots can provide a good indication of the ejection force. More detailed treatments of such studies are now in the open literature, to which the interested reader is referred [118-120],... 

Another approach to determining the contribution being made by each of the possible com-pression/decompression mechanisms involves monitoring the degree and rate of relaxation in tablets immediately after the point of maximum applied force has been reached. Once a powder bed exceeds a certain yield stress, it behaves as a fluid and exhibits plastic flow [121,122], Certain investigators [122] have studied plastic flow in terms of viscous and elastic elements and have derived the following equation ... 

Indentation hardness using modified tests based on Brinell hardness measurements have been used by some researchers [148] to provide information on the surface hardness of tablets. In addition, these tests are capable of providing a measure of a tablet s plasticity or elasticity. For the most part, such tests have been confined to basic research applications in a few laboratories, but their value is beginning to be more widely recognized. 

Methods for characterizing the elastic, plastic, and brittle properties of compacts of organic materials have been developed by Hiestand and coworkers [29-33]. These indices of tableting performance measure the mechanical properties of compacted materials. 

The mechanical properties of materials, though not often studied in detail, can have a profound effect on solids processing. Clearly, tableting properties are influenced by the elastic and plastic deformation properties as well as the viscoelastic properties of a material. As we have pointed out, the powder flow properties are also affected, as well as the tendency of materials to set up on storage. Because of the importance of mechanical properties, it is important to be able to... 

Indices are dimensionless parameters derived from various mechanical and physical properties of the tablet blend and resulting compacts. Mechanical properties typically measured include indentation hardness (kinetic and static), elastic modulus, and tensile strength (10,11). Physical properties include particle size, shape, and size distribution, density (true, bulk, and tapped), flow properties and cohesive properties. 

The specific material properties of most import to the compaction operation are elastic deformation behavior, plastic deformation behavior, and viscoelastic properties. These are also referred to as mechanisms of deformation. As mentioned earlier, they are equally important during compression and decompression i.e., the application of the compressional load to form the tablet, and the removal of the compressional load to allow tablet ejection. Elastic recovery during this decompression stage can result in tablet capping and lamination. 

During compaction, primary particles are packed, re-arranged and can undergo deformation and possibly breakage. These events can occur sequentially or in parallel. The mechanical strength of a tablet may strongly depend on the mechanical properties of the primary particles and the particle-particle interactions within it. It is essential that the particles deform plastically or rupture since the stored elastic strains can weaken the tablet on release (Roberts and Rowe, 1987). 

Adolfsson, A., and Nystrom, C. (1996), Tablet strength, porosity, elasticity and solid state structure of tablets compressed at high loads, Int. I. Pharm., 132, 95-106. 

As already described, the tablet is not completely formed during compression [35,36], When the punches leave each other, the tablet relaxes during decompression, further relaxing when one punch leaves the tablet and continuing relaxation after ejection from the die. The process is called decompression as long as a force is measurable. Afterward the process is called elastic recovery or relaxation of the tablet. 

One possibility to analyze the tableting process is to describe the areas under the curve during compression and decompression and to draw conclusions on plastic and elastic parts of deformation. Emschermann and Muller [91] applied this method to data from eccentric machines (Figure 14). Similar area comparisons were performed by the research group of Schmidt [92-94] for rotary machines (Figure 15). They tried to gain information on elasticity by calculating differences between the area under the plot in the compression phase and the area under the plot in the decompression phase. A sophisticated technique to interpret area data under one... 

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