Tablet plastic deformation

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... 

Binders improve the strength of compacts through increased plastic deformation or chemical bonding. They may be classified as matrix type, film type, and chemical. Komarek [Chem. Eng., 74(25), 154 (1967)] provides a classification of binders and lubricants used in the tableting of various materials. 

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],... 

Plastic deformation is the permanent change in shape of a specimen due to applied stress. The onset of plastic deformation is seen as curvature in the stress—strain curve. Plastic deformation is important because it allows pharmaceutical excipients and drugs to establish large true areas of contact during compaction that can remain on decompression. In this way, good, intact, tablets can be prepared. 

Just as metals can be ductile or brittle, so can organic materials. The Brittle Fracture Index is a measure of the brittleness of a material. It is a measure of the ability of a compact of material to relieve stress by plastic deformation. The Brittle Fracture Index (BFI) is determined [29,31] by comparing the tensile strength of a compact, stress concentrator) in it, o-T0, using the tensile test we have described. A hole in the center of the compact generally weakens a tablet. If a material is very brittle, theoretical considerations show that the tensile strength of a tablet with a hole in it will be about one-third that of a solid tablet. If, however, the material can relieve stress by plastic deformation, then the strength of the compact with a hole in it will approach that of a compact with no hole. The Brittle Fracture... 

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... 

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. 

Some models of these tablet presses are equipped with a precompression station. This is an additional set of pressure wheels that can apply force to the material in the die prior to the final (normal) compaction step i.e., the tablet is compressed twice. When used, the force apphed is usually lower than that in the final compaction. A precompression step can densify the material, allow more time for plastic deformation, and allow air to escape rather than being trapped inside the compact. 

Strain rate sensitivity of (or the effect of press speed on) the formulation is of primary concern in scale-up. Whether the product development work was performed on a single-stroke press or a smaller rotary press, the objective in operations will be to increase efficiency, in this case the tablet output rate and, therefore, the speed of the press. For a material that deforms exclusively by brittle fracture, there will be no concern. Materials that exhibit plastic deformation, which is a kinetic phenomenon, do exhibit strain rate sensitivity, and the effect of press speed will be significant. One must be aware that although specific ingredients (such as calcium phosphate and lactose) may exhibit predominately brittle fracture behavior, almost everything has some plastic deformation component, and for some materials (such as microcrystalline cellulose) plastic deformation is the predominant behavior. The usual parameter indication is that target tablet hardness cannot be achieved at the faster press speed. Slowing the press may be the only option to correct the problem. 

A technique generally applied to characterize and prevent the capping and lamination of a material intended to be compacted is using the brittle fracture index (BFI). The BFI was designed by Hiestand et al. [31] and measures the ability of a material to relieve stress by plastic deformation around a defect. It is obtained by applying Equation (8) and compares the tensile strength of a tablet with a hole in its center (To), which acts as a built-in stress concentrator defect, with the tensile strength of a similar tablet without a hole (T), both at the same relative density ... 

Binders are commonly used when making conventional tablets. The binders are either added in dry form or dissolved in a suitable solvent and then added in connection with a wet-granulation process. Most binders are polymers and increase the plastic deformation of the formulation. 

Fig. 24 Differences in the deformation behavior of substances during tableting demonstrated by sorbitol instant as an example for a plastically deforming substance and ascorbic acid as a more brittle one. Tablet compressed at 5 kN (10 mm in diameter) (A) and 30 kN (B). Inner part of a broken tablet showing an ascorbic acid crystal being totally embedded into plastically deformed sorbitol (C) and a crystal being partially removed from the sorbitol matrix through the break down of the tablet (D). A cracked ascorbic acid crystal in a sorbitol matrix (E).

Tablets. SEM was widely used to investigate the structure of tablets. An excellent review is presented by Hess. " Imaging of tablets is a useful tool to demonstrate differences in compression behavior of substances. Fig. 24 shows an example where the plastically deforming spray-dried sorbitol instant was compressed together with the more brittle ascorbic acid in one tablet. At low compressional force of 5 kN for a 10 mm tablet, the rectangular ascorbic acid crystals as well as the partially deformed sorbitol particles are visible (Fig. 24A). In Figs. 24A and B, the surfaces of two tablets compressed at 5 kN and 30 kN are compared. At higher compressional forces, a uniform, flat, and smooth tablet surface is formed, but within this surface a single unchanged ascorbic acid crystal could be detected. Observation of a broken tablet (Fig. 24C), which was prepared at a high compressional force of 30 kN, reveals that the ascorbic acid crystal is totally fixed within a matrix of plastically...

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