Powder elastic recovery

Armstrong, N. A., and Haines-Nutt, R. F. (1972), Elastic recovery and surface area changes in compacted powder systems, I. Pharm. Pharmacol., 24S, 135P-136P. 

Hiestand Tableting Indices Likelihood of failure during decompression depends on the ability 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 likely to cap or delaminate. Hiestand and Smith [Powder Technol., 38, 145 (1984)] developed three pharmaceutical tableting indices, which are applicable for gener characterization of powder com-pactiability. The strain index (SI) is a measure of the elastic recovery following plastic deformation, the bonding index (BI) is a measure of plastic deFormation at contacts and bond survival, and the brittle fracture index (BFI) is a measure of compact brittleness. 

Figure 5.10 shows the various steps of the sintering process. At first the preform completes its elastic recovery and begins to thermally expand past the PTFE melting point. The expansion can reach up to 25%-30% by volume depending on the type of resin, powder, preforming pressure, and temperature. Above 342°C, PTFE is a transparent gel due to the absence of a crystalline phase. 

In one study, the formulation efficiency of several direct compression materials was evaluated using instrumented press methodology.It was found that subtle changes in the structure of the component particles could lead to the observation of significantly different behaviors upon compression. The tablet hardness and compressibility of differently sourced sucrose materials, obtained at comparable compressional forces, was found to vary significantly with the source of the compound. To quantitative the results, an elastic recovery index was defined, and it was found that the indices of direct compressible materials were lower than those of poorly compressible powders. 

Pressure-cast bodies of ceramics also have problems, when they are cast from either a dispersed or a flocculated slurry. As the last portion of the sluny is consolidated, the pressure gradient across the cast becomes zero, so that the total applied pressure is transferred to the cast. Therefore, upon the applied pressure is removed, the cast expands, i.e., undergoes strain recovery, due to the stored elastic energy. However, the nature of the strain recovery is different from that in the die compaction of dry powders, in a way that the strain recovery for the compacts of the pressure casting is time dependent. This time-dependent strain recovery is attributed to the fact that the fluid, either liquid or gas, must flow into the compact to allow the particle network to expand and relieve the stored strain. The magnitude of the recovered strain increases with increasing consolidation pressure nonUnearly, which can be described by the following Hertzian elastic stress-strain relation ... 

Elastic creep recovery can be observed after short term loading (10 sec) as illustrated in Figure 7. Since this recovery is primarily induced by the filler, one can observe distinct differences in the properties of the RAC s as the filler content is increased. Addition of 10% powdered rubber to an AC10 produced a mixture with a creep recovery better than that of an AC30 from the same source. The stress relaxation process is rather slow creep recovery after medium term loading (100... 

---