Optimization of Experimental Conditions for the Compression and Stress Relaxation Test of Spray-Dried Granules.

Abstract

Mechanical propertiesof spray-dried granules have been characterized in compression by repeatedly ap plying and stopping stress. In order to shorten the experimental time and obtain optimal conditions for relia bly evaluating the relaxation behavior, we discussed the influence of compression rate, compression number, relaxation time and initial height of the granule bed, respectively. It was found that the stress relaxation of granules increased with increasing compression rate. It was also demonstrated that the stress relaxation was almost independent of the initial height of the granule bed. Furthermore on the one time compression and stress relaxation test, the results were found to be in good agreement with those obtained from the mul tiple compression and stress relaxation test. These results show that the experimental time can be shortened from a few hours to about ten minutes.[ 1. Introduction In the press forming process of ceramics, it is important to accurately characterize the mechanical properties of spray-dried granules. In many researches on the characteri zation of granules, the mechanical properties were often evaluated by applying the conventional compression test,1)-3) in which the packing density of granule bed was measured as a function of applied pressure, or the single granule compression test,4),5) in which the strength of a granule was measured. These methods give us information only on the density of green body, but not on the homogenei ty of green body.In pharmaceutical science, the static compression behav iors and the tablettabilities of the agglomerated crystals and the original crystals were investigated and compared by measuring the stress relaxation.6) In this report it was found that the relaxation pressure of the agglomerated crystals was higher than that of the original crystals. It was also shown that the tensile strength of the tablet of agglomerated crystals was greater than that of the original crystals. However several kinds of agglomerated powders were not compared, therefor it is not clear the difference of the break ing and/or deforming behavior of agglomerated powders and how to prepare the optimum agglomerated powders for the press forming. Furthermore the effect of the experimen tal conditions on the stress relaxation behavior has not been discussed sufficiently. In food science, compression and stress relaxation tests have been used for the textual evalua tion of various fruits and other food commodities.7) However there is no report to adapt a compression and stress relaxation test for characterization of ceramics gram ules.In our previous works8)-10) a new evaluation technique has been developed for characterizing compaction behavior of spray-dried granules, using the compression and stress relaxation test. In previous reports we discussed the differ ence of the breaking and/or deforming behavior between binder contained granules and binder less granules. It was shown that the relaxation ratio (the ratio of the relaxation pressure to the applied pressure) of binde...