"Apparent" Young's elastic modulus and radial recovery for some tableted pharmaceutical excipients

The effects of compact size and of powder particle size on the determination of "apparent" compressive Young's modulus of elasticity, E, were evaluated, for three pharmaceutical excipients (Microcrystalline cellulose, MCC, Calcium hydrogen phosphate dihydrate, CHPD, and pregelatinized starch, PGS) differing in deformational behaviour during compression. One- and two-compact (composite) specimens were employed and the equations of Spriggs [J. Am. Ceram. Soc. 44 (1961) 628] and Phani & Niyogi [J. Mater. Sci. 22 (1987) 257] were employed for prediction of E, at zero and 0.15 porosity (E(0) and E(0.15)). It was found that E(0) and E(0.15) are affected by the particle size, in the case of PGS, but by the compact size and assembly in the case of MCC and CHPD. Reduction in E(0) and E(0.15) values for two-compact assembly corresponded to large and medium compact size only of MCC and CHPD, while specimen assembly was not affecting significantly the experimental data if small compacts of MCC and CHPD or any size compacts of PGS were used. The exponent f of the Phani and Niyogi equation has been considered as parameter of pore structure and particle morphology. It was significantly affected by the compact size of all the excipients, by the two-compact assembly of MCC and CHPD and by the particle size of CHPD and PGS. Absence of significant particle size effect on the parameter f, for the case of MCC is attributed to the elongated particle shape, while the highly significant one for the case of PGS is explained by the extensive elasto-plastic deformation during compression, which is greatly dependent on the grain size. Values of the % radial elastic recovery (%RR), at porosity 0 and 0.15, were determined and correlation of the experimental data was attempted to Young's modulus (E). A simple linear equation is proposed for prediction of Young's modulus, E, from %RR, at least in the porosity range between 0.1 and 0.3 (representative of commercial pharmaceutical tablets), but use of %RR instead to E is applicable only under many limitations.