Time-dependent deformation of some direct compression excipients.

Three techniques were used to compare the time-dependent deformation of microfine cellulose (Elcema G250), anhydrous lactose, dicalcium phosphate dihydrate (Emcompress), modified starch (Sta-Rx 1500) and sodium chloride. (1) In stress-relaxation experiments using a reciprocating tablet machine, none of the materials behaved as a Maxwell body in contrast to recent published work (David & Augsburger, 1977). Possible reasons for this disagreement are discussed. (2) Heckel plots showed that increasing the time for which a material was under compression (contact times of 0.17 and 10 s) had no effect on dicalcium phosphate compacts but increased the consolidation of other materials in the rank order sodium chloride less than lactose less than cellulose less than starch. (3) Deformation tests on preformed compacts were carried out in diametral compression by loading compacts to 75% of their breaking force at four different strain rates between 0.05 and 6.5 mm min-1. The deformation of Sta-Rx compacts was time-dependent. Sodium chloride compacts exhibited brittle behaviour in the diametral compression test and in the 10 s contact time experiment. This was apparently due to work-hardening, following the extensive plastic deformation of crystals during compaction as indicated by the stress relaxation results.