Plasticisation of amylodextrin by moisture. Consequences for compaction behaviour and tablet properties.

Amylodextrin, a starch-based controlled release excipient, spontaneously absorbs moisture during storage. The aim of this study was to investigate plasticisation of amylodextrin by moisture and its effect on compaction and tablet characteristics. The glass transition temperature (T(g)) of amylodextrin powders with moisture fractions (x(w)) 0.070<x(w)<0.40 was studied by conventional and modulated DSC. Elastic modulus and yield stress were determined from compressive stress-strain experiments. Compaction behaviour was studied at 3 and 300 mm/s using a compaction simulator. The T(g) of amylodextrin-water blends showed a smooth reduction with increasing x(w), equalling room temperature at x(w)=0.19. Experimentally obtained T(g) values were close to temperatures as predicted by the Gordon-Taylor/Kelley-Bueche model and the modified Couchman-Karasz model. The elastic modulus decreased steeply between x(w)=0.17 and 0.23. Compaction experiments showed that moisture facilitated consolidation due to increasing powder compressibility and reduced compact relaxation. However, at x(w)=0.23, compressibility was reduced and relaxation significantly higher due to the rubbery character of this powder. Consequently, the lowest tablet porosities were obtained around x(w)=0.15. Although decreasing porosities enhanced tablet strengths, the maximum obtainable tablet strengths decreased with moisture due to reduced particle bonding and lowering of the elastic modulus. Moisture largely affects the visco-elastic and compaction characteristics of amylodextrin. Hence, control over moisture content is essential to produce tablets with reproducible porosity, strength and dissolution characteristics.