EFFECT OF PARTICLE-SIZE AND SORBED MOISTURE ON THE TENSILE-STRENGTH OF SOME TABLETED HYDROXYPROPYL METHYLCELLULOSE (HPMC) POLYMERS

The tensile strengths of compacts produced from powdered HPMC polymers with different particle size, methoxy/hydropropoxy substitution ratio, molecular size and moisture content achieved by equilibration at different relative humidities have been determined either immediately after their compression or after storage. It has been found that the tensile strength of tablets compressed at certain pressure level increases with moisture content, reaches a maximum at about 10 wt% moisture and then decreases while the tensile strength of tablets at fixed packing fraction exhibits an initial plateau up to 6 wt% moisture content and then decreases. The changes in tensile strength are related with the changes in the compressional characteristics and the moisture distribution effects mainly due to alterations in the particle size and in the methoxy/hydropropoxy substitution ratio while no dependence on the molecular size is evident. The profiles of particle packing vs moisture content were found to be sigmoidal, possibly due to the lubrication effect of tightly bound moisture up to 6 wt%, and the tensile strength maxima are attributed to the combined effect of closer packing of particles and softening of interparticle bonds. Our previous finding that tensile strength begins to decrease when the moisture content is about double that corresponding to tightly bound or 'monomolecular' water has been confirmed.