Cohesion of particulate solids. VI. Improvement of apparatus and application to measurement of cohesiveness at various levels of humidity.

Abstract

An electrical device was introduced in place of a torsion balance in the vertical tensile strength method, and all forces applied to the surface of the powder bed were recorded automatically. The complex peaks of tensile stresses recorded for irregular particles were consistent with the interparticulate interactions discussed in the preceding report in relation to cohesiveness. The tensile strength between particles at various humidities was measured and the influence of environmental humidity on the cohesiveness of pharmaceutical powder is discussed. Hydrophilic and water-soluble powders showed an abrupt and large increase in cohesiveness with increasing humidity and a large amount of water was adsorbed in the vicinity of the critical relative humidity (R.H.). Hydrophilic and water-insoluble powders showed moderate changes of cohesiveness and amount of water adsorbed with increasing humidity. Some powders showed a progressive and exceptional increase of cohesiveness with increasing humidity due to swelling. A large amount of water was absorbed by such powders as starchs, silica gel, microcrystalline cellulose, and methylcellulose compared with other powders. Contact angle did not appear to be related to cohesiveness at various humidities. The slightly larger cohesiveness of hydrophobic powders than hydrophilic powders may have been a result of the irregular shapes of particles. The cohesiveness of particles became increasingly sensitive to humidity changes as the diameters of the particles was decreased, except in the case of water-insoluble powders.