Adhesive Force and Fluidity of Powder in Heated State

Abstract

The adhesive force between powder particles was measured in the heated state and the effect of adhesive force on fluidity was studied. The samples mainly used here were glass beads and alkali halide NaCl. Calcium carbonate and coal were also used. The adhesive force was determined by the tensile strength to cause layer-fracture of the packed powder. Since the sintering occurred at initial contact points between particles when the powder was heated in the packing state, the variation of adhesive force with temperature could not be measured accurately. In order to overcome this difficulty and obtain reproducible data, a new device (hereafter called a cell) was made. The sample powder in the cell can be heated in the fluid state so as that individual particles remain mobile. After keeping the sample powder at a certain temperature for a fixed time, the powder was closepacked at the same temperature and immediately the fracture force was measured. The fluidity of powder under heating was measured by the device reported previously.
The following results were obtained from the above experiments.
(1) Above 100°C, the adhesive force increased with increasing temperature.
(2) Even in the state of fluidity, the apparent size of particles increased as a result of aggregation due to an increase in adhesive force.
(3) Since the apparent size of the aggregated particles measured by a sieve increased rapidly from T/T_m≅0.7, the magnitude of adhesive force between the particles became small. Therefore, the fluidity of the powder increased in this region. T_m quoted above is the melting point or softening point of powder samples.
(4) At higher temperatures, the powder itself begins to aggregate and solidify, resulting in rapid decrease of fluidity.