粉体粒子の凝集性に対する湿度の影響

抄録

In order to investigate the effect of humidity in the atmosphere on the cohesion of powder particles, the adsorption weight of water and the cohesion forces between particles were measured in various humidities. Spherical glass powders with three kinds of particle sizes (17-100μ) and hydrophobic glass powder made by silicon treatment of the former powders, were used as samples. Cohesive forces were calculated from the tensile strengths of powder beds measured by Du Noüy surface tension balance.
In the adsorption isotherms at 25°C, the adsorption weights were nearly constant until the relative humidity reached about 60%. Beyond this point, the weights increased with increasing humidity. When the adsorption isotherms were represented in the form of adsorption weight per unit surface area of particles, the isotherm curves for the powders of different particle size roughly coincided in the constant range, but they were different in the untreated samples and the hydrophobic samples. These constant values corresponded to 13 water molecular layers and 6 layers, respectively. On the other hand, the tensile strengths were low and roughly constant in this constant adsorption range, but they increased sharply with an increase in humidity, and then decreased.
On the basis of the above results, it was considered that the water vapor was uniformly adsorbed on the surface of particles first, and then condensed at the contact points between particles, forming the water bridges. The liquid bridge forces were calculated and compared with the theoretical results by Rumpf on the assumption that the tensile strengths were given by the product of the number of contact points and the cohesive force of a liquid bridge. The experimental results corresponded to the theoretical curves with the relative distance of about 10^-2 and a contact angle of 0°for the untreated samples and 40-60°for the hydrophobic samples. The relative distance is the ratio of the distance between the particle surfaces at a contact point to the particle diameter. The bridge forces, however, had maximum values which were about one order of magnitude lower than the theoretical values and decreased sharply. It might be considered that irregular distribution of adsorbed water or the formation of secondary structure of particles in the powder bed occured.