Effect of glidants in binary powder mixtures

Abstract

The intention of this study is to investigate on a particulate level the flow properties of dry powder mixtures consisting of cornstarch and a second nanoscaled material. Special attention is paid to the question on the working mechanism of glidants. In 1974, Rumpf showed that a roughness on the surface of a smooth particle leads to a reduction of its forces of interaction with another particle. The interaction forces are reduced as the surface roughness increases the distance between the centers of gravity of the two interacting particles. Agglomerates as well as the primary particles of materials used as glidants are characterized by diameters in the lower nanometer range. In consequence they are strongly adsorbed at the surface of larger particles and act as a surface roughness. If the effect of a glidant would be due to its ability to act as a surface roughness then all nanoparticles being small enough to reduce the net interaction forces could be used as glidants almost irrespective of their chemical nature. Indeed we have been able to demonstrate that nanoparticles of titanium dioxide, aluminum oxide, silicon dioxide or of carbon black act as glidants. Mixing time directly influences the efficiency of a nanomaterial to act as a glidant. Due to increasing ratio of adhesive force to particle weight with decreasing particle radius, nanomaterials tend to aggregate and agglomerate. With increasing mixing time the size of agglomerates decreases. At the same time the number of primary particles available for adsorption on the surfaces of the cornstarch particles increases. An optimum in flow properties is achieved at a characteristic mixing time. At a further increase in mixing time, the size of agglomerates decreases and the coverage of the cornstarch particles by nanoparticles increases. Eventually cornstarch particles are obtained being completely coated with nanoparticles. The surfaces of these coated particles are smooth. Accordingly they show a poor flow behavior. The property of the nanomaterial to act as a glidant is lost.