Friction and adhesion of single spray-dried granules containing a hygroscopic polymeric binder

The atomic force microscope has been used to study the friction and adhesion of single spray-dried granules containing a mixture of fine tungsten carbide and cobalt powders and various amounts of a polymeric binder, polyethylene glycol (PEG). The pull-off and friction forces between two single granules (representing intergranular friction) and between a granule and a hard metal substrate (representing die-wall friction) have been determined as a function of relative humidity. It was found that the granule-wall friction increased with binder content and relative humidity. The small friction force at the lowest addition of PEG was related to a small contact area due to the high surface roughness of the granules. The substantial increase in the friction coefficient at PEG-addition > 1 wt.% was related to the plasticity of the binder-rich granule surface where an increase in binder content or relative humidity increases the deformability. The granule-granule friction and adhesion was independent of the relative humidity and substantially lower than the granule-wall friction at all PEG contents, which has important implications for the handling of granular matter.