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.