Dispersing WC–Co powders in aqueous media with polyethylenimine

The use of aqueous WC–Co suspensions for powder metallurgical processing of hard metal powders is expected to have environmental as well as economic advantages over the currently used systems with organic solvents. Without a suitable dispersing agent, however, poor mixing and aggregation of the different particulate constituents will occur in water and requirements on homogeneity and colloidal stability of the hard metal slurry cannot be met. We evaluated the effect of polyethylenimine (PEI) on the flow properties of concentrated aqueous WC–Co suspensions. The investigated parameters were polymer concentration, polymer molecular weight, pH, powder solubility, and ageing time. We found that simple mixing of powder, water, and PEI (at concentrations above 0.3 wt%) is sufficient to obtain well-dispersed, low-viscous suspensions without the need for pH adjustment or further additives. Almost identical flow behavior was observed over the investigated pH range 7–10.8. Effects of ageing and powder solubility on the colloidal stability were negligible over the investigated time span of 36 h. Adsorption of PEI with M_w=10,000 results in thinner adsorbed layers as compared to PEI with M_w=25,000, which is reflected by a lower suspension viscosity at high solids loading (40 vol%).