Mixing Mechanics: Nanopowder blends -- preventing segregation
Nanopowders can be very difficult to blend and keep from segregating. One reason is that nanoparticles tend to be attracted to each other because of their small mass, static electicity, and Van der Waals forces (natural interparticle forces of attraction and repulsion). These various attraction forces can cause nanoparticles to form agglomerates rather than blend uniformly with other particles. In this column, I’m going to explore some tactics to overcome the problem of segregation in nanopowder blends.
The first thing to know is that when you’re incorporating nanoparticles into a blend, your blender should have one (or more) high-speed, high-shear chopper. On a horizontal blender (such as a paddle mixer or rotary drum blender), a chopper is typically a rotating paddle or blade mounted on the blending trough’s sidewall. Depending on the blending trough’s length, the unit will probably have several choppers. On a fixed-shell blender (such as a Forberg or Nauta mixer) or rotating blender (such as a V- or double-cone blender), the chopper is a high-speed rotating beater bar with protruding pegs. Whatever its type, the chopper breaks up the agglomerates and fluidizes the material bed. It also keeps the nanoparticles moving so they’re less likely to attach to each other and form agglomerates.