Improving polymer blend dispersions in mini-mixers

The simple cup and rotor mini-mixer, designed to blend very small polymer batches (0.3 g, MiniMAX), was compared to larger lab scale mixers: an internal batch mixer (50 g, Haake): a conical, recirculating twin screw extruder (5 g, DACA); and a 16 mm co-rotating twin screw (300 g/hr, PRISM), All were compared at the maximum shear rate in the cup and rotor mixer, 110 s(-1). Particle sizes of poly(propylene) (PP) dispersed in poly(styrene) (80 wt% PS) were measured by dissolving the PS, filtering and using scanning electron microscopy. The 16 mm twin screw gave somewhat smaller particle sizes than the lab scale mixers (1.2 mum vs 1.7 and 1.9 mum), but dispersion in the cup and rotor mini-mixer was much poorer. Simply adding three steel balls to the cup as suggested by Marechal et al. (Polym Networks Blends, 1997) greatly improved the dispersion (1.8 mum). Modifying the rotor design to allow recirculation yielded similar improvement. The benefit of adding three balls was confirmed in blends of low viscosity poly(dimethyl siloxane) PDMS in PS. When anhydride terminal PDMS was blended with amino terminal PS, the particle sizes were much smaller (10 vs. 0.3 mum) and the differences between the three versions of the cup and rotor were much less pronounced.