Improvement of a gram-scale mixer for polymer blending

A small-scale mixer designed by Bryce Maxwell is commercialized by Custom Scientific Instruments, Inc. under the name of MINI-MAX Molder. It is very useful for the study of polymer blends, especially when the available amount of polymer specimen is limited, for example, less than a few grams. However, it gives essentially shear flow and suffers from poor distribution and dispersion capabilities in comparison with large scale extruder and internal mixers. We propose here an improvement of the mixing and dispersing capabilities of the Maxwell small-scale mixer by the addition of Teflon disk and steel balls together with the mixed materials. When polypropylene and high impact polystyrene were mixed at 70/30 wt. ratio at 180°C without the disk and balls, the high impact polystyrene particle size (D) was 6.27 μm. A finer dispersion (D = 1.44 μm) was achieved by the introduction of one Teflon disk at the center of mixing cup and three steel balls. Furthermore, with increasing the number of steel balls from three to seven, and with using different sizes of balls, much finer dispersions were achieved (D = 0.58 μm and 0.47 μm). This may be caused by: (1) the addition of Teflon disk eliminates the low shear regions in the mixing cavity, and (2) the addition of steel balls induces asymmetric circulation of the materials, some changes in the flow lines going from the center to the border or from the top to the bottom, some reorientation of the materials, and higher shear fields.