THE RECIRCULATING SCREW MIXER - A NEW SMALL-VOLUME MIXER FOR THE POLYMER LABORATORY

The recirculating screw mixer (RSM), a new small-volume intensive mixe r for the polymer laboratory, is designed, built, modeled, and tested. This type of batch mixer is intended for the mixing of 1 to 30 cm 3 o f viscous material at high shear rates. A material element in the mixe r experiences alternating screw pump and tubular flows with reorientat ion between these flows. A mixer with a 10 cm3 sample capacity is buil t for testing and evaluation. Flow visualization experiments are used to investigate the quality of the distributive mixing achieved. The fl ows in the mixer are modeled for the cases of a Newtonian fluid and a power law fluid. The Newtonian model accurately predicts the recircula tion time for particles suspended in Newtonian silicone oils. The powe r law model accurately predicts the screw torque obtained with a polys tyrene and polyethylene. A method for the measurement of fluid rheolog y from the operating conditions of the RSM is proposed and tested. The mixing achieved by the RSM is compared to that obtained by a batch mi xer with roller blades. Both mixers are used to prepare blends of ethy lene-propylene rubber in polystyrene. The morphologies of the resultan t blends are compared and differences in the mixing action are discuss ed. The mixers are also used to prepare composites of fumed silica in polyethylene. The quality of mixing obtained in the RSM compares quite favorably with that obtained in the batch mixer with roller blades fo r polystyrene/ethylene-propylene rubber reactive blends and polyethyle ne/silica composites.