The effect of scaleup on the processing behavior of a blend exhibiting phase inversion during compounding

The scaleup behavior of blends exhibiting phase inversion during compoundin g in batch mixers was studied. Similar morphological changes were observed during compounding of polystyrene/polyethylene blends of different batch si zes ranging from 12g to 240g. The time to achieve a continuous phase of the major component, polystyrene, was shown to depend on the scale of the mixi ng device. Based on visual observation of the morphological changes, a cons tant nominal-maximum-shear-rate scaleup condition was used. Upon a five-fol d increase in batch size the time to phase inversion increased by a factor of 3. This change is explained using a combination of the reduced specific area and reduced mechanical energy input under the experimental conditions. A novel blade design using modular triangular elements was constructed and results from radial and axial scaleup using the new blades are presented. Similarities between the triangular and roller blades are used to highlight the importance of the high-shear region in determining the softening rate of the polystyrene pellets. The flexibility of the blade design was exploit ed to study the effect of blade configuration on the time to phase inversio n. A relative stagger parameter is introduced to explain the observed depen dence. Increasing the relative stagger decreased the stress transfer to the batch and increased the time to phase inversion. Implications of these res ults for mixing in the kneading sections of twin-screw extruders is discuss ed.