Polymer Blending in Single Screw Extrusion Overall Mechanism

7 Polymer Blending in Single Screw Extrusion Overall Mechanism 

The process is promoted by instability. Surface tension promotes change to minimum surface area the ultimate most stable configuration being a sphere (i.e., a droplet). This effect causes the ribbons to break up into nets and fibrils, and also causes the fibrils to break up into droplets overall a very efficient process. 

The critical question is Does this mechanism mean that single screw extruders are equally as good as twin screw extruders for polymer blending  

With no elongational shear there could be limits related to viscosity ratio, particularly as there can be difficulties in laminar shear mixing of viscous fluids with widely differing viscosities. However, results by Yu and co-workers [25] for trials using a viscosity ratio of 34 at 180 °C and 45 at 160 °C indicates that this may not be a limitation for many applications. 

These trials were carried out using 5 95 wt% PS LDPE which had viscosity ratios of 34 at 180 °C and 45 at 160 °C. The apparatus comprised two separately extruded polymer streams feeding a barrel flighted plain mandrel screw evaluator [25] fitted with a pineapple mixer. The overall arrangement was similar to that described in Section 8.6. The blends were also prepared with 0.5 wt% Kraton compatibiliser precompounded into the PS. To facilitate comparison of the results in their graphs, those for 30 and 90 rpm are presented in Table 14.2. 

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