>> Tuesday, 16 September 2008
Once more, technical issue, from Massachusetts Institute of Technology web page click right on
Conclusion and complement for part 1/2, polymer viscoelasticity effects: Weissenberg and Barus.
Weissenberg effect during shear flow between two coaxial cylinders appear climbing up along the rotating rod for no-newtonian fluids, that is melted polymers. The same effect can be observe during stain an paint mixing and can not be observe for newtonian fluids, e.g. water.Weissenberg effect is a result in generating during shear flow extra stain, called normal stresses. Barus effect appears during extrusion and it is observed by swelling extruded material leaving nozzle. For melted polymers the effect is big and describe as relation between flow and nozzle diameter, it is beetwen 1,2-2,5. Depends on flow capacity, tool geometry, but the most on relation channel length/diameter and polymer properties, less observed for bimodal polyethylene about it in the next act. The same as Weissenberg effect, Barus effect it is due to a non-zero first normal stress difference.