The importance of extensional flows in processability of polymer melts

The shear flow properties of polymer melts are used to characterize them and to study their processing performance. However, there are polymer melts which exhibit indistinguishable shear flow properties, yet significantly different extensional flow properties which can affect their processing performance. This means that one can observe two polymers have the same shear properties but perform differently during certain types of process operations, indicate that shear properties may not be sufficient to characterize polymeric materials and therefore, a knowledge of uniaxial, biaxial and planar extensional viscosity is most crucial to understanding the processing behavior of polymer melts. In order to show this subject, it has been compared the extensional and shear behavior of three different type of polyethylene. For example, in the film blowing process of the three LDPE resins, two of the PE resins may perform identically while the third can not be drawn as much [3] as well as the melt flow indices and zero shear viscosity of the resins are very similar which is in contrast to their processing performance. The result of comparing the three types of polyethylene the shows that extensional viscosity measurements are sensitive to factors not readily detectable by means of shear flow measurements. This also means that it is necessary that constitutive equations be available which are able to handle polymer melts which have no apparent differences in shear flow, but exhibit different extensional behavior.