Rheological properties of different film blowing polyethylene samples under shear and elongational flow

The rheological behavior of polyethylenes is mainly dominated by the molecular weight, the molecular weight distribution and by the type, the amount and the distribution of the chain branches. In this work a linear metallocene catalyzed polyethylene (m-PE), a branched metallocene catalyzed polyethylene (m-bPE), a conventional linear low density polyethylene (LLDPE) and a low density polyethylene (LDPE) have been investigated in order to compare their rheological behavior in shear and in elongational flow. The four samples have similar melt flow index and in particular a value typical of film blowing grade. The melt viscosity has been studied both in shear and in isothermal and non-isothermal elongational flow. The most important features of the results are that in shear flow the m-PE sample shows less pronounced non Newtonian behavior while in the elongational flow the behavior of m-PE is very similar to that of the linear low density polyethylene: the narrower molecular weight distribution and the better homogeneity of the branching distribution are reasonably responsible for this behavior. Of course the most pronounced nonlinear behavior is shown, as expected, by the LDPE sample and by the branched metallocene sample. This similar behavior has to be attributed to the presence of branching. Similar comments hold in non-isothermal elongational flow; the LDPE sample shows the highest values of the melt strength and the other two samples show very similar values. As for the breaking stretching ratio the opposite is true for LDPE while m-PE and LLDPE show higher values. The transient isothermal elongational viscosity curves show that the branched samples show a strain hardening effect, while LLDPE and m-PE samples present a linear behavior.