Elongation rheology of polyolefins and its relation to processability

The rheology of a variety of polyolefin resins, and resin blends, has been examined under conditions of constant strain rate elongation. These include high, low, and linear low density polyethylene, as well as polypropylene, commercially modified polypropylene, and elastomer blends with polypropylene. Two distinctly different classes of behavior are apparent in the viscosity growth patterns during uniaxial extension. Some resins display unbounded stress growth even at low strain rates. Others quickly attain a time independent value of extensional viscosity even at increased strain rates. The former behavior is identified with favorable blow molding behavior, while the latter was characteristic of resins unsuitable for such processing. The class of elongation behavior depends not only on resin type, but may be influenced by molecular weight distribution or blend morphology.