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Two pairs of polyethylenes (HDPE, LDPE) were studied in fourteen laboratories. The experiments concentrated on film blowing and on laboratory tests. Laboratory tests were performed on crystallization from the melt, shear viscosity (steady and time dependent), storage modulus, loss modulus, relaxation modulus, entrance pressure correction, melt flow index, extrudate swell, uniaxial extensional creep, recovery after uniaxial extension, and tensile test on extrudate. Th samples were chosen so that their film blowing behavior is significantly different, but the behavior in shear flow is similar. The goal of the study is to select laboratory tests which are as sensitive to material differences as the actual film blowing process is. A correlation of such sensitive laboratory tests with film blowing will be a basis for predicting the technological behavior of commercial polymers. -The two LDPE samples were polymerized in different batches. The two HDPE samples were prepared from the same powder lot and differ by the kind of processing aid (zinc or calcium stearate) which was added before granulating. -The crystallization behavior was found to be different within each pair or about the same, depending on the participating laboratories; no satisfactory agreement could be achieved. -The most sensitive rheological tests were found to involve extensional flow. Other sensitive rheological tests were in shear when dominated by long time constants (zero viscosity, stress relaxation). For both pairs, the sample with the lower extensional viscosity can be extended the most in the tensile test and can be blown into the thinnest film. -Crystallization and extensional rheology seem to be the two most important areas in laboratory testing, as used for distinguishing between polyethylene film blowing materials.

pure and applied chemistry

A collaborative study on the relation between film blowing performance and rheological properties of two low-density and two high-density polyethylene samples