Journal of Rheology

A novel processing aid for polymer extrusion: Rheology and processing of polyethylene and hyperbranched polymer blends

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The use of hyperbranched polymers (HBPs) as a processing aid for linear low density polyethylene (LLDPE) was investigated. Various generation (or pseudo-generation) HBPs were used which had either 16 carbon atom alkanes or a mixture of 20/22 carbon atom alkanes on the end groups. In addition, the degree of end group substitution was studied. Blends of up to 10% HBP content were mixed via extrusion at 170°C to produce 1 mm diameter fibers. Processability, surface appearance and the rheological properties of the blends were evaluated. It was found the power requirement for extrusion was significantly decreased as a result of reduced blend viscosity, and also, the mass flow rate for a given extruder speed was greater than virgin LLDPE for all HBP blends. Melt fracture and sharkskin of the blends was successfully eliminated, and minimal preprocessing time was required for the effect to take place. Surface analysis using x-ray photoelectron spectroscopy and transmission electron microscope techniques were performed with both showing that the HBP had a preference to accumulate at the fiber surface. Rheological experiments were similarly affected, therefore, the blend viscosity is really a composite of a HBP rich phase and a neat LLDPE phase. It is hypothesized that the HBP rich phase acted as a lubricating layer at the polymer/die wall interface. The HBP with a greater degree of end group substitution acted better as a processing/ rheological property aid. Blends of LLDPE and paraffin wax were also studied. The surface appearance of HBPs/LLDPE blends was superior to those blends mixed with paraffin wax, as was the extruder performance. The results suggest that HBPs, at trace levels (≈ 500 ppm), may offer a number of advantages when used as a processing aid for LLDPE. © 1999 The Society of Rheology.