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

The use of hyperbranched polymers (HBPs) as a processing aid for linear low density polyethylene (LLDPE) was investigated. Various generation (or pseu do-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 de gree of end group substitution was studied. Blends of up to 10% HBP content were mixed via extrusion at 170 degrees C to produce 1 mm diameter fibers. Processability, surface appearance and the rheological properties of the b lends were evaluated. It was found the power requirement for extrusion was significantly decreased as a result of reduced blend viscosity, and also, t he mass how rate for a given extruder speed was greater than virgin LLDPE f or all HBP blends. Melt fracture and sharkskin of the blends was successful ly eliminated, and minimal preprocessing time was required for the effect t o take place. Surface analysis using x-ray photoelectron spectroscopy and t ransmission electron microscope techniques were performed with both showing that the HBP had a preference to accumulate at the fiber surface. Rheologi cal experiments were similarly affected, therefore, the blend viscosity is really a composite of a HBP rich phase and a neat LLDPE phase. It is hypoth esized 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 an d paraffin wax were also studied. The surface appearance of HBPs/LLDPE blen ds was superior to those blends mixed with paraffin wax, as was the extrude r performance. The results suggest that HBPs, at trace levels ( approximate to 500 ppm), may offer a number of advantages when used as a processing ai d for LLDPE. (C) 1999 The Society of Rheology. [S0148-6055(99)02203-8].