Extensional flow characterisation and extrusion blow moulding of high density polyethylene modified by calcium carbonate

Free surface elongational flow properties, including transient state stress growth and melt rupture data, have been measured on a range of calcium carbonate filled high density polyethylene (HDPE) compounds, using a Rutherford elongational rheometer operating at constant strain rate in the range 0·1–6·0 s−1. Results show that polymer molecular weight has a strong influence on both tensile stress growth and melt rupture data; increasing elongational strain rate decreases the stress growth coefficient, in all compounds. At any specific elongational strain rate, the stress growth coefficient increases with volume fraction of particulate additives and also when using additives of fine particle size, as a result of packing fraction effects. Acid coatings added at super-monolayer levels influence the viscosity of highly filled compounds, by internal lubrication. Dynamic extrusion data obtained by experimental blow moulding of calcium carbonate modified HDPE materials show that modifications to melt state shear modulus result in reduced die swell, to an extent determined by filler coating. In combination with die swell, higher extensional viscosity of filled HDPE contributes significantly to enhanced parison sag resistance. Solid state modulus enhancement was consistently observed in calcium carbonate filled HDPE containers and if excellent filler dispersion can be achieved, it is feasible to retain the ductile mode failure of unfilled HDPE when containers are subjected to high velocity impact loads.