EXPERIMENTAL CHARACTERIZATION OF SHARKSKIN IN POLYETHYLENES

The sharkskin defect appearing during the capillary extrusion of three low-density polyethylene resins with different molecular structures h as been characterized. Using complementary techniques, profilometry, o ptical microscopy, and observation of cross sections, the amplitude an d the period of the defects have been measured accurately. The influen ce of flow rate, temperature, and die geometry has been quantified. Th e specific behavior of the orifice die has been put in evidence. It sh ows that, if the role played by the stress field is evident, the wall shear stress is not the unique determinant of the sharkskin process. T he influences of molecular structure and elongational behavior on shar kskin are discussed. It appears that resins exhibiting long chain bran ching and strain hardening are less sensitive to sharkskin. (C) 1997 T he Society of Rheology.