Stuart Jacob Kurtz

THE DYNAMICS OF SHARKSKIN MELT FRACTURE: EFFECT OF DIE GEOMETRY

We have proposed a general mechanism for the process of sharkskin melt fracture which not only is consistent with observed frequency and depth but also suggests that some forms of gross melt fracture can be viewed as an extension of the sharkskin process. This mechanism is supported by studies on LLDPE as a model for narrow molecular weight linear polymers showing the effect of shear stress, molecular weight, temperature and, in the current study, die diameter, on the severity of sharkskin melt fracture. Experimentally, the frequency of SSMF ranged from 10 Hz for high molecular weight, low shear stress conditions to over 400 Hz for low molecular weight, high shear stress conditions. These frequencies appear to be unaffected by die diameter but strongly dependent on shear stress in the die. The fracture depth however is a strong function of die diameter, being directly related to the stretching field outside the die. The results of our study imply that explanations based solely on either land slip or exit stretching are inadequate.