Dynamic slip and nonlinear viscoelasticity

Abstract

AbstractSeveral significant problems arise when film is fabricated on a large scale. One of these is the appearance of irregularities on the extrudate surfaces when the polymer melt is extruded at high rates. These irregularities vary in intensity and form and are generally known as sharkskin melt fracture. This phenomenon, which occurs when the wall shear stress exceeds a critical value, is a limiting factor for production rates in many industrial extrusion operations such as film blowing of polyethylene. We used a sliding plate rheometer incorporating a shear stress transducer to study slip in both steady and unsteady flows. By combining a dynamic slip model with a nonlinear viscoelastic constitutive model, we determined the slip model parameters for LLDPE film resin with and without a fluoropolymer sharkskin suppressant. The models give good prediction of our slip data in steady shear but show insufficient gap dependence in exponential shear. Our own film blowing studies demonstrated the efficiency of the sharkskin suppressant; it has more than doubled the throughput in our laboratory setup. The fluoropolymer additive was found to profoundly affect both the steady and dynamic slip parameters. Hence, the sharkskin suppressant alters how the LLDPE remembers its past slipping motions.