Experimental characterization of extrudate swell behavior of linear polybutadiene

Abstract

This paper describes an experimental study of the time-dependent extrudate swell behavior of a series of linear polybutadienes as a function of molecular weight (MW) and molecular weight distribution (MWD). The “initial” extrudate swell, i.e., the extrudate swell measured near the die exit, is universally found to be around 1.10, independent of MW, MWD and the extrusion rate. Like the initial extrudate swell, the extrudate surface velocity at the die exit, Vs, when normalized by the average capillary flow velocity, Vp, is also a relatively universal function of the distance X from the exit plane. It increases from 0.3 to 1 over a normalized distance from X/D=0 to X/D=0.035 regardless of MW, MWD, and the extrusion rate. In the presence of exit slip, which can be produced by coating the exit wall with a fluoropolymer, Vs is nearly equal to Vp even at x=0. The extrudate swell at a given distance from the exit is shown to be independent of the molecular weight for monodisperse samples. A small tail in the MWD at the high end produces considerably larger extrudate swell. Although the extrudate swell increases with increasing applied stress for a given sample, a series of polymer solutions of different concentrations show that the real controlling variable is the applied stress normalized by the material’s elastic plateau modulus. The extrudate swell is also found to depend systematically on the die diameter at all stages of its growth. Moreover, the extrudate swell is greater when it is allowed to grow in media (other than the air) such as oil and water. These two last effects can be understood in terms of the free-surface-to-volume ratio of the extrudate: the larger this ratio is more rapidly the residual molecular deformation in the extrudate can relax. A proper understanding of these interfacial effects will have important implications in processes such as injection/blow molding and film blowing from the viewpoints of both industrial practice and numerical simulations.