Experimental and theoretical investigation of multiaxial elongational flow and processing of polymer melts

Several of the most important polymer processing operations involve the deformation of melt streams in the absence of walls. Such processes in which applied stresses act in the absence of walls result in elongation of polymer melts. This includes uniaxial elongation for melt spinning, multiaxial elongation for film casting, tubular blown film extrusion, thermoforming, blow molding and stretch blow molding. One of the most important problems associated with these operations is the influence of the rheological properties of the melts on the ability to form the desired products. To understand these processes we must be concerned with the behavior of polymer melts in elongational flow, especially multiaxial elongational flow.

Instabilities undergoing multiaxial elongational flow (including uniaxial extension) were considered in both isolated (batch) systems and continuous systems. Major attention was given to the differences in geometry, conservation equations (such as the continuity and force balance equation), and rheological properties.

The growth of disturbances in isolated sheets and annular cylinders of viscoelastic fluids, Newtonian fluids, and elastic solids undergoing multiaxial elongational flow was considered. Both stable and unstable flow were of concern in the film extrusion processes (including a melt spinning process). Unstable flows in the draw down and blowing processes were considered. An attempt was made vi to develop a general perspective of the response or processability undergoing continuous multiaxial elongation (including uniaxial extension) of polymer melts as functions of their rheological properties (such as steady shear flow behavior and uniaxial delongational flow behavior) and in terms of a convected Maxwell model. This was correlated with molecular structure such as molecular weight, molecular weight distribution, and degree of long chain branching.