Capillary Rheometry - Elongational Viscosity and Beyond !

Abstract

A browse through the origins of the polymer processing industry provides interesting reading, if only to remind us that extrusion, as one of the most prolific polymer processing techniques of today, was hijacked from the food industry over one hundred years ago. This fact also underlines the notion that many polymer processing techniques are relatively crude and remain arguably quite unchanged. In contrast, the complex nature of the polymers used was seldom in doubt but, and fortuitously for most, remained solely the domain of the academic. Albeit complex, polymeric materials have been quite forgiving, negating the need for in-depth rheological study. However, in recent years the introduction of computational techniques available for the simulation of polymer processing, has put a new complexion on the use of rheology in industry and posed an interesting dilemma. In order to predict the flow behaviour of viscoelastic materials, the mathematics has had to be revisited with a vengeance by the programmers. Consequently these packages require the input of an abundance of material parameters, most of which are well known, in name at least, but notoriously difficult to measure. No longer can we relax in the safety of knowing only the MFI value of our material. Having appreciated the need for further parameters we run into the next problem of how to generate them. Some of these techniques have been in existence for many years but, due to the lack of need, academic debate over which model to employ and not least, the marketing campaigns of instrument manufacturers, have remained the matter of rarely visited papers. This article revisits two such measurements which certainly embody the above description.