Polymer Melt Rheological Property Measurement in a Novel Composite-Geometry Flow Rheometer

Abstract

A novel rheometer is described, in which extensional viscosity and elastic shear compliance are determined simultaneously from pressure measurements taken on steady state polymer melt flows through an open-ended die with successive, similarly shaped, converging and diverging zones. Equations incorporating practically realistic simplifying approximations are developed for the calculation of extensional viscosity and shear compliance from the measured pressure data. The construction of a prototype rheometer is described, and results from its operation are presented. Agreement between these results and published melt elasticity data for a low-density polyethylene material gives evidence to confirm the basic rheometric principle and flow analysis. The implications of pressure measurement errors are discussed. Assembly of the new composite-geometry rheometer in series with a conventional capillary viscometer would give in-line shear viscosity characterization simultaneously and facilitate on-line processing of pressure data from the converging and diverging flows by microcomputer to provide complete characterizations for practical use in polymer process machinery design. The work represents a significant advance on the basic empirical power law modelling process used for many years by practising engineers to represent shear viscosity phenomena as a basis for design. No other methodology is currently available to suit the engineer.