Analysis of rheological properties and molecular weight distributions in continuous polymerization reactors

Abstract

This work explores the possibility of exploiting structure-property relationships to manufacture tailor-made polymers with target end-use properties. A novel framework which aims to improve upon current industrial practices in polymerization process and product quality control is proposed. The strong inter-relationship between the molecular architecture and rheological properties of polymers is the basis of this framework.

The melt index is one of the most commonly used industrial measures of a polymer's processibilty. However, this single-point non-Newtonian viscosity is inadequate to accurately reflect the polymer melt's flow behavior. This justifies monitoring the entire viscosity-shear rate behavior during the polymerization stage. In addition, the crucial role played by the polymer melt's elastic characteristics is not reflected in it's shear viscosity and so elasticity meaurements are also warranted. In this study, rheological models available in the open literature are utillized to demonstrate these critical issues at industrially relevant operating conditions. The observations made are also compared with published experimental results and found to be qualitatively similar.

Two case studies are presented. The first one is the free-radical solution polymerization of styrene with binary initiators in a cascade of two CSTRs. In the second case, the solution polymerization of ethylene in a single CSTR with a mixture of two single-site transition metal catalysts is considered. The feasibility of the proposed framework to tailor the product's MWD, irrespective of the underlying reactor configuration or kinetic mechanism, is demonstrated via steady state simulations. Relative gain analysis reveals the non-linearity and interactions in the control loops.

Although the main contributions of this study primarily deal with the viscoelastic behavior of linear homopolymers, potential extensions to systems involving polymers with small amounts of long chain branching or the control of other end-use properties are also discussed.