(508c) Modelling of High-Pressure Ethylene Polymerization – From Reactor to Rheology

Developing a new polymer grade normally requires numerous experiments in pilot plants. Modeling studies can help to reduce the number of these expensive experiments and allow assessing the potential of different approaches.

For extrusion coating applications the processability of the polymers is of great importance. The processability is strongly coupled to the polymeric microstructure which in turn is governed by the reaction conditions during the polymerization. Not only pressure and temperature but also the reactor type (tubular or autoclave) have a strong influence on the polymer properties. Due to the differences in the polymer microstructure (e.g. molar mass distribution, branching density distribution, topology, etc.) for extrusion coating applications autoclave grades are predominantly used. However, the heat transfer and therefore as a direct consequence the conversion in autoclave reactors is inferior to tubular reactors. Thus it is desirable to use modeling techniques to assess potential routes to produce autoclave-like grades on tubular reactors.

 As outlined above such a model has to be able to

• solve the heat balance for tubular reactors with counter-current cooling
• provide detailed information on the polymer microstructure
• capture and visualize the main differences between autoclave and tubular grades which are relevant for the processability in extrusion coating applications

Within the software Predici^® [1] a model has been developed which meets these requirements. Based on a simplistic topological approach a characteristic function is derived which quantifies whether a polymer possesses a more comb-like or tree-like structure. It is found that tubular reactors produce more comb-like shaped polymers whereas autoclave reactors yield tree-like structures, which is documented in literature long time ago. However, the quantitative approach allows to assess whether modifications on the reaction conditions lead to autoclave-like grades from tubular reactors. The model clearly shows that by feeding a functionalized comonomer to the tubular reactor more tree-like polymers can be created.

A direct coupling to rheological quantities is highly desirable. Coupling the Predici simulation with Monte Carlo techniques yields a representative set of sample molecules for which the complete polymer architecture is known. This information is required to use the rheology models of the McLeish group [2] to predict shear viscosity functions. The computed shear viscosity functions of commercial tubular grades agree well with the experimental values.

[1]           M. Wulkow, Macromol. React. Eng. 2008, 2, 461 – 494.

[2]           D. J. Read, D. Auhl, C. Das, J. den Doelder, M. Kapnistos, I. Vittorias, T. C. B. McLeish, Science 2011, 333, 1871 – 1874.