(255az) The Task of Modeling Long Chain Branching in the LDPE Synthesis

Although the high-pressure LDPE process is very energy consuming and highly demanding concerning construction material, it is still widely applied in the plastic industry. In 2014 more than 90 million tons of polyethylene have been produced worldwide with a significant amount being LDPE.

One of the main reasons why LDPE stays competitive on the market despite the extreme process conditions of temperatures up to 300 °C and pressures up to 3,000 bars are the special properties of the product. The comparatively low density of the product can be ascribed to the short-chain branches (SCBs) present in the polymer, which are induced by the free-radical polymerization process. SCBs are created during an internal backbiting step of a growing polymer chain.

At the same time long-chain branches (LCBs) are formed when a radical functionality of a growing chain is transferred to another macromolecule and monomer is added subsequently. These LCBs account for the extraordinary rheological properties of LDPE. Thus describing this reaction step correctly when modeling the LDPE process is crucial to determine the LCB density as a significant product property. However, expressing it consistently over a wide range of reactors, process conditions and miniplants as well as industrial world scale plants has proved to be challenging. M. Busch demonstrated that the rate coefficient for the transfer of the radical functionality to a dead polymer molecule depends on the conversion in laboratory experiments.[1] In contrast T. Herrmann showed that this observation cannot be transferred to industrial processes directly.[2] Further investigations in this area are highly desirable, especially when the structure-properties relationship of LDPE is to be used for an enhanced and reliable simulation-based product design.

The aim of this work is therefore to gain a deeper insight into the transfer to polymer reaction and the underlying microscopic concept as well as the macroscopic effects/consequences. This ambitious target requires a combination of different approaches and disciplines. High-pressure polymerizations of ethene have to be conducted under well-defined conditions. The resulting product has to be analyzed analytically preferably including multi-angle light scattering, ¹³C-NMR and rheometry in order to gain a deep insight into the LCB density of the polymer. Additionally, conducting matching simulations might prove useful to understand long-chain branching on a molecular level. While deterministic kinetic simulations yield average LCB densities, Monte Carlo simulations provide exact topologies of single macromolecules, which can be then used in rheological models.[3]

Combining these ideas/approaches several polymerization experiments will be examined with a special focus on the analytic evaluation of long-chain branching and the applicability of existing kinetic models.

References

[1] M. Busch, Habilitation, University of Göttingen 2003.

[2] T. Herrmann, PhD thesis, Technical University Darmstadt 2011.

[3] D. J. Read, Journal of Polymer Science, Part B: Polymer Physics, 2015, 53, 123-141.