(53c) Modelling of Phase Behavior of Polyethylene Solutions

Christoph Walowski, Sabine Enders

Karlsruhe Institute of Technology, Institute of Technical Thermodynamics and Refrigeration Engineering, Engler-Bunte-Ring 21, D-76131 Karlsruhe, Germany.

Email: christoph.walowski@kit.edu; sabine.enders@kit.edu

Key words: Liquid-Liquid Equilibria, Impact of branching, Impact of asymmetric molecular weight distribution functions, Impact of Solvents

Abstract:

Polyolefins, i.e. polyethylene (PE), polypropylene (PP), poly-1-butene, ethylene/1-alkene copolymers (also known as linear-low density PE) and propene/alkene copolymers, are the worldâ??s most widely used synthetic polymers and their production still continues to grow exponentially. They are known to be cost-effective and good performing materials used in a broad range of commodity applications that influence our everyday lives. Folie and Radosz [1] explained in detail the significance of the involved phase behavior for the industrial production. These authors [1] emphasize the complexity of the phase behavior of this system, which is caused by a) the materials behaving highly non-ideal at high pressure, b) the polymer and the solvent greatly differing in size, and c) commercial polymers being composed of many molecules differing in molar mass and chemical composition. This contribution is closely related to an earlier work by us [2], in which we proposed an alternative method for phase equilibria calculations for the system polyethylene + ethylene.

The basic idea of the new method [2] is the combination of a cubic equations of state, namely the Sako-Wu-Prausnitz-equation of state, with the Lattice Cluster Theory [3], where the last is used for the development of the combining rules for the pure-component parameters. This method permits to consider the degree of branching by the use of the involved architecture coefficientâ??s given by the experimental degree of branching. Applying continuous thermodynamics [4] allows to take into account the molecular weight distribution function directly in the thermodynamics framework.

However, until now only the Schulz-Flory distribution function were incooperated in the model. Usually, synthetic polyethylene can have a very broad and asymmetric distribution function with respect to the molecular weight. One important feature of this contribution is the investigation of the impact of an asymmetric distribution function on the phase behavior. High asymmetric distribution functions can be coursed a shoulder in the cloud point curve and three phase equilibria. This situation is related to the occurrence of heterogeneous double plait points that separate the branch of stable critical points and the branch of unstable critical points.

The second feature of this contribution consists in the application of the developed framework to other systems. We focus our attention to the system polyethylene + hexane, because several experimental data are available in the literature [5,6]. These experimental data could be modelled with the Sanchez-Lacombe EOS, where the polydispersity was approximated by the use of pseudo-components [6]. Unfortunately, the binary interaction parameter depends on temperature and molecular weight. In this contribution, we will compare both methods, where we pay special attention to the required model parameter.

[1] B. Folie, M. Radosz, Ind. Eng. Chem. Res. 34 (1995) 1501-1516.

[2] C. Walowski, K. Langenbach, D. Browarzik, S. Enders, Fluid Phase Equilibria, submitted.

[3] K.F. Freed, J. Dudowicz, Adv. Polym. Sci. 183 (2005) 63-126.

[4] M.T. Rätzsch, H. Kehlen, Prog. Polym. Sci. 14 (1989) 1-46.

[5] M. Haruki, S. Mano, Y. Koga, S.I. Kihara, S. Takishima, Fluid Phase Equilibria 295 (2010) 137â??147.

[6] M. Haruki, K. Nakanishi, S. Mano, S.I. Kihara, S.Takishima, Fluid Phase Equilibria 305 (2011) 152â??160.