(169a) Modeling of n-Butane Solubility in Semi-Crystalline Polyethylene

In polymer processing the solubility of gases in melted and solid polymers strongly influence the quality of the product [1]. Physical foaming conducted by injection molding is a typical technology, where it is highly relevant to predict the solubility of CO₂ in the polymer melt as well as in the solid state of the polymer at various pressures and temperatures, to optimize the process regarding to product quality, energy efficiency and costs.

Hence, models with a high prediction performance of the gas solubility in melted and solid semi-crystalline polymers are required to enable an optimized and tailored product and process design.

Lattice theories, e.g. Sanchez-Lacombe EOS [2] are typically applied to predict solubilities of gases in polymer melts in a wide range of temperature and pressure. Modeling the gas solubility in semi-crystalline polymers for both states, the solid and the melted polymer state, is a challenging task. This is due to the semi-crystalline nature, where crystallites are embedded in the amorphous polymer phase and consequently act as physical constraints. This phenomenon directly influences the gas solubility behavior [3]. Hence, the semi-crystalline character of the polymer has to be taken into account by the thermodynamic model to enable high prediction quality regarding gas solubility.

In this work, cavitation stress theory combined with a mesoscopic continuum mechanics approach is coupled with a Lattice Theory, i.e., Sanchez-Lacombe Theory. Cavitation in the amorphous solid polymer phase is considered and linked to continuum mechanical eigenstress fields generated by the mesophase structure of the polymer. This allows to directly calculate gas solubility induced eigenstresses in the semicrystalline polymer and the simultaneous influence of cavitation and mesoscopic eigenstress on the gas solubility itself. Based on this model the impact of different degrees of crystallinity of the polymer on the gas solubility are studied and the behaviour of eigenstress evolution in the polymer phase is investigated. As an example, the solubility of n-butane in semi-crystalline low density polyethylene is predicted and compared with experimental data given in [4].

[1] J. Nagy, L. Reith, M. Fischlschweiger, G. Steinbichler, Chem. Eng. Sci. 111 (2014) 85-93.

[2] I.C. Sanchez, R.H. Lacombe, J. Chem. Phys. 80 (1976) 2352-2362.

[3] M. Minelli, M.G. Angelis, Fluid Phase Equilibria 367 (2014) 173-181.

[4] M. Wang, Y. Sato, T. Iketani, S. Takishima, H. Masuoka, T. Watanabe, Y. Fukasawa, Fluid Phase Equilibria 232 (2005) 1-8.