Sorption of Gases in Semicrystalline Polyethylene

Polyethylene is the most popular man-made polymer. One way of making polyethylene consists in producing the polymer in the form of grains suspended by a gas flow of olefins (monomers), nitrogen and other hydrocarbons (alkanes). Industrials want to know the solubility of the monomers in the polyethylene grains, as the rate of the polymerization reaction is a direct function of this solubility. The sorption of gas in polyethylene does not only occur at the surface of the grains, but also in the whole polyethylene sample. It is hence an absorption process (not an adsorption), and classical theories for adsorption such as Langmuir or B.E.T theory cannot be applied in this case. In this study, we have used the SAFT-VR (Statistical Associated Fluid Theory for Variable Ranges) equation of state to represent the thermodynamic properties of the fluid phases. In the SAFT approach, macroscopic properties are obtained from a microscopic description of the molecular interactions: molecules are represented by flexible chains of tangent spherical segments, and dispersion forces are represented as site-site interactions. At temperatures above the melting point of the polymer, a good representation of solubility data is obtained with the SAFT EOS. [Paricaud et al., Ind. Eng. Chem. Res., 43, 6871-6889 (2004]). Strong synergies are observed when the gas phase is composed of several hydrocarbons. In particular, it is observed that the presence of nitrogen in the gas mixture gives rise to a decrease of the solubility of the monomers, at fixed partial pressure. However, by replacing N2 by an alkane such as butane, the opposite effect is obtained and the solubility of the monomers is strongly increased. At temperatures around 80 degrees Celsius, below the melting point of polyethylene, (reactor conditions), polyethylene is at the semicrystalline state. Crystallinity has strong effects on solubility. On one hand, crystalline regions (crystallites), which exhibit a lamellar structure, act as barriers against the diffusion of gas molecules. As a result, it is assumed that gas molecules only absorb in the amorphous regions of the grains. As the amorphous regions exhibit a liquid-like microstructure, one can also assume a vapor-liquid equilibrium between the vapor phase and the amorphous regions. On the other hand, when the semicrystalline polymer absorbs hydrocarbons, the amorphous areas located between crystalline lamellas swell, but this swelling is limited by the elongation of tie chain molecules linking the lamellas. This effect is commonly called the plasticizing effect. We have developed a model based on Guth-Flory’s theory of chain elasticity, to take this effect into account. The plasticizing effect is represented by a perturbation term added to the SAFT equation of state, which is a function of the volume expansion of the polymer sample. Good predictions of solubility data are obtained for different gas/polyethylene systems.