(241f) Thermodynamics of Sorption and Swelling in Polyethylene at Gas-Phase Polymerization Conditions

Thermodynamics of sorption and diffusion of monomer(s) and/or diluents in semi-crystalline polyolefins is one of the most important phenomena affecting catalytic polymerization processes, down-stream processing as well as application properties of these materials. In the atmosphere of suitable solvent the swelling also occurs. Swelling causes the prolongation of the diffusion path and thus it has to be investigated together with sorption and diffusion. Despite the importance, thermodynamically consistent and predictive tools for sorption equilibria are available only for a few specific cases and the attempts to predict the swelling of semi-crystalline polyolefins using equations of state have not been successful so far. Prediction of diffusion in polyolefins is another open problem. This situation is caused by several reasons: (i) scattered experimental data for the broad range of various PE grades, temperature, pressure and composition of penetrant mixtures, (ii) elastic constrains by tie chains connecting crystallites affecting the thermodynamics of amorphous phases, (iii) misunderstanding of the semi-crystalline morphology of lower crystallinity PE, (iv) neglecting the importance of relaxation dynamics in PE on the diffusion, and (v) temperature-dependence of crystallinity.

In our work, sorption and diffusion of low molecular weight penetrants is measured gravimetrically, where we measure the mass increase of polymer due to the gas sorption. The gravimetric data have to be corrected to the buoyancy force of the pendulous swollen polymer. Experimental measurement of swelling was carried out in video-microscopic apparatus. The method is based on the visual observation of a polymer particle by microscope with attached digital camera. The measured swelling isotherms were used in the evaluation of the gravimetric data.

This contribution presents results for sorption and swelling of ethylene, co-monomers (1-hexene, 1-butene) and inert diluents (propane) in various PE grades at conditions relevant to catalytic polymerization of olefins. This work stands out of similar studies in the open literature due to: (i) systematic measurement for a broad range of PE densities from 0.90 to 0.96 g/cm³ covering practically all relevant grades, (ii) reporting a comprehensive set of swelling data for several penetrants, (iii) systematic measurements for several penetrants at a range of industrially relevant temperatures and pressures, and (iv) modelling the sorption data by the advanced PC-SAFT equation of state.

The experimental data and observed trends presented in this work contribute to the complex understanding of the mass transport and sorption equilibria of penetrants in polymers.