(544dy) Thermodynamics of Sorption in Polyolefins in Gaseous and Liquid Media

Polyolefins, especially polyethylene (PE) and polypropylene (PP) are materials with very useful and consumer-attractive features, such as high chemical resistance, low weight, mechanical strength, flexibility, low cost, zero toxicity and ease of manufacture. Therefore, polyolefins represents major part of the polymer production. Polymerization can be operated either by gaseous or liquid-dispersion processes. Here, the polymerization reactor contains solid catalyst particles with growing polymer suspended in a gaseous or liquid diluent and gaseous monomer units, which pass through the polymer powder to the active site of the catalyst. The monomer concentration at the active catalytic sites is affected by the solubility of the monomer in polymer at the polymerization temperature and pressure, thus the monomer solubility and the associated monomer diffusion in the polymer powder influences the polymerization process. The polymerization process is also affected by swelling of the polymer particles. In general, the increasing volume of the polymer particle causes the increasing amount of sorbed monomer, which can polymerize here and results in an increasing polymerization rate. Also the increasing volume of the polymer particle causes the longer diffusion path of the monomer. The knowledge of sorption, swelling and transport properties in polyolefins is important not only for the polymerization rate, but also subsequent processes are affected by these phenomena, thus it is important to determine the amount of sorbed diluent and unreacted monomer in the polymer particle after the polymerization in the reactor. This amount is affected by the solubility of the diluent and monomer in the polyolefin particle.

In our work, the 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. Swelling isotherms were used in the evaluation of the gravimetric data. Furthermore, several interesting features were observed. The shape of the swelling isotherms follows the shape of the relevant sorption isotherms. Also the degree of swelling correlates well with the solubilities. The thermodynamic behaviour in liquid penetrant is studied using newly developed methods. The liquid-sorption measurement is based on recording the mass decrease during desorption. For the evaluation of swelling in liquid, modified video-microscopic method was used.

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. In addition, the results are interpreted in the context of our recently proposed phase structure of PE [1]. The systematic study of penetrant sorption of ethylene in PE [2] over a broad range of densities 0.90 to 0.96 g/cm3 is extended to heavier gaseous [3] and liquid penetrants. We demonstrate that the phase structure of PE proposed in [1] offers a fundamentally new and self-consistent approach to the thermodynamics of penetrant sorption in PE.

References:

[1] Chmelař J., Pokorný R., Schneider P., Smolná K., Bělský P., Kosek J.: Free and constrained amorphous phases in polyethylene: Interpretation of 1H NMR and SAXS data over a broad range of crystallinity. Polymer 2015, 58, 189-198.

[2] Chmelař J., Smolná K., Haškovcová K., Podivinská M., Maršálek J., Kosek J.: Equilibrium sorption of ethylene: Experimental study and PC-SAFT simulations. Polymer 2015, 59, 270-277.

[3] Chmelař J., Haškovcová K., Podivinská M., Kosek J.: Equilibrium Sorption of Propane and 1-Hexene in Polyethylene: Experiments and Perturbed-Chain Statistical Associating Fluid Theory Simulations. Industrial & Engineering Chemistry Research 2017, 56, 6820-682.