(255bn) Thermodynamics of Sorption in Polyolefins in 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 represent major part of the polymer production. The polymerization can be operated either by gaseous or liquid-dispersion processes, whereas the thermodynamics of liquid penetrant sorption is relevant for the liquid-dispersion polymerization processes. Here, the polymerization reactor contains solid catalyst particle 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 the sorbed monomer, which can polymerize here and is resulting 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 the 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 the monomer in the polyolefin particle.

In this work, 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, where the equilibrium sorption is extrapolated by the initial slope method. For the evaluation of swelling in liquid, modified video-microscopic method was used.

These measurements are not common in the literature, even though the diluents sorption and particle swelling in the liquid is more pronounced than in the gaseous diluents. Based on the new methodology of sorption measurements the solubility for a wide range of PE density and temperatures was measured. Also the swelling of polyethylene particles by liquid penetrant is presented in this work. The new experimental swelling data together with the solubilities are unique in the polyolefin research and could be usable as input data for the control and optimization of the polyolefin production process itself. In addition, the results are interpreted in the context of our recently proposed phase structure of PE [1]. 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] ChmelaÅ? J., Pokorný R., Schneider P., Smolná K., BÄ?lský 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.