(668e) Light-Hydrocarbons Diffusion Coefficients in Semi-Crystalline Polyolefins: Automated Experiments with High Performance Differential Pressure-Decay

Transport of monomer(s) is one of essential topics in polymer reaction engineering. Understanding the diffusion of gaseous penetrants in complex semicrystalline polymer structure is still not complete despite of the benefits possibly leading to a better reactor control. The aim of this work is the investigation of diffusion in semicrystalline polyolefins by experimental and theoretical methods. Experiments were conducted in our “self-made” apparatus – differential pressure-decay.

Pressure decay is the method used to observe transport dynamics of gaseous species in solid material or their melts. The solid sample – the polymer in our case is places in the closed cell. For short time are connections to cell opened and the penetrating gas is injected there. When the connection into cell is closed again pressure evolution is observed and recorded. Pressure is decreasing due to sorption into a polymer sample. The equilibrium state is indicated by a stable pressure and the experimental procedure continues with fast cell degassing. The rising pressure evolution now represents the penetrant desorption process from polymer sample. These two main subprocesses are repeated periodically to obtain high number of experimental diffusion curves needed for the diffusion coefficients estimation procedure.

The apparatus is designed to hold up pressures occurring commonly in industry (up to 30 bar) and sufficient temperature range (30 °C – 110 °C). The overall system (e.g. solenoid valves, cells, connections, pressure transducers) are placed in the insulated box and are heated by hot air circulation to achieve uniform temperature.

Fast pressure changes during experimental procedure are associated with temperature fluctuations affecting the actual pressure in the investigated system. To reduce this phenomenon the pressure decay apparatus was recently rebuilt to add reference cell with high precious differential pressure transducer measuring pressure difference between cells. The reference cell is empty and the differential pressure evolution is not affected by temperature changes after the step change of pressure.

The assembled apparatus will serve for systematic and automated examination of diffusion coefficients in polymers. The control and data acquisition is realized by LabView script.

The objective of our research is to investigate the diffusion of light-hydrocarbon olefins such as ethylene, propylene or n-hexane in semi-crystalline polymers. Polyolefin morphology with stacked crystalline lamellae and the presence of two different amorphous regions is not commonly known and its scheme is shown in Figure 1 below. During the diffusion the penetrating molecules have to go through these amorphous regions having a different permeability. In this poster we will present diffusion coefficients of light-hydrocarbon olefins in various density polyethylenes (PE) and polypropylenes (PP). Density of polymer affects the physical properties and is directly connected to polymer crystallinity. We present estimated diffusion coefficients as a function of penetrant volume fraction in amorphous regions, as this allows a good comparison for various penetrants. Equilibrium sorption isotherms of penetrants in polyolefins are obtained by our magnetic-suspension gravimetric apparatus. The calculations must also include the crystallinity correction according to [1] at current temperature. In previous work [2] we have shown that for ethylene sorption isotherms the swelling phenomenon cannot be neglected and thus we must correct also geometric dimensions of evaluated polymer sample during diffusion coefficient computation.

Diffusion coefficient evaluation is accomplished with Fickian approach which is the core of the advanced evaluation program which is also reconstructing the beginning and end of the diffusion curve at the time where the solenoid valves are opened, and data cannot be collected. The only output parameter is then diffusion coefficient computed by Finite Volume Method.

The literature often proposes that the diffusion of light-hydrocarbons in polymers is concentration dependent. Fickian approach is not capable to describe this dependency by theoretically-supported model. The often employed approach is Free Volume Theory (FVT) [3] which supports the concentration dependent diffusion coefficient. The principle of this theory requires to estimate two fundamental constants of the polymer sample. By employing of FVT we shall be able to predict other penetrants diffusion coefficients in previously examined polymer samples.