(17a) Diffusion of Dense Gases in Thermoplastics and Elastomers

In numerous industrial processes compressed gases are in contact with polymers. In some cases, controlled mass transfer is intended to occur, e.g. through membranes in separation processes. In other cases this is undesired like in sealings or pipelines. As a consequence of the sorption of gases, material properties like the specific volume may change which is also due to thermal effects and mechanical compression. The relation of mass transfer and swelling behaviour during sorption of pure gases and gas mixtures in solid polymers is of great importance to engineering applications.

The swelling behaviour of solid polymers under compressed gas atmosphere was investigated by use of a high pressure view cell connected to a CCD camera and microscopic lens. In this way, it is possible to investigate sealing materials under operating conditions, also detecting minor degrees of swelling like in case of polyimide or polytetrafluorethylene. Special attention is paid to the installation situation of the sealing elements  during operation. The degree of swelling is related to the fixation of the polymer. A tight fixation influences the kinetic as well as the equilibrium solubility of the gas within the polymer. This is important for dimensioning of polymeric sealing elements in particular for new applications with supercritical carbon dioxide, e.g. in refrigeration engineering.

The sorption and desorption of gas is an unsteady mass tranport procedure. With the help of a magnetic coupled balance the mass change over time was determined. In a pressure and temperature range of up to 30 MPa and 120° C solubilities and sorption/desorption kinetics of commercial polymers have been measured. The fugacity turned out to be decisive for predicting the solubility of gas mixtures in polymers.

Diffusion coefficients have been determined numerically assuming Fickian diffusion for cylindrical geometries.

The desorption of gases from polymers is of special interest because of the superposition of concentration and pressure gradients. In order to separate both effects, a highly soluble gas (e.g. CO₂) was replaced at isobaric conditions after reaching equilibrium sorption by a gas showing low solubility within the polymer (e.g. N₂). In this way, there is only a driving force in concentration and no superposed pressure gradient. In the attached figure, the mass change of the desorption of carbon dioxide from fluororubber at ambient pressure is compared to the desorption in nitrogen atmophere at constant (sorption) pressure. The desorption curve when replacing CO₂ by nitrogen at constant pressure has an ideal (Fickian) shape. There is no explosive decompression effect visible and it has been shown by REM analysis that no damage of the fluororubber takes place.

Figure 1: Desorption of carbon dioxide (CO₂) from fluororubber at 50° C at ambient pressure and in nitrogen (N₂) atmosphere at the pressure of sorption. Sorption conditions: CO₂, 5 MPa, 50° C.