Mass transfer through elongated membranes
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Description
The effect of mechanical deformation on the barrier properties of rubbers and LLDPE geomembranes was analyzed from both an experimental and a theoretical perspective In the experimental approach, an apparatus for studying the permeation of organic solvents through mechanically deformed polymeric materials was custom built and tested. The apparatus consisted of a biaxial membrane stretching device and a modified ASTM F739 permeation cell. Capillary gas chromatography was used as the analytical technique to measure, in real time, the concentration of organic compounds at the downstream side of the membrane. The equipment was used to monitor in real time the permeation of mixtures of organic liquids through rubbers and linear low-density polyethylene membranes at several elongations From the theoretical perspective, a mathematical model that modifies Fick's law of diffusion including a stress related diffusion coefficient was developed and tested. The model uses a linear viscoelasticity approach to account for the effect of stress on the transport of the solvents through the deformed membrane. The model yields satisfactory agreement with the experimental data and provides an indication of whether the transport process is diffusion or stress controlled A decrease in breakthrough times with elongation was observed for the permeation of acetone, hexane and mixtures thereof through rubber membranes at several degrees of uni and biaxial elongation. The decrease in breakthrough times was related to a decrease in thickness of the membrane as it was elongated. An increase in the steady state permeation rate of acetone and hexane with elongation was also observed. This increase in permeation rates was associated with the solubility of acetone and hexane into the rubber membranes. In addition, stress enhanced transport as well as facilitated transport of acetone through butyl rubber membranes was observed A decrease in breakthrough times with elongation was observed for the permeation of methylene chloride, trichloroethylene and mixtures thereof through LLDPE geomembranes. The decrease in breakthrough times was related to a decrease in thickness of the membrane as it was elongated. An increase in permeation rates with elongation was also noted and related to the solubility of the solvents into the geomembranes