Water Vapour Resistance of Hydrophobic Microporous Membranes under Reduced Pressures at a Constant Temperature

The water vapour permeability resistance of a commercial polytetrafluoroethylene (PTFE) membrane was measured under reduced pressures at constant temperature. The PTFE membrane is hydrophobic, wind-proof and water vapour permeable. The device used for this investigation consisted of two concentric cylinders; one was filled with distilled water, the other was kept at a low water vapour concentration which was 20% relative humidity at 293 K by means of a potassium acetate saturated solution. The apparatus was devised so that no air layer existed on either side of the test specimen. The water vapour permeability resistance of the microporous PTFE was almost unchanged at the reduced pressures examined. To confirm the present experimental method, an additional measurement was made with the use of a polyester (PET) net, which was selected because of its open structure. The water vapour permeability resistance of the PET net decreased with decreasing atmospheric pressure. The reasons for this difference are discussed in relation to the sizes of the micropores and the mean free path of the water vapour molecules.