Water vapor permeability of ultrathin polyurethan films

Measurements were made on the flux of water through unsupported polyurethan thin films ranging jn thickness from 0.7 to 275 μ (11 mils), using cup cells under the conditions of water in contact on the inside film surface and 45% or 35% relative humidity outside. The measured film impedance (reciprocal flux) 1/J could be represented by a two-parameter equation where m is the reciprocal of the bulk permeability, L the film thickness, and b the boundary layer impedance. The value of b depends on the relative humidity and is closely approximated by the evaporation rate of water measured under the test conditions. This eliminates the need to make measurements as a function of film thickness in order to correct for the boundary layer effect on the measured permeability of thin or highly water-permeable films.