Water transport and condensation in fluoropolymer films

This report explores phenomena related to the undesired condensation of wat er in or under fluoropolymer films. Condensation of water in or under polym er films will occur when the local vapor pressure is higher than that corre sponding to the local dew point vapor pressure. This can happen for any typ e of polymer film, including fluoropolymer films, even though solubility of water in the film may be extremely low. A "cold'' substrate, for example, can readily lead to water condensation under a fluoropolymer film, and ther efore to a reduction of adhesion or to complete delamination. These problem s will not be so severe and may, indeed, disappear for acid-resistant fluor opolymer films exposed to hot and/or concentrated acids, such as sulfuric a cid. The vapor pressure (activity) of the water in the acid is so reduced t hat condensation in or under the film will not occur at the usual temperatu res encountered in practice. Diffusion coefficients, permeation coefficients, and solubilities for water in selected fluoropolymers (PVDF, ETFE, ECTFE, PFA, MIFA, FEP, and PTFE) h ave been measured at 90 degreesC. The diffusion coefficients are greater th an 3 x 10(-7) cm(2)/s at this temperature. This means that a change in the environment will result in a new equilibrium condition for absorption, deso rption, and/or permeation in practical fluoropolymer coatings within a maxi mum of a day or so for all of the materials tested. Practical free film thi cknesses near 2 mm were used. Approximate diffusion coefficients have also been measured by a new techniq ue at 20 degreesC for comparison. When the environment changes, a new condi tion for equilibrium is established at room temperature after times varying from several days to 2 months in these experiments which used the same pra ctical film thicknesses. The technique involves following release of dissol ved tritiated water (HTO) from a film while it is immersed in a scintillati on liquid (gel) in a glass counting vial. The counting rate increases to an equilibrium level in proportion to the extent of diffusion of HTO out of t he sample. (C) 2001 Elsevier Science B.V. All rights reserved.