The permeability of thin films can be reduced by using a new polymer, by adding impermeable and aligned flakes, or by incorporating reactive groups. This paper discusses the latter two mechanisms, those of flakes and reactive groups. Flakes reduce both the steady permeability and increase the lag before permeation, typically by a factor of ten. The effect of flakes has in the past been correlated with theories which assume the flakes are aligned like long ribbons, so that diffusion is two dimensional. Alternatively, this effect can be estimated with theories developed here, which assume finite flakes and three-dimensional diffusion. The old and new theories bracket observed behavior. The permeability changes caused by reactive groups are very different. Reactive groups do not alter steady-state permeability, but increase the lag before penetration only if the groups are immobile; the increase will be greatest if the reaction is irreversible. When both aligned flakes and reactive groups are added to a thin film, the effects are cumulative, so that the lag can increase over a thousand times. Predictions of this increase are supported by experiment. Because all experiments here are on model systems, the extension to real coatings is also discussed.
Bibliographical noteFunding Information:
This work was primarily supported by the United States Air Force (AFSOR F49620-01-1-0333). Other support came from the Petroleum Research Fund (PRF 36528-AC9). G.D. Moggridge would like to thank the Department of Chemical Engineering, University of Minnesota, for a visiting Piercy professorship. Nancy K. Lape holds a National Science Foundation Graduate Fellowship.
- Barrier films