Effect of High Hydrostatic Pressure on the Permeability of Elastomers to Water

Abstract

The permeability of butyl rubber to pressurized water was measured at 10,000 psi hydrostatic pressure using a relatively simple method which consisted of constructing pouches from rubber, filling these with desiccant and exposing them to pressurized water. At this high hydrostatic pressure the permeability was found to be smaller than at atmospheric pressure which suggests the suitability of butyl rubber for the construction of hydrophone boots used at great depths. Similar results found by other investigators for other polymeric materials are cited. A thermodynamic analysis of the process of permeation by activated diffusion was made and a mechanism was discussed which explains the decrease in permeability of butyl rubber to water with increased pressure.

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“…The permeation of most polymers by liquids takes place by a mechanism of activated diffusion (Lebovits, 1966a(Lebovits, ,b, 1968. This is a three-step process.…”

“…The first step involves absorption of the permeating molecule at its ingoing surface and dissolving of the molecule into the polymer. The rate of solution will be governed by a vapor pressure of the permeant on the high-pressure side of the polymer barrier (Lebovits, 1966a(Lebovits, ,b, 1968.…”

“…Factors affecting the rate of diffusion are concentration gradient with the polymer, activation energy for the process, and temperature. The concentration gradient is responsible for diffusion primarily in the direction of flow, because the probability of a given molecule moving will be proportional to the concentration at the site of the molecule (Lebovits, 1966a(Lebovits, ,b, 1968). The activation energy is that energy needed to separate the polymer chains and permit passage of the migrating molecule.…”

“…A detailed thermodynamic argument has been given by Lebovits (1968) to show that the vapor pressure, and not the hydrostatic pressure, determines concentration of the permeant within the polymer. Additional complications, due to such factors as permeants that swell the membrane, second-order transition points in the polymer, or two membranes in series, are treated by Lebovits (1966a).…”

“…The rate of permeation through the polymer barrier will be influenced by various aspects of the material. These include the chemical composition of the polymer, chemical similarities between the polymer and permeant, degree of cross-linking, crystallinity of the polymer, degree of plasticization by a foreign plasticizer (or the permeant itself), and sometimes by the previous history of the polymer (Lebovits, 1966a;Felder and Huvard, 1980). For a polymer to possess good barrier properties, the molecular structure must be such that it hinders the diffusion process, and the polymer should not possess chain structures or functional groups chemically similar to the penetrant molecule.…”