Measurement and Modeling of Water-Vapor Diffusion in Elastomers with Impact in Humidity and Vacuum Measurements

Abstract

The dynamics of water-vapor dissolution in Viton O-rings is measured with a gravimetric method using a precise mass comparator. A sample gasket was degassed in high vacuum for a sufficiently long period to remove more than 99 % of the dissolved water vapor. After that, it was exposed to the ambient atmosphere with a controlled temperature, and relative humidity and water-vapor uptake curves were measured gravimetrically with a precise balance. The dynamics of a water-vapor release into vacuum from another sample that was previously saturated with water vapor at room temperature was determined. The sample was placed in a vacuum outgassing rate measurement apparatus. The time dependence of the evolved water vapor was calculated by integrating the measured outgassing rate. The physical process of water absorption can be described by the diffusion equation. The geometry of the samples required solving the diffusion equation in cylindrical coordinates. This was done numerically using a finite-difference method. As a result of the modeling, room temperature values of the diffusion constant $D$, the solubility $s$, and the permeability $K = D\times s$ of water vapor in the sample material (Viton A-401C) were obtained. For sample 1, we obtained $D = 8.0 \times 10 ^{-8}$ cm$^{2}\,\,{\cdot }\,\,$s$^{-1}$ and $s = 6.5 \times 10^{-7}$ g$\,{\cdot }\,$cm$^{-3}\,{\cdot }\,$Pa$^{-1}$, while for sample 2, $D = 3.0 \times 10^{-7}$ cm$^{2}\,{\cdot }\,$s$^{-1}$ and $s = 3.5 \times 10^{-7}$ g$\,{\cdot }\,$cm$^{-3}\,{\cdot }\,$Pa$^{-1}$.