Outgassing of Glass

Abstract

The gas evolved from glass at temperatures below the softening point, which is of interest in bake-out problems, is primarily water. The water evolved from unit surface at constant temperature above 300°C is linear with respect to the square root of the time of bake-out. The intercept of the linear plot, which can be altered by different surface treatments, is a measure of the easily removed water residing at the surface. The slope is a measure of the rate of evolution of water that has diffused to the surface from the interior. Values of the diffusion constant for water and concentration gradients after bake-out have been calculated for a soda-lime glass. The diffusion constant is an exponential function of the reciprocal of the absolute bake-out temperature. Values of the activation energy for the diffusion process are given for eight glasses. A method of calculating the amount of water that will diffuse from glass for any time-temperature conditions following any bake-out is presented.