Measurement of Gas Diffusion in Closed-Cell Foams

Abstract

The aging process in closed-cell thermal insulation foams refers to that phenomenon which results in a degradation of insulation value of the foam over a period of time and is generally attributed to diffusion of atmospheric components into the foam and diffusion of expansion agent out of the foam. In this paper an apparatus is described for measuring the effective or apparent diffusion coefficient of atmospheric and other rapidly permeating gases in closed-cell polymeric foams. The technique is based on a solution of the transient diffusion equation, which is outlined along with the method of data reduction. The diffusion cell is designed to accommodate cylindrical foam specimens of varying sizes up to 7.6 cm (3 in.) in diameter and up to 5.1 cm (2 in.) thick. Other features of the apparatus include quick and convenient interchange of foam samples as well as automatic data acquisition. Major sources of experimental error are discussed; those aspects of the experimental technique designed to reduce the magnitude of the experimental uncertainty are also mentioned. Results obtained with the apparatus for a number of different foams are presented and discussed in terms of various foam diffusion models described in the literature. Interpretation of the data in this manner gives some insight into the cellular structure of the foam and also provides a correlation for estimating the diffusion coefficient of other gases in the foam sample.