Estimation of the effective diffusivity of blowing agents in closed-cell low-density polyurethane foams based on thermal aging data

Abstract

Low-density closed-cell polyurethane (PU) foams are applied as thermal insulation materials due to their low thermal conductivity imparted by that of the physical blowing agent (PBA) used in foam production. However, foam conductivity tends to gradually increase with time, primarily due to changes in the gas composition in foam cells brought about by gas diffusion. To enable predicting the variation of conductivity during the service life of foam insulation, gas diffusivities are usually determined by measuring the gas composition in foams at different aging times. This study considers an alternative approach of estimating the effective diffusivities of gases in PU foams, which is based on the experimentally determined evolution of foam conductivity. The proposed approach is shown to provide close estimates of the effective diffusion coefficients for PBAs in PU foams when the specimen size and aging duration are such that a significant fraction, ca. 36%, of the PBA leaves the foams during the aging test.