Effective Diffusion Coefficients of CO2 and HCFC-22 in Polyurethane and Polyisocyanurate Foams

Abstract

Thin-slice accelerated desorption of gases using a gravimetric methodology has been used to determine effective diffusion coefficients of both carbon dioxide and HCFC-22 in thermoset foam structures. Foam samples consisting of a polyurethane appliance foam and a polyisocyanurate boardstock foam both blown with CFC-11, and polyurethane foams blown with HCFC-22 and carbondioxide have been employed as polymer substrates for the desorption measurements.

A modification of the sorption equations has been developed to describe the special case of two component simultaneous desorption from a uniformly heterogeneous continuum. One component must desorb significantly faster than the other component to separate the individual weight-loss slopes.

For those foam samples that were manufactured with CFC-11 and the diffusion of the blowing agents is still occurring, the initial slope must be determined. Core slices from each sample, 0.25 cm thick, were allowed to saturate with air. The sample weights were monitored daily for several weeks. The initial blowing agent loss slopes, weight against scaled age, were determined for each slice.

The slices were then saturated with a gas mixture, of 1 atmosphere air and 0.5 atmosphere of the secondary gas, either CO₂ or HCFC-22, for one month. After the gas pressure was vented to the ambient atmospheric pressure, the slice weight was monitored until the weight curve for a slice coincided with the continuation of the initial blowing agent slope for that slice. The difference (increase) in the slope of weight loss during the desorption of the recharged blowing agent was used to compute the effective diffusion coefficient for the sample. The average diffusion coefficient of CO₂ was found to be approximately one order of magnitude higher than that of HCFC-22 (i.e., 7.0 × 10^-8 compared to 6.6 × 10^-9 cm²/sec) through a foam co-blown with HCFC-22 and CO₂. The ratio of the effective diffusion coefficients of CO₂ and HCFC-22 was not, however, constant from one polymer substrate to another.

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