The Effective Diffusivity of CO2 and HCFC-141 b in PU Foam Prepared with HCFC-141b as Insulating Blowing Agent at 20, 25 and 40 °C

The temperature dependent effective diffusivities of gas components diffusing from experimental insulating panels are required to correctly model the transient composition change of the diffusing components with temperature. The polyurethane reaction by-product CO₂ and insulating blowing agent HCFC-141b have a significantly different influence on the aging of polyurethane foam insulation because of the specific magnitudes of their effective diffusivity values. Foam panels prepared with HCFC-141b as insulating blowing agent were initially bank-sawed and then sliced using a precision sheer. The precision sliced foam specimens accumulated oxygen and nitrogen from the atmosphere while depleting the CO₂ and HCFC-141b from the preparation of the foam panels. The gravimetric method for measuring the effective diffusivity of gases and vapors in closed-cell plastic foams is well-developed and has been used to measure other rapidly and slowly diffusing blowing agents in polyurethane and other polymeric foam systems. To prepare the slices for measurement of the effective diffiisivity, a single slice of air-filled CO₂ and HCFC-141b depleted foam was placed into a saturation (pressure) vessel and sufficient material, gaseous CO₂ or liquid HCFC-141b was added to produce a 6 psig saturation pressure within the foam at room temperature. After 14 days, the saturation material is vented, the saturation vessel is opened and the slice moved to the pan of an electronic analytical balance. A computer data-acquisition and control system monitors and controls the atmospheric conditions inside the insulated environmental chamber. The computer records the weight of the foam slice at specific time intervals during the depletion of the single diffusing component. The slope of the dimensionless weight ratio with the scaled-age uses the short-term approximation of the one-dimensional diffusion equation to estimate the effective diffusivity. Using die effective diffusivity values determined at 10, 25 and 40°C, the zero intercept of the Arrhenius plot provides the frequency factor while the slope provides a measure of the molar activation energy.