The Acceleration of Foam Aging by Thin-Slicing: Some Interpretations and Limitations
AbstractThin-slice accelerated aging methodologies have not been ag gressively pursued because of concerns about the reliability and applicability of the results. The use of gases such as helium, nitrogen and carbon dioxide, which diffuse rapidly and are only sparingly soluble in the polymer, may be used to obtain a diffu sion measurement in the foam matrix. These diffusion measurements provide a basis for examining the reliability of the scaling technique for more soluble blowing agents. Results of numerical solutions of the diffusion equation suggest that there may be a thickness limit for thin-slicing, depending on cell size. The existence of this limit may be evaluated by examining the effects of slice thickness on the effective diffusion coefficients of the pure sparingly soluble gases in foam samples. In this investigation, diffusion will be monitored by two different measurements: weight change (gain or loss) and thermal conductivity increase during air uptake and blowing agent loss. The effective diffusion coefficients from both measurements will be compared to demonstrate a consistent diffusion mechanism. Both polyurethane (PU) and polyurethane modified polyisocyanurate (PUIR) foams will be used for this demonstration. The goal is to show that thin-slice methodology balances the need for precise measurements with the desire for a significant reduction in testing time. The desire for a significant reduction in testing times is particularly important as environmentally damaging blowing agents (chlorofluorocarbons) are being replaced by less environmentally damaging blowing agents (such as the hydrochlorofluoro carbons HCFC-141b, HCFC-22 and HCFC-123). The effective diffusion coeffi cients of slices as thin as 0.0013 m agree with those up to 0.025 m thick, based on ad justed thermal conductivities for PUIR foams blown with CFC-11.